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Introduction

This book contains the Polkadot Fellowship Requests for Comments (RFCs) +detailing proposed changes to the technical implementation of the Polkadot network.

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(source)

Table of Contents

RFC-1: Agile Coretime +
- Summary
- Motivation +
  - Present System
  - Problems
  +
- Requirements
- Stakeholders
- Explanation +
  +
- Performance, Ergonomics and Compatibility
- Testing, Security and Privacy
- Future Directions and Related Material
- Drawbacks, Alternatives and Unknowns
- Prior Art and References
+

RFC-1: Agile Coretime

+ + + +


Start Date	30 June 2023
Description	Agile periodic-sale-based model for assigning Coretime on the Polkadot Ubiquitous Computer.
Authors	Gavin Wood

Summary

This proposes a periodic, sale-based method for assigning Polkadot Coretime, the analogue of "block space" within the Polkadot Network. The method takes into account the need for long-term capital expenditure planning for teams building on Polkadot, yet also provides a means to allow Polkadot to capture long-term value in the resource which it sells. It supports the possibility of building rich and dynamic secondary markets to optimize resource allocation and largely avoids the need for parameterization.

Motivation

Present System

The Polkadot Ubiquitous Computer, or just Polkadot UC, represents the public service provided by the Polkadot Network. It is a trust-free, WebAssembly-based, multicore, internet-native omnipresent virtual machine which is highly resilient to interference and corruption.

The present system of allocating the limited resources of the Polkadot Ubiquitous Computer is through a process known as parachain slot auctions. This is a parachain-centric paradigm whereby a single core is long-term allocated to a single parachain which itself implies a Substrate/Cumulus-based chain secured and connected via the Relay-chain. Slot auctions are on-chain candle auctions which proceed for several days and result in the core being assigned to the parachain for six months at a time up to 24 months in advance. Practically speaking, we only see two year periods being bid upon and leased.

Funds behind the bids made in the slot auctions are merely locked, they are not consumed or paid and become unlocked and returned to the bidder on expiry of the lease period. A means of sharing the deposit trustlessly known as a crowdloan is available allowing token holders to contribute to the overall deposit of a chain without any counterparty risk.

Problems

The present system is based on a model of one-core-per-parachain. This is a legacy interpretation of the Polkadot platform and is not a reflection of its present capabilities. By restricting ownership and usage to this model, more dynamic and resource-efficient means of utilizing the Polkadot Ubiquitous Computer are lost.

More specifically, it is impossible to lease out cores at anything less than six months, and apparently unrealistic to do so at anything less than two years. This removes the ability to dynamically manage the underlying resource, and generally experimentation, iteration and innovation suffer. It bakes into the platform an assumption of permanence for anything deployed into it and restricts the market's ability to find a more optimal allocation of the finite resource.

There is no ability to determine capital requirements for hosting a parachain beyond two years from the point of its initial deployment onto Polkadot. While it would be unreasonable to have perfect and indefinite cost predictions for any real-world platform, not having any clarity whatsoever beyond "market rates" two years hence can be a very off-putting prospect for teams to buy into.

However, quite possibly the most substantial problem is both a perceived and often real high barrier to entry of the Polkadot ecosystem. By forcing innovators to either raise seven-figure sums through investors or appeal to the wider token-holding community, Polkadot makes it difficult for a small band of innovators to deploy their technology into Polkadot. While not being actually permissioned, it is also far from the barrierless, permissionless ideal which an innovation platform such as Polkadot should be striving for.

Requirements

The solution SHOULD provide an acceptable value-capture mechanism for the Polkadot network.
The solution SHOULD allow parachains and other projects deployed on to the Polkadot UC to make long-term capital expenditure predictions for the cost of ongoing deployment.
The solution SHOULD minimize the barriers to entry in the ecosystem.
The solution SHOULD work well when the Polkadot UC has up to 1,000 cores.
The solution SHOULD work when the number of cores which the Polkadot UC can support changes over time.
The solution SHOULD facilitate the optimal allocation of work to cores of the Polkadot UC, including by facilitating the trade of regular core assignment at various intervals and for various spans.
The solution SHOULD avoid creating additional dependencies on functionality which the Relay-chain need not strictly provide for the delivery of the Polkadot UC.

Furthermore, the design SHOULD be implementable and deployable in a timely fashion; three months from the acceptance of this RFC should not be unreasonable.

Stakeholders

Primary stakeholder sets are:

Protocol researchers and developers, largely represented by the Polkadot Fellowship and Parity Technologies' Engineering division.
Polkadot Parachain teams both present and future, and their users.
Polkadot DOT token holders.

Socialization:

The essensials of this proposal were presented at Polkadot Decoded 2023 Copenhagen on the Main Stage. A small amount of socialization at the Parachain Summit preceeded it and some substantial discussion followed it. Parity Ecosystem team is currently soliciting views from ecosystem teams who would be key stakeholders.

Explanation

Overview

Upon implementation of this proposal, the parachain-centric slot auctions and associated crowdloans cease. Instead, Coretime on the Polkadot UC is sold by the Polkadot System in two separate formats: Bulk Coretime and Instantaneous Coretime.

When a Polkadot Core is utilized, we say it is dedicated to a Task rather than a "parachain". The Task to which a Core is dedicated may change at every Relay-chain block and while one predominant type of Task is to secure a Cumulus-based blockchain (i.e. a parachain), other types of Tasks are envisioned.

Bulk Coretime is sold periodically on a specialised system chain known as the Coretime-chain and allocated in advance of its usage, whereas Instantaneous Coretime is sold on the Relay-chain immediately prior to usage on a block-by-block basis.

This proposal does not fix what should be done with revenue from sales of Coretime and leaves it for a further RFC process.

Owners of Bulk Coretime are tracked on the Coretime-chain and the ownership status and properties of the owned Coretime are exposed over XCM as a non-fungible asset.

At the request of the owner, the Coretime-chain allows a single Bulk Coretime asset, known as a Region, to be used in various ways including transferal to another owner, allocated to a particular task (e.g. a parachain) or placed in the Instantaneous Coretime Pool. Regions can also be split out, either into non-overlapping sub-spans or exactly-overlapping spans with less regularity.

The Coretime-Chain periodically instructs the Relay-chain to assign its cores to alternative tasks as and when Core allocations change due to new Regions coming into effect.

Renewal and Migration

There is a renewal system which allows a Bulk Coretime assignment of a single core to be renewed unchanged with a known price increase from month to month. Renewals are processed in a period prior to regular purchases, effectively giving them precedence over a fixed number of cores available.

Renewals are only enabled when a core's assignment does not include an Instantaneous Coretime allocation and has not been split into shorter segments.

Thus, renewals are designed to ensure only that committed parachains get some guarantees about price for predicting future costs. This price-capped renewal system only allows cores to be reused for their same tasks from month to month. In any other context, Bulk Coretime would need to be purchased regularly.

As a migration mechanism, pre-existing leases (from the legacy lease/slots/crowdloan framework) are initialized into the Coretime-chain and cores assigned to them prior to Bulk Coretime sales. In the sale where the lease expires, the system offers a renewal, as above, to allow a priority sale of Bulk Coretime and ensure that the Parachain suffers no downtime when transitioning from the legacy framework.

Instantaneous Coretime

Processing of Instantaneous Coretime happens in part on the Polkadot Relay-chain. Credit is purchased on the Coretime-chain for regular DOT tokens, and this results in a DOT-denominated Instantaneous Coretime Credit account on the Relay-chain being credited for the same amount.

Though the Instantaneous Coretime Credit account records a balance for an account identifier (very likely controlled by a collator), it is non-transferable and non-refundable. It can only be consumed in order to purchase some Instantaneous Coretime with immediate availability.

The Relay-chain reports this usage back to the Coretime-chain in order to allow it to reward the providers of the underlying Coretime, either the Polkadot System or owners of Bulk Coretime who contributed to the Instantaneous Coretime Pool.

Specifically the Relay-chain is expected to be responsible for:

holding non-transferable, non-refundable DOT-denominated Instantaneous Coretime Credit balance information.
setting and adjusting the price of Instantaneous Coretime based on usage.
allowing collators to consume their Instantaneous Coretime Credit at the current pricing in exchange for the ability to schedule one PoV for near-immediate usage.
ensuring the Coretime-Chain has timely accounting information on Instantaneous Coretime Sales revenue.

Coretime-chain

The Coretime-chain is a new system parachain. It has the responsibility of providing the Relay-chain via UMP with information of:

The number of cores which should be made available.
Which tasks should be running on which cores and in what ratios.
Accounting information for Instantaneous Coretime Credit.

It also expects information from the Relay-chain via DMP:

The number of cores available to be scheduled.
Account information on Instantaneous Coretime Sales.

The specific interface is properly described in RFC-5.

Detail

Parameters

This proposal includes a number of parameters which need not necessarily be fixed. Their usage is explained below, but their values are suggested or specified in the later section Parameter Values.

Reservations and Leases

The Coretime-chain includes some governance-set reservations of Coretime; these cover every System-chain. Additionally, governance is expected to initialize details of the pre-existing leased chains.

Regions

A Region is an assignable period of Coretime with a known regularity.

All Regions are associated with a unique Core Index, to identify which core the assignment of which ownership of the Region controls.

All Regions are also associated with a Core Mask, an 80-bit bitmap, to denote the regularity at which it may be scheduled on the core. If all bits are set in the Core Mask value, it is said to be Complete. 80 is selected since this results in the size of the datatype used to identify any Region of Polkadot Coretime to be a very convenient 128-bit. Additionally, if TIMESLICE (the number of Relay-chain blocks in a Timeslice) is 80, then a single bit in the Core Mask bitmap represents exactly one Core for one Relay-chain block in one Timeslice.

All Regions have a span. Region spans are quantized into periods of TIMESLICE blocks; BULK_PERIOD divides into TIMESLICE a whole number of times.

The Timeslice type is a u32 which can be multiplied by TIMESLICE to give a BlockNumber value representing the same quantity in terms of Relay-chain blocks.

Regions can be tasked to a TaskId (aka ParaId) or pooled into the Instantaneous Coretime Pool. This process can be Provisional or Final. If done only provisionally or not at all then they are fresh and have an Owner which is able to manipulate them further including reassignment. Once Final, then all ownership information is discarded and they cannot be manipulated further. Renewal is not possible when only provisionally tasked/pooled.

Bulk Sales

A sale of Bulk Coretime occurs on the Coretime-chain every BULK_PERIOD blocks.

In every sale, a BULK_LIMIT of individual Regions are offered for sale.

Each Region offered for sale has a different Core Index, ensuring that they each represent an independently allocatable resource on the Polkadot UC.

The Regions offered for sale have the same span: they last exactly BULK_PERIOD blocks, and begin immediately following the span of the previous Sale's Regions. The Regions offered for sale also have the complete, non-interlaced, Core Mask.

The Sale Period ends immediately as soon as span of the Coretime Regions that are being sold begins. At this point, the next Sale Price is set according to the previous Sale Price together with the number of Regions sold compared to the desired and maximum amount of Regions to be sold. See Price Setting for additional detail on this point.

Following the end of the previous Sale Period, there is an Interlude Period lasting INTERLUDE_PERIOD of blocks. After this period is elapsed, regular purchasing begins with the Purchasing Period.

This is designed to give at least two weeks worth of time for the purchased regions to be partitioned, interlaced, traded and allocated.

The Interlude

The Interlude period is a period prior to Regular Purchasing where renewals are allowed to happen. This has the effect of ensuring existing long-term tasks/parachains have a chance to secure their Bulk Coretime for a well-known price prior to general sales.

Regular Purchasing

Any account may purchase Regions of Bulk Coretime if they have the appropriate funds in place during the Purchasing Period, which is from INTERLUDE_PERIOD blocks after the end of the previous sale until the beginning of the Region of the Bulk Coretime which is for sale as long as there are Regions of Bulk Coretime left for sale (i.e. no more than BULK_LIMIT have already been sold in the Bulk Coretime Sale). The Purchasing Period is thus roughly BULK_PERIOD - INTERLUDE_PERIOD blocks in length.

The Sale Price varies during an initial portion of the Purchasing Period called the Leadin Period and then stays stable for the remainder. This initial portion is LEADIN_PERIOD blocks in duration. During the Leadin Period the price decreases towards the Sale Price, which it lands at by the end of the Leadin Period. The actual curve by which the price starts and descends to the Sale Price is outside the scope of this RFC, though a basic suggestion is provided in the Price Setting Notes, below.

Renewals

At any time when there are remaining Regions of Bulk Coretime to be sold, including during the Interlude Period, then certain Bulk Coretime assignmnents may be Renewed. This is similar to a purchase in that funds must be paid and it consumes one of the Regions of Bulk Coretime which would otherwise be placed for purchase. However there are two key differences.

Firstly, the price paid is the minimum of RENEWAL_PRICE_CAP more than what the purchase/renewal price was in the previous renewal and the current (or initial, if yet to begin) regular Sale Price.

Secondly, the purchased Region comes preassigned with exactly the same workload as before. It cannot be traded, repartitioned, interlaced or exchanged. As such unlike regular purchasing the Region never has an owner.

Renewal is only possible for either cores which have been assigned as a result of a previous renewal, which are migrating from legacy slot leases, or which fill their Bulk Coretime with an unsegmented, fully and finally assigned workload which does not include placement in the Instantaneous Coretime Pool. The renewed workload will be the same as this initial workload.

Manipulation

Regions may be manipulated in various ways by its owner:

Transferred in ownership.
Partitioned into quantized, non-overlapping segments of Bulk Coretime with the same ownership.
Interlaced into multiple Regions over the same period whose eventual assignments take turns to be scheduled.
Assigned to a single, specific task (identified by TaskId aka ParaId). This may be either provisional or final.
Pooled into the Instantaneous Coretime Pool, in return for a pro-rata amount of the revenue from the Instantaneous Coretime Sales over its period.

Enactment

Specific functions of the Coretime-chain

Several functions of the Coretime-chain SHALL be exposed through dispatchables and/or a nonfungible trait implementation integrated into XCM:

1. `transfer`

Regions may have their ownership transferred.

A transfer(region: RegionId, new_owner: AccountId) dispatchable shall have the effect of altering the current owner of the Region identified by region from the signed origin to new_owner.

An implementation of the nonfungible trait SHOULD include equivalent functionality. RegionId SHOULD be used for the AssetInstance value.

2. `partition`

Regions may be split apart into two non-overlapping interior Regions of the same Core Mask which together concatenate to the original Region.

A partition(region: RegionId, pivot: Timeslice) dispatchable SHALL have the effect of removing the Region identified by region and adding two new Regions of the same owner and Core Mask. One new Region will begin at the same point of the old Region but end at pivot timeslices into the Region, whereas the other will begin at this point and end at the end point of the original Region.

Also:

owner field of region must the equal to the Signed origin.
pivot must equal neither the begin nor end fields of the region.

3. `interlace`

Regions may be decomposed into two Regions of the same span whose eventual assignments take turns on the core by virtue of having complementary Core Masks.

An interlace(region: RegionId, mask: CoreMask) dispatchable shall have the effect of removing the Region identified by region and creating two new Regions. The new Regions will each have the same span and owner of the original Region, but one Region will have a Core Mask equal to mask and the other will have Core Mask equal to the XOR of mask and the Core Mask of the original Region.

Also:

owner field of region must the equal to the Signed origin.
mask must have some bits set AND must not equal the Core Mask of the old Region AND must only have bits set which are also set in the old Region's' Core Mask.

4. `assign`

Regions may be assigned to a core.

A assign(region: RegionId, target: TaskId, finality: Finality) dispatchable shall have the effect of placing an item in the workplan corresponding to the region's properties and assigned to the target task.

If the region's end has already passed (taking into account any advance notice requirements) then this operation is a no-op. If the region's begining has already passed, then it is effectively altered to become the next schedulable timeslice.

finality may have the value of either Final or Provisional. If Final, then the operation is free, the region record is removed entirely from storage and renewal may be possible: if the Region's span is the entire BULK_PERIOD, then the Coretime-chain records in storage that the allocation happened during this period in order to facilitate the possibility for a renewal. (Renewal only becomes possible when the full Core Mask of a core is finally assigned for the full BULK_PERIOD.)

Also:

owner field of region must the equal to the Signed origin.

5. `pool`

Regions may be consumed in exchange for a pro rata portion of the Instantaneous Coretime Sales Revenue from its period and regularity.

A pool(region: RegionId, beneficiary: AccountId, finality: Finality) dispatchable shall have the effect of placing an item in the workplan corresponding to the region's properties and assigned to the Instantaneous Coretime Pool. The details of the region will be recorded in order to allow for a pro rata share of the Instantaneous Coretime Sales Revenue at the time of the Region relative to any other providers in the Pool.

finality may have the value of either Final or Provisional. If Final, then the operation is free and the region record is removed entirely from storage.

Also:

owner field of region must the equal to the Signed origin.

6. Purchases

A dispatchable purchase(price_limit: Balance) shall be provided. Any account may call purchase to purchase Bulk Coretime at the maximum price of price_limit.

This may be called successfully only:

during the regular Purchasing Period;
when the caller is a Signed origin and their account balance is reducible by the current sale price;
when the current sale price is no greater than price_limit; and
when the number of cores already sold is less than BULK_LIMIT.

If successful, the caller's account balance is reduced by the current sale price and a new Region item for the following Bulk Coretime span is issued with the owner equal to the caller's account.

7. Renewals

A dispatchable renew(core: CoreIndex) shall be provided. Any account may call renew to purchase Bulk Coretime and renew an active allocation for the given core.

This may be called during the Interlude Period as well as the regular Purchasing Period and has the same effect as purchase followed by assign, except that:

The price of the sale is the Renewal Price (see next).
The Region is allocated exactly the given core is currently allocated for the present Region.

Renewal is only valid where a Region's span is assigned to Tasks (not placed in the Instantaneous Coretime Pool) for the entire unsplit BULK_PERIOD over all of the Core Mask and with Finality. There are thus three possibilities of a renewal being allowed:

Purchased unsplit Coretime with final assignment to tasks over the full Core Mask.
Renewed Coretime.
A legacy lease which is ending.

Renewal Price

The Renewal Price is the minimum of the current regular Sale Price (or the initial Sale Price if in the Interlude Period) and:

If the workload being renewed came to be through the Purchase and Assignment of Bulk Coretime, then the price paid during that Purchase operation.
If the workload being renewed was previously renewed, then the price paid during this previous Renewal operation plus RENEWAL_PRICE_CAP.
If the workload being renewed is a migation from a legacy slot auction lease, then the nominal price for a Regular Purchase (outside of the Lead-in Period) of the Sale during which the legacy lease expires.

8. Instantaneous Coretime Credits

A dispatchable purchase_credit(amount: Balance, beneficiary: RelayChainAccountId) shall be provided. Any account with at least amount spendable funds may call this. This increases the Instantaneous Coretime Credit balance on the Relay-chain of the beneficiary by the given amount.

This Credit is consumable on the Relay-chain as part of the Task scheduling system and its specifics are out of the scope of this proposal. When consumed, revenue is recorded and provided to the Coretime-chain for proper distribution. The API for doing this is specified in RFC-5.

Notes on the Instantaneous Coretime Market

For an efficient market to form around the provision of Bulk-purchased Cores into the pool of cores available for Instantaneous Coretime purchase, it is crucial to ensure that price changes for the purchase of Instantaneous Coretime are reflected well in the revenues of private Coretime providers during the same period.

In order to ensure this, then it is crucial that Instantaneous Coretime, once purchased, cannot be held indefinitely prior to eventual use since, if this were the case, a nefarious collator could purchase Coretime when cheap and utilize it some time later when expensive and deprive private Coretime providers of their revenue.

It must therefore be assumed that Instantaneous Coretime, once purchased, has a definite and short "shelf-life", after which it becomes unusable. This incentivizes collators to avoid purchasing Coretime unless they expect to utilize it imminently and thus helps create an efficient market-feedback mechanism whereby a higher price will actually result in material revenues for private Coretime providers who contribute to the pool of Cores available to service Instantaneous Coretime purchases.

Notes on Economics

The specific pricing mechanisms are out of scope for the present proposal. Proposals on economics should be properly described and discussed in another RFC. However, for the sake of completeness, I provide some basic illustration of how price setting could potentially work.

Bulk Price Progression

The present proposal assumes the existence of a price-setting mechanism which takes into account several parameters:

OLD_PRICE: The price of the previous sale.
BULK_TARGET: the target number of cores to be purchased as Bulk Coretime Regions or renewed during the previous sale.
BULK_LIMIT: the maximum number of cores which could have been purchased/renewed during the previous sale.
CORES_SOLD: the actual number of cores purchased/renewed in the previous sale.
SELLOUT_PRICE: the price at which the most recent Bulk Coretime was purchased (not renewed) prior to selling more cores than BULK_TARGET (or immediately after, if none were purchased before). This may not have a value if no Bulk Coretime was purchased.

In general we would expect the price to increase the closer CORES_SOLD gets to BULK_LIMIT and to decrease the closer it gets to zero. If it is exactly equal to BULK_TARGET, then we would expect the price to remain the same.

In the edge case that no cores were purchased yet more cores were sold (through renewals) than the target, then we would also avoid altering the price.

A simple example of this would be the formula:

IF SELLOUT_PRICE == NULL AND CORES_SOLD > BULK_TARGET THEN
+    RETURN OLD_PRICE
+END IF
+EFFECTIVE_PRICE := IF CORES_SOLD > BULK_TARGET THEN
+    SELLOUT_PRICE
+ELSE
+    OLD_PRICE
+END IF
+NEW_PRICE := IF CORES_SOLD < BULK_TARGET THEN
+    EFFECTIVE_PRICE * MAX(CORES_SOLD, 1) / BULK_TARGET
+ELSE
+    EFFECTIVE_PRICE + EFFECTIVE_PRICE *
+        (CORES_SOLD - BULK_TARGET) / (BULK_LIMIT - BULK_TARGET)
+END IF
+

This exists only as a trivial example to demonstrate a basic solution exists, and should not be intended as a concrete proposal.

Intra-Leadin Price-decrease

During the Leadin Period of a sale, the effective price starts higher than the Sale Price and falls to end at the Sale Price at the end of the Leadin Period. The price can thus be defined as a simple factor above one on which the Sale Price is multiplied. A function which returns this factor would accept a factor between zero and one specifying the portion of the Leadin Period which has passed.

Thus we assume SALE_PRICE, then we can define PRICE as:

PRICE := SALE_PRICE * FACTOR((NOW - LEADIN_BEGIN) / LEADIN_PERIOD)
+

We can define a very simple progression where the price decreases monotonically from double the Sale Price at the beginning of the Leadin Period.

FACTOR(T) := 2 - T
+

Parameter Values

Parameters are either suggested or specified. If suggested, it is non-binding and the proposal should not be judged on the value since other RFCs and/or the governance mechanism of Polkadot is expected to specify/maintain it. If specified, then the proposal should be judged on the merit of the value as-is.

+ + + + + + + +

Name	Value
`BULK_PERIOD`	`28 * DAYS`	specified
`INTERLUDE_PERIOD`	`7 * DAYS`	specified
`LEADIN_PERIOD`	`7 * DAYS`	specified
`TIMESLICE`	`8 * MINUTES`	specified
`BULK_TARGET`	`30`	suggested
`BULK_LIMIT`	`45`	suggested
`RENEWAL_PRICE_CAP`	`Perbill::from_percent(2)`	suggested

Instantaneous Price Progression

This proposal assumes the existence of a Relay-chain-based price-setting mechanism for the Instantaneous Coretime Market which alters from block to block, taking into account several parameters: the last price, the size of the Instantaneous Coretime Pool (in terms of cores per Relay-chain block) and the amount of Instantaneous Coretime waiting for processing (in terms of Core-blocks queued).

The ideal situation is to have the size of the Instantaneous Coretime Pool be equal to some factor of the Instantaneous Coretime waiting. This allows all Instantaneous Coretime sales to be processed with some limited latency while giving limited flexibility over ordering to the Relay-chain apparatus which is needed for efficient operation.

If we set a factor of three, and thus aim to retain a queue of Instantaneous Coretime Sales which can be processed within three Relay-chain blocks, then we would increase the price if the queue goes above three times the amount of cores available, and decrease if it goes under.

Let us assume the values OLD_PRICE, FACTOR, QUEUE_SIZE and POOL_SIZE. A simple definition of the NEW_PRICE would be thus:

NEW_PRICE := IF QUEUE_SIZE < POOL_SIZE * FACTOR THEN
+    OLD_PRICE * 0.95
+ELSE
+    OLD_PRICE / 0.95
+END IF
+

This exists only as a trivial example to demonstrate a basic solution exists, and should not be intended as a concrete proposal.

Notes on Types

This exists only as a short illustration of a potential technical implementation and should not be treated as anything more.

Regions

This data schema achieves a number of goals:

Coretime can be individually traded at a level of a single usage of a single core.
Coretime Regions, of arbitrary span and up to 1/80th interlacing can be exposed as NFTs and exchanged.
Any Coretime Region can be contributed to the Instantaneous Coretime Pool.
Unlimited number of individual Coretime contributors to the Instantaneous Coretime Pool. (Effectively limited only in number of cores and interlacing level; with current values this would allow 80,000 individual payees per timeslice).
All keys are self-describing.
Workload to communicate core (re-)assignments is well-bounded and low in weight.
All mandatory bookkeeping workload is well-bounded in weight.

#![allow(unused)]
+fn main() {
+type Timeslice = u32; // 80 block amounts.
+type CoreIndex = u16;
+type CoreMask = [u8; 10]; // 80-bit bitmap.
+
+// 128-bit (16 bytes)
+struct RegionId {
+    begin: Timeslice,
+    core: CoreIndex,
+    mask: CoreMask,
+}
+// 296-bit (37 bytes)
+struct RegionRecord {
+    end: Timeslice,
+    owner: AccountId,
+}
+
+map Regions = Map<RegionId, RegionRecord>;
+
+// 40-bit (5 bytes). Could be 32-bit with a more specialised type.
+enum CoreTask {
+    Off,
+    Assigned { target: TaskId },
+    InstaPool,
+}
+// 120-bit (15 bytes). Could be 14 bytes with a specialised 32-bit `CoreTask`.
+struct ScheduleItem {
+    mask: CoreMask, // 80 bit
+    task: CoreTask, // 40 bit
+}
+
+/// The work we plan on having each core do at a particular time in the future.
+type Workplan = Map<(Timeslice, CoreIndex), BoundedVec<ScheduleItem, 80>>;
+/// The current workload of each core. This gets updated with workplan as timeslices pass.
+type Workload = Map<CoreIndex, BoundedVec<ScheduleItem, 80>>;
+
+enum Contributor {
+    System,
+    Private(AccountId),
+}
+
+struct ContributionRecord {
+    begin: Timeslice,
+    end: Timeslice,
+    core: CoreIndex,
+    mask: CoreMask,
+    payee: Contributor,
+}
+type InstaPoolContribution = Map<ContributionRecord, ()>;
+
+type SignedTotalMaskBits = u32;
+type InstaPoolIo = Map<Timeslice, SignedTotalMaskBits>;
+
+type PoolSize = Value<TotalMaskBits>;
+
+/// Counter for the total CoreMask which could be dedicated to a pool. `u32` so we don't ever get
+/// an overflow.
+type TotalMaskBits = u32;
+struct InstaPoolHistoryRecord {
+    total_contributions: TotalMaskBits,
+    maybe_payout: Option<Balance>,
+}
+/// Total InstaPool rewards for each Timeslice and the number of core Mask which contributed.
+type InstaPoolHistory = Map<Timeslice, InstaPoolHistoryRecord>;
+}

CoreMask tracks unique "parts" of a single core. It is used with interlacing in order to give a unique identifier to each component of any possible interlacing configuration of a core, allowing for simple self-describing keys for all core ownership and allocation information. It also allows for each core's workload to be tracked and updated progressively, keeping ongoing compute costs well-bounded and low.

Regions are issued into the Regions map and can be transferred, partitioned and interlaced as the owner desires. Regions can only be tasked if they begin after the current scheduling deadline (if they have missed this, then the region can be auto-trimmed until it is).

Once tasked, they are removed from there and a record is placed in Workplan. In addition, if they are contributed to the Instantaneous Coretime Pool, then an entry is placing in InstaPoolContribution and InstaPoolIo.

Each timeslice, InstaPoolIo is used to update the current value of PoolSize. A new entry in InstaPoolHistory is inserted, with the total_contributions field of InstaPoolHistoryRecord being informed by the PoolSize value. Each core's has its Workload mutated according to its Workplan for the upcoming timeslice.

When Instantaneous Coretime Market Revenues are reported for a particular timeslice from the Relay-chain, this information gets placed in the maybe_payout field of the relevant record of InstaPoolHistory.

Payments can be requested made for any records in InstaPoolContribution whose begin is the key for a value in InstaPoolHistory whose maybe_payout is Some. In this case, the total_contributions is reduced by the ContributionRecord's mask and a pro rata amount paid. The ContributionRecord is mutated by incrementing begin, or removed if begin becomes equal to end.

Example:

#![allow(unused)]
+fn main() {
+// Simple example with a `u16` `CoreMask` and bulk sold in 100 timeslices.
+Regions:
+{ core: 0u16, begin: 100, mask: 0b1111_1111_1111_1111u16 } => { end: 200u32, owner: Alice };
+// First split @ 50
+Regions:
+{ core: 0u16, begin: 100, mask: 0b1111_1111_1111_1111u16 } => { end: 150u32, owner: Alice };
+{ core: 0u16, begin: 150, mask: 0b1111_1111_1111_1111u16 } => { end: 200u32, owner: Alice };
+// Share half of first 50 blocks
+Regions:
+{ core: 0u16, begin: 100, mask: 0b1111_1111_0000_0000u16 } => { end: 150u32, owner: Alice };
+{ core: 0u16, begin: 100, mask: 0b0000_0000_1111_1111u16 } => { end: 150u32, owner: Alice };
+{ core: 0u16, begin: 150, mask: 0b1111_1111_1111_1111u16 } => { end: 200u32, owner: Alice };
+// Sell half of them to Bob
+Regions:
+{ core: 0u16, begin: 100, mask: 0b1111_1111_0000_0000u16 } => { end: 150u32, owner: Alice };
+{ core: 0u16, begin: 100, mask: 0b0000_0000_1111_1111u16 } => { end: 150u32, owner: Bob };
+{ core: 0u16, begin: 150, mask: 0b1111_1111_1111_1111u16 } => { end: 200u32, owner: Alice };
+// Bob splits first 10 and assigns them to himself.
+Regions:
+{ core: 0u16, begin: 100, mask: 0b1111_1111_0000_0000u16 } => { end: 150u32, owner: Alice };
+{ core: 0u16, begin: 100, mask: 0b0000_0000_1111_1111u16 } => { end: 110u32, owner: Bob };
+{ core: 0u16, begin: 110, mask: 0b0000_0000_1111_1111u16 } => { end: 150u32, owner: Bob };
+{ core: 0u16, begin: 150, mask: 0b1111_1111_1111_1111u16 } => { end: 200u32, owner: Alice };
+// Bob shares first 10 3 ways and sells smaller shares to Charlie and Dave
+Regions:
+{ core: 0u16, begin: 100, mask: 0b1111_1111_0000_0000u16 } => { end: 150u32, owner: Alice };
+{ core: 0u16, begin: 100, mask: 0b0000_0000_1100_0000u16 } => { end: 110u32, owner: Charlie };
+{ core: 0u16, begin: 100, mask: 0b0000_0000_0011_0000u16 } => { end: 110u32, owner: Dave };
+{ core: 0u16, begin: 100, mask: 0b0000_0000_0000_1111u16 } => { end: 110u32, owner: Bob };
+{ core: 0u16, begin: 110, mask: 0b0000_0000_1111_1111u16 } => { end: 150u32, owner: Bob };
+{ core: 0u16, begin: 150, mask: 0b1111_1111_1111_1111u16 } => { end: 200u32, owner: Alice };
+// Bob assigns to his para B, Charlie and Dave assign to their paras C and D; Alice assigns first 50 to A
+Regions:
+{ core: 0u16, begin: 150, mask: 0b1111_1111_1111_1111u16 } => { end: 200u32, owner: Alice };
+Workplan:
+(100, 0) => vec![
+    { mask: 0b1111_1111_0000_0000u16, task: Assigned(A) },
+    { mask: 0b0000_0000_1100_0000u16, task: Assigned(C) },
+    { mask: 0b0000_0000_0011_0000u16, task: Assigned(D) },
+    { mask: 0b0000_0000_0000_1111u16, task: Assigned(B) },
+]
+(110, 0) => vec![{ mask: 0b0000_0000_1111_1111u16, task: Assigned(B) }]
+// Alice assigns her remaining 50 timeslices to the InstaPool paying herself:
+Regions: (empty)
+Workplan:
+(100, 0) => vec![
+    { mask: 0b1111_1111_0000_0000u16, task: Assigned(A) },
+    { mask: 0b0000_0000_1100_0000u16, task: Assigned(C) },
+    { mask: 0b0000_0000_0011_0000u16, task: Assigned(D) },
+    { mask: 0b0000_0000_0000_1111u16, task: Assigned(B) },
+]
+(110, 0) => vec![{ mask: 0b0000_0000_1111_1111u16, task: Assigned(B) }]
+(150, 0) => vec![{ mask: 0b1111_1111_1111_1111u16, task: InstaPool }]
+InstaPoolContribution:
+{ begin: 150, end: 200, core: 0, mask: 0b1111_1111_1111_1111u16, payee: Alice }
+InstaPoolIo:
+150 => 16
+200 => -16
+// Actual notifications to relay chain.
+// Assumes:
+// - Timeslice is 10 blocks.
+// - Timeslice 0 begins at block #1000.
+// - Relay needs 10 blocks notice of change.
+//
+Workload: 0 => vec![]
+PoolSize: 0
+
+// Block 990:
+Relay <= assign_core(core: 0u16, begin: 1000, assignment: vec![(A, 8), (C, 2), (D, 2), (B, 4)])
+Workload: 0 => vec![
+    { mask: 0b1111_1111_0000_0000u16, task: Assigned(A) },
+    { mask: 0b0000_0000_1100_0000u16, task: Assigned(C) },
+    { mask: 0b0000_0000_0011_0000u16, task: Assigned(D) },
+    { mask: 0b0000_0000_0000_1111u16, task: Assigned(B) },
+]
+PoolSize: 0
+
+// Block 1090:
+Relay <= assign_core(core: 0u16, begin: 1100, assignment: vec![(A, 8), (B, 8)])
+Workload: 0 => vec![
+    { mask: 0b1111_1111_0000_0000u16, task: Assigned(A) },
+    { mask: 0b0000_0000_1111_1111u16, task: Assigned(B) },
+]
+PoolSize: 0
+
+// Block 1490:
+Relay <= assign_core(core: 0u16, begin: 1500, assignment: vec![(Pool, 16)])
+Workload: 0 => vec![
+    { mask: 0b1111_1111_1111_1111u16, task: InstaPool },
+]
+PoolSize: 16
+InstaPoolIo:
+200 => -16
+InstaPoolHistory:
+150 => { total_contributions: 16, maybe_payout: None }
+
+// Sometime after block 1500:
+InstaPoolHistory:
+150 => { total_contributions: 16, maybe_payout: Some(P) }
+
+// Sometime after block 1990:
+InstaPoolIo: (empty)
+PoolSize: 0
+InstaPoolHistory:
+150 => { total_contributions: 16, maybe_payout: Some(P0) }
+151 => { total_contributions: 16, maybe_payout: Some(P1) }
+152 => { total_contributions: 16, maybe_payout: Some(P2) }
+...
+199 => { total_contributions: 16, maybe_payout: Some(P49) }
+
+// Sometime later still Alice calls for a payout
+InstaPoolContribution: (empty)
+InstaPoolHistory: (empty)
+// Alice gets rewarded P0 + P1 + ... P49.
+}

Rollout

Rollout of this proposal comes in several phases:

Finalise the specifics of implementation; this may be done through a design document or through a well-documented prototype implementation.
Implement the design, including all associated aspects such as unit tests, benchmarks and any support software needed.
If any new parachain is required, launch of this.
Formal audit of the implementation and any manual testing.
Announcement to the various stakeholders of the imminent changes.
Software integration and release.
Governance upgrade proposal(s).
Monitoring of the upgrade process.

Performance, Ergonomics and Compatibility

No specific considerations.

Parachains already deployed into the Polkadot UC must have a clear plan of action to migrate to an agile Coretime market.

While this proposal does not introduce documentable features per se, adequate documentation must be provided to potential purchasers of Polkadot Coretime. This SHOULD include any alterations to the Polkadot-SDK software collection.

Testing, Security and Privacy

Regular testing through unit tests, integration tests, manual testnet tests, zombie-net tests and fuzzing SHOULD be conducted.

A regular security review SHOULD be conducted prior to deployment through a review by the Web3 Foundation economic research group.

Any final implementation MUST pass a professional external security audit.

The proposal introduces no new privacy concerns.

+ +

RFC-3 proposes a means of implementing the high-level allocations within the Relay-chain.

RFC-5 proposes the API for interacting with Relay-chain.

Additional work should specify the interface for the instantaneous market revenue so that the Coretime-chain can ensure Bulk Coretime placed in the instantaneous market is properly compensated.

Drawbacks, Alternatives and Unknowns

Unknowns include the economic and resource parameterisations:

The initial price of Bulk Coretime.
The price-change algorithm between Bulk Coretime sales.
The price increase per Bulk Coretime period for renewals.
The price decrease graph in the Leadin period for Bulk Coretime sales.
The initial price of Instantaneous Coretime.
The price-change algorithm for Instantaneous Coretime sales.
The percentage of cores to be sold as Bulk Coretime.
The fate of revenue collected.

Prior Art and References

Robert Habermeier initially wrote on the subject of Polkadot blockspace-centric in the article Polkadot Blockspace over Blockchains. While not going into details, the article served as an early reframing piece for moving beyond one-slot-per-chain models and building out secondary market infrastructure for resource allocation.

(source)

Table of Contents

RFC-5: Coretime Interface +
+

RFC-5: Coretime Interface

+ + + +


Start Date	06 July 2023
Description	Interface for manipulating the usage of cores on the Polkadot Ubiquitous Computer.
Authors	Gavin Wood, Robert Habermeier

Summary

In the Agile Coretime model of the Polkadot Ubiquitous Computer, as proposed in RFC-1 and RFC-3, it is necessary for the allocating parachain (envisioned to be one or more pallets on a specialised Brokerage System Chain) to communicate the core assignments to the Relay-chain, which is responsible for ensuring those assignments are properly enacted.

This is a proposal for the interface which will exist around the Relay-chain in order to communicate this information and instructions.

Motivation

The background motivation for this interface is splitting out coretime allocation functions and secondary markets from the Relay-chain onto System parachains. A well-understood and general interface is necessary for ensuring the Relay-chain receives coretime allocation instructions from one or more System chains without introducing dependencies on the implementation details of either side.

Requirements

The interface MUST allow the Relay-chain to be scheduled on a low-latency basis.
Individual cores MUST be schedulable, both in full to a single task (a ParaId or the Instantaneous Coretime Pool) or to many unique tasks in differing ratios.
Typical usage of the interface SHOULD NOT overload the VMP message system.
The interface MUST allow for the allocating chain to be notified of all accounting information relevant for making accurate rewards for contributing to the Instantaneous Coretime Pool.
The interface MUST allow for Instantaneous Coretime Market Credits to be communicated.
The interface MUST allow for the allocating chain to instruct changes to the number of cores which it is able to allocate.
The interface MUST allow for the allocating chain to be notified of changes to the number of cores which are able to be allocated by the allocating chain.

Stakeholders

Primary stakeholder sets are:

Developers of the Relay-chain core-management logic.
Developers of the Brokerage System Chain and its pallets.

Socialization:

This content of this RFC was discussed in the Polkdot Fellows channel.

Explanation

The interface has two sections: The messages which the Relay-chain is able to receive from the allocating parachain (the UMP message types), and messages which the Relay-chain is able to send to the allocating parachain (the DMP message types). These messages are expected to be able to be implemented in a well-known pallet and called with the XCM Transact instruction.

Future work may include these messages being introduced into the XCM standard.

UMP Message Types

`request_core_count`

Prototype:

fn request_core_count(
+    count: u16,
+)
+

Requests the Relay-chain to alter the number of schedulable cores to count. Under normal operation, the Relay-chain SHOULD send a notify_core_count(count) message back.

`request_revenue_info_at`

Prototype:

fn request_revenue_at(
+    when: BlockNumber,
+)
+

Requests that the Relay-chain send a notify_revenue message back at or soon after Relay-chain block number when whose until parameter is equal to when.

The period in to the past which when is allowed to be may be limited; if so the limit should be understood on a channel outside of this proposal. In the case that the request cannot be serviced because when is too old a block then a notify_revenue message must still be returned, but its revenue field may be None.

`credit_account`

Prototype:

fn credit_account(
+    who: AccountId,
+    amount: Balance,
+)
+

Instructs the Relay-chain to add the amount of DOT to the Instantaneous Coretime Market Credit account of who.

It is expected that Instantaneous Coretime Market Credit on the Relay-chain is NOT transferrable and only redeemable when used to assign cores in the Instantaneous Coretime Pool.

`assign_core`

Prototype:

type PartsOf57600 = u16;
+enum CoreAssignment {
+    InstantaneousPool,
+    Task(ParaId),
+}
+fn assign_core(
+    core: CoreIndex,
+    begin: BlockNumber,
+    assignment: Vec<(CoreAssignment, PartsOf57600)>,
+    end_hint: Option<BlockNumber>,
+)
+

Requirements:

assert!(core < core_count);
+assert!(targets.iter().map(|x| x.0).is_sorted());
+assert_eq!(targets.iter().map(|x| x.0).unique().count(), targets.len());
+assert_eq!(targets.iter().map(|x| x.1).sum(), 57600);
+

Where:

core_count is assumed to be the sole parameter in the last received notify_core_count message.

Instructs the Relay-chain to ensure that the core indexed as core is utilised for a number of assignments in specific ratios given by assignment starting as soon after begin as possible. Core assignments take the form of a CoreAssignment value which can either task the core to a ParaId value or indicate that the core should be used in the Instantaneous Pool. Each assignment comes with a ratio value, represented as the numerator of the fraction with a denominator of 57,600.

If end_hint is Some and the inner is greater than the current block number, then the Relay-chain should optimize in the expectation of receiving a new assign_core(core, ...) message at or prior to the block number of the inner value. Specific functionality should remain unchanged regardless of the end_hint value.

On the choice of denominator: 57,600 is a very composite number which factors into: 2 ** 8, 3 ** 2, 5 ** 2. By using it as the denominator we allow for various useful fractions to be perfectly represented including thirds, quarters, fifths, tenths, 80ths, percent and 256ths.

DMP Message Types

`notify_core_count`

Prototype:

fn notify_core_count(
+    count: u16,
+)
+

Indicate that from this block onwards, the range of acceptable values of the core parameter of assign_core message is [0, count). assign_core will be a no-op if provided with a value for core outside of this range.

`notify_revenue_info`

Prototype:

fn notify_revenue_info(
+    until: BlockNumber,
+    revenue: Option<Balance>,
+)
+

Provide the amount of revenue accumulated from Instantaneous Coretime Sales from Relay-chain block number last_until to until, not including until itself. last_until is defined as being the until argument of the last notify_revenue message sent, or zero for the first call. If revenue is None, this indicates that the information is no longer available.

This explicitly disregards the possibility of multiple parachains requesting and being notified of revenue information. The Relay-chain must be configured to ensure that only a single revenue information destination exists.

Realistic Limits of the Usage

For request_revenue_info, a successful request should be possible if when is no less than the Relay-chain block number on arrival of the message less 100,000.

For assign_core, a successful request should be possible if begin is no less than the Relay-chain block number on arrival of the message plus 10 and workload contains no more than 100 items.

Performance, Ergonomics and Compatibility

No specific considerations.

Testing, Security and Privacy

Standard Polkadot testing and security auditing applies.

The proposal introduces no new privacy concerns.

+ +

RFC-1 proposes a means of determining allocation of Coretime using this interface.

RFC-3 proposes a means of implementing the high-level allocations within the Relay-chain.

Drawbacks, Alternatives and Unknowns

None at present.

Prior Art and References

None.

(source)

Table of Contents

RFC-0007: System Collator Selection +
- Summary
- Motivation +
  - Requirements
  +
- Stakeholders
- Explanation +
  - Set Size
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References +
  - Written Discussions
  - Prior Feedback and Input From
  +
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0007: System Collator Selection

+ + + +


Start Date	07 July 2023
Description	Mechanism for selecting collators of system chains.
Authors	Joe Petrowski

Summary

As core functionality moves from the Relay Chain into system chains, so increases the reliance on +the liveness of these chains for the use of the network. It is not economically scalable, nor +necessary from a game-theoretic perspective, to pay collators large rewards. This RFC proposes a +mechanism -- part technical and part social -- for ensuring reliable collator sets that are +resilient to attemps to stop any subsytem of the Polkadot protocol.

Motivation

In order to guarantee access to Polkadot's system, the collators on its system chains must propose +blocks (provide liveness) and allow all transactions to eventually be included. That is, some +collators may censor transactions, but there must exist one collator in the set who will include a +given transaction. In fact, all collators may censor varying subsets of transactions, but as long +as no transaction is in the intersection of every subset, it will eventually be included. The +objective of this RFC is to propose a mechanism to select such a set on each system chain.

While the network as a whole uses staking (and inflationary rewards) to attract validators, +collators face different challenges in scale and have lower security assumptions than validators. +Regarding scale, there exist many system chains, and it is economically expensive to pay collators +a premium. Likewise, any staked DOT for collation is not staked for validation. Since collator +sets do not need to meet Byzantine Fault Tolerance criteria, staking as the primary mechanism for +collator selection would remove stake that is securing BFT assumptions, making the network less +secure.

Another problem with economic scalability relates to the increasing number of system chains, and +corresponding increase in need for collators (i.e., increase in collator slots). "Good" (highly +available, non-censoring) collators will not want to compete in elections on many chains when they +could use their resources to compete in the more profitable validator election. Such dilution +decreases the required bond on each chain, leaving them vulnerable to takeover by hostile +collator groups.

This RFC proposes a system whereby collation is primarily an infrastructure service, with the +on-chain Treasury reimbursing costs of semi-trusted node operators, referred to as "Invulnerables". +The system need not trust the individual operators, only that as a set they would be resilient to +coordinated attempts to stop a single chain from halting or to censor a particular subset of +transactions.

In the case that users do not trust this set, this RFC also proposes that each chain always have +available collator positions that can be acquired by anyone by placing a bond.

Requirements

System MUST have at least one valid collator for every chain.
System MUST allow anyone to become a collator, provided they reserve/hold enough DOT.
System SHOULD select a set of collators with reasonable expectation that the set will not collude +to censor any subset of transactions.
Collators selected by governance SHOULD have a reasonable expectation that the Treasury will +reimburse their operating costs.

Stakeholders

Infrastructure providers (people who run validator/collator nodes)
Polkadot Treasury

Explanation

This protocol builds on the existing +Collator Selection pallet +and its notion of Invulnerables. Invulnerables are collators (identified by their AccountIds) who +will be selected as part of the collator set every session. Operations relating to the management +of the Invulnerables are done through privileged, governance origins. The implementation should +maintain an API for adding and removing Invulnerable collators.

In addition to Invulnerables, there are also open slots for "Candidates". Anyone can register as a +Candidate by placing a fixed bond. However, with a fixed bond and fixed number of slots, there is +an obvious selection problem: The slots fill up without any logic to replace their occupants.

This RFC proposes that the collator selection protocol allow Candidates to increase (and decrease) +their individual bonds, sort the Candidates according to bond, and select the top N Candidates. +The selection and changeover should be coordinated by the session manager.

A FRAME pallet already exists for sorting ("bagging") "top N" groups, the +Bags List pallet. +This pallet's SortedListProvider should be integrated into the session manager of the Collator +Selection pallet.

Despite the lack of apparent economic incentives (i.e., inflation), several reasons exist why one +may want to bond funds to participate in the Candidates election, for example:

They want to build credibility to be selected as Invulnerable;
They want to ensure availability of an application, e.g. a stablecoin issuer might run a collator +on Asset Hub to ensure transactions in its asset are included in blocks;
They fear censorship themselves, e.g. a voter might think their votes are being censored from +governance, so they run a collator on the governance chain to include their votes.

Unlike the fixed-bond mechanism that fills up its Candidates, the election mechanism ensures that +anyone can join the collator set by placing the Nth highest bond.

Set Size

In order to achieve the requirements listed under Motivation, it is reasonable to have +approximately:

20 collators per system chain,
of which 15 are Invulnerable, and
five are elected by bond.

Drawbacks

The primary drawback is a reliance on governance for continued treasury funding of infrastructure +costs for Invulnerable collators.

Testing, Security, and Privacy

The vast majority of cases can be covered by unit testing. Integration test should ensure that the +Collator Selection UpdateOrigin, which has permission to modify the Invulnerables and desired +number of Candidates, can handle updates over XCM from the system's governance location.

Performance, Ergonomics, and Compatibility

This proposal has very little impact on most users of Polkadot, and should improve the performance +of system chains by reducing the number of missed blocks.

Performance

As chains have strict PoV size limits, care must be taken in the PoV impact of the session manager. +Appropriate benchmarking and tests should ensure that conservative limits are placed on the number +of Invulnerables and Candidates.

Ergonomics

The primary group affected is Candidate collators, who, after implementation of this RFC, will need +to compete in a bond-based election rather than a race to claim a Candidate spot.

Compatibility

This RFC is compatible with the existing implementation and can be handled via upgrades and +migration.

Prior Art and References

Written Discussions

Prior Feedback and Input From

Kian Paimani
Jeff Burdges
Rob Habermeier
SR Labs Auditors
Current collators including Paranodes, Stake Plus, Turboflakes, Peter Mensik, SIK, and many more.

Unresolved Questions

None at this time.

+ +

There may exist in the future system chains for which this model of collator selection is not +appropriate. These chains should be evaluated on a case-by-case basis.

(source)

Table of Contents

RFC-0008: Store parachain bootnodes in relay chain DHT +
- Summary
- Motivation
- Stakeholders
- Explanation +
  - DHT provider registration
  - New networking protocol
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0008: Store parachain bootnodes in relay chain DHT

+ + + +


Start Date	2023-07-14
Description	Parachain bootnodes shall register themselves in the DHT of the relay chain
Authors	Pierre Krieger

Summary

The full nodes of the Polkadot peer-to-peer network maintain a distributed hash table (DHT), which is currently used for full nodes discovery and validators discovery purposes.

This RFC proposes to extend this DHT to be used to discover full nodes of the parachains of Polkadot.

Motivation

The maintenance of bootnodes has long been an annoyance for everyone.

When a bootnode is newly-deployed or removed, every chain specification must be updated in order to take the update into account. This has lead to various non-optimal solutions, such as pulling chain specifications from GitHub repositories. +When it comes to RPC nodes, UX developers often have trouble finding up-to-date addresses of parachain RPC nodes. With the ongoing migration from RPC nodes to light clients, similar problems would happen with chain specifications as well.

Furthermore, there exists multiple different possible variants of a certain chain specification: with the non-raw storage, with the raw storage, with just the genesis trie root hash, with or without checkpoint, etc. All of this creates confusion. Removing the need for parachain developers to be aware of and manage these different versions would be beneficial.

Since the PeerId and addresses of bootnodes needs to be stable, extra maintenance work is required from the chain maintainers. For example, they need to be extra careful when migrating nodes within their infrastructure. In some situations, bootnodes are put behind domain names, which also requires maintenance work.

Because the list of bootnodes in chain specifications is so annoying to modify, the consequence is that the number of bootnodes is rather low (typically between 2 and 15). In order to better resist downtimes and DoS attacks, a better solution would be to use every node of a certain chain as potential bootnode, rather than special-casing some specific nodes.

While this RFC doesn't solve these problems for relay chains, it aims at solving it for parachains by storing the list of all the full nodes of a parachain on the relay chain DHT.

Assuming that this RFC is implemented, and that light clients are used, deploying a parachain wouldn't require more work than registering it onto the relay chain and starting the collators. There wouldn't be any need for special infrastructure nodes anymore.

Stakeholders

This RFC has been opened on my own initiative because I think that this is a good technical solution to a usability problem that many people are encountering and that they don't realize can be solved.

Explanation

The content of this RFC only applies for parachains and parachain nodes that are "Substrate-compatible". It is in no way mandatory for parachains to comply to this RFC.

Note that "Substrate-compatible" is very loosely defined as "implements the same mechanisms and networking protocols as Substrate". The author of this RFC believes that "Substrate-compatible" should be very precisely specified, but there is controversy on this topic.

While a lot of this RFC concerns the implementation of parachain nodes, it makes use of the resources of the Polkadot chain, and as such it is important to describe them in the Polkadot specification.

This RFC adds two mechanisms: a registration in the DHT, and a new networking protocol.

DHT provider registration

This RFC heavily relies on the functionalities of the Kademlia DHT already in use by Polkadot. +You can find a link to the specification here.

Full nodes of a parachain registered on Polkadot should register themselves onto the Polkadot DHT as the providers of a key corresponding to the parachain that they are serving, as described in the Content provider advertisement section of the specification. This uses the ADD_PROVIDER system of libp2p-kademlia.

This key is: sha256(concat(scale_compact(para_id), randomness)) where the value of randomness can be found in the randomness field when calling the BabeApi_currentEpoch function. +For example, for a para_id equal to 1000, and at the time of writing of this RFC (July 14th 2023 at 09:13 UTC), it is sha(0xa10f12872447958d50aa7b937b0106561a588e0e2628d33f81b5361b13dbcf8df708), which is equal to 0x483dd8084d50dbbbc962067f216c37b627831d9339f5a6e426a32e3076313d87.

In order to avoid downtime when the key changes, parachain full nodes should also register themselves as a secondary key that uses a value of randomness equal to the randomness field when calling BabeApi_nextEpoch.

Implementers should be aware that their implementation of Kademlia might already hash the key before XOR'ing it. The key is not meant to be hashed twice.

The compact SCALE encoding has been chosen in order to avoid problems related to the number of bytes and endianness of the para_id.

New networking protocol

A new request-response protocol should be added, whose name is /91b171bb158e2d3848fa23a9f1c25182fb8e20313b2c1eb49219da7a70ce90c3/paranode (that hexadecimal number is the genesis hash of the Polkadot chain, and should be adjusted appropriately for Kusama and others).

The request consists in a SCALE-compact-encoded para_id. For example, for a para_id equal to 1000, this is 0xa10f.

Note that because this is a request-response protocol, the request is always prefixed with its length in bytes. While the body of the request is simply the SCALE-compact-encoded para_id, the data actually sent onto the substream is both the length and body.

The response consists in a protobuf struct, defined as:

syntax = "proto2";
+
+message Response {
+    // Peer ID of the node on the parachain side.
+    bytes peer_id = 1;
+
+    // Multiaddresses of the parachain side of the node. The list and format are the same as for the `listenAddrs` field of the `identify` protocol.
+    repeated bytes addrs = 2;
+
+    // Genesis hash of the parachain. Used to determine the name of the networking protocol to connect to the parachain. Untrusted.
+    bytes genesis_hash = 3;
+
+    // So-called "fork ID" of the parachain. Used to determine the name of the networking protocol to connect to the parachain. Untrusted.
+    optional string fork_id = 4;
+};
+

The maximum size of a response is set to an arbitrary 16kiB. The responding side should make sure to conform to this limit. Given that fork_id is typically very small and that the only variable-length field is addrs, this is easily achieved by limiting the number of addresses.

Implementers should be aware that addrs might be very large, and are encouraged to limit the number of addrs to an implementation-defined value.

Drawbacks

The peer_id and addrs fields are in theory not strictly needed, as the PeerId and addresses could be always equal to the PeerId and addresses of the node being registered as the provider and serving the response. However, the Cumulus implementation currently uses two different networking stacks, one of the parachain and one for the relay chain, using two separate PeerIds and addresses, and as such the PeerId and addresses of the other networking stack must be indicated. Asking them to use only one networking stack wouldn't feasible in a realistic time frame.

The values of the genesis_hash and fork_id fields cannot be verified by the requester and are expected to be unused at the moment. Instead, a client that desires connecting to a parachain is expected to obtain the genesis hash and fork ID of the parachain from the parachain chain specification. These fields are included in the networking protocol nonetheless in case an acceptable solution is found in the future, and in order to allow use cases such as discovering parachains in a not-strictly-trusted way.

Testing, Security, and Privacy

Because not all nodes want to be used as bootnodes, implementers are encouraged to provide a way to disable this mechanism. However, it is very much encouraged to leave this mechanism on by default for all parachain nodes.

This mechanism doesn't add or remove any security by itself, as it relies on existing mechanisms. +However, if the principle of chain specification bootnodes is entirely replaced with the mechanism described in this RFC (which is the objective), then it becomes important whether the mechanism in this RFC can be abused in order to make a parachain unreachable.

Due to the way Kademlia works, it would become the responsibility of the 20 Polkadot nodes whose sha256(peer_id) is closest to the key (described in the explanations section) to store the list of bootnodes of each parachain. +Furthermore, when a large number of providers (here, a provider is a bootnode) are registered, only the providers closest to the key are kept, up to a certain implementation-defined limit.

For this reason, an attacker can abuse this mechanism by randomly generating libp2p PeerIds until they find the 20 entries closest to the key representing the target parachain. They are then in control of the parachain bootnodes. +Because the key changes periodically and isn't predictable, and assuming that the Polkadot DHT is sufficiently large, it is not realistic for an attack like this to be maintained in the long term.

Furthermore, parachain clients are expected to cache a list of known good nodes on their disk. If the mechanism described in this RFC went down, it would only prevent new nodes from accessing the parachain, while clients that have connected before would not be affected.

Performance, Ergonomics, and Compatibility

Performance

The DHT mechanism generally has a low overhead, especially given that publishing providers is done only every 24 hours.

Doing a Kademlia iterative query then sending a provider record shouldn't take more than around 50 kiB in total of bandwidth for the parachain bootnode.

Assuming 1000 parachain full nodes, the 20 Polkadot full nodes corresponding to a specific parachain will each receive a sudden spike of a few megabytes of networking traffic when the key rotates. Again, this is relatively negligible. If this becomes a problem, one can add a random delay before a parachain full node registers itself to be the provider of the key corresponding to BabeApi_next_epoch.

Maybe the biggest uncertainty is the traffic that the 20 Polkadot full nodes will receive from light clients that desire knowing the bootnodes of a parachain. Light clients are generally encouraged to cache the peers that they use between restarts, so they should only query these 20 Polkadot full nodes at their first initialization. +If this every becomes a problem, this value of 20 is an arbitrary constant that can be increased for more redundancy.

Ergonomics

Irrelevant.

Compatibility

Irrelevant.

Prior Art and References

None.

Unresolved Questions

While it fundamentally doesn't change much to this RFC, using BabeApi_currentEpoch and BabeApi_nextEpoch might be inappropriate. I'm not familiar enough with good practices within the runtime to have an opinion here. Should it be an entirely new pallet?

+ +

It is possible that in the future a client could connect to a parachain without having to rely on a trusted parachain specification.

(source)

Table of Contents

RFC-0012: Process for Adding New System Collectives +
- Summary
- Motivation
- Stakeholders
- Explanation +
  - Removing Collectives
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility
- Prior Art and References
- Unresolved Questions
+

RFC-0012: Process for Adding New System Collectives

+ + + +


Start Date	24 July 2023
Description	A process for adding new (and removing existing) system collectives.
Authors	Joe Petrowski

Summary

Since the introduction of the Collectives parachain, many groups have expressed interest in forming +new -- or migrating existing groups into -- on-chain collectives. While adding a new collective is +relatively simple from a technical standpoint, the Fellowship will need to merge new pallets into +the Collectives parachain for each new collective. This RFC proposes a means for the network to +ratify a new collective, thus instructing the Fellowship to instate it in the runtime.

Motivation

Many groups have expressed interest in representing collectives on-chain. Some of these include:

Parachain technical fellowship (new)
Fellowship(s) for media, education, and evangelism (new)
Polkadot Ambassador Program (existing)
Anti-Scam Team (existing)

Collectives that form part of the core Polkadot protocol should have a mandate to serve the +Polkadot network. However, as part of the Polkadot protocol, the Fellowship, in its capacity of +maintaining system runtimes, will need to include modules and configurations for each collective.

Once a group has developed a value proposition for the Polkadot network, it should have a clear +path to having its collective accepted on-chain as part of the protocol. Acceptance should direct +the Fellowship to include the new collective with a given initial configuration into the runtime. +However, the network, not the Fellowship, should ultimately decide which collectives are in the +interest of the network.

Stakeholders

Polkadot stakeholders who would like to organize on-chain.
Technical Fellowship, in its role of maintaining system runtimes.

Explanation

The group that wishes to operate an on-chain collective should publish the following information:

Charter, including the collective's mandate and how it benefits Polkadot. This would be similar +to the +Fellowship Manifesto.
Seeding recommendation.
Member types, i.e. should members be individuals or organizations.
Member management strategy, i.e. how do members join and get promoted, if applicable.
How much, if at all, members should get paid in salary.
Any special origins this Collective should have outside its self. For example, the Fellowship +can whitelist calls for referenda via the WhitelistOrigin.

This information could all be in a single document or, for example, a GitHub repository.

After publication, members should seek feedback from the community and Technical Fellowship, and +make any revisions needed. When the collective believes the proposal is ready, they should bring a +remark with the text APPROVE_COLLECTIVE("{collective name}, {commitment}") to a Root origin +referendum. The proposer should provide instructions for generating commitment. The passing of +this referendum would be unequivocal direction to the Fellowship that this collective should be +part of the Polkadot runtime.

Note: There is no need for a REJECT referendum. Proposals that have not been approved are simply +not included in the runtime.

Removing Collectives

If someone believes that an existing collective is not acting in the interest of the network or in +accordance with its charter, they should likewise have a means to instruct the Fellowship to +remove that collective from Polkadot.

An on-chain remark from the Root origin with the text +REMOVE_COLLECTIVE("{collective name}, {para ID}, [{pallet indices}]") would instruct the +Fellowship to remove the collective via the listed pallet indices on paraId. Should someone want +to construct such a remark, they should have a reasonable expectation that a member of the +Fellowship would help them identify the pallet indices associated with a given collective, whether +or not the Fellowship member agrees with removal.

Collective removal may also come with other governance calls, for example voiding any scheduled +Treasury spends that would fund the given collective.

Drawbacks

Passing a Root origin referendum is slow. However, given the network's investment (in terms of code +maintenance and salaries) in a new collective, this is an appropriate step.

Testing, Security, and Privacy

No impacts.

Performance, Ergonomics, and Compatibility

Generally all new collectives will be in the Collectives parachain. Thus, performance impacts +should strictly be limited to this parachain and not affect others. As the majority of logic for +collectives is generalized and reusable, we expect most collectives to be instances of similar +subsets of modules. That is, new collectives should generally be compatible with UIs and other +services that provide collective-related functionality, with little modifications to support new +ones.

Prior Art and References

The launch of the Technical Fellowship, see the +initial forum post.

Unresolved Questions

None at this time.

(source)

Table of Contents

RFC-0014: Improve locking mechanism for parachains +
- Summary
- Motivation
- Requirements
- Stakeholders
- Explanation +
  - Status quo
  - Proposed changes
  - Migration
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance
- Ergonomics
- Compatibility
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0014: Improve locking mechanism for parachains

+ + + +


Start Date	July 25, 2023
Description	Improve locking mechanism for parachains
Authors	Bryan Chen

Summary

This RFC proposes a set of changes to the parachain lock mechanism. The goal is to allow a parachain manager to self-service the parachain without root track governance action.

This is achieved by remove existing lock conditions and only lock a parachain when:

A parachain manager explicitly lock the parachain
OR a parachain block is produced successfully

Motivation

The manager of a parachain has permission to manage the parachain when the parachain is unlocked. Parachains are by default locked when onboarded to a slot. This requires the parachain wasm/genesis must be valid, otherwise a root track governance action on relaychain is required to update the parachain.

The current reliance on root track governance actions for managing parachains can be time-consuming and burdensome. This RFC aims to address this technical difficulty by allowing parachain managers to take self-service actions, rather than relying on general public voting.

The key scenarios this RFC seeks to improve are:

Rescue a parachain with invalid wasm/genesis.

While we have various resources and templates to build a new parachain, it is still not a trivial task. It is very easy to make a mistake and resulting an invalid wasm/genesis. With lack of tools to help detect those issues¹, it is very likely that the issues are only discovered after the parachain is onboarded on a slot. In this case, the parachain is locked and the parachain team has to go through a lengthy governance process to rescue the parachain.

Perform lease renewal for an existing parachain.

One way to perform lease renewal for a parachain is by doing a least swap with another parachain with a longer lease. This requires the other parachain must be operational and able to perform XCM transact call into relaychain to dispatch the swap call. Combined with the overhead of setting up a new parachain, this is an time consuming and expensive process. Ideally, the parachain manager should be able to perform the lease swap call without having a running parachain².

Requirements

A parachain manager SHOULD be able to rescue a parachain by updating the wasm/genesis without root track governance action.
A parachain manager MUST NOT be able to update the wasm/genesis if the parachain is locked.
A parachain SHOULD be locked when it successfully produced the first block.
A parachain manager MUST be able to perform lease swap without having a running parachain.

Stakeholders

Parachain teams
Parachain users

Explanation

Status quo

A parachain can either be locked or unlocked³. With parachain locked, the parachain manager does not have any privileges. With parachain unlocked, the parachain manager can perform following actions with the paras_registrar pallet:

deregister: Deregister a Para Id, freeing all data and returning any deposit.
swap: Initiate or confirm lease swap with another parachain.
add_lock: Lock the parachain.
schedule_code_upgrade: Schedule a parachain upgrade to update parachain wasm.
set_current_head: Set the parachain's current head.

Currently, a parachain can be locked with following conditions:

From add_lock call, which can be dispatched by relaychain Root origin, the parachain, or the parachain manager.
When a parachain is onboarded on a slot⁴.
When a crowdloan is created.

Only the relaychain Root origin or the parachain itself can unlock the lock⁵.

This creates an issue that if the parachain is unable to produce block, the parachain manager is unable to do anything and have to rely on relaychain Root origin to manage the parachain.

Proposed changes

This RFC proposes to change the lock and unlock conditions.

A parachain can be locked only with following conditions:

Relaychain governance MUST be able to lock any parachain.
A parachain MUST be able to lock its own lock.
A parachain manager SHOULD be able to lock the parachain.
A parachain SHOULD be locked when it successfully produced a block for the first time.

A parachain can be unlocked only with following conditions:

Relaychain governance MUST be able to unlock any parachain.
A parachain MUST be able to unlock its own lock.

Note that create crowdloan MUST NOT lock the parachain and onboard a parachain SHOULD NOT lock it until a new block is successfully produced.

Migration

A one off migration is proposed in order to apply this change retrospectively so that existing parachains can also be benefited from this RFC. This migration will unlock parachains that confirms with following conditions:

Parachain is locked.
Parachain never produced a block. Including from expired leases.
Parachain manager never explicitly lock the parachain.

Drawbacks

Parachain locks are designed in such way to ensure the decentralization of parachains. If parachains are not locked when it should be, it could introduce centralization risk for new parachains.

For example, one possible scenario is that a collective may decide to launch a parachain fully decentralized. However, if the parachain is unable to produce block, the parachain manager will be able to replace the wasm and genesis without the consent of the collective.

It is considered this risk is tolerable as it requires the wasm/genesis to be invalid at first place. It is not yet practically possible to develop a parachain without any centralized risk currently.

Another case is that a parachain team may decide to use crowdloan to help secure a slot lease. Previously, creating a crowdloan will lock a parachain. This means crowdloan participants will know exactly the genesis of the parachain for the crowdloan they are participating. However, this actually providers little assurance to crowdloan participants. For example, if the genesis block is determined before a crowdloan is started, it is not possible to have onchain mechanism to enforce reward distributions for crowdloan participants. They always have to rely on the parachain team to fulfill the promise after the parachain is alive.

Existing operational parachains will not be impacted.

Testing, Security, and Privacy

The implementation of this RFC will be tested on testnets (Rococo and Westend) first.

An audit maybe required to ensure the implementation does not introduce unwanted side effects.

There is no privacy related concerns.

Performance

This RFC should not introduce any performance impact.

Ergonomics

This RFC should improve the developer experiences for new and existing parachain teams

Compatibility

This RFC is fully compatibility with existing interfaces.

Prior Art and References

Parachain Slot Extension Story: https://github.com/paritytech/polkadot/issues/4758
Allow parachain to renew lease without actually run another parachain: https://github.com/paritytech/polkadot/issues/6685
Always treat parachain that never produced block for a significant amount of time as unlocked: https://github.com/paritytech/polkadot/issues/7539

Unresolved Questions

None at this stage.

+ +

This RFC is only intended to be a short term solution. Slots will be removed in future and lock mechanism is likely going to be replaced with a more generalized parachain manage & recovery system in future. Therefore long term impacts of this RFC are not considered.

¹ +

https://github.com/paritytech/cumulus/issues/377 +²: https://github.com/paritytech/polkadot/issues/6685 +³: https://github.com/paritytech/polkadot/blob/994af3de79af25544bf39644844cbe70a7b4d695/runtime/common/src/paras_registrar.rs#L51-L52C15 +⁴: https://github.com/paritytech/polkadot/blob/994af3de79af25544bf39644844cbe70a7b4d695/runtime/common/src/paras_registrar.rs#L473-L475 +⁵: https://github.com/paritytech/polkadot/blob/994af3de79af25544bf39644844cbe70a7b4d695/runtime/common/src/paras_registrar.rs#L333-L340

(source)

Table of Contents

RFC-0022: Adopt Encointer Runtime +
+

RFC-0022: Adopt Encointer Runtime

+ + + +


Start Date	Aug 22nd 2023
Description	Permanently move the Encointer runtime into the Fellowship runtimes repo.
Authors	@brenzi for Encointer Association, 8000 Zurich, Switzerland

Summary

Encointer is a system chain on Kusama since Jan 2022 and has been developed and maintained by the Encointer association. This RFC proposes to treat Encointer like any other system chain and include it in the fellowship repo with this PR.

Motivation

Encointer does not seek to be in control of its runtime repository. As a decentralized system, the fellowship has a more suitable structure to maintain a system chain runtime repo than the Encointer association does.

Also, Encointer aims to update its runtime in batches with other system chains in order to have consistency for interoperability across system chains.

Stakeholders

Fellowship: Will continue to take upon them the review and auditing work for the Encointer runtime, but the process is streamlined with other system chains and therefore less time-consuming compared to the separate repo and CI process we currently have.
Kusama Network: Tokenholders can easily see the changes of all system chains in one place.
Encointer Association: Further decentralization of the Encointer Network necessities like devops.
Encointer devs: Being able to work directly in the Fellowship runtimes repo to streamline and synergize with other developers.

Explanation

Our PR has all details about our runtime and how we would move it into the fellowship repo.

Noteworthy: All Encointer-specific pallets will still be located in encointer's repo for the time being: https://github.com/encointer/pallets

It will still be the duty of the Encointer team to keep its runtime up to date and provide adequate test fixtures. Frequent dependency bumps with Polkadot releases would be beneficial for interoperability and could be streamlined with other system chains but that will not be a duty of fellowship. Whenever possible, all system chains could be upgraded jointly (including Encointer) with a batch referendum.

Further notes:

Encointer will publish all its crates crates.io
Encointer does not carry out external auditing of its runtime nor pallets. It would be beneficial but not a requirement from our side if Encointer could join the auditing process of other system chains.

Drawbacks

Other than all other system chains, development and maintenance of the Encointer Network is mainly financed by the KSM Treasury and possibly the DOT Treasury in the future. Encointer is dedicated to maintaining its network and runtime code for as long as possible, but there is a dependency on funding which is not in the hands of the fellowship. The only risk in the context of funding, however, is that the Encointer runtime will see less frequent updates if there's less funding.

Testing, Security, and Privacy

No changes to the existing system are proposed. Only changes to how maintenance is organized.

Performance, Ergonomics, and Compatibility

No changes

Prior Art and References

Existing Encointer runtime repo

Unresolved Questions

None identified

+ +

More info on Encointer: encointer.org

(source)

Table of Contents

RFC-0032: Minimal Relay +
- Summary
- Motivation
- Stakeholders
- Explanation +
  - Migrations
  - Interfaces
  - Functional Architecture
  - Resource Allocation
  - Deployment
  - Kusama
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0032: Minimal Relay

+ + + +


Start Date	20 September 2023
Description	Proposal to minimise Relay Chain functionality.
Authors	Joe Petrowski, Gavin Wood

Summary

The Relay Chain contains most of the core logic for the Polkadot network. While this was necessary +prior to the launch of parachains and development of XCM, most of this logic can exist in +parachains. This is a proposal to migrate several subsystems into system parachains.

Motivation

Polkadot's scaling approach allows many distinct state machines (known generally as parachains) to +operate with common guarantees about the validity and security of their state transitions. Polkadot +provides these common guarantees by executing the state transitions on a strict subset (a backing +group) of the Relay Chain's validator set.

However, state transitions on the Relay Chain need to be executed by all validators. If any of +those state transitions can occur on parachains, then the resources of the complement of a single +backing group could be used to offer more cores. As in, they could be offering more coretime (a.k.a. +blockspace) to the network.

By minimising state transition logic on the Relay Chain by migrating it into "system chains" -- a +set of parachains that, with the Relay Chain, make up the Polkadot protocol -- the Polkadot +Ubiquitous Computer can maximise its primary offering: secure blockspace.

Stakeholders

Parachains that interact with affected logic on the Relay Chain;
Core protocol and XCM format developers;
Tooling, block explorer, and UI developers.

Explanation

The following pallets and subsystems are good candidates to migrate from the Relay Chain:

Identity
Balances
Staking +
- Staking
- Election Provider
- Bags List
- NIS
- Nomination Pools
- Fast Unstake
+
Governance +
- Treasury and Bounties
- Conviction Voting
- Referenda
+

Note: The Auctions and Crowdloan pallets will be replaced by Coretime, its system chain and +interface described in RFC-1 and RFC-5, respectively.

Migrations

Some subsystems are simpler to move than others. For example, migrating Identity can be done by +simply preventing state changes in the Relay Chain, using the Identity-related state as the genesis +for a new chain, and launching that new chain with the genesis and logic (pallet) needed.

Other subsystems cannot experience any downtime like this because they are essential to the +network's functioning, like Staking and Governance. However, these can likely coexist with a +similarly-permissioned system chain for some time, much like how "Gov1" and "OpenGov" coexisted at +the latter's introduction.

Specific migration plans will be included in release notes of runtimes from the Polkadot Fellowship +when beginning the work of migrating a particular subsystem.

Interfaces

The Relay Chain, in many cases, will still need to interact with these subsystems, especially +Staking and Governance. These subsystems will require making some APIs available either via +dispatchable calls accessible to XCM Transact or possibly XCM Instructions in future versions.

For example, Staking provides a pallet-API to register points (e.g. for block production) and +offences (e.g. equivocation). With Staking in a system chain, that chain would need to allow the +Relay Chain to update validator points periodically so that it can correctly calculate rewards.

A pub-sub protocol may also lend itself to these types of interactions.

Functional Architecture

This RFC proposes that system chains form individual components within the system's architecture and +that these components are chosen as functional groups. This approach allows synchronous +composibility where it is most valuable, but isolates logic in such a way that provides flexibility +for optimal resource allocation (see Resource Allocation). For the +subsystems discussed in this RFC, namely Identity, Governance, and Staking, this would mean:

People Chain, for identity and personhood logic, providing functionality related to the attributes +of single actors;
Governance Chain, for governance and system collectives, providing functionality for pluralities +to express their voices within the system;
Staking Chain, for Polkadot's staking system, including elections, nominations, reward +distribution, slashing, and non-interactive staking; and
Asset Hub, for fungible and non-fungible assets, including DOT.

The Collectives chain and Asset Hub already exist, so implementation of this RFC would mean two new +chains (People and Staking), with Governance moving to the currently-known-as Collectives chain +and Asset Hub being increasingly used for DOT over the Relay Chain.

Note that one functional group will likely include many pallets, as we do not know how pallet +configurations and interfaces will evolve over time.

Resource Allocation

The system should minimise wasted blockspace. These three (and other) subsystems may not each +consistently require a dedicated core. However, core scheduling is far more agile than functional +grouping. While migrating functionality from one chain to another can be a multi-month endeavour, +cores can be rescheduled almost on-the-fly.

Migrations are also breaking changes to some use cases, for example other parachains that need to +route XCM programs to particular chains. It is thus preferable to do them a single time in migrating +off the Relay Chain, reducing the risk of needing parachain splits in the future.

Therefore, chain boundaries should be based on functional grouping where synchronous composibility +is most valuable; and efficient resource allocation should be managed by the core scheduling +protocol.

Many of these system chains (including Asset Hub) could often share a single core in a semi-round +robin fashion (the coretime may not be uniform). When needed, for example during NPoS elections or +slashing events, the scheduler could allocate a dedicated core to the chain in need of more +throughput.

Deployment

Actual migrations should happen based on some prioritization. This RFC proposes to migrate Identity, +Staking, and Governance as the systems to work on first. A brief discussion on the factors involved +in each one:

Identity

Identity will be one of the simpler pallets to migrate into a system chain, as its logic is largely +self-contained and it does not "share" balances with other subsystems. As in, any DOT is held in +reserve as a storage deposit and cannot be simultaneously used the way locked DOT can be locked for +multiple purposes.

Therefore, migration can take place as follows:

The pallet can be put in a locked state, blocking most calls to the pallet and preventing updates +to identity info.
The frozen state will form the genesis of a new system parachain.
Functions will be added to the pallet that allow migrating the deposit to the parachain. The +parachain deposit is on the order of 1/100th of the Relay Chain's. Therefore, this will result in +freeing up Relay State as well as most of each user's reserved balance.
The pallet and any leftover state can be removed from the Relay Chain.

User interfaces that render Identity information will need to source their data from the new system +parachain.

Note: In the future, it may make sense to decommission Kusama's Identity chain and do all account +identities via Polkadot's. However, the Kusama chain will serve as a dress rehearsal for Polkadot.

Staking

Migrating the staking subsystem will likely be the most complex technical undertaking, as the +Staking system cannot stop (the system MUST always have a validator set) nor run in parallel (the +system MUST have only one validator set) and the subsystem itself is made up of subsystems in the +runtime and the node. For example, if offences are reported to the Staking parachain, validator +nodes will need to submit their reports there.

Handling balances also introduces complications. The same balance can be used for staking and +governance. Ideally, all balances stay on Asset Hub, and only report "credits" to system chains like +Staking and Governance. However, staking mutates balances by issuing new DOT on era changes and for +rewards. Allowing DOT directly on the Staking parachain would simplify staking changes.

Given the complexity, it would be pragmatic to include the Balances pallet in the Staking parachain +in its first version. Any other systems that use overlapping locks, most notably governance, will +need to recognise DOT held on both Asset Hub and the Staking parachain.

There is more discussion about staking in a parachain in Moving Staking off the Relay +Chain.

Governance

Migrating governance into a parachain will be less complicated than staking. Most of the primitives +needed for the migration already exist. The Treasury supports spending assets on remote chains and +collectives like the Polkadot Technical Fellowship already function in a parachain. That is, XCM +already provides the ability to express system origins across chains.

Therefore, actually moving the governance logic into a parachain will be simple. It can run in +parallel with the Relay Chain's governance, which can be removed when the parachain has demonstrated +sufficient functionality. It's possible that the Relay Chain maintain a Root-level emergency track +for situations like parachains +halting.

The only complication arises from the fact that both Asset Hub and the Staking parachain will have +DOT balances; therefore, the Governance chain will need to be able to credit users' voting power +based on balances from both locations. This is not expected to be difficult to handle.

Kusama

Although Polkadot and Kusama both have system chains running, they have to date only been used for +introducing new features or bodies, for example fungible assets or the Technical Fellowship. There +has not yet been a migration of logic/state from the Relay Chain into a parachain. Given its more +realistic network conditions than testnets, Kusama is the best stage for rehearsal.

In the case of identity, Polkadot's system may be sufficient for the ecosystem. Therefore, Kusama +should be used to test the migration of logic and state from Relay Chain to parachain, but these +features may be (at the will of Kusama's governance) dropped from Kusama entirely after a successful +migration on Polkadot.

For Governance, Polkadot already has the Collectives parachain, which would become the Governance +parachain. The entire group of DOT holders is itself a collective (the legislative body), and +governance provides the means to express voice. Launching a Kusama Governance chain would be +sensible to rehearse a migration.

The Staking subsystem is perhaps where Kusama would provide the most value in its canary capacity. +Staking is the subsystem most constrained by PoV limits. Ensuring that elections, payouts, session +changes, offences/slashes, etc. work in a parachain on Kusama -- with its larger validator set -- +will give confidence to the chain's robustness on Polkadot.

Drawbacks

These subsystems will have reduced resources in cores than on the Relay Chain. Staking in particular +may require some optimizations to deal with constraints.

Testing, Security, and Privacy

Standard audit/review requirements apply. More powerful multi-chain integration test tools would be +useful in developement.

Performance, Ergonomics, and Compatibility

Describe the impact of the proposal on the exposed functionality of Polkadot.

Performance

This is an optimization. The removal of public/user transactions on the Relay Chain ensures that its +primary resources are allocated to system performance.

Ergonomics

This proposal alters very little for coretime users (e.g. parachain developers). Application +developers will need to interact with multiple chains, making ergonomic light client tools +particularly important for application development.

For existing parachains that interact with these subsystems, they will need to configure their +runtimes to recognize the new locations in the network.

Compatibility

Implementing this proposal will require some changes to pallet APIs and/or a pub-sub protocol. +Application developers will need to interact with multiple chains in the network.

Prior Art and References

Unresolved Questions

There remain some implementation questions, like how to use balances for both Staking and +Governance. See, for example, Moving Staking off the Relay +Chain.

+ +

Ideally the Relay Chain becomes transactionless, such that not even balances are represented there. +With Staking and Governance off the Relay Chain, this is not an unreasonable next step.

With Identity on Polkadot, Kusama may opt to drop its People Chain.

(source)

Table of Contents

RFC-0050: Fellowship Salaries +
- Summary
- Motivation
- Stakeholders
- Explanation +
  - Salary Asset
  - Projections
  - Updates
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
+

RFC-0050: Fellowship Salaries

+ + + +


Start Date	15 November 2023
Description	Proposal to set rank-based Fellowship salary levels.
Authors	Joe Petrowski, Gavin Wood

Summary

The Fellowship Manifesto states that members should receive a monthly allowance on par with gross +income in OECD countries. This RFC proposes concrete amounts.

Motivation

One motivation for the Technical Fellowship is to provide an incentive mechanism that can induct and +retain technical talent for the continued progress of the network.

In order for members to uphold their commitment to the network, they should receive support to +ensure that their needs are met such that they have the time to dedicate to their work on Polkadot. +Given the high expectations of Fellows, it is reasonable to consider contributions and requirements +on par with a full-time job. Providing a livable wage to those making such contributions makes it +pragmatic to work full-time on Polkadot.

Note: Goals of the Fellowship, expectations for each Dan, and conditions for promotion and demotion +are all explained in the Manifesto. This RFC is only to propose concrete values for allowances.

Stakeholders

Fellowship members
Polkadot Treasury

Explanation

This RFC proposes agreeing on salaries relative to a single level, the III Dan. As such, changes to +the amount or asset used would only be on a single value, and all others would adjust relatively. A +III Dan is someone whose contributions match the expectations of a full-time individual contributor. +The salary at this level should be reasonably close to averages in OECD countries.

+ + + + + + + + + +

Dan	Factor
I	0.125
II	0.25
III	1
IV	1.5
V	2.0
VI	2.5
VII	2.5
VIII	2.5
IX	2.5

Note that there is a sizable increase between II Dan (Proficient) and III Dan (Fellow). By the third +Dan, it is generally expected that one is working on Polkadot as their primary focus in a full-time +capacity.

Salary Asset

Although the Manifesto (Section 8) specifies a monthly allowance in DOT, this RFC proposes the use +of USDT instead. The allowance is meant to provide members stability in meeting their day-to-day +needs and recognize contributions. Using USDT provides more stability and less speculation.

This RFC proposes that a III Dan earn 80,000 USDT per year. The salary at this level is commensurate +with average salaries in OECD countries (note: 77,000 USD in the U.S., with an average engineer at +100,000 USD). The other ranks would thus earn:

+ + + + + + + + + +

Dan	Annual Salary
I	10,000
II	20,000
III	80,000
IV	120,000
V	160,000
VI	200,000
VII	200,000
VIII	200,000
IX	200,000

The salary levels for Architects (IV, V, and VI Dan) are typical of senior engineers.

Allowances will be managed by the Salary pallet.

Projections

Based on the current membership, the maximum yearly and monthly costs are shown below:

+ + + + + + + + + +

Dan	Salary	Members	Yearly	Monthly
I	10,000	27	270,000	22,500
II	20,000	11	220,000	18,333
III	80,000	8	640,000	53,333
IV	120,000	3	360,000	30,000
V	160,000	5	800,000	66,667
VI	200,000	3	600,000	50,000
> VI	200,000	0	0	0

Total			2,890,000	240,833

Note that these are the maximum amounts; members may choose to take a passive (lower) level. On the +other hand, more people will likely join the Fellowship in the coming years.

Updates

Updates to these levels, whether relative ratios, the asset used, or the amount, shall be done via +RFC.

Drawbacks

By not using DOT for payment, the protocol relies on the stability of other assets and the ability +to acquire them. However, the asset of choice can be changed in the future.

Testing, Security, and Privacy

N/A.

Performance, Ergonomics, and Compatibility

Performance

N/A

Ergonomics

N/A

Compatibility

N/A

Prior Art and References

Unresolved Questions

None at present.

(source)

Table of Contents

RFC-0056: Enforce only one transaction per notification +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0056: Enforce only one transaction per notification

+ + + +


Start Date	2023-11-30
Description	Modify the transactions notifications protocol to always send only one transaction at a time
Authors	Pierre Krieger

Summary

When two peers connect to each other, they open (amongst other things) a so-called "notifications protocol" substream dedicated to gossiping transactions to each other.

Each notification on this substream currently consists in a SCALE-encoded Vec<Transaction> where Transaction is defined in the runtime.

This RFC proposes to modify the format of the notification to become (Compact(1), Transaction). This maintains backwards compatibility, as this new format decodes as a Vec of length equal to 1.

Motivation

There exists three motivations behind this change:

+
It is technically impossible to decode a SCALE-encoded Vec<Transaction> into a list of SCALE-encoded transactions without knowing how to decode a Transaction. That's because a Vec<Transaction> consists in several Transactions one after the other in memory, without any delimiter that indicates the end of a transaction and the start of the next. Unfortunately, the format of a Transaction is runtime-specific. This means that the code that receives notifications is necessarily tied to a specific runtime, and it is not possible to write runtime-agnostic code.
+
+
Notifications protocols are already designed to be optimized to send many items. Currently, when it comes to transactions, each item is a Vec<Transaction> that consists in multiple sub-items of type Transaction. This two-steps hierarchy is completely unnecessary, and was originally written at a time when the networking protocol of Substrate didn't have proper multiplexing.
+
+
It makes the implementation way more straight-forward by not having to repeat code related to back-pressure. See explanations below.
+

Stakeholders

Low-level developers.

Explanation

To give an example, if you send one notification with three transactions, the bytes that are sent on the wire are:

concat(
+    leb128(total-size-in-bytes-of-the-rest),
+    scale(compact(3)), scale(transaction1), scale(transaction2), scale(transaction3)
+)
+

But you can also send three notifications of one transaction each, in which case it is:

concat(
+    leb128(size(scale(transaction1)) + 1), scale(compact(1)), scale(transaction1),
+    leb128(size(scale(transaction2)) + 1), scale(compact(1)), scale(transaction2),
+    leb128(size(scale(transaction3)) + 1), scale(compact(1)), scale(transaction3)
+)
+

Right now the sender can choose which of the two encoding to use. This RFC proposes to make the second encoding mandatory.

The format of the notification would become a SCALE-encoded (Compact(1), Transaction). +A SCALE-compact encoded 1 is one byte of value 4. In other words, the format of the notification would become concat(&[4], scale_encoded_transaction). +This is equivalent to forcing the Vec<Transaction> to always have a length of 1, and I expect the Substrate implementation to simply modify the sending side to add a for loop that sends one notification per item in the Vec.

As explained in the motivation section, this allows extracting scale(transaction) items without having to know how to decode them.

By "flattening" the two-steps hierarchy, an implementation only needs to back-pressure individual notifications rather than back-pressure notifications and transactions within notifications.

Drawbacks

This RFC chooses to maintain backwards compatibility at the cost of introducing a very small wart (the Compact(1)).

An alternative could be to introduce a new version of the transactions notifications protocol that sends one Transaction per notification, but this is significantly more complicated to implement and can always be done later in case the Compact(1) is bothersome.

Testing, Security, and Privacy

Irrelevant.

Performance, Ergonomics, and Compatibility

Performance

Irrelevant.

Ergonomics

Irrelevant.

Compatibility

The change is backwards compatible if done in two steps: modify the sender to always send one transaction per notification, then, after a while, modify the receiver to enforce the new format.

Prior Art and References

Irrelevant.

Unresolved Questions

None.

+ +

None. This is a simple isolated change.

(source)

Table of Contents

RFC-0062: Lowering Existential Deposit on Asset Hub for Polkadot +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0062: Lowering Existential Deposit on Asset Hub for Polkadot

+ + + +


Start Date	28 December 2023
Description	A proposal to reduce the existential deposit required for Asset Hub for Polkadot, making (a) asset minting to all DOT token holders more affordable for Asset Minters and (b) asset conversion on Asset Hub for Polkadot more accessible for all DOT Token holders.
Authors	Sourabh Niyogi

Summary

This RFC proposes lowering the existential deposit requirements on Asset Hub for Polkadot by a factor of 25, from 0.1 DOT to .004 DOT. The objective is to lower the barrier to entry for asset minters to mint a new asset to the entire DOT token holder base, and make Asset Hub on Polkadot a place where everyone can do small asset conversions.

Motivation

The current Existential deposit is 0.1 DOT on Asset Hub for Polkadot. While this is not does not appear to be a significant financial barrier for most people (only $0.80), this value makes Asset Hub impractical for Asset Hub Minters, specifically for the case where the Asset Hub Minters wishes to mint a new asset for the entire community of DOT holders (e.g. 1.25MM DOT holders would cost 125K DOT @ $8 = $1MM).

By lowering the existential deposit requirements from 0.1 DOT to 0.004 DOT, the cost of minting to the entire community of DOT holders goes from an unmanagable number [125K DOT, the value of several houses circa December 2023] down to a manageable number [5K DOT, the value of a car circa December 2023].

Stakeholders

Asset Hub Minters: Those who call asset.mint.
DOT Token Holders: Those who hold DOT on the Polkadot Relay Chain, Asset Hub for Polkadot or other chains.

Explanation

The exact amount of the existential deposit (ED) is proposed to be 0.004 DOT based on

asset.transfer costing 0.00124 DOT
asset.transferKeepAlive costing 0.00092 DOT +This implies that the new ED can support 3 asset.transfer or 4 asset.transferKeepAlive operations.

Empirically, asset.transferKeepAlive is the lowest valued extrinsic at this time, so there is no value to lowering the ED below 0.001 DOT. Lowering further would be unnecessary invite account spam attacks common to EVM chains, which have no ED.

By RFC #32 Minimal Relay Chain, believed to be implemented within the next couple of years, Asset Hub should be able to support the entire DOT existing token holder base. If there is any doubt that Substrate chains can store 10x-100x as many elements, then this change should test Asset Hub for Polkadot's capabilities.

The implementation is believed to be trivial:

https://github.com/polkadot-fellows/runtimes/blob/30e0dbfdcb78722ed61325c0ebf1efdcdb6033ba/system-parachains/asset-hubs/asset-hub-polkadot/src/constants.rs#L21

from

pub const EXISTENTIAL_DEPOSIT: Balance = constants::currency::EXISTENTIAL_DEPOSIT / 10;
+

pub const EXISTENTIAL_DEPOSIT: Balance = constants::currency::EXISTENTIAL_DEPOSIT / 250;
+

Given this change, once Asset Hub Minter 1 spends approximately 5K DOT to cover the ED for the entire DOT Token Holder base, then Asset Hub Minter 2 who subsequently wishes to mint to the same DOT Tokenholder will not pay anything (assuming no new DOT Tokenholders); however, both the first and second holder will need to spend 2,485 DOT to conduct their asset.mint operations (0.001988 DOT per asset.mint) on the entire 1.25MM DOT Token holders. If Minter 3 does the same thing when there are 1.26MM DOT Token holders (10K new DOT holders), then Minter 3 will bear the cost of 40 DOT. This is summarized here:

+ + + +

Minter	Cost to fund ED for 1.25MM users	Cost to call `asset.mint` for 1.25MM users
Minter 1	5K DOT (instead of 125K DOT)	2,485 DOT
Minter 2	0 DOT	2,485 DOT
Minter 3	40 DOT	2,485 DOT

As new DOT Token Holders always enter the system, this lower ED will reduce costs for all new minters, not just Minter 1. Given this reduced cost for Asset Hub Minters (Minter 2, 3, ...), this will enable a greater number of DOT Token Holders to use the assetconversion pallet for newly minted assets.

It is believed that having a greater number of assetconversion end-users will be massively beneficial for DOT ecosystem growth, especially for key asset pools of DOT/USDC and DOT/USDT, which can be reliably predicted to be the most widely used pools on the Asset Hub for Polkadot.

It is assumed that the estimated cost to store a single account is less than 0.004 DOT. If this assumption is challenged by Polkadot Fellows, we request the Fellows provided a empirical determination of what the actual cost of storing a single account is, at present day numbers of DOT Token Holders (approximately 1-2MM) and then to support a factor or 10-1000x growth over the next 5 years. This assumption has been discussed on the forum: Polkadot AssetHub - high NFT collection deposit

First, the cost has to be mapped from DOT into real world USD storage costs of running an Asset Hub on Polkadot node, and the DOT / USD ratio itself has varied widely in the past and will continue to do so in the future. Second, according to this analysis, at present the pragmatic cost of estimating storage is approximated by what it costs to store accounts for 1 or 2 years at most. Underestimates on this cost is believed to be an economic subsidy while overestimates on this cost is believe to be an economic depressant on activity.

Given the relatively underused AssetHub for Polkadot, we believe the correct thing to do is to aim to subsidize Asset Hub activity with a lower ED.

Drawbacks

The primary drawback for subsidize Asset Hub activity with a 25x lower ED is borne by Asset Hub users in the distant future who will pay for the subsidized activity by lowering the ED.

Testing, Security, and Privacy

Lowering the ED from 0.004 DOT to 0 DOT would clearly unnecessarily invite account spam attacks common to EVM chains, which have no ED.

Lowering ED from 0.004 DOT to 0.002 DOT or 0.001 DOT would threaten user experience wherein just 1 or 2 asset pallet operation would reap the account.

Performance, Ergonomics, and Compatibility

Performance

This change is not expected to have a significant impact on the overall performance of the Asset Hub for Polkadot.

Ergonomics

The proposed change aims to enhance the user experience for:

Asset Creators/Minters, making the cost to mint an asset for all DOT Token holders around 5K DOT.
DOT Token Holders, who will enjoy many new assets on Asset Hub created by the above minters

Compatibility

It is believed that Asset Hub for Kusama can undergo the same logic change without issue.

For Asset Hub for Polkadot, it is extremely desirable that this change be approved in early 2024 with some urgency.

Unresolved Questions

It is desirable to know the cost to store an account on Asset Hub for Polkadot when the number of accounts is 10MM, 100MM, 1B to better the cost of the subsidy. We do not believe a precise answer to this merits delaying a subsidy at present. However, if approved, we believe once the number of accounts reaches 10MM-25MM or exponential growth is observed, this ED be reevaluated.

+ +

If accepted, this RFC could pave the way for other accessibility improvements:

EVM Contracts on Asset Hub for Polkadot/Kusama
ink! Contracts on Asset Hub for Polkadot/Kusama
CorePlay activity on Asset Hub for Polkadot/Kusama

(source)

Table of Contents

RFC-0004: Remove the host-side runtime memory allocator +
- Summary
- Motivation
- Stakeholders
- Explanation +
  - New host functions
  - Other changes
  +
- Drawbacks
- Prior Art
- Unresolved Questions
- Future Possibilities
+

RFC-0004: Remove the host-side runtime memory allocator

+ + + +


Start Date	2023-07-04
Description	Update the runtime-host interface to no longer make use of a host-side allocator
Authors	Pierre Krieger

Summary

Update the runtime-host interface to no longer make use of a host-side allocator.

Motivation

The heap allocation of the runtime is currently controlled by the host using a memory allocator on the host side.

The API of many host functions consists in allocating a buffer. For example, when calling ext_hashing_twox_256_version_1, the host allocates a 32 bytes buffer using the host allocator, and returns a pointer to this buffer to the runtime. The runtime later has to call ext_allocator_free_version_1 on this pointer in order to free the buffer.

Even though no benchmark has been done, it is pretty obvious that this design is very inefficient. To continue with the example of ext_hashing_twox_256_version_1, it would be more efficient to instead write the output hash to a buffer that was allocated by the runtime on its stack and passed by pointer to the function. Allocating a buffer on the stack in the worst case scenario simply consists in decreasing a number, and in the best case scenario is free. Doing so would save many Wasm memory reads and writes by the allocator, and would save a function call to ext_allocator_free_version_1.

Furthermore, the existence of the host-side allocator has become questionable over time. It is implemented in a very naive way, and for determinism and backwards compatibility reasons it needs to be implemented exactly identically in every client implementation. Runtimes make substantial use of heap memory allocations, and each allocation needs to go twice through the runtime <-> host boundary (once for allocating and once for freeing). Moving the allocator to the runtime side, while it would increase the size of the runtime, would be a good idea. But before the host-side allocator can be deprecated, all the host functions that make use of it need to be updated to not use it.

Stakeholders

No attempt was made at convincing stakeholders.

Explanation

New host functions

This section contains a list of new host functions to introduce.

(func $ext_storage_read_version_2
+    (param $key i64) (param $value_out i64) (param $offset i32) (result i64))
+(func $ext_default_child_storage_read_version_2
+    (param $child_storage_key i64) (param $key i64) (param $value_out i64)
+    (param $offset i32) (result i64))
+

The signature and behaviour of ext_storage_read_version_2 and ext_default_child_storage_read_version_2 is identical to their version 1 counterparts, but the return value has a different meaning. +The new functions directly return the number of bytes that were written in the value_out buffer. If the entry doesn't exist, a value of -1 is returned. Given that the host must never write more bytes than the size of the buffer in value_out, and that the size of this buffer is expressed as a 32 bits number, a 64bits value of -1 is not ambiguous.

The runtime execution stops with an error if value_out is outside of the range of the memory of the virtual machine, even if the size of the buffer is 0 or if the amount of data to write would be 0 bytes.

(func $ext_storage_next_key_version_2
+    (param $key i64) (param $out i64) (return i32))
+(func $ext_default_child_storage_next_key_version_2
+    (param $child_storage_key i64) (param $key i64) (param $out i64) (return i32))
+

The behaviour of these functions is identical to their version 1 counterparts. +Instead of allocating a buffer, writing the next key to it, and returning a pointer to it, the new version of these functions accepts an out parameter containing a pointer-size to the memory location where the host writes the output. The runtime execution stops with an error if out is outside of the range of the memory of the virtual machine, even if the function wouldn't write anything to out. +These functions return the size, in bytes, of the next key, or 0 if there is no next key. If the size of the next key is larger than the buffer in out, the bytes of the key that fit the buffer are written to out and any extra byte that doesn't fit is discarded.

Some notes:

It is never possible for the next key to be an empty buffer, because an empty key has no preceding key. For this reason, a return value of 0 can unambiguously be used to indicate the lack of next key.
The ext_storage_next_key_version_2 and ext_default_child_storage_next_key_version_2 are typically used in order to enumerate keys that starts with a certain prefix. Given that storage keys are constructed by concatenating hashes, the runtime is expected to know the size of the next key and can allocate a buffer that can fit said key. When the next key doesn't belong to the desired prefix, it might not fit the buffer, but given that the start of the key is written to the buffer anyway this can be detected in order to avoid calling the function a second time with a larger buffer.

(func $ext_hashing_keccak_256_version_2
+    (param $data i64) (param $out i32))
+(func $ext_hashing_keccak_512_version_2
+    (param $data i64) (param $out i32))
+(func $ext_hashing_sha2_256_version_2
+    (param $data i64) (param $out i32))
+(func $ext_hashing_blake2_128_version_2
+    (param $data i64) (param $out i32))
+(func $ext_hashing_blake2_256_version_2
+    (param $data i64) (param $out i32))
+(func $ext_hashing_twox_64_version_2
+    (param $data i64) (param $out i32))
+(func $ext_hashing_twox_128_version_2
+    (param $data i64) (param $out i32))
+(func $ext_hashing_twox_256_version_2
+    (param $data i64) (param $out i32))
+(func $ext_trie_blake2_256_root_version_3
+    (param $data i64) (param $version i32) (param $out i32))
+(func $ext_trie_blake2_256_ordered_root_version_3
+    (param $data i64) (param $version i32) (param $out i32))
+(func $ext_trie_keccak_256_root_version_3
+    (param $data i64) (param $version i32) (param $out i32))
+(func $ext_trie_keccak_256_ordered_root_version_3
+    (param $data i64) (param $version i32) (param $out i32))
+(func $ext_default_child_storage_root_version_3
+    (param $child_storage_key i64) (param $out i32))
+(func $ext_crypto_ed25519_generate_version_2
+    (param $key_type_id i32) (param $seed i64) (param $out i32))
+(func $ext_crypto_sr25519_generate_version_2
+    (param $key_type_id i32) (param $seed i64) (param $out i32) (return i32))
+(func $ext_crypto_ecdsa_generate_version_2
+    (param $key_type_id i32) (param $seed i64) (param $out i32) (return i32))
+

The behaviour of these functions is identical to their version 1 or version 2 counterparts. Instead of allocating a buffer, writing the output to it, and returning a pointer to it, the new version of these functions accepts an out parameter containing the memory location where the host writes the output. The output is always of a size known at compilation time. The runtime execution stops with an error if out is outside of the range of the memory of the virtual machine.

(func $ext_default_child_storage_root_version_3
+    (param $child_storage_key i64) (param $out i32))
+(func $ext_storage_root_version_3
+    (param $out i32))
+

The behaviour of these functions is identical to their version 1 and version 2 counterparts. Instead of allocating a buffer, writing the output to it, and returning a pointer to it, the new versions of these functions accepts an out parameter containing the memory location where the host writes the output. The output is always of a size known at compilation time. The runtime execution stops with an error if out is outside of the range of the memory of the virtual machine.

I have taken the liberty to take the version 1 of these functions as a base rather than the version 2, as a PPP deprecating the version 2 of these functions has previously been accepted: https://github.com/w3f/PPPs/pull/6.

(func $ext_storage_clear_prefix_version_3
+    (param $prefix i64) (param $limit i64) (param $removed_count_out i32)
+    (return i32))
+(func $ext_default_child_storage_clear_prefix_version_3
+    (param $child_storage_key i64) (param $prefix i64)
+    (param $limit i64)  (param $removed_count_out i32) (return i32))
+(func $ext_default_child_storage_kill_version_4
+    (param $child_storage_key i64) (param $limit i64)
+    (param $removed_count_out i32) (return i32))
+

The behaviour of these functions is identical to their version 2 and 3 counterparts. Instead of allocating a buffer, writing the output to it, and returning a pointer to it, the version 3 and 4 of these functions accepts a removed_count_out parameter containing the memory location to a 8 bytes buffer where the host writes the number of keys that were removed in little endian. The runtime execution stops with an error if removed_count_out is outside of the range of the memory of the virtual machine. The functions return 1 to indicate that there are keys remaining, and 0 to indicate that all keys have been removed.

Note that there is an alternative proposal to add new host functions with the same names: https://github.com/w3f/PPPs/pull/7. This alternative doesn't conflict with this one except for the version number. One proposal or the other will have to use versions 4 and 5 rather than 3 and 4.

(func $ext_crypto_ed25519_sign_version_2
+    (param $key_type_id i32) (param $key i32) (param $msg i64) (param $out i32) (return i32))
+(func $ext_crypto_sr25519_sign_version_2
+    (param $key_type_id i32) (param $key i32) (param $msg i64) (param $out i32) (return i32))
+func $ext_crypto_ecdsa_sign_version_2
+    (param $key_type_id i32) (param $key i32) (param $msg i64) (param $out i32) (return i32))
+(func $ext_crypto_ecdsa_sign_prehashed_version_2
+    (param $key_type_id i32) (param $key i32) (param $msg i64) (param $out i32) (return i64))
+

The behaviour of these functions is identical to their version 1 counterparts. The new versions of these functions accept an out parameter containing the memory location where the host writes the signature. The runtime execution stops with an error if out is outside of the range of the memory of the virtual machine, even if the function wouldn't write anything to out. The signatures are always of a size known at compilation time. On success, these functions return 0. If the public key can't be found in the keystore, these functions return 1 and do not write anything to out.

Note that the return value is 0 on success and 1 on failure, while the previous version of these functions write 1 on success (as it represents a SCALE-encoded Some) and 0 on failure (as it represents a SCALE-encoded None). Returning 0 on success and non-zero on failure is consistent with common practices in the C programming language and is less surprising than the opposite.

(func $ext_crypto_secp256k1_ecdsa_recover_version_3
+    (param $sig i32) (param $msg i32) (param $out i32) (return i64))
+(func $ext_crypto_secp256k1_ecdsa_recover_compressed_version_3
+    (param $sig i32) (param $msg i32) (param $out i32) (return i64))
+

The behaviour of these functions is identical to their version 2 counterparts. The new versions of these functions accept an out parameter containing the memory location where the host writes the signature. The runtime execution stops with an error if out is outside of the range of the memory of the virtual machine, even if the function wouldn't write anything to out. The signatures are always of a size known at compilation time. On success, these functions return 0. On failure, these functions return a non-zero value and do not write anything to out.

The non-zero value written on failure is:

1: incorrect value of R or S
2: incorrect value of V
3: invalid signature

These values are equal to the values returned on error by the version 2 (see https://spec.polkadot.network/chap-host-api#defn-ecdsa-verify-error), but incremented by 1 in order to reserve 0 for success.

(func $ext_crypto_ed25519_num_public_keys_version_1
+    (param $key_type_id i32) (return i32))
+(func $ext_crypto_ed25519_public_key_version_2
+    (param $key_type_id i32) (param $key_index i32) (param $out i32))
+(func $ext_crypto_sr25519_num_public_keys_version_1
+    (param $key_type_id i32) (return i32))
+(func $ext_crypto_sr25519_public_key_version_2
+    (param $key_type_id i32) (param $key_index i32) (param $out i32))
+(func $ext_crypto_ecdsa_num_public_keys_version_1
+    (param $key_type_id i32) (return i32))
+(func $ext_crypto_ecdsa_public_key_version_2
+    (param $key_type_id i32) (param $key_index i32) (param $out i32))
+

The functions superceded the ext_crypto_ed25519_public_key_version_1, ext_crypto_sr25519_public_key_version_1, and ext_crypto_ecdsa_public_key_version_1 host functions.

Instead of calling ext_crypto_ed25519_public_key_version_1 in order to obtain the list of all keys at once, the runtime should instead call ext_crypto_ed25519_num_public_keys_version_1 in order to obtain the number of public keys available, then ext_crypto_ed25519_public_key_version_2 repeatedly. +The ext_crypto_ed25519_public_key_version_2 function writes the public key of the given key_index to the memory location designated by out. The key_index must be between 0 (included) and n (excluded), where n is the value returned by ext_crypto_ed25519_num_public_keys_version_1. Execution must trap if n is out of range.

The same explanations apply for ext_crypto_sr25519_public_key_version_1 and ext_crypto_ecdsa_public_key_version_1.

Host implementers should be aware that the list of public keys (including their ordering) must not change while the runtime is running. This is most likely done by copying the list of all available keys either at the start of the execution or the first time the list is accessed.

(func $ext_offchain_http_request_start_version_2
+  (param $method i64) (param $uri i64) (param $meta i64) (result i32))
+

The behaviour of this function is identical to its version 1 counterpart. Instead of allocating a buffer, writing the request identifier in it, and returning a pointer to it, the version 2 of this function simply returns the newly-assigned identifier to the HTTP request. On failure, this function returns -1. An identifier of -1 is invalid and is reserved to indicate failure.

(func $ext_offchain_http_request_write_body_version_2
+  (param $method i64) (param $uri i64) (param $meta i64) (result i32))
+(func $ext_offchain_http_response_read_body_version_2
+  (param $request_id i32) (param $buffer i64) (param $deadline i64) (result i64))
+

The behaviour of these functions is identical to their version 1 counterpart. Instead of allocating a buffer, writing two bytes in it, and returning a pointer to it, the new version of these functions simply indicates what happened:

For ext_offchain_http_request_write_body_version_2, 0 on success.
For ext_offchain_http_response_read_body_version_2, 0 or a non-zero number of bytes on success.
-1 if the deadline was reached.
-2 if there was an I/O error while processing the request.
-3 if the identifier of the request is invalid.

These values are equal to the values returned on error by the version 1 (see https://spec.polkadot.network/chap-host-api#defn-http-error), but tweaked in order to reserve positive numbers for success.

When it comes to ext_offchain_http_response_read_body_version_2, the host implementers must not read too much data at once in order to not create ambiguity in the returned value. Given that the size of the buffer is always inferior or equal to 4 GiB, this is not a problem.

(func $ext_offchain_http_response_wait_version_2
+    (param $ids i64) (param $deadline i64) (param $out i32))
+

The behaviour of this function is identical to its version 1 counterpart. Instead of allocating a buffer, writing the output to it, and returning a pointer to it, the new version of this function accepts an out parameter containing the memory location where the host writes the output. The runtime execution stops with an error if out is outside of the range of the memory of the virtual machine.

The encoding of the response code is also modified compared to its version 1 counterpart and each response code now encodes to 4 little endian bytes as described below:

100-999: the request has finished with the given HTTP status code.
-1 if the deadline was reached.
-2 if there was an I/O error while processing the request.
-3 if the identifier of the request is invalid.

The buffer passed to out must always have a size of 4 * n where n is the number of elements in the ids.

(func $ext_offchain_http_response_header_name_version_1
+    (param $request_id i32) (param $header_index i32) (param $out i64) (result i64))
+(func $ext_offchain_http_response_header_value_version_1
+    (param $request_id i32) (param $header_index i32) (param $out i64) (result i64))
+

These functions supercede the ext_offchain_http_response_headers_version_1 host function.

Contrary to ext_offchain_http_response_headers_version_1, only one header indicated by header_index can be read at a time. Instead of calling ext_offchain_http_response_headers_version_1 once, the runtime should call ext_offchain_http_response_header_name_version_1 and ext_offchain_http_response_header_value_version_1 multiple times with an increasing header_index, until a value of -1 is returned.

These functions accept an out parameter containing a pointer-size to the memory location where the header name or value should be written. The runtime execution stops with an error if out is outside of the range of the memory of the virtual machine, even if the function wouldn't write anything to out.

These functions return the size, in bytes, of the header name or header value. If request doesn't exist or is in an invalid state (as documented for ext_offchain_http_response_headers_version_1) or the header_index is out of range, a value of -1 is returned. Given that the host must never write more bytes than the size of the buffer in out, and that the size of this buffer is expressed as a 32 bits number, a 64bits value of -1 is not ambiguous.

If the buffer in out is too small to fit the entire header name of value, only the bytes that fit are written and the rest are discarded.

(func $ext_offchain_submit_transaction_version_2
+    (param $data i64) (return i32))
+(func $ext_offchain_http_request_add_header_version_2
+    (param $request_id i32) (param $name i64) (param $value i64) (result i32))
+

Instead of allocating a buffer, writing 1 or 0 in it, and returning a pointer to it, the version 2 of these functions return 0 or 1, where 0 indicates success and 1 indicates failure. The runtime must interpret any non-0 value as failure, but the client must always return 1 in case of failure.

(func $ext_offchain_local_storage_read_version_1
+    (param $kind i32) (param $key i64) (param $value_out i64) (param $offset i32) (result i64))
+

This function supercedes the ext_offchain_local_storage_get_version_1 host function, and uses an API and logic similar to ext_storage_read_version_2.

It reads the offchain local storage key indicated by kind and key starting at the byte indicated by offset, and writes the value to the pointer-size indicated by value_out.

The function returns the number of bytes that were written in the value_out buffer. If the entry doesn't exist, a value of -1 is returned. Given that the host must never write more bytes than the size of the buffer in value_out, and that the size of this buffer is expressed as a 32 bits number, a 64bits value of -1 is not ambiguous.

(func $ext_offchain_network_peer_id_version_1
+    (param $out i64))
+

This function writes the PeerId of the local node to the memory location indicated by out. A PeerId is always 38 bytes long. +The runtime execution stops with an error if out is outside of the range of the memory of the virtual machine.

(func $ext_input_size_version_1
+    (return i64))
+(func $ext_input_read_version_1
+    (param $offset i64) (param $out i64))
+

When a runtime function is called, the host uses the allocator to allocate memory within the runtime where to write some input data. These two new host functions provide an alternative way to access the input that doesn't make use of the allocator.

The ext_input_size_version_1 host function returns the size in bytes of the input data.

The ext_input_read_version_1 host function copies some data from the input data to the memory of the runtime. The offset parameter indicates the offset within the input data where to start copying, and must be inferior or equal to the value returned by ext_input_size_version_1. The out parameter is a pointer-size containing the buffer where to write to. +The runtime execution stops with an error if offset is strictly superior to the size of the input data, or if out is outside of the range of the memory of the virtual machine, even if the amount of data to copy would be 0 bytes.

Other changes

In addition to the new host functions, this RFC proposes two changes to the runtime-host interface:

The following function signature is now also accepted for runtime entry points: (func (result i64)).
Runtimes no longer need to expose a constant named __heap_base.

All the host functions that are being superceded by new host functions are now considered deprecated and should no longer be used. +The following other host functions are similarly also considered deprecated:

ext_storage_get_version_1
ext_default_child_storage_get_version_1
ext_allocator_malloc_version_1
ext_allocator_free_version_1
ext_offchain_network_state_version_1

Drawbacks

This RFC might be difficult to implement in Substrate due to the internal code design. It is not clear to the author of this RFC how difficult it would be.

Prior Art

The API of these new functions was heavily inspired by API used by the C programming language.

Unresolved Questions

The changes in this RFC would need to be benchmarked. This involves implementing the RFC and measuring the speed difference.

It is expected that most host functions are faster or equal speed to their deprecated counterparts, with the following exceptions:

+
ext_input_size_version_1/ext_input_read_version_1 is inherently slower than obtaining a buffer with the entire data due to the two extra function calls and the extra copying. However, given that this only happens once per runtime call, the cost is expected to be negligible.
+
+
The ext_crypto_*_public_keys, ext_offchain_network_state, and ext_offchain_http_* host functions are likely slightly slower than their deprecated counterparts, but given that they are used only in offchain workers this is acceptable.
+
+
It is unclear how replacing ext_storage_get with ext_storage_read and ext_default_child_storage_get with ext_default_child_storage_read will impact performances.
+
+
It is unclear how the changes to ext_storage_next_key and ext_default_child_storage_next_key will impact performances.
+

Future Possibilities

After this RFC, we can remove from the source code of the host the allocator altogether in a future version, by removing support for all the deprecated host functions. +This would remove the possibility to synchronize older blocks, which is probably controversial and requires a some preparations that are out of scope of this RFC.

(source)

Table of Contents

RFC-0026: Sassafras Consensus Protocol +
+

RFC-0026: Sassafras Consensus Protocol

+ + + +


Start Date	September 06, 2023
Description	Sassafras consensus protocol description and structures
Authors	Davide Galassi

Abstract

Sassafras is a novel consensus protocol designed to address the recurring +fork-related challenges encountered in other lottery-based protocols.

Sassafras aims to establish a unique association between each epoch's slots and +the validators, ensuring that there is one and only one validator per slot.

The protocol ensures the anonymity of the validator associated to a slot until +the slot is not claimed at block production time.

1. Motivation

Sassafras Protocol has been extensively documented in a comprehensive research +paper from the Web3 foundation +research team.

This RFC serves the purpose of conveying most of the essential implementation +details that are crucial for interoperability and clarifying aspects left open +for implementation discretion.

1.1. Relevance to Implementors

This RFC focuses on providing implementors with the necessary insights into the +protocol's operation.

To avoid ambiguities and interoperability issues, this document takes precedence +over the research paper in cases where discrepancies arise between the two.

1.2. Supporting Sassafras for Polkadot

In addition to fostering interoperability, another objective of this RFC is to +facilitate the implementation of Sassafras within the Polkadot ecosystem. While +the specifics of deployment mechanics are beyond the scope of this document, it +paves the way for integrating Sassafras into the Polkadot network.

2. Stakeholders

2.1. Developers of Blockchains

Developers responsible for creating blockchains who intend to leverage the +benefits offered by the Sassafras Protocol.

2.2. Contributors to the Polkadot Ecosystem

Developers contributing to the Polkadot ecosystem, both relay-chain and para-chains. +The protocol will have a central role in the next generation Polkadot relay chain +block authoring system.

3. Notation and Convention

This section outlines the notation and conventions used throughout the document +to ensure clarity and consistency.

3.1. Data Structures Definitions and Encoding

Data structures are primarily defined using ASN.1, +with a few exceptions:

Integer types are not explicitly defined in ASN.1 and in the context of +this document U<n> should be interpreted as a n-bit unsigned integers

If no context-specific instructions are given, all types must be serialized +using SCALE codec.

To ensure interoperability of serialized structures, the order of the single +fields is required to match the structures definitions found in this document.

3.2. Pseudo-Code

Through this document it is advantageous to make use of code snippets as part +of the comprehensive description. These snippets shall adhere to the subsequent +conventions:

+
For simplicity, code snippets are presented in a Rust-like pseudocode format.
+
+
The function BYTES(x: T) returns an OCTET_STRING representing the raw +byte array representation of the object x with type T.
+
- If T is VisibleString (i.e. an ASCII string): it returns the sequence +of octets of its ASCII representation.
- If T is U<n>: it returns the little-endian encoding of the integer +U<n> as n/8 octets.
+
+
The function U<n>(x: OCTET_STRING) returns a U<n> interpreting x as +the little-endian encoding of a n bits unsigned integer.
+
+
The function SCALE(x: T) returns an OCTET_STRING representing the +SCALE encoding of +x with type T.
+
+
The function BLAKE2(n: U32, x: OCTET_STRING) returns n bytes of the +standard blake2b hash of x as an OCTET_STRING.
+
+
The function CONCAT(x₀: OCTET_STRING, ..., xₖ: OCTET_STRING) returns the +concatenation of the inputs as an OCTET_STRING.
+
+
The function LENGTH(x: OCTET_STRING) returns a U32 representing the +number of octets in x.
+

3.3. Incremental Introduction of Types and Functions

Types and helper functions will be introduced incrementally as they become +relevant within the document's context.

We find this approach more agile, especially given that the set of types used is +not extensive or overly complex.

This incremental presentation enhances readability and comprehension.

4. Protocol Introduction

Timeline is partitioned in epochs, epochs are partitioned in slots.

The Sassafras protocol employs a binding mechanism between validators and slots +through the use of a ticketing system.

The protocol can be divided into five discrete and asynchronous phases:

4.1. Submission of Candidate Tickets

Validators generate and submit their candidate tickets to the blockchain. Each +ticket comes with an anonymous validity proof.

4.2. Validation of Candidate Tickets

Each candidate tickets undergo a validation process for the associated validity +proof and compliance with other protocol-specific constraints.

4.3. Tickets and Slots Binding

After collecting all candidate tickets, a deterministic method is employed to +uniquely associate a subset of these tickets to the next epoch slots.

4.4. Claim of Ticket Ownership

Validators prove ownership of tickets during the block production phase. This +step establishes a secure binding between validators and their respective slots.

4.5. Validation of Ticket Ownership

During block verification, the claims of ticket ownership are validated to +uphold the protocol's integrity.

5. Bandersnatch VRFs Cryptographic Primitives

This chapter provides a high-level overview of the Bandersnatch VRF primitive as +it relates to the Sassafras protocol.

It's important to note that this section is not intended to serve as an +exhaustive exploration of the mathematically intensive foundations of the +cryptographic primitive. Instead, its primary purpose is to offer a concise and +comprehensible interpretation of the primitive within the context of this RFC.

For a more detailed understanding we recommend referring to the Ring-VRF +research paper from W3F.

5.1. VRF Input

The VRF Input, denoted as VrfInput, is constructed by combining a domain identifier +with arbitrary data using the vrf_input function:

#![allow(unused)]
+fn main() {
+    fn vrf_input(domain: OCTET_STRING, buf: OCTET_STRING) -> VrfInput;
+}

The specific implementation details of this function are intentionally omitted +here, you can find a complete reference implementation in the +bandersnatch_vrfs +project.

Helper function to construct a VrfInput from a sequence of data items:

#![allow(unused)]
+fn main() {
+    fn vrf_input_from_items(domain: OCTET_STRING, data: SEQUENCE_OF OCTET_STRING) -> VrfInput {
+        buf = OCTET_STRING(SIZE(0));
+        for item in data {
+            buf.append(item);
+            buf.append(LENGTH(item) as U8);
+        }
+        return vrf_input(domain, buf);
+    }
+}

Note that we cast the length of each item to a U8. In the context of the +protocol we never have to append strings longer than 255. The function is +internal and not designed to be generic.

+ +Or we should provide a generic one in bandersnatch primitive wrapper to be +used in other contexts? + +

5.2. VRF Output

A VrfOutput in this context is computed in function of a VrfInput and a +BandersnatchSecretKey.

A VrfOutput can be created in two ways: as a standalone object or as part of a +VRF signature. In both scenarios, the resulting VrfOutput remains the same, but +the primary difference lies in the inclusion of a signature in the latter, which +serves to confirm its validity.

In practice, the VrfOutput is a verifiable seed to produce a variable number +of pseudo-random bytes. These bytes are considered valid when VrfOutput is +accompanied by a valid signature.

When constructed as a standalone object, VrfOutput is primarily employed +in situations where the secret key owner needs to check if the generated +pseudo-random bytes fulfill some criteria before applying the signature.

To facilitate the construction of VrfOutput from a secret key and VrfInput, +the following helper function is provided:

#![allow(unused)]
+fn main() {
+    fn vrf_output(secret: BandernatchSecretKey, input: VrfInput) -> VrfOutput;
+}

Additionally, a helper function is provided for producing len bytes from +VrfInput and VrfOutput:

#![allow(unused)]
+fn main() {
+    fn vrf_bytes(len: U32, input: VrfInput, output: VrfOuput) -> OCTET_STRING;
+}

Just like the VrfInput support function, we have intentionally excluded the +detailed implementation of this function in this document. A reference implementation +is provided in the dleq_vrfs library:

vrf_output
vrf_bytes

5.3. VRF Signature Data

This section defines the data to be signed using the VRF primitive:

#![allow(unused)]
+fn main() {
+    VrfSignatureData ::= SEQUENCE {
+        transcript: Transcript,
+        inputs: SEQUENCE_OF VrfInput
+    }
+}

transcript: an ark-transcript +object. In practice, this is a special hash of some protocol-specific data +to sign which should not influence the VrfOutput.
inputs: sequence of VrfInputs to be signed.

To simplify the construction of a VrfSignatureData object, a helper function is provided:

#![allow(unused)]
+fn main() {
+    fn vrf_signature_data(
+        transcript_label: OCTET_STRING,
+        transcript_data: SEQUENCE_OF OCTET_STRING,
+        inputs: SEQUENCE_OF VrfInput
+    ) -> VrfSignatureData {
+        let mut transcript = Transcript::new_labeled(transcript_label);
+        for data in transcript_data {
+            transcript.append(data);
+        }
+        VrfSignatureData { transcript, inputs }
+    }
+}

5.4. VRF Signature

Bandersnatch VRF offers two signature flavors:

plain signature, which is much like a traditional Schnorr signature,
ring signature which leverages a zk-SNARK to allows for anonymous signatures +using a key from a predefined set of enabled keys, known as the ring.

5.4.1. Plain VRF Signature

This section describes the signature process for VrfSignatureData using the +plain Bandersnatch signature flavor.

#![allow(unused)]
+fn main() {
+    PlainSignature ::= OCTET_STRING;
+
+    VrfSignature ::= SEQUENCE {
+        signature: PlainSignature,
+        outputs: SEQUENCE-OF VrfOutput
+    }
+}

signature: the actual signature.
outputs: a sequence of VrfOutputs corresponding to the VrfInputs values.

Helper function to create a VrfPlainSignature from VrfSignatureData:

#![allow(unused)]
+fn main() {
+    BandersnatchSecretKey ::= OCTET_STRING;
+
+    fn vrf_sign(
+        secret: BandernatchSecretKey,
+        signature_data: VrfSignatureData
+    ) -> VrfSignature
+}

Helper function for validating the signature and returning a BOOLEAN value +indicating the validity of the signature.

#![allow(unused)]
+fn main() {
+    BandersnatchPublicKey ::= OCTET_STRING;
+
+    fn vrf_verify(
+        public: BandersnatchPublicKey,
+        signature: VrfSignature
+    ) -> BOOLEAN;
+}

In this document, the types BandersnatchSecretKey, BandersnatchPublicKey +and PlainSignature are intentionally left undefined. Their definitions can be +found in the bandersnatch_vrfs reference implementation.

5.4.2. Ring VRF Signature

This section deals with the signature process for VrfSignatureData using the +Bandersnatch ring signature flavor.

#![allow(unused)]
+fn main() {
+    RingSignature ::= OCTET_STRING;
+
+    RingVrfSignature ::= SEQUENCE {
+        signature: RingSignature,
+        outputs: SEQUENCE_OF VrfOutput
+    }
+}

signature: the actual signature.
outputs: sequence of VrfOutput objects corresponding to the VrfInput values.

Helper function to create a RingVrfSignature from VrfSignatureData:

#![allow(unused)]
+fn main() {
+    BandersnatchRingProverKey ::= OCTET_STRING;
+    
+    fn ring_vrf_sign(
+        secret: BandersnatchRingProverKey,
+        signature_data: VrfSignatureData,
+    ) -> RingVrfSignature;
+}

Helper function for validating the signature and returning a BOOLEAN +indicating the validity of the signature (True if it's valid). It's important +to note that this function does not require the signer's public key.

#![allow(unused)]
+fn main() {
+    BandersnatchRingVerifierKey ::= OCTET_STRING;
+
+    fn ring_vrf_verify(
+        verifier: BandersnatchRingVerifierKey,
+        signature: RingVrfSignature,
+    ) -> BOOLEAN;
+}

In this document, the types BandersnatchRingProverKey, +BandersnatchRingVerifierKey, and RingSignature are intentionally left +undefined. Their definitions can be found in the bandersnatch_vrfs reference +implementation.

6. Sassafras Protocol

6.1. Epoch's First Block

The first block produced for epoch N is required to include the descriptor for +the next epoch N+1.

The descriptor for next epoch is NextEpochDescriptor.

#![allow(unused)]
+fn main() {
+    AuthorityId ::= BandersnatchPublicKey;
+
+    Randomness ::= OCTET_STRING(SIZE(32));
+
+    NextEpochDescriptor ::= SEQUENCE {
+        randomness: Randomness,
+        authorities: SEQUENCE_OF AuthorityId,
+        configuration: ProtocolConfiguration OPTIONAL
+    }
+}

randomness: randomness value.
authorities: list of authorities.
configuration: optional protocol configuration.

The NextEpochDescriptor must be SCALE encoded and embedded in the block +header digest log.

The identifier for the digest element is BYTES("SASS").

Security Consideration: Instances of NextEpochDescriptor are generated +through on-chain code whenever a block is identified as the first of an epoch. +Consequently, every node executing the block should verify that the descriptor +locally generated during block execution matches the one produced by the block +author, which is found in the digest data before block import.

6.1.1. Epoch Randomness

Each block ships with some entropy source in the form of bandersnatch +VrfOutput. Per block randomness is accumulated in the protocol's on-chain +randomness accumulator after block import.

The exact procedure to accumulate per-block randomness is described in detail +later, in the randomness accumulator paragraph (6.7).

Next epoch randomness is computed as:

#![allow(unused)]
+fn main() {
+    next_epoch_randomness = BLAKE2(32, CONCAT(randomness_accumulator, BYTES(next_epoch_index)));
+}

6.1.2. Protocol Configuration

The ProtocolConfiguration primarily influences certain checks carried out +during tickets validation. It is defined as follows:

#![allow(unused)]
+fn main() {
+    ProtocolConfiguration ::= SEQUENCE {
+        attempts_number: U32,
+        redundancy_factor: U32
+    }
+}

attempts_number: max number of tickets that can be submitted by each +next epoch authority.
redundancy_factor: controls the expected number of extra tickets produced +beyond epoch_length.

The attempts number influences the anonymity of block producers. As all +published tickets have a public attempt number less than attempts_number, +all the tickets which share the attempt number value must belong to different +block producers, which reduces anonymity late in the epoch.

We do not mind max_attempts < epoch_length though because this loss of +anonymity already becomes small when attempts_number = 64 or 128 and larger +values requires more computation.

Details about how exactly these parameters drives the ticket validity +probability can be found in the section dedicated to candidate ticket validation +(6.2.2).

ProtocolConfiguration values can be adjusted via a dedicated extrinsic which +should have origin set to Root. A valid configuration proposal submitted on +epoch K will be propagated in the NextEpochDescriptor at the beginning of +epoch K+1 and will be effectively enacted on epoch K+2.

6.1.3. Startup Parameters

Some parameters for first epoch (index = 0) are configurable via genesis configuration.

#![allow(unused)]
+fn main() {
+    GenesisConfig ::= SEQUENCE {
+        authorities: SEQUENCE_OF AuthorityId,
+        configuration: ProtocolConfiguration OPTIONAL
+    }
+}

Randomness for first epoch is set to all zeros.

As block #0 is locally produced by every node by processing the genesis configuration, +the first block explicitly produced by a validator for the first epoch is block #1.

Block #1 must embed the NextEpochDescriptor for next epoch. This is +constructed re-using the same values used for the first epoch.

6.2. Creation and Submission of Candidate Tickets

As a shorthand notation, in this section we refer to one of the next epoch +validators as 'the validator'.

Upon the beginning of a new epoch N, the validator will construct a set of +'tickets' to be submitted on-chain. These tickets aim to secure ownership of one +or more slots in the upcoming epoch N+1.

Each validator is allowed to submit a maximum number of tickets whose value is +found in the next epoch ProtocolConfiguration attempts_number field.

The expected ratio between the attempts and the number of tickets which are +assigned to the next epoch slots is driven by the ticket threshold +(6.2.2).

Each ticket has an associated unique identifier, denoted as TicketId.

#![allow(unused)]
+fn main() {
+    TicketId ::= U128
+}

6.2.1. Ticket Identifier Value

The value of the TicketId is determined by the output of the Bandersnatch VRF +when using the following inputs:

Next epoch randomness: Randomness obtained from the NextEpochDescriptor.
Next epoch index: U64 computed as epoch start slot divided epoch duration.
Attempt index: U32 value going from 0 to attempts_number.

Let next_epoch be an object with the information associated to the next epoch.

#![allow(unused)]
+fn main() {
+    ticket_id_vrf_input = vrf_input_from_items(
+        BYTES("sassafras-ticket-v1.0"),
+        [ 
+            next_epoch.randomness,
+            BYTES(next_epoch.epoch_index),
+            BYTES(attempt_index)
+        ]
+    );
+
+    ticket_id_vrf_output = vrf_output(AUTHORITY_SECRET_KEY, ticket_id_vrf_input);
+
+    ticket_bytes = vrf_bytes(16, ticket_id_vrf_input, ticket_id_vrf_output);
+    ticket_id = U128(ticket_bytes);
+}

6.2.2. Tickets Threshold

A TicketId value is valid if its value is less than the ticket threshold.

T = (r·s)/(a·v)
+

Where:

v: the number of authorities (aka validators) in the epoch
s: number of slots in the epoch
r: the redundancy factor
a: number of attempts
T: ticket threshold value (0 ≤ T ≤ 1)

6.2.2.1 Formula Derivation

For an epoch of s slots we want to have a number of tickets in expectation for +block production equal to the r·s.

We need that there is a very small probability of their being less than s +winning tickets, even if up to 1/3 of authorities are offline.

First we set the probability of a ticket winning as T = (r·s)/(a·v).

Let n be the number of validators who actually participate and so v·2/3 ≤ n ≤ v.

These n validators make a attempts each, for a total of a·n attempts.

Let X be the random variable associated to the number of winning tickets, then +its expected value is:

E[X] = T·a·n = (r·s·n)/v
+

By setting r = 2, we get

s·4/3 ≤ E[X] ≤ s·2
+

Using Bernestein's inequality we get Pr[X < s] ≤ exp(-s/21).

For s = 600 this gives Pr[X < s] < 4·10⁻¹³, and thus we end up with a great +tolerance over offline nodes and we end-up filling all the slots with tickets +with high probability.

For more details about threshold formula please refer to the +probabilities and parameters +paragraph of the w3f description of the protocol.

6.2.3. Ticket Body

Every candidate ticket identifier has an associated body.

#![allow(unused)]
+fn main() {
+    TicketBody ::= SEQUENCE {
+        attempt_index: U32,
+        erased_pub: Ed25519PublicKey,
+        revealed_pub: Ed25519PublicKey
+    }
+}

attempt_index: attempt index used to generate the associated TicketId.
erased_pub: Ed25519 ephemeral public key which gets erased as soon as the +ticket is claimed.
revealed_pub: Ed25519 ephemeral public key which gets exposed as soon as the +ticket is claimed.

The process of generating an erased key pair is intentionally left undefined, +allowing the implementor the freedom to choose the most suitable strategy.

Revealed key pair is generated using bytes produced by the VRF with input +parameters equal to those employed in TicketId generation, only the label +is different.

Let next_epoch be an object with the information associated to the next epoch:

#![allow(unused)]
+fn main() {
+    revealed_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-revealed-v1.0"),
+        data: [ 
+            next_epoch.randomness,
+            BYTES(next_epoch.epoch_index),
+            BYTES(attempt_index)
+        ]
+    );
+
+    revealed_vrf_output = vrf_output(AUTHORITY_SECRET_KEY, revealed_vrf_input);
+
+    revealed_seed = vrf_bytes(32, revealed_vrf_input, revealed_vrf_output);
+    revealed_pub = ed25519_secret_from_seed(revealed_seed).public();
+}

The usage of the ephemeral public keys will be clarified in the ticket claiming +section (6.5).

6.2.4. Ring Signature Production

TicketBody must be signed using the Bandersnatch ring VRF flavor (5.4.2).

#![allow(unused)]
+fn main() {
+    sign_data = vrf_signature_data(
+        transcript_label: BYTES("sassafras-ticket-body-v1.0"),
+        transcript_data: [
+            SCALE(ticket_body)
+        ],
+        inputs: [
+            ticket_id_vrf_input
+        ]
+    )
+  
+    ring_signature = ring_vrf_sign(RING_PROVER_KEY, sign_data)
+}

RING_PROVER object is constructed using the authority secret key, the set +public keys which belong to the next epoch authorities and the zk-SNARK +context parameters (more details in the +bandersnatch_vrfs +reference implementation).

The body and the ring signature are combined in the TicketEnvelope:

#![allow(unused)]
+fn main() {
+    TicketEnvelope ::= SEQUENCE {
+        ticket_body: TicketBody,
+        ring_signature: RingVrfSignature
+    }   
+}

All the ticket envelopes corresponding to valid tickets are submitted on-chain +via a dedicated unsigned extrinsic.

6.3. Validation of candidate tickets

All the actions in the steps described by this paragraph are executed by +on-chain code.

The tickets are received via a dedicated extrinsic call.

Generic validation rules:

Tickets submissions must occur within the first half of the epoch.
For unsigned extrinsics, it must be submitted by one of the current session +validators.

Ticket specific validation rules:

Ring signature is verified using the on-chain BandersnatchRingVerifierKey.
Ticket identifier is locally computed from the VrfOutput contained in the +RingVrfSignature and its value is checked to be less than the ticket-threshold.

Valid tickets bodies are persisted on-chain.

6.4. Ticket-Slot assignment

Before the beginning of the next epoch, the on-chain list of tickets must be +associated with the next epoch's slots.

The assignment process happens in the second half of the submission epoch.

In the end, there must be at most one ticket per slot.

Initially, the complete list of tickets is sorted based on their ticket-id, +with smaller values coming first.
In cases where there are more tickets than available slots, the list is pruned +by removing the larger value.
Tickets are then assigned to the slots using an outside-in assignment strategy.

6.4.1. Outside-In Assignment

Given an ordered sequence of tickets [t0, t1, t2, ..., tk] to be assigned to +n slots, where n ≥ k, the tickets are allocated according to the following +strategy:

    slot-index  : [  0,  1,  2, ............ , n ]
+    tickets     : [ t1, t3, t5, ... , t4, t2, t0 ]
+

Here slot-index is a relative value computed as:

slot-index = absolute_slot_index - epoch_start_slot
+

The association between each ticket and a slot is recorded on-chain and thus +is public. What remains confidential is the identity of the ticket owner, and +consequently, who possesses the authority to claim the corresponding slot. This +information is known only to the author of the ticket.

6.4.2. Fallback Assignment

In case the number of available tickets is less than the number of epoch slots, +some (orphan) slots in the middle of the epoch will remain unbounded to any +ticket.

In such situation, these unassigned slots are allocated using a fallback +assignment strategy.

The authorities registered on-chain are kept in a sorted buffer.

The index of the authority which has the privilege to claim an unbounded slot is +calculated as follows:

#![allow(unused)]
+fn main() {
+    index_bytes = BLAKE2(4, CONCAT(epoch_randomness, BYTES(slot)));
+    index = U32(index_bytes) mod authorities_number;
+}

6.5. Claim of ticket ownership during block production

With tickets bound to epoch slots, every validator acquires information about +the slots for which they are supposed to produce a block.

The procedure for block authoring varies based on whether a given slot has an +associated ticket according to the on-chain state.

If a slot is associated with a ticket, we will employ the primary authoring +method. Conversely, if the slot lacks an associated ticket, we will resort to +the secondary authoring method as a fallback.

6.5.1. Primary Claim Method

Let ticket_body represent the TicketBody that has been committed to the on- +chain state, curr_epoch denote an object containing information about the +current epoch, and slot represent the absolute monotonic slot number.

Follows the construction of VrfSignatureData:

#![allow(unused)]
+fn main() {
+    randomness_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-randomness-v1.0"),
+        data: [
+            curr_epoch.randomness,
+            BYTES(curr_epoch.epoch_index),
+            BYTES(slot)
+        ]
+    );
+
+    revealed_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-revealed-v1.0"),
+        data: [
+            curr_epoch.randomness,
+            BYTES(curr_epoch.epoch_index),
+            BYTES(ticket_body.attempt_index)
+        ]
+    );
+    
+    sign_data = vrf_signature_data(
+        transcript_label: BYTES("sassafras-claim-v1.0"),
+        transcript_data: [
+            SCALE(ticket_body)
+        ],
+        inputs: [
+            randomness_vrf_input,
+            revealed_vrf_input
+        ]
+    );
+}

The inclusion of revealed_vrf_input will generate a VrfSignature with a +VrfOutput allowing the verifier to reconstruct a revealed_pub key +which is expected to be equal to the one committed into the TicketBody.

6.5.1.1. (Optional) Ed25519 Erased Ephemeral Key Claim

As the ticket ownership can be claimed by reconstructing the revealed_pub +entry of the ticket, this step is purely optional and serves only to enforce +the claim.

+ +Is this step really necessary? +- Isn't better to keep it simple if this step doesn't offer any extra security? +- We already have a strong method to claim ticket ownership. + +

The Fiat-Shamir transform is used to obtain a 32-byte challenge associated +with the VrfSignData transcript.

Validators employ the secret key associated with erased_pub, which has been +committed in the TicketBody, to sign this challenge.

#![allow(unused)]
+fn main() {
+    challenge = sign_data.transcript.challenge();
+    erased_signature = ed25519_sign(ERASED_SECRET_KEY, challenge)
+}

6.5.2. Secondary Claim Method

If the slot doesn't have any associated ticket then the validator is the one +with index equal to the rule exposed in the fallback assignment section +(6.4.2).

Given randomness_vrf_input constructed as shown for the primary method, the +VrfSignatureData is constructed as:

#![allow(unused)]
+fn main() {
+    sign_data = vrf_signature_data(
+        transcript_label: BYTES("sassafras-slot-claim-transcript-v1.0"),
+        transcript_data: [ ],
+        inputs: [
+            randomness_vrf_input
+        ]
+    )
+}

6.5.3. Slot Claim object

To establish ownership of a slot, the block author must construct a SlotClaim object +which contains all the necessary information to assert ownership of the slot.

#![allow(unused)]
+fn main() {
+    SlotClaim ::= SEQUENCE {
+        authority_index: U32,
+        slot: U64,
+        signature: VrfSignature,
+        erased_signature: Ed25519Signature OPTIONAL
+    }
+}

+
authority_index: index of the block author in the on-chain authorities list.
+
+
slot: absolute slot number (not relative with respect to the epoch start)
+
+
signature: signature that includes one or two VrfOutputs.
+
- The first VrfOutput is always present and is used to generate per-block +randomness. This is used to claim ticket ownership.
- The second VrfOutput is included if the slot is associated with a ticket. +This is relevant to claim ticket ownership.
+
+
erased_signature: optional signature providing an additional proof of ticket +ownership (see 6.5.1.1).
+

#![allow(unused)]
+fn main() {
+    signature = vrf_sign(AUTHORITY_SECRET_KEY, sign_data);
+
+    claim = SlotClaim {
+        authority_index,
+        slot,
+        signature,
+        erased_signature
+    }
+}

The claim object is SCALE encoded and sent in the block's header digest log.

6.6. Validation of the claim during block verification

Validation of SlotClaim object found in the block's header.

The procedure depends on whether the slot has an associated ticket or not +according to the on-chain state.

If there is a ticket linked to the slot, the primary verification method will be +used; otherwise, the protocol resorts to the secondary one.

In both scenarios, the signature within the SlotClaim is verified using +a VrfSignData constructed as specified by paragraph 6.5.

Given claim an instance of SlotClaim:

#![allow(unused)]
+fn main() {
+    public_key = AUTHORITIES[claim.authority_index];
+
+    vrf_verify(public_key, sign_data, claim.signature);
+}

If signature verification fails then the claim is not legit.

6.6.1. Primary Claim Method Verification

This verification is performed to confirm ticket ownership and is performed +utilizing the second VrfOutput contained within the SlotClaim signature.

By using the VrfOutput object together with the associated expected VrfInput +the verifier should be able to reconstruct the revealed_pub key committed in +the TicketBody. If there is a mismatch, the claim is not legit.

#![allow(unused)]
+fn main() {
+    revealed_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-revealed-v1.0"),
+        data: [
+            curr_epoch.randomness,
+            BYTES(curr_epoch.epoch_index),
+            BYTES(ticket_body.attempt_index)
+        ]
+    );
+
+    reveled_vrf_output = claim.signature.outputs[1];
+
+    revealed_seed = vrf_bytes(32, revealed_vrf_input, revealed_vrf_output);
+    revealed_pub = ed25519_secret_from_seed(revealed_seed).public();
+
+    assert(revealed_pub == ticket_body.revealed_pub);
+}

6.6.1.1. (Optional) Ephemeral Key Signature Check

If the erased_signature element within the SlotClaim is present the +erased_pub key is used to verify it.

The signed challenge is generated with identical steps as outlined in section +6.5.1.1.

#![allow(unused)]
+fn main() {
+    challenge = sign_data.transcript.challenge();
+    result = ed25519_verify(ticket_body.erased_pub, challenge, claim.erased_signature);
+
+    assert(result == true);
+}

6.6.2. Secondary Claim Method Verification

If the slot doesn't have any associated ticket then the validator index contained in +the claim should match the one given by the rule outlined in the fallback assignment +section (6.4.2)

6.7. Randomness Accumulator

The first VrfOutput which ships with the block's SlotClaim signature +is mandatory and must be used as the entropy source for the randomness which +gets accumulated on-chain after block processing.

Given claim the instance of SlotClaim within the block header, and +accumulator the current value for the current epoch randomness accumulator, +the accumulator value is updated as follows:

#![allow(unused)]
+fn main() {
+    randomness_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-randomness-v1.0"),
+        data: [
+            curr_epoch.randomness,
+            BYTES(curr_epoch.epoch_index),
+            BYTES(slot)
+        ]
+    );
+
+    randomness_vrf_output = claim.signature.outputs[0];
+
+    randomness = vrf_bytes(32, randomness_vrf_input, randomness_vrf_output);
+
+    randomness_accumulator = BLAKE2(32, CONCAT(randomness_accumulator, randomness));
+}

The updated accumulator value is stored on-chain.

The randomness accumulated during epoch N will be used, at the start of the +next epoch (N+1), as an input to compute the NextEpochDescriptor +randomness element (see section 6.1).

As outlined throughout the document, epoch randomness value secures various +protocol-specific functions, including ticket generation and assignment of +fallback slots (refer to section 6.4.2). Additionally, users may utilize this +value for other purposes as needed.

7. Drawbacks

None

8. Testing, Security, and Privacy

The reference implementation for this RFC will be tested on testnets first.

An audit may be required to ensure the implementation does not introduce unwanted side effects

9. Performance, Ergonomics, and Compatibility

9.1. Performance

The utilization of Sassafras consensus represents a significant advancement in +the mitigation of short-lived fork occurrences.

Generation of forks are not possible when following the protocol and the only source +of forks is network partitioning. In this case, on recovery, the decision of +which fork to follow is not opinionated and there is only one choice.

9.2. Ergonomics

No specific considerations.

9.3. Compatibility

The adoption of Sassafras impacts native client code and thus can't be +introduced via a simple runtime upgrade.

A deployment strategy should be carefully engineered for live networks.

This subject is left open for a dedicated RFC.

10. Prior Art and References

Web3 Foundation research page: https://research.web3.foundation/Polkadot/protocols/block-production/SASSAFRAS
Sassafras whitepaper: https://eprint.iacr.org/2023/031.pdf
Ring-VRF whitepaper: https://eprint.iacr.org/2023/002.pdf
Sassafras reference implementation tracking issue: https://github.com/paritytech/substrate/issues/11515
Sassafras reference implementation main PR: https://github.com/paritytech/substrate/pull/11879

11. Unresolved Questions

None

+ +

While this RFC lays the groundwork and outlines the core aspects of the +protocol, several crucial topics remain to be addressed in future RFCs to ensure +the protocol's completeness and security.

These topics include:

12.1. Interactions with the Runtime

+
Outbound Interface. Interfaces exposed by the host which are required by the runtime. +These are commonly dubbed Host Functions.
+
+
Unrecorded Inbound Interfaces. Interfaces exposed by the runtime which are required by the host. +These are commonly dubbed Runtime APIs.
+
+
Transactional Inbound Interfaces. Interfaces exposed by the runtime which alter the state. +These are commonly dubbed Extrinsics and Inherents.
+

12.2. Deployment Strategies

Protocol Migration. Exploring how this protocol can seamlessly replace +an already operational instance of another protocol is essential. Future RFCs +should delve into the deployment strategy, including considerations for a smooth +transition process.

12.3. ZK-SNARK SRS Initialization Ceremony.

+
Timing and Procedure: Determining the timing and procedure for the ZK-SNARK +SRS (Structured Reference String) initialization ceremony. Future RFCs should +provide insights into whether this process should be performed before the +deployment of Sassafras and the steps involved.
+
+
Sharing with Para-chains: Considering the complexity of the ceremony, we +must understand whether the SRS is shared with para-chains or maintained +independently.
+

12.4. Anonymous Submission of Tickets.

Mixnet Integration: Submitting tickets directly can pose a risk of +potential deanonymization through traffic analysis. Subsequent RFCs should +investigate the potential for incorporating Mixnet technology or other +privacy-enhancing mechanisms to address this concern.

(source)

Table of Contents

RFC-34: XCM Absolute Location Account Derivation +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
+

RFC-34: XCM Absolute Location Account Derivation

+ + + +


Start Date	05 October 2023
Description	XCM Absolute Location Account Derivation
Authors	Gabriel Facco de Arruda

Summary

This RFC proposes changes that enable the use of absolute locations in AccountId derivations, which allows protocols built using XCM to have static account derivations in any runtime, regardless of its position in the family hierarchy.

Motivation

These changes would allow protocol builders to leverage absolute locations to maintain the exact same derived account address across all networks in the ecosystem, thus enhancing user experience.

One such protocol, that is the original motivation for this proposal, is InvArch's Saturn Multisig, which gives users a unifying multisig and DAO experience across all XCM connected chains.

Stakeholders

Ecosystem developers

Explanation

This proposal aims to make it possible to derive accounts for absolute locations, enabling protocols that require the ability to maintain the same derived account in any runtime. This is done by deriving accounts from the hash of described absolute locations, which are static across different destinations.

The same location can be represented in relative form and absolute form like so:

#![allow(unused)]
+fn main() {
+// Relative location (from own perspective)
+{
+    parents: 0,
+    interior: Here
+}
+
+// Relative location (from perspective of parent)
+{
+    parents: 0,
+    interior: [Parachain(1000)]
+}
+
+// Relative location (from perspective of sibling)
+{
+    parents: 1,
+    interior: [Parachain(1000)]
+}
+
+// Absolute location
+[GlobalConsensus(Kusama), Parachain(1000)]
+}

Using DescribeFamily, the above relative locations would be described like so:

#![allow(unused)]
+fn main() {
+// Relative location (from own perspective)
+// Not possible.
+
+// Relative location (from perspective of parent)
+(b"ChildChain", Compact::<u32>::from(*index)).encode()
+
+// Relative location (from perspective of sibling)
+(b"SiblingChain", Compact::<u32>::from(*index)).encode()
+
+}

The proposed description for absolute location would follow the same pattern, like so:

#![allow(unused)]
+fn main() {
+(
+    b"GlobalConsensus",
+    network_id,
+    b"Parachain",
+    Compact::<u32>::from(para_id),
+    tail
+).encode()
+}

This proposal requires the modification of two XCM types defined in the xcm-builder crate: The WithComputedOrigin barrier and the DescribeFamily MultiLocation descriptor.

WithComputedOrigin

The WtihComputedOrigin barrier serves as a wrapper around other barriers, consuming origin modification instructions and applying them to the message origin before passing to the inner barriers. One of the origin modifying instructions is UniversalOrigin, which serves the purpose of signaling that the origin should be a Universal Origin that represents the location as an absolute path prefixed by the GlobalConsensus junction.

In it's current state the barrier transforms locations with the UniversalOrigin instruction into relative locations, so the proposed changes aim to make it return absolute locations instead.

DescribeFamily

The DescribeFamily location descriptor is part of the HashedDescription MultiLocation hashing system and exists to describe locations in an easy format for encoding and hashing, so that an AccountId can be derived from this MultiLocation.

This implementation contains a match statement that does not match against absolute locations, so changes to it involve matching against absolute locations and providing appropriate descriptions for hashing.

Drawbacks

No drawbacks have been identified with this proposal.

Testing, Security, and Privacy

Tests can be done using simple unit tests, as this is not a change to XCM itself but rather to types defined in xcm-builder.

Security considerations should be taken with the implementation to make sure no unwanted behavior is introduced.

This proposal does not introduce any privacy considerations.

Performance, Ergonomics, and Compatibility

Performance

Depending on the final implementation, this proposal should not introduce much overhead to performance.

Ergonomics

The ergonomics of this proposal depend on the final implementation details.

Compatibility

Backwards compatibility should remain unchanged, although that depend on the final implementation.

Prior Art and References

DescirbeFamily type: https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/xcm/xcm-builder/src/location_conversion.rs#L122
WithComputedOrigin type: https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/xcm/xcm-builder/src/barriers.rs#L153

Unresolved Questions

Implementation details and overall code is still up to discussion.

(source)

Table of Contents

RFC-0042: Add System version that replaces StateVersion on RuntimeVersion +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0042: Add System version that replaces StateVersion on RuntimeVersion

+ + + +


Start Date	25th October 2023
Description	Add System Version and remove State Version
Authors	Vedhavyas Singareddi

Summary

At the moment, we have system_version field on RuntimeVersion that derives which state version is used for the +Storage. +We have a use case where we want extrinsics root is derived using StateVersion::V1. Without defining a new field +under RuntimeVersion, +we would like to propose adding system_version that can be used to derive both storage and extrinsic state version.

Motivation

Since the extrinsic state version is always StateVersion::V0, deriving extrinsic root requires full extrinsic data. +This would be problematic when we need to verify the extrinsics root if the extrinsic sizes are bigger. This problem is +further explored in https://github.com/polkadot-fellows/RFCs/issues/19

For Subspace project, we have an enshrined rollups called Domain with optimistic verification and Fraud proofs are +used to detect malicious behavior. +One of the Fraud proof variant is to derive Domain block extrinsic root on Subspace's consensus chain. +Since StateVersion::V0 requires full extrinsic data, we are forced to pass all the extrinsics through the Fraud proof. +One of the main challenge here is some extrinsics could be big enough that this variant of Fraud proof may not be +included in the Consensus block due to Block's weight restriction. +If the extrinsic root is derived using StateVersion::V1, then we do not need to pass the full extrinsic data but +rather at maximum, 32 byte of extrinsic data.

Stakeholders

Technical Fellowship, in its role of maintaining system runtimes.

Explanation

In order to use project specific StateVersion for extrinsic roots, we proposed +an implementation that introduced +parameter to frame_system::Config but that unfortunately did not feel correct. +So we would like to propose adding this change to +the RuntimeVersion +object. The system version, if introduced, will be used to derive both storage and extrinsic state version. +If system version is 0, then both Storage and Extrinsic State version would use V0. +If system version is 1, then Storage State version would use V1 and Extrinsic State version would use V0. +If system version is 2, then both Storage and Extrinsic State version would use V1.

If implemented, the new RuntimeVersion definition would look something similar to

#![allow(unused)]
+fn main() {
+/// Runtime version (Rococo).
+#[sp_version::runtime_version]
+pub const VERSION: RuntimeVersion = RuntimeVersion {
+		spec_name: create_runtime_str!("rococo"),
+		impl_name: create_runtime_str!("parity-rococo-v2.0"),
+		authoring_version: 0,
+		spec_version: 10020,
+		impl_version: 0,
+		apis: RUNTIME_API_VERSIONS,
+		transaction_version: 22,
+		system_version: 1,
+	};
+}

Drawbacks

There should be no drawbacks as it would replace state_version with same behavior but documentation should be updated +so that chains know which system_version to use.

Testing, Security, and Privacy

AFAIK, should not have any impact on the security or privacy.

Performance, Ergonomics, and Compatibility

These changes should be compatible for existing chains if they use state_version value for system_verision.

Performance

I do not believe there is any performance hit with this change.

Ergonomics

This does not break any exposed Apis.

Compatibility

This change should not break any compatibility.

Prior Art and References

We proposed introducing a similar change by introducing a +parameter to frame_system::Config but did not feel that +is the correct way of introducing this change.

Unresolved Questions

I do not have any specific questions about this change at the moment.

+ +

IMO, this change is pretty self-contained and there won't be any future work necessary.

(source)

Table of Contents

RFC-0043: Introduce storage_proof_size Host Function for Improved Parachain Block Utilization +
- Summary
- Motivation
- Stakeholders
- Explanation
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
+

RFC-0043: Introduce `storage_proof_size` Host Function for Improved Parachain Block Utilization

+ + + +


Start Date	30 October 2023
Description	Host function to provide the storage proof size to runtimes.
Authors	Sebastian Kunert

Summary

This RFC proposes a new host function for parachains, storage_proof_size. It shall provide the size of the currently recorded storage proof to the runtime. Runtime authors can use the proof size to improve block utilization by retroactively reclaiming unused storage weight.

Motivation

The number of extrinsics that are included in a parachain block is limited by two constraints: execution time and proof size. FRAME weights cover both concepts, and block-builders use them to decide how many extrinsics to include in a block. However, these weights are calculated ahead of time by benchmarking on a machine with reference hardware. The execution-time properties of the state-trie and its storage items are unknown at benchmarking time. Therefore, we make some assumptions about the state-trie:

Trie Depth: We assume a trie depth to account for intermediary nodes.
Storage Item Size: We make a pessimistic assumption based on the MaxEncodedLen trait.

These pessimistic assumptions lead to an overestimation of storage weight, negatively impacting block utilization on parachains.

In addition, the current model does not account for multiple accesses to the same storage items. While these repetitive accesses will not increase storage-proof size, the runtime-side weight monitoring will account for them multiple times. Since the proof size is completely opaque to the runtime, we can not implement retroactive storage weight correction.

A solution must provide a way for the runtime to track the exact storage-proof size consumed on a per-extrinsic basis.

Stakeholders

Parachain Teams: They MUST include this host function in their runtime and node.
Light-client Implementors: They SHOULD include this host function in their runtime and node.

Explanation

This RFC proposes a new host function that exposes the storage-proof size to the runtime. As a result, runtimes can implement storage weight reclaiming mechanisms that improve block utilization.

This RFC proposes the following host function signature:

#![allow(unused)]
+fn main() {
+fn ext_storage_proof_size_version_1() -> u64;
+}

The host function MUST return an unsigned 64-bit integer value representing the current proof size. In block-execution and block-import contexts, this function MUST return the current size of the proof. To achieve this, parachain node implementors need to enable proof recording for block imports. In other contexts, this function MUST return 18446744073709551615 (u64::MAX), which represents disabled proof recording.

Performance, Ergonomics, and Compatibility

Performance

Parachain nodes need to enable proof recording during block import to correctly implement the proposed host function. Benchmarking conducted with balance transfers has shown a performance reduction of around 0.6% when proof recording is enabled.

Ergonomics

The host function proposed in this RFC allows parachain runtime developers to keep track of the proof size. Typical usage patterns would be to keep track of the overall proof size or the difference between subsequent calls to the host function.

Compatibility

Parachain teams will need to include this host function to upgrade.

Prior Art and References

Pull Request including proposed host function: PoV Reclaim (Clawback) Node Side.
Issue with discussion: [FRAME core] Clawback PoV Weights For Dispatchables

(source)

Table of Contents

RFC-0044: Rent based registration model +
- Summary
- Motivation
- Requirements
- Stakeholders
- Explanation +
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0044: Rent based registration model

+ + + +


Start Date	6 November 2023
Description	A new rent based parachain registration model
Authors	Sergej Sakac

Summary

This RFC proposes a new model for a sustainable on-demand parachain registration, involving a smaller initial deposit and periodic rent payments. The new model considers that on-demand chains may be unregistered and later re-registered. The proposed solution also ensures a quick startup for on-demand chains on Polkadot in such cases.

Motivation

With the support of on-demand parachains on Polkadot, there is a need to explore a new, more cost-effective model for registering validation code. In the current model, the parachain manager is responsible for reserving a unique ParaId and covering the cost of storing the validation code of the parachain. These costs can escalate, particularly if the validation code is large. We need a better, sustainable model for registering on-demand parachains on Polkadot to help smaller teams deploy more easily.

This RFC suggests a new payment model to create a more financially viable approach to on-demand parachain registration. In this model, a lower initial deposit is required, followed by recurring payments upon parachain registration.

This new model will coexist with the existing one-time deposit payment model, offering teams seeking to deploy on-demand parachains on Polkadot a more cost-effective alternative.

Requirements

The solution SHOULD NOT affect the current model for registering validation code.
The solution SHOULD offer an easily configurable way for governance to adjust the initial deposit and recurring rent cost.
The solution SHOULD provide an incentive to prune validation code for which rent is not paid.
The solution SHOULD allow anyone to re-register validation code under the same ParaId without the need for redundant pre-checking if it was already verified before.
The solution MUST be compatible with the Agile Coretime model, as described in RFC#0001
The solution MUST allow anyone to pay the rent.
The solution MUST prevent the removal of validation code if it could still be required for disputes or approval checking.

Stakeholders

Future Polkadot on-demand Parachains

Explanation

This RFC proposes a set of changes that will enable the new rent based approach to registering and storing validation code on-chain. +The new model, compared to the current one, will require periodic rent payments. The parachain won't be pruned automatically if the rent is not paid, but by permitting anyone to prune the parachain and rewarding the caller, there will be an incentive for the removal of the validation code.

On-demand parachains should still be able to utilize the current one-time payment model. However, given the size of the deposit required, it's highly likely that most on-demand parachains will opt for the new rent-based model.

Importantly, this solution doesn't require any storage migrations in the current system nor does it introduce any breaking changes. The following provides a detailed description of this solution.

Registering an on-demand parachain

In the current implementation of the registrar pallet, there are two constants that specify the necessary deposit for parachains to register and store their validation code:

#![allow(unused)]
+fn main() {
+trait Config {
+	// -- snip --
+
+	/// The deposit required for reserving a `ParaId`.
+	#[pallet::constant]
+	type ParaDeposit: Get<BalanceOf<Self>>;
+
+	/// The deposit to be paid per byte stored on chain.
+	#[pallet::constant]
+	type DataDepositPerByte: Get<BalanceOf<Self>>;
+}
+}

This RFC proposes the addition of three new constants that will determine the payment amount and the frequency of the recurring rent payment:

#![allow(unused)]
+fn main() {
+trait Config {
+	// -- snip --
+
+	/// Defines how frequently the rent needs to be paid.
+	///
+	/// The duration is set in sessions instead of block numbers.
+	#[pallet::constant]
+	type RentDuration: Get<SessionIndex>;
+
+	/// The initial deposit amount for registering validation code.
+	///
+	/// This is defined as a proportion of the deposit that would be required in the regular
+	/// model.
+	#[pallet::constant]
+	type RentalDepositProportion: Get<Perbill>;
+
+	/// The recurring rental cost defined as a proportion of the initial rental registration deposit.
+	#[pallet::constant]
+	type RentalRecurringProportion: Get<Perbill>;
+}
+}

Users will be able to reserve a ParaId and register their validation code for a proportion of the regular deposit required. However, they must also make additional rent payments at intervals of T::RentDuration.

For registering using the new rental system we will have to make modifications to the paras-registrar pallet. We should expose two new extrinsics for this:

#![allow(unused)]
+fn main() {
+mod pallet {
+	// -- snip --
+
+	pub fn register_rental(
+		origin: OriginFor<T>,
+		id: ParaId,
+		genesis_head: HeadData,
+		validation_code: ValidationCode,
+	) -> DispatchResult { /* ... */ }
+
+	pub fn pay_rent(origin: OriginFor<T>, id: ParaId) -> DispatchResult {
+		/* ... */ 
+	}
+}
+}

A call to register_rental will require the reservation of only a percentage of the deposit that would otherwise be required to register the validation code when using the regular model. +As described later in the Quick para re-registering section below, we will also store the code hash of each parachain to enable faster re-registration after a parachain has been pruned. For this reason the total initial deposit amount is increased to account for that.

#![allow(unused)]
+fn main() {
+// The logic for calculating the initial deposit for parachain registered with the 
+// new rent-based model:
+
+let validation_code_deposit = per_byte_fee.saturating_mul((validation_code.0.len() as u32).into());
+
+let head_deposit = per_byte_fee.saturating_mul((genesis_head.0.len() as u32).into())
+let hash_deposit = per_byte_fee.saturating_mul(HASH_SIZE);
+
+let deposit = T::RentalDepositProportion::get().mul_ceil(validation_code_deposit)
+	.saturating_add(T::ParaDeposit::get())
+	.saturating_add(head_deposit)
+	.saturating_add(hash_deposit)
+}

Once the ParaId is reserved and the validation code is registered the rent must be periodically paid to ensure the on-demand parachain doesn't get removed from the state. The pay_rent extrinsic should be callable by anyone, removing the need for the parachain to depend on the parachain manager for rent payments.

On-demand parachain pruning

If the rent is not paid, anyone has the option to prune the on-demand parachain and claim a portion of the initial deposit reserved for storing the validation code. This type of 'light' pruning only removes the validation code, while the head data and validation code hash are retained. The validation code hash is stored to allow anyone to register it again as well as to enable quicker re-registration by skipping the pre-checking process.

The moment the rent is no longer paid, the parachain won't be able to purchase on-demand access, meaning no new blocks are allowed. This stage is called the "hibernation" stage, during which all the parachain-related data is still stored on-chain, but new blocks are not permitted. The reason for this is to ensure that the validation code is available in case it is needed in the dispute or approval checking subsystems. Waiting for one entire session will be enough to ensure it is safe to deregister the parachain.

This means that anyone can prune the parachain only once the "hibernation" stage is over, which lasts for an entire session after the moment that the rent is not paid.

The pruning described here is a light form of pruning, since it only removes the validation code. As with all parachains, the parachain or para manager can use the deregister extrinsic to remove all associated state.

Ensuring rent is paid

The paras pallet will be loosely coupled with the para-registrar pallet. This approach enables all the pallets tightly coupled with the paras pallet to have access to the rent status information.

Once the validation code is stored without having its rent paid the assigner_on_demand pallet will ensure that an order for that parachain cannot be placed. This is easily achievable given that the assigner_on_demand pallet is tightly coupled with the paras pallet.

On-demand para re-registration

If the rent isn't paid on time, and the parachain gets pruned, the new model should provide a quick way to re-register the same validation code under the same ParaId. This can be achieved by skipping the pre-checking process, as the validation code hash will be stored on-chain, allowing us to easily verify that the uploaded code remains unchanged.

#![allow(unused)]
+fn main() {
+/// Stores the validation code hash for parachains that successfully completed the 
+/// pre-checking process.
+///
+/// This is stored to enable faster on-demand para re-registration in case its pvf has been earlier
+/// registered and checked.
+///
+/// NOTE: During a runtime upgrade where the pre-checking rules change this storage map should be
+/// cleared appropriately.
+#[pallet::storage]
+pub(super) type CheckedCodeHash<T: Config> =
+	StorageMap<_, Twox64Concat, ParaId, ValidationCodeHash>;
+}

To enable parachain re-registration, we should introduce a new extrinsic in the paras-registrar pallet that allows this. The logic of this extrinsic will be same as regular registration, with the distinction that it can be called by anyone, and the required deposit will be smaller since it only has to cover for the storage of the validation code.

Drawbacks

This RFC does not alter the process of reserving a ParaId, and therefore, it does not propose reducing it, even though such a reduction could be beneficial.

Even though this RFC doesn't delve into the specifics of the configuration values for parachain registration but rather focuses on the mechanism, configuring it carelessly could lead to potential problems.

Since the validation code hash and head data are not removed when the parachain is pruned but only when the deregister extrinsic is called, the T::DataDepositPerByte must be set to a higher value to create a strong enough incentive for removing it from the state.

Testing, Security, and Privacy

The implementation of this RFC will be tested on Rococo first.

Proper research should be conducted on setting the configuration values of the new system since these values can have great impact on the network.

An audit is required to ensure the implementation's correctness.

The proposal introduces no new privacy concerns.

Performance, Ergonomics, and Compatibility

Performance

This RFC should not introduce any performance impact.

Ergonomics

This RFC does not affect the current parachains, nor the parachains that intend to use the one-time payment model for parachain registration.

Compatibility

This RFC does not break compatibility.

Prior Art and References

Prior discussion on this topic: https://github.com/paritytech/polkadot-sdk/issues/1796

Unresolved Questions

None at this time.

+ +

As noted in this GitHub issue, we want to raise the per-byte cost of on-chain data storage. However, a substantial increase in this cost would make it highly impractical for on-demand parachains to register on Polkadot. +This RFC offers an alternative solution for on-demand parachains, ensuring that the per-byte cost increase doesn't overly burden the registration process.

(source)

Table of Contents

RFC-0000: Metadata for offline signers +
- Summary
- Motivation +
  - Background
  - Solution requirements
  +
- Stakeholders
- Explanation +
  +
- Drawbacks +
  - Increased transaction size
  - Transition overhead
  +
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0000: Metadata for offline signers

+ + + +


Start Date	2023-10-31
Description	Add SignedExtension to check Metadata Root Hash
Authors	Alzymologist Oy, Zondax LLC, Parity GmbH

Summary

Add a metadata digest value (33-byte constant within fixed spec_version) to Signed Extensions to supplement signer party with proof of correct extrinsic interpretation. The digest value is generated once before release and is well-known and deterministic. The digest mechanism is designed to be modular and flexible. It also supports partial metadata transfer as needed by the signing party's extrinsic decoding mechanism. This considers signing devices potentially limited communication bandwidth and/or memory capacity.

Motivation

Background

While all blockchain systems support (at least in some sense) offline signing used in air-gapped wallets and lightweight embedded devices, only few allow simultaneously complex upgradeable logic and full message decoding on the cold off-line signer side; Substrate is one of these heartening few, and therefore - we should build on this feature to greatly improve transaction security, and thus in general, network resilience.

As a starting point, it is important to recognise that prudence and due care are naturally required. As we build further reliance on this feature we should be very careful to make sure it works correctly every time so as not to create false sense of security.

In order to enable decoding that is small and optimized for chain storage transactions, a metadata entity is used, which is not at all small in itself (on the order of half-MB for most networks). This is a dynamic data chunk which completely describes chain interfaces and properties that could be made into a portable scale-encoded string for any given network version and passed along into an off-chain device to familiarize it with latest network updates. Of course, compromising this metadata anywhere in the path could result in differences between what user sees and signs, thus it is essential that we protect it.

Therefore, we have 2 problems to be solved:

Metadata is large, takes long time to be passed into a cold storage device with memory insufficient for its storage; metadata SHOULD be shortened and transmission SHOULD be optimized.
Metadata authenticity SHOULD be ensured.

As of now, there is no working solution for (1), as the whole metadata has to be passed to the device. On top of this, the solution for (2) heavily relies on a trusted party managing keys and ensuring metadata is indeed authentic: creating poorly decentralized points of potential failure.

Solution requirements

Include metadata digest into signature

Some cryptographically strong digest of metadata MAY be included into signable blob. There SHALL NOT be storage overhead for this blob, nor computational overhead, on the node side; thus MUST be a constant within given runtime version, deterministically defined by metadata.

Metadata information that could be used in signable extrinsic decoding MAY be included in digest, its inclusion MUST be indicated in signed extensions;
Digest MUST be deterministic with respect to metadata;
Digest MUST be cryptographically strong against pre-image, both first and second;
Extra-metadata information necessary for extrinsic decoding and constant within runtime version MUST be included in digest;
It SHOULD be possible to quickly withdraw offline signing mechanism without access to cold signing devices;
Digest format SHOULD be versioned.
Work necessary for proving metadata authenticity MAY be omitted at discretion of signer device design (to support automation tools).

Reduce metadata size

Metadata should be stripped from parts that are not necessary to parse a signable extrinsic, then it should be separated into a finite set of self-descriptive chunks. Thus, a subset of chunks necessary for signable extrinsic decoding and rendering could be sent, possibly in small portions (ultimately - one at a time), to cold device together with proof.

Single chunk with proof payload size SHOULD fit within few kB;
Chunks handling mechanism SHOULD support chunks being sent in any order without memory utilization overhead;
Unused enum variants MUST be stripped (this has great impact on transmitted metadata size; examples: era enum, enum with all calls for call batching).

Stakeholders

All chain teams are stakeholders, as implementing this feature would require timely effort on their side and would impact compatibility with older tools.

This feature is essential for all offline signer tools; many regular signing tools might make use of it. In general, this RFC greatly improves security of any network implementing it, as many governing keys are used with offline signers.

Implementing this RFC would remove requirement to maintain metadata portals manually, as task of metadata verification would be effectively moved to consensus mechanism of the chain.

Explanation

Detailed description of metadata shortening and digest process is provided in metadata-shortener crate (see cargo doc --open and examples). Below are presented algorithms of the process.

Definitions

Metadata structure

Metadata in general consists of four sections:

Types registry
Pallets
Extrinsic metadata
Runtime type

Of these, only sections 1-3 contain information required for extrinsic decoding. The most important section is (1) Types registry, that is mostly used in extrinsic decoding. It is also the largest part, thus it is modularized for fractional transmission. Part (2) contains runtime version and is otherwise useless for transaction decoding; thus its contents are reduced to this parameter and included into Metadata Descriptor. Part (3) is included into Metadata Descriptor verbatim.

Metadata descriptor

Values for:

u8 metadata shortening protocol version,
SCALE-encoded ExtrinsicMetadata,
SCALE-encoded spec_version String,
SCALE-encoded spec_name String,
u16 base58 prefix,
u8 decimals value or 0u8 if no units are defined,
SCALE-encoded tokenSymbol String defined on chain to identify the name of currency (available for example through system.properties() RPC call) or empty string if no base units are defined,

struct MetadataDescriptor { // really a scale-encoded enum, thus first field is enum value - only 0x01 currently supported.
+  protocol_version: u8,
+  extrinsic_metadata: Vec<u8>, // SCALE from `ExtrinsicMetadata
+  spec_version: Vec<u8>, // SCALE form `String`
+  spec_name: Vec<u8>, // SCALE from `String`
+  base58_prefix: u16,
+  decimals: u8,
+  token_symbol: Vec<u8>, // SCALE from `String`
+}
+

constitute metadata descriptor. This is minimal information that is, together with (shortened) types registry, sufficient to decode any signable transaction.

Merkle tree

A Complete Binary Merkle Tree (CBMT) is proposed as digest structure.

Every node of the proposed tree has a 32-bit value.

A terminal node of the tree we call leaf. Its value is input for digest.

The top node of the tree we call root.

All node values for non-leave nodes are not terminal are computed through non-commutative merge procedure of child nodes.

In CBMT, all layers must be populated, except for the last one, that must have complete filling from the left.

Nodes are numbered top-down and left-to-right starting with 0 at the top of tree.

Example 8-node tree
+
+        0
+     /     \
+    1       2
+   / \     / \
+  3   4   5   6
+ / \
+7   8
+
+Nodes 4, 5, 6, 7, 8 are leaves
+Node 0 is root
+
+

General flow

The metadata is converted into lean modular form (vector of chunks)
A Merkle tree is constructed from the metadata chunks
A root of tree (as a left element) is merged with Metadata Descriptor (as a right element)
Resulting value is a constant to be included in additionalSigned to prove that the metadata seen by cold device is genuine

Metadata modularization

Types registry is stripped from docs fields.
Types records are separated into chunks, with enum variants being individual chunks differing by variant index; each chunk consisting of id (same as in full metadata registry) and SCALE-encoded 'Type' description (reduced to 1-variant enum for enum variants). Enums with 0 variants are treated as regular types.
Chunks are sorted by id in ascending order; chunks with same id are sorted by enum variant index in ascending order.

types_registry = metadataV14.types
+modularized_registry = EmptyVector<id, type>
+for (id, type) in types.registry.iterate_enumerate {
+  type.doc = Null
+  if (type is ReduceableEnum) { // false for 0-variant enums
+    for variant in type.variants.iterate {
+      variant_type = Type {
+        path: type.path
+        type_params: Null
+        type_def: TypeDef::Variant(variants: [variant])
+      }
+      modularized_registry.push(id, variant_type)
+    }
+  } else {
+    modularized_registry.push(id, type)
+  }
+}
+
+modularized_registry.sort(|a, b| {
+    if a.id == b.id { //only possible for variants
+      a.variant_index > b.variant_index
+    } else { a.id > b.id }
+  }
+)
+
+

Merging protocol

blake3 transformation of concatenated child nodes (blake3(left + right)) as merge procedure;

Complete Binary Merkle Tree construction protocol

Leaves are numbered in ascending order. Leaf index is associated with corresponding chunk.
Merge is performed using the leaf with highest index as right and node with second to highest index as left children; result is pushed to the end of nodes queue and leaves are discarded.
Step (2) is repeated until no leaves or just one leaf remains; in latter case, the last leaf is pushed to the front of the nodes queue.
Right node and then left node is popped from the front of the nodes queue and merged; the result is sent to the end of the queue.
Step (4) is repeated until only one node remains; this is tree root.

Resulting tree for metadata consisting of 5 nodes (numbered from 0 to 4):
+
+       root
+     /     \
+    *       *
+   / \     / \
+  *   0   1   2
+ / \
+3   4
+

Digest

Blake3 hash is computed for each chunk of modular short metadata registry.
Complete Binary Merkle Tree is constructed as described above.
Root hash of this tree (left) is merged with metadata descriptor blake3 hash (right); this is metadata digest.

Version number and corresponding resulting metadata digest MUST be included into Signed Extensions as specified in Chain Verification section below.

Shortening

For shortening, an attempt to decode transaction completely using provided metadata is performed with the same algorithm that would be used on the cold side. All chunks are associated with their leaf indices. An example of this protocol is proposed in metadata-shortener that is based on substrate-parser decoding protocol; any decoding protocol could be used here as long as cold signer's design finds it appropriate for given security model.

Transmission

Shortened metadata chunks MAY be trasmitted into cold device together with Merkle proof in its entirety or in parts, depending on memory capabilities of the cold device and it ability to reconstruct larger fraction of tree. This document does not specify the manner of transmission. The order of metadata chunks MAY be arbitrary, the only requirement is that indices of leaf nodes in Merkle tree corresponding to chunks MUST be communicated. Community MAY handle proof format standartization independently.

Offline verification

The transmitted metadata chunks are hashed together with proof lemmas to obtain root that MAY be transmitted along with the rest of payload. Verification that the root transmitted with message matches with calculated root is optional; the transmitted root SHOULD NOT be used in signature, calculated root MUST be used; however, there is no mechanism to enforce this - it should be done during cold signers code audit.

Chain verification

The root of metadata computed by cold device MAY be included into Signed Extensions; this way the transaction will pass as valid iff hash of metadata as seen by cold storage device is identical to consensus hash of metadata, ensuring fair signing protocol.

The Signed Extension representing metadata digest is a single byte representing both digest vaule inclusion and shortening protocol version; this MUST be included in Signed Extensions set. Depending on its value, a digest value is included as additionalSigned to signature computation according to following specification:

+ + + +

signed extension value	digest value	comment
`0x00`		digest is not included
`0x01`	32-byte digest	this represents protocol version 1
`0x02` - `0xFF`	reserved	reserved for future use

Drawbacks

Increased transaction size

A 1-byte increase in transaction size due to signed extension value. Digest is not included in transferred transaction, only in signing process.

Transition overhead

Some slightly out of spec systems might experience breaking changes as new content of signed extensions is added. It is important to note, that there is no real overhead in processing time nor complexity, as the metadata checking mechanism is voluntary. The only drawbacks are expected for tools that do not implement MetadataV14 self-descripting features.

Testing, Security, and Privacy

The metadata shortening protocol should be extensively tested on all available examples of metadata before releasing changes to either metadata or shortener. Careful code review should be performed on shortener implementation code to ensure security. The main metadata tree would inevitably be constructed on runtime build which would also ensure correctness.

To be able to recall shortener protocol in case of vulnerability issues, a version byte is included.

Performance, Ergonomics, and Compatibility

Performance

This is negligibly short pessimization during build time on the chain side. Cold wallets performance would improve mostly as metadata validity mechanism that was taking most of effort in cold wallet support would become trivial.

Ergonomics

The proposal was optimized for cold storage wallets usage with minimal impact on all other parts of the ecosystem

Compatibility

Proposal in this form is not compatible with older tools that do not implement proper MetadataV14 self-descriptive features; those would have to be upgraded to include a new signed extensions field.

Prior Art and References

This project was developed upon a Polkadot Treasury grant; relevant development links are located in metadata-offline-project repository.

Unresolved Questions

How would polkadot-js handle the transition?
Where would non-rust tools like Ledger apps get shortened metadata content?

+ +

Changes to code of all cold signers to implement this mechanism SHOULD be done when this is enabled; non-cold signers may perform extra metadata check for better security. Ultimately, signing anything without decoding it with verifiable metadata should become discouraged in all situations where a decision-making mechanism is involved (that is, outside of fully automated blind signers like trade bots or staking rewards payout tools).

(source)

Table of Contents

RFC-0047: Assignment of availability chunks to validators +
- Summary
- Motivation
- Stakeholders
- Explanation +
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
- Appendix A
+

RFC-0047: Assignment of availability chunks to validators

+ + + +


Start Date	03 November 2023
Description	An evenly-distributing indirection layer between availability chunks and validators.
Authors	Alin Dima

Summary

Propose a way of permuting the availability chunk indices assigned to validators for a given core and relay +chain block, in the context of +recovering available data from systematic chunks, with the +purpose of fairly distributing network bandwidth usage.

Motivation

Currently, the ValidatorIndex is always identical to the ChunkIndex. Since the validator array is only shuffled once +per session, naively using the ValidatorIndex as the ChunkIndex would pose an unreasonable stress on the first N/3 +validators during an entire session, when favouring availability recovery from systematic chunks.

Therefore, the relay chain node needs a deterministic way of evenly distributing the first ~(N_VALIDATORS / 3) +systematic availability chunks to different validators, based on the relay chain block and core. +The main purpose is to ensure fair distribution of network bandwidth usage for availability recovery in general and in +particular for systematic chunk holders.

Stakeholders

Relay chain node core developers.

Explanation

Systematic erasure codes

An erasure coding algorithm is considered systematic if it preserves the original unencoded data as part of the +resulting code. +The implementation of the erasure coding algorithm used for polkadot's availability data is systematic. +Roughly speaking, the first N_VALIDATORS/3 chunks of data can be cheaply concatenated to retrieve the original data, +without running the resource-intensive and time-consuming reconstruction algorithm.

Here's the concatenation procedure of systematic chunks for polkadot's erasure coding algorithm +(minus error handling, for briefness):

#![allow(unused)]
+fn main() {
+pub fn reconstruct_from_systematic<T: Decode>(
+	n_validators: usize,
+	chunks: Vec<&[u8]>,
+) -> T {
+	let threshold = systematic_threshold(n_validators);
+	let shard_len = chunks.iter().next().unwrap().len();
+	let mut systematic_bytes = Vec::with_capacity(shard_len * threshold);
+
+	for i in (0..shard_len).step_by(2) {
+		for chunk in chunks.iter().take(threshold) {
+			systematic_bytes.push(chunk[i]);
+			systematic_bytes.push(chunk[i + 1]);
+		}
+	}
+
+	Decode::decode(&mut &systematic_bytes[..]).unwrap()
+}
+
+fn systematic_threshold(n_validators: usize) -> usize {
+	let mut threshold = (n_validators - 1) / 3;
+	if !is_power_of_two(threshold) {
+		threshold = next_lower_power_of_2(threshold);
+	}
+
+	threshold
+}
+}

In a nutshell, it performs a column-wise concatenation with 2-byte chunks. +The output could be zero-padded at the end, so scale decoding must be aware of the expected length in bytes and ignore +trailing zeros.

Availability recovery at present

According to the polkadot protocol spec:

+
A validator should request chunks by picking peers randomly and must recover at least f+1 chunks, where +n=3f+k and k in {1,2,3}.
+

For parity's polkadot node implementation, the process was further optimised. At this moment, it works differently based +on the estimated size of the available data:

(a) for small PoVs (up to 128 Kib), sequentially try requesting the unencoded data from the backing group, in a random +order. If this fails, fallback to option (b).

(b) for large PoVs (over 128 Kib), launch N parallel requests for the erasure coded chunks (currently, N has an upper +limit of 50), until enough chunks were recovered. Validators are tried in a random order. Then, reconstruct the +original data.

All options require that after reconstruction, validators then re-encode the data and re-create the erasure chunks trie +in order to check the erasure root.

Availability recovery from systematic chunks

As part of the effort of +increasing polkadot's resource efficiency, scalability and performance, +work is under way to modify the Availability Recovery protocol by leveraging systematic chunks. See +this comment for preliminary +performance results.

In this scheme, the relay chain node will first attempt to retrieve the ~N/3 systematic chunks from the validators that +should hold them, before falling back to recovering from regular chunks, as before.

A re-encoding step is still needed for verifying the erasure root, so the erasure coding overhead cannot be completely +brought down to 0.

Not being able to retrieve even one systematic chunk would make systematic reconstruction impossible. Therefore, backers +can be used as a backup to retrieve a couple of missing systematic chunks, before falling back to retrieving regular +chunks.

Chunk assignment function

Properties

The function that decides the chunk index for a validator should be parameterized by at least +(validator_index, block_number, core_index) +and have the following properties:

deterministic
relatively quick to compute and resource-efficient.
when considering the other params besides validator_index as fixed, the function should describe a permutation +of the chunk indices
considering block_number as a fixed argument, the validators that map to the first N/3 chunk indices should +have as little overlap as possible for different paras scheduled on that relay parent.

In other words, we want a uniformly distributed, deterministic mapping from ValidatorIndex to ChunkIndex per block +per core.

It's desirable to not embed this function in the runtime, for performance and complexity reasons. +However, this means that the function needs to be kept very simple and with minimal or no external dependencies. +Any change to this function could result in parachains being stalled and needs to be coordinated via a runtime upgrade +or governance call.

Proposed function

Pseudocode:

#![allow(unused)]
+fn main() {
+pub fn get_chunk_index(
+  n_validators: u32,
+  validator_index: ValidatorIndex,
+  block_number: BlockNumber,
+  core_index: CoreIndex
+) -> ChunkIndex {
+  let threshold = systematic_threshold(n_validators); // Roughly n_validators/3
+  let core_start_pos = abs(core_index - block_number) * threshold;
+
+  (core_start_pos + validator_index) % n_validators
+}
+}

Network protocol

The request-response /req_chunk protocol will be bumped to a new version (from v1 to v2). +For v1, the request and response payloads are:

#![allow(unused)]
+fn main() {
+/// Request an availability chunk.
+pub struct ChunkFetchingRequest {
+	/// Hash of candidate we want a chunk for.
+	pub candidate_hash: CandidateHash,
+	/// The index of the chunk to fetch.
+	pub index: ValidatorIndex,
+}
+
+/// Receive a requested erasure chunk.
+pub enum ChunkFetchingResponse {
+	/// The requested chunk data.
+	Chunk(ChunkResponse),
+	/// Node was not in possession of the requested chunk.
+	NoSuchChunk,
+}
+
+/// This omits the chunk's index because it is already known by
+/// the requester and by not transmitting it, we ensure the requester is going to use his index
+/// value for validating the response, thus making sure he got what he requested.
+pub struct ChunkResponse {
+	/// The erasure-encoded chunk of data belonging to the candidate block.
+	pub chunk: Vec<u8>,
+	/// Proof for this chunk's branch in the Merkle tree.
+	pub proof: Proof,
+}
+}

Version 2 will add an index field to ChunkResponse:

#![allow(unused)]
+fn main() {
+#[derive(Debug, Clone, Encode, Decode)]
+pub struct ChunkResponse {
+	/// The erasure-encoded chunk of data belonging to the candidate block.
+	pub chunk: Vec<u8>,
+	/// Proof for this chunk's branch in the Merkle tree.
+	pub proof: Proof,
+	/// Chunk index.
+	pub index: ChunkIndex
+}
+}

An important thing to note is that in version 1, the ValidatorIndex value is always equal to the ChunkIndex. +Until the feature is enabled, this will also be true for version 2. However, after the feature is enabled, +this will generally not be true.

The requester will send the request to validator with index V. The responder will map the V validator index to the +C chunk index and respond with the C-th chunk.

The protocol implementation MAY check the returned ChunkIndex against the expected mapping to ensure that +it received the right chunk. +In practice, this is desirable during availability-distribution and systematic chunk recovery. However, regular +recovery may not check this index, which is particularly useful when participating in disputes that don't allow +for easy access to the validator->chunk mapping. See Appendix A for more details.

In any case, the requester MUST verify the chunk's proof using the provided index.

During availability-recovery, given that the requester may not know (if the mapping is not available) whether the received chunk corresponds to +the requested validator index, it has to keep track of received chunk indices and ignore duplicates. Such duplicates +should be considered the same as an invalid/garbage response (drop it and move on to the next validator - we can't +punish via reputation changes, because we don't know which validator misbehaved).

Upgrade path

Step 1: Enabling new network protocol

In the beginning, both /req_chunk/1 and /req_chunk/2 will be supported, until all validators and +collators have upgraded to use the new version. V1 will be considered deprecated. During this step, the mapping will +still be 1:1 (ValidatorIndex == ChunkIndex), regardless of protocol. +Once all nodes are upgraded, a new release will be cut that removes the v1 protocol. Only once all nodes have upgraded +to this version will step 2 commence.

Step 2: Enabling the new validator->chunk mapping

Considering that the Validator->Chunk mapping is critical to para consensus, the change needs to be enacted atomically +via governance, only after all validators have upgraded the node to a version that is aware of this mapping, +functionality-wise. +It needs to be explicitly stated that after the governance enactment, validators that run older client versions that +don't support this mapping will not be able to participate in parachain consensus.

Additionally, an error will be logged when starting a validator with an older version, after the feature was enabled.

On the other hand, collators will not be required to upgrade in this step, as regular chunk recovery will work as before, +granted that version 1 of the networking protocol has been removed. Note that collators only perform +availability-recovery in rare, adversarial scenarios, so it is fine to not optimise for this case and let them upgrade +at their own pace.

To support enabling this feature via the runtime, we will use the NodeFeatures bitfield of the HostConfiguration +struct (added in https://github.com/paritytech/polkadot-sdk/pull/2177). Adding and enabling a feature +with this scheme does not require a runtime upgrade, but only a referendum that issues a +Configuration::set_node_feature extrinsic. Once the feature is enabled and new configuration is live, the +validator->chunk mapping ceases to be a 1:1 mapping and systematic recovery may begin.

Drawbacks

Getting access to the core_index that used to be occupied by a candidate in some parts of the dispute protocol is +very complicated (See appendix A). This RFC assumes that availability-recovery processes initiated during +disputes will only use regular recovery, as before. This is acceptable since disputes are rare occurrences in practice +and is something that can be optimised later, if need be. Adding the core_index to the CandidateReceipt would +mitigate this problem and will likely be needed in the future for CoreJam. +Related discussion about CandidateReceipt
It's a breaking change that requires all validators and collators to upgrade their node version.

Testing, Security, and Privacy

Extensive testing will be conducted - both automated and manual. +This proposal doesn't affect security or privacy.

Performance, Ergonomics, and Compatibility

Performance

This is a necessary data availability optimisation, as reed-solomon erasure coding has proven to be a top consumer of +CPU time in polkadot as we scale up the parachain block size and number of availability cores.

With this optimisation, preliminary performance results show that CPU time used for reed-solomon coding/decoding can be +halved and total POV recovery time decrease by 80% for large POVs. See more +here.

Ergonomics

Not applicable.

Compatibility

This is a breaking change. See upgrade path section above. +All validators need to have upgraded their node versions before the feature will be enabled via a runtime upgrade +governance call.

Prior Art and References

See comments on the tracking issue and the +in-progress PR

Unresolved Questions

Is there a better upgrade path that would preserve backwards compatibility?

+ +

This enables future optimisations for the performance of availability recovery, such as retrieving batched systematic +chunks from backers/approval-checkers.

Appendix A

This appendix details the intricacies of getting access to the core index of a candidate in parity's polkadot node.

Here, core_index refers to the index of the core that a candidate was occupying while it was pending availability +(from backing to inclusion).

Availability-recovery can currently be triggered by the following phases in the polkadot protocol:

During the approval voting process.
By other collators of the same parachain.
During disputes.

Getting the right core index for a candidate can be troublesome. Here's a breakdown of how different parts of the +node implementation can get access to it:

+
The approval-voting process for a candidate begins after observing that the candidate was included. Therefore, the +node has easy access to the block where the candidate got included (and also the core that it occupied).
+
+
The pov_recovery task of the collators starts availability recovery in response to noticing a candidate getting +backed, which enables easy access to the core index the candidate started occupying.
+
+
Disputes may be initiated on a number of occasions:
+
3.a. is initiated by the validator as a result of finding an invalid candidate while participating in the +approval-voting protocol. In this case, availability-recovery is not needed, since the validator already issued their +vote.
+
3.b is initiated by the validator noticing dispute votes recorded on-chain. In this case, we can safely +assume that the backing event for that candidate has been recorded and kept in memory.
+
3.c is initiated as a result of getting a dispute statement from another validator. It is possible that the dispute +is happening on a fork that was not yet imported by this validator, so the subsystem may not have seen this candidate +being backed.
+

A naive attempt of solving 3.c would be to add a new version for the disputes request-response networking protocol. +Blindly passing the core index in the network payload would not work, since there is no way of validating that +the reported core_index was indeed the one occupied by the candidate at the respective relay parent.

Another attempt could be to include in the message the relay block hash where the candidate was included. +This information would be used in order to query the runtime API and retrieve the core index that the candidate was +occupying. However, considering it's part of an unimported fork, the validator cannot call a runtime API on that block.

(source)

Table of Contents

RFC-0059: Add a discovery mechanism for nodes based on their capabilities +
- Summary
- Motivation
- Stakeholders
- Explanation +
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0059: Add a discovery mechanism for nodes based on their capabilities

+ + + +


Start Date	2023-12-18
Description	Nodes having certain capabilities register themselves in the DHT to be discoverable
Authors	Pierre Krieger

Summary

This RFC proposes to make the mechanism of RFC #8 more generic by introducing the concept of "capabilities".

Implementations can implement certain "capabilities", such as serving old block headers or being a parachain bootnode.

The discovery mechanism of RFC #8 is extended to be able to discover nodes of specific capabilities.

Motivation

The Polkadot peer-to-peer network is made of nodes. Not all these nodes are equal. Some nodes store only the headers of recently blocks, some nodes store all the block headers and bodies since the genesis, some nodes store the storage of all blocks since the genesis, and so on.

It is currently not possible to know ahead of time (without connecting to it and asking) which nodes have which data available, and it is not easily possible to build a list of nodes that have a specific piece of data available.

If you want to download for example the header of block 500, you have to connect to a randomly-chosen node, ask it for block 500, and if it says that it doesn't have the block, disconnect and try another randomly-chosen node. +In certain situations such as downloading the storage of old blocks, nodes that have the information are relatively rare, and finding through trial and error a node that has the data can take a long time.

This RFC attempts to solve this problem by giving the possibility to build a list of nodes that are capable of serving specific data.

Stakeholders

Low-level client developers. +People interested in accessing the archive of the chain.

Explanation

Reading RFC #8 first might help with comprehension, as this RFC is very similar.

Please keep in mind while reading that everything below applies for both relay chains and parachains, except mentioned otherwise.

Capabilities

This RFC defines a list of so-called capabilities:

Head of chain provider. An implementation with this capability must be able to serve to other nodes block headers, block bodies, justifications, calls proofs, and storage proofs of "recent" (see below) blocks, and, for relay chains, to serve to other nodes warp sync proofs where the starting block is a session change block and must participate in Grandpa and Beefy gossip.
History provider. An implementation with this capability must be able to serve to other nodes block headers and block bodies of any block since the genesis, and must be able to serve to other nodes justifications of any session change block since the genesis up until and including their currently finalized block.
Archive provider. This capability is a superset of History provider. In addition to the requirements of History provider, an implementation with this capability must be able to serve call proofs and storage proof requests of any block since the genesis up until and including their currently finalized block.
Parachain bootnode (only for relay chains). An implementation with this capability must be able to serve the network request described in RFC 8.

In the context of the head of chain provider, the word "recent" means: any not-finalized-yet block that is equal to or an ancestor of a block that it has announced through a block announce, and any finalized block whose height is superior to its current finalized block minus 16. +This does not include blocks that have been pruned because they're not a descendant of its current finalized block. In other words, blocks that aren't a descendant of the current finalized block can be thrown away. +A gap of blocks is required due to race conditions: when a node finalizes a block, it takes some time for its peers to be made aware of this, during which they might send requests concerning older blocks. The exact gap is arbitrary.

Substrate is currently by default a head of chain provider provider. After it has finished warp syncing, it downloads the list of old blocks, after which it becomes a history provider. +If Substrate is instead configured as an archive node, then it downloads the state of all blocks since the genesis, after which it becomes an archive provider, history provider, and head of chain provider. +If blocks pruning is enabled and the chain is a relay chain, then Substrate unfortunately doesn't implement any of these capabilities, not even head of chain provider. This is considered as a bug that should be fixed, see https://github.com/paritytech/polkadot-sdk/issues/2733.

DHT provider registration

This RFC heavily relies on the functionalities of the Kademlia DHT already in use by Polkadot. You can find a link to the specification here.

Implementations that have the history provider capability should register themselves as providers under the key sha256(concat("history", randomness)).

Implementations that have the archive provider capability should register themselves as providers under the key sha256(concat("archive", randomness)).

Implementations that have the parachain bootnode capability should register themselves as provider under the key sha256(concat(scale_compact(para_id), randomness)), as described in RFC 8.

"Register themselves as providers" consists in sending ADD_PROVIDER requests to nodes close to the key, as described in the Content provider advertisement section of the specification.

The value of randomness can be found in the randomness field when calling the BabeApi_currentEpoch function.

In order to avoid downtimes when the key changes, nodes should also register themselves as a secondary key that uses a value of randomness equal to the randomness field when calling BabeApi_nextEpoch.

Implementers should be aware that their implementation of Kademlia might already hash the key before XOR'ing it. The key is not meant to be hashed twice.

Implementations must not register themselves if they don't fulfill the capability yet. For example, a node configured to be an archive node but that is still building its archive state in the background must register itself only after it has finished building its archive.

Secondary DHTs

Implementations that have the history provider capability must also participate in a secondary DHT that comprises only of nodes with that capability. The protocol name of that secondary DHT must be /<genesis-hash>/kad/history.

Similarly, implementations that have the archive provider capability must also participate in a secondary DHT that comprises only of nodes with that capability and whose protocol name is /<genesis-hash>/kad/archive.

Just like implementations must not register themselves if they don't fulfill their capability yet, they must also not participate in the secondary DHT if they don't fulfill their capability yet.

Head of the chain providers

Implementations that have the head of the chain provider capability do not register themselves as providers, but instead are the nodes that participate in the main DHT. In other words, they are the nodes that serve requests of the /<genesis_hash>/kad protocol.

Any implementation that isn't a head of the chain provider (read: light clients) must not participate in the main DHT. This is already presently the case.

Implementations must not participate in the main DHT if they don't fulfill the capability yet. For example, a node that is still in the process of warp syncing must not participate in the main DHT. However, assuming that warp syncing doesn't last more than a few seconds, it is acceptable to ignore this requirement in order to avoid complicating implementations too much.

Drawbacks

None that I can see.

Testing, Security, and Privacy

The content of this section is basically the same as the one in RFC 8.

This mechanism doesn't add or remove any security by itself, as it relies on existing mechanisms.

Due to the way Kademlia works, it would become the responsibility of the 20 Polkadot nodes whose sha256(peer_id) is closest to the key (described in the explanations section) to store the list of nodes that have specific capabilities. +Furthermore, when a large number of providers are registered, only the providers closest to the key are kept, up to a certain implementation-defined limit.

For this reason, an attacker can abuse this mechanism by randomly generating libp2p PeerIds until they find the 20 entries closest to the key representing the target capability. They are then in control of the list of nodes with that capability. While doing this can in no way be actually harmful, it could lead to eclipse attacks.

Because the key changes periodically and isn't predictable, and assuming that the Polkadot DHT is sufficiently large, it is not realistic for an attack like this to be maintained in the long term.

Performance, Ergonomics, and Compatibility

Performance

The DHT mechanism generally has a low overhead, especially given that publishing providers is done only every 24 hours.

Doing a Kademlia iterative query then sending a provider record shouldn't take more than around 50 kiB in total of bandwidth for the parachain bootnode.

Assuming 1000 nodes with a specific capability, the 20 Polkadot full nodes corresponding to that capability will each receive a sudden spike of a few megabytes of networking traffic when the key rotates. Again, this is relatively negligible. If this becomes a problem, one can add a random delay before a node registers itself to be the provider of the key corresponding to BabeApi_next_epoch.

Maybe the biggest uncertainty is the traffic that the 20 Polkadot full nodes will receive from light clients that desire knowing the nodes with a capability. If this every becomes a problem, this value of 20 is an arbitrary constant that can be increased for more redundancy.

Ergonomics

Irrelevant.

Compatibility

Irrelevant.

Prior Art and References

Unknown.

Unresolved Questions

+ +

This RFC would make it possible to reliably discover archive nodes, which would make it possible to reliably send archive node requests, something that isn't currently possible. This could solve the problem of finding archive RPC node providers by migrating archive-related request to using the native peer-to-peer protocol rather than JSON-RPC.

If we ever decide to break backwards compatibility, we could divide the "history" and "archive" capabilities in two, between nodes capable of serving older blocks and nodes capable of serving newer blocks. +We could even add to the peer-to-peer network nodes that are only capable of serving older blocks (by reading from a database) but do not participate in the head of the chain, and that just exist for historical purposes.

(source)

Table of Contents

RFC-0061: Support allocator inside of runtime +
- Summary
- Motivation
- Stakeholders
- Explanation +
  - Runtime side spec
  - Client side spec
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0061: Support allocator inside of runtime

+ + + +


Start Date	27 December 2023
Description	Supporting runtime built-in allocator makes the substrate runtime more versatile
Authors	Jiahao Ye

Summary

Currently, substrate runtime use an simple allocator defined by host side. Every runtime MUST +import these allocator functions for normal execution. This situation make runtime code not versatile enough.

So this RFC proposes to define a new spec for allocator part to make substrate runtime more generic.

Motivation

Since this RFC define a new way for allocator, we now regard the old one as legacy allocator. +As we all know, since the allocator implementation details are defined by the substrate client, parachain/parathread cannot customize memory allocator algorithm, so the new specification allows the runtime to customize memory allocation, and then export the allocator function according to the specification for the client side to use. +Another benefit is that some new host functions can be designed without allocating memory on the client, which may have potential performance improvements. Also it will help provide a unified and clean specification if substrate runtime support multi-targets(e.g. RISC-V). +There is also a potential benefit. Many programming languages that support compilation to wasm may not be friendly to supporting external allocator. This is beneficial for other programming languages to enter the substrate runtime ecosystem. +The last and most important benefit is that for offchain context execution, the runtime can fully support pure wasm. What this means here is that all imported host functions could not actually be called (as stub functions), then the various verification logic of the runtime can be converted into pure wasm, which provides the possibility for the substrate runtime to run block verification in other environments (such as in browsers and other non-substrate environments).

Stakeholders

No attempt was made at convincing stakeholders.

Explanation

Runtime side spec

This section contains a list of functions should be exported by substrate runtime.

We define the spec as version 1, so the following dummy function v1 MUST be exported to hint +client that runtime is using version 1 spec, otherwise rollback to legacy allocator. +The function should never be used, and its name is only for version checking.

  (export "v1" (func $v1))
+

Choose this way is more generic than custom section since many other tools do not support custom section very well. But if an environment want to run it, it should always be possible to parse +the export section.

The allocator functions are:

(export "alloc" (func $alloc))
+(export "dealloc" (func $dealloc))
+(export "realloc" (func $realloc))
+

Their signatures are:

(func $alloc (param $size i32) (result i32))
+(func $dealloc (param $addr i32) (param $size i32))
+(func $realloc (param $addr i32) (param $size i32) (param $new_size i32) (result i32))
+

Note: dealloc/realloc is not used in substrate side currently, but for the functional integrity.

The following imports are disabled.

The two kind of allocators(leagcy and v1) cannot know each other, and importing them will cause abnormal memory allocation.

(import "env" "ext_allocator_free_version_1" (func $ext_allocator_free_version_1 (type 0)))
+(import "env" "ext_allocator_malloc_version_1" (func $ext_allocator_malloc_version_1 (type 1)))
+

The following export could be removed. The client side no need to know heap base.

(export "__heap_base" (global 2))
+

Client side spec

During instantiating time, add a version checking stage for wasm executor before any other wasm module checking. +Check if parsed wasm module contains a exported v1 function:

If not exist, we predicate it using legacy allocator, just do normal checking like before. Set legacy allocator be Some while set v1 allocator be None.
If exist, we predicate it using v1 allocator. And then we lookup and hold the exported alloc function for the total lifestyle of instance, return error if not exist. Set legacy allocator be None while set v1 allocator be Some.
When wasm host functions or other entrypoint call(e.g. runtime_apis/validate_block) need to allocate memory, check if instance hold the alloc, if hold just call it otherwise call the legacy +allocator.

Detail-heavy explanation of the RFC, suitable for explanation to an implementer of the changeset. This should address corner cases in detail and provide justification behind decisions, and provide rationale for how the design meets the solution requirements.

Drawbacks

The allocator inside of the runtime will make code size bigger, but it's not obvious. +The allocator inside of the runtime maybe slow down(or speed up) the runtime, still not obvious.

We could ignore these drawbacks since they are not prominent. And the execution efficiency is highly decided by runtime developer. We could not prevent a poor efficiency if developer want to do it.

Testing, Security, and Privacy

Keep the legacy allocator runtime test cases, and add new feature to compile test cases for v1 allocator spec. And then update the test asserts.

Update template runtime to enable v1 spec. Once the dev network runs well, it seems that the spec is implmented correctly.

Performance, Ergonomics, and Compatibility

Performance

As the above says, not obvious impact about performance. And polkadot-sdk could offer the best practice allocator for all chains. +Third party also could customized by theirself. So the performance could be improved over time.

Ergonomics

Only for runtime developer, Just need to import a new crate and enable a new feature. Maybe it's convienient for other wasm-target language to implment.

Compatibility

It's 100% compatible. Only Some runtime configs and executor configs need to be depreacted.

For support new runtime spec, we MUST upgrade the client binary to support new spec of client part firstly.

We SHALL add an optional primtive crate to enable the version 1 spec and disable the legacy allocator by cargo feature. +For the first year, we SHALL disable the v1 by default, and enable it by default start in the next year.

Prior Art and References

Unresolved Questions

None at this time.

+ +

The content discussed with RFC-0004 is basically orthogonal, but it could still be considered together, and it is preferred that this rfc be implmentented first.

This feature could make substrate runtime be easier supported by other languages and integreted into other ecosystem.

(source)

Table of Contents

RFC-0006: Dynamic Pricing for Bulk Coretime Sales +
- Summary
- Motivation +
  - Requirements
  +
- Stakeholders
- Explanation +
  - Overview
  - Parameters
  - Function
  - Pseudo-code
  - Properties of the Curve
  - Example Configurations
  +
- Drawbacks
- Prior Art and References
- Future Possibilities
- References
+

RFC-0006: Dynamic Pricing for Bulk Coretime Sales

+ + + + +


Start Date	July 09, 2023
Description	A dynamic pricing model to adapt the regular price for bulk coretime sales
Authors	Tommi Enenkel (Alice und Bob)
License	MIT

Summary

This RFC proposes a dynamic pricing model for the sale of Bulk Coretime on the Polkadot UC. The proposed model updates the regular price of cores for each sale period, by taking into account the number of cores sold in the previous sale, as well as a limit of cores and a target number of cores sold. It ensures a minimum price and limits price growth to a maximum price increase factor, while also giving govenance control over the steepness of the price change curve. It allows governance to address challenges arising from changing market conditions and should offer predictable and controlled price adjustments.

Accompanying visualizations are provided at [1].

Motivation

RFC-1 proposes periodic Bulk Coretime Sales as a mechanism to sell continouos regions of blockspace (suggested to be 4 weeks in length). A number of Blockspace Regions (compare RFC-1 & RFC-3) are provided for sale to the Broker-Chain each period and shall be sold in a way that provides value-capture for the Polkadot network. The exact pricing mechanism is out of scope for RFC-1 and shall be provided by this RFC.

A dynamic pricing model is needed. A limited number of Regions are offered for sale each period. The model needs to find the price for a period based on supply and demand of the previous period.

The model shall give Coretime consumers predictability about upcoming price developments and confidence that Polkadot governance can adapt the pricing model to changing market conditions.

Requirements

The solution SHOULD provide a dynamic pricing model that increases price with growing demand and reduces price with shrinking demand.
The solution SHOULD have a slow rate of change for price if the number of Regions sold is close to a given sales target and increase the rate of change as the number of sales deviates from the target.
The solution SHOULD provide the possibility to always have a minimum price per Region.
The solution SHOULD provide a maximum factor of price increase should the limit of Regions sold per period be reached.
The solution should allow governance to control the steepness of the price function

Stakeholders

The primary stakeholders of this RFC are:

Protocol researchers and evelopers
Polkadot DOT token holders
Polkadot parachains teams
Brokers involved in the trade of Bulk Coretime

Explanation

Overview

The dynamic pricing model sets the new price based on supply and demand in the previous period. The model is a function of the number of Regions sold, piecewise-defined by two power functions.

The left side ranges from 0 to the target. It represents situations where demand was lower than the target.
The right side ranges from the target to limit. It represents situations where demand was higher than the target.

The curve of the function forms a plateau around the target and then falls off to the left and rises up to the right. The shape of the plateau can be controlled via a scale factor for the left side and right side of the function respectively.

Parameters

From here on, we will also refer to Regions sold as 'cores' to stay congruent with RFC-1.

+ + + + + + +

Name	Suggested Value	Description	Constraints
`BULK_LIMIT`	45	The maximum number of cores being sold	`0 < BULK_LIMIT`
`BULK_TARGET`	30	The target number of cores being sold	`0 < BULK_TARGET <= BULK_LIMIT`
`MIN_PRICE`	1	The minimum price a core will always cost.	`0 < MIN_PRICE`
`MAX_PRICE_INCREASE_FACTOR`	2	The maximum factor by which the price can change.	`1 < MAX_PRICE_INCREASE_FACTOR`
`SCALE_DOWN`	2	The steepness of the left side of the function.	`0 < SCALE_DOWN`
`SCALE_UP`	2	The steepness of the right side of the function.	`0 < SCALE_UP`

Function

P(n) = \begin{cases} 
+    (P_{\text{old}} - P_{\text{min}}) \left(1 - \left(\frac{T - n}{T}\right)^d\right) + P_{\text{min}} & \text{if } n \leq T \\
+    ((F - 1) \cdot P_{\text{old}} \cdot \left(\frac{n - T}{L - T}\right)^u) + P_{\text{old}} & \text{if } n > T 
+\end{cases}
+

$P_{\text{old}}$ is the old_price, the price of a core in the previous period.
$P_{\text{min}}$ is the MIN_PRICE, the minimum price a core will always cost.
$F$ is the MAX_PRICE_INCREASE_FACTOR, the factor by which the price maximally can change from one period to another.
$d$ is the SCALE_DOWN, the steepness of the left side of the function.
$u$ is the SCALE_UP, the steepness of the right side of the function.
$T$ is the BULK_TARGET, the target number of cores being sold.
$L$ is the BULK_LIMIT, the maximum number of cores being sold.
$n$ is cores_sold, the number of cores being sold.

Left side

The left side is a power function that describes an increasing concave downward curvature that approaches old_price. We realize this by using the form $y = a(1 - x^d)$, usually used as a downward sloping curve, but in our case flipped horizontally by letting the argument $x = \frac{T-n}{T}$ decrease with $n$, doubly inversing the curve.

This approach is chosen over a decaying exponential because it let's us a better control the shape of the plateau, especially allowing us to get a straight line by setting SCALE_DOWN to $1$.

Ride side

The right side is a power function of the form $y = a(x^u)$.

Pseudo-code

NEW_PRICE := IF CORES_SOLD <= BULK_TARGET THEN
+    (OLD_PRICE - MIN_PRICE) * (1 - ((BULK_TARGET - CORES_SOLD)^SCALE_DOWN / BULK_TARGET^SCALE_DOWN)) + MIN_PRICE
+ELSE
+    ((MAX_PRICE_INCREASE_FACTOR - 1) * OLD_PRICE * ((CORES_SOLD - BULK_TARGET)^SCALE_UP / (BULK_LIMIT - BULK_TARGET)^SCALE_UP)) + OLD_PRICE
+END IF
+

Properties of the Curve

Minimum Price

We introduce MIN_PRICE to control the minimum price.

The left side of the function shall be allowed to come close to 0 if cores sold approaches 0. The rationale is that if there are actually 0 cores sold, the previous sale price was too high and the price needs to adapt quickly.

Price forms a plateau around the target

If the number of cores is close to BULK_TARGET, less extreme price changes might be sensible. This ensures that a drop in sold cores or an increase doesn’t lead to immediate price changes, but rather slowly adapts. Only if more extreme changes in the number of sold cores occur, does the price slope increase.

We introduce SCALE_DOWN and SCALE_UP to control for the steepness of the left and the right side of the function respectively.

Max price increase factor

We introduce MAX_PRICE_INCREASE_FACTOR as the factor that controls how much the price may increase from one period to another.

Introducing this variable gives governance an additional control lever and avoids the necessity for a future runtime upgrade.

Example Configurations

Baseline

This example proposes the baseline parameters. If not mentioned otherwise, other examples use these values.

The minimum price of a core is 1 DOT, the price can double every 4 weeks. Price change around BULK_TARGET is dampened slightly.

BULK_TARGET = 30
+BULK_LIMIT = 45
+MIN_PRICE = 1
+MAX_PRICE_INCREASE_FACTOR = 2
+SCALE_DOWN = 2
+SCALE_UP = 2
+OLD_PRICE = 1000
+

More aggressive pricing

We might want to have a more aggressive price growth, allowing the price to triple every 4 weeks and have a linear increase in price on the right side.

BULK_TARGET = 30
+BULK_LIMIT = 45
+MIN_PRICE = 1
+MAX_PRICE_INCREASE_FACTOR = 3
+SCALE_DOWN = 2
+SCALE_UP = 1
+OLD_PRICE = 1000
+

Conservative pricing to ensure quick corrections in an affluent market

If governance considers the risk that a sudden surge in DOT price might price chains out from bulk coretime markets, it can ensure the model quickly reacts to a quick drop in demand, by setting 0 < SCALE_DOWN < 1 and setting the max price increase factor more conservatively.

BULK_TARGET = 30
+BULK_LIMIT = 45
+MIN_PRICE = 1
+MAX_PRICE_INCREASE_FACTOR = 1.5
+SCALE_DOWN = 0.5
+SCALE_UP = 2
+OLD_PRICE = 1000
+

Linear pricing

By setting the scaling factors to 1 and potentially adapting the max price increase, we can achieve a linear function

BULK_TARGET = 30
+BULK_LIMIT = 45
+MIN_PRICE = 1
+MAX_PRICE_INCREASE_FACTOR = 1.5
+SCALE_DOWN = 1
+SCALE_UP = 1
+OLD_PRICE = 1000
+

Drawbacks

None at present.

Prior Art and References

This pricing model is based on the requirements from the basic linear solution proposed in RFC-1, which is a simple dynamic pricing model and only used as proof. The present model adds additional considerations to make the model more adaptable under real conditions.

Future Possibilities

This RFC, if accepted, shall be implemented in conjunction with RFC-1.

References

[1] Polkadot forum post with visualizations: Dynamic Pricing for Bulk Coretime Sales

(source)

Table of Contents

RFC-0009: Improved light client requests networking protocol +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0009: Improved light client requests networking protocol

+ + + +


Start Date	2023-07-19
Description	Modify the networking storage read requests to solve some problems with the existing one
Authors	Pierre Krieger

Summary

Improve the networking messages that query storage items from the remote, in order to reduce the bandwidth usage and number of round trips of light clients.

Motivation

Clients on the Polkadot peer-to-peer network can be divided into two categories: full nodes and light clients. So-called full nodes are nodes that store the content of the chain locally on their disk, while light clients are nodes that don't. In order to access for example the balance of an account, a full node can do a disk read, while a light client needs to send a network message to a full node and wait for the full node to reply with the desired value. This reply is in the form of a Merkle proof, which makes it possible for the light client to verify the exactness of the value.

Unfortunately, this network protocol is suffering from some issues:

It is not possible for the querier to check whether a key exists in the storage of the chain except by querying the value of that key. The reply will thus include the value of the key, only for that value to be discarded by the querier that isn't interested by it. This is a waste of bandwidth.
It is not possible for the querier to know whether a value in the storage of the chain has been modified between two blocks except by querying this value for both blocks and comparing them. Only a few storage values get modified in a block, and thus most of the time the comparison will be equal. This leads to a waste of bandwidth as the values have to be transferred.
While it is possible to ask for multiple specific storage keys at the same time, it is not possible to ask for a list of keys that start with a certain prefix. Due to the way FRAME works, storage keys are grouped by "prefix", for example all account balances start with the same prefix. It is thus a common necessity for a light client to obtain the list of all keys (and possibly their values) that start with a specific prefix. This is currently not possible except by performing multiple queries serially that "walk down" the trie.

Once Polkadot and Kusama will have transitioned to state_version = 1, which modifies the format of the trie entries, it will be possible to generate Merkle proofs that contain only the hashes of values in the storage. Thanks to this, it is already possible to prove the existence of a key without sending its entire value (only its hash), or to prove that a value has changed or not between two blocks (by sending just their hashes). +Thus, the only reason why aforementioned issues exist is because the existing networking messages don't give the possibility for the querier to query this. This is what this proposal aims at fixing.

Stakeholders

This is the continuation of https://github.com/w3f/PPPs/pull/10, which itself is the continuation of https://github.com/w3f/PPPs/pull/5.

Explanation

The protobuf schema of the networking protocol can be found here: https://github.com/paritytech/substrate/blob/5b6519a7ff4a2d3cc424d78bc4830688f3b184c0/client/network/light/src/schema/light.v1.proto

The proposal is to modify this protocol in this way:

@@ -11,6 +11,7 @@ message Request {
+                RemoteReadRequest remote_read_request = 2;
+                RemoteReadChildRequest remote_read_child_request = 4;
+                // Note: ids 3 and 5 were used in the past. It would be preferable to not re-use them.
++               RemoteReadRequestV2 remote_read_request_v2 = 6;
+        }
+ }
+ 
+@@ -48,6 +49,21 @@ message RemoteReadRequest {
+        repeated bytes keys = 3;
+ }
+ 
++message RemoteReadRequestV2 {
++       required bytes block = 1;
++       optional ChildTrieInfo child_trie_info = 2;  // Read from the main trie if missing.
++       repeated Key keys = 3;
++       optional bytes onlyKeysAfter = 4;
++       optional bool onlyKeysAfterIgnoreLastNibble = 5;
++}
++
++message ChildTrieInfo {
++       enum ChildTrieNamespace {
++               DEFAULT = 1;
++       }
++
++       required bytes hash = 1;
++       required ChildTrieNamespace namespace = 2;
++}
++
+ // Remote read response.
+ message RemoteReadResponse {
+        // Read proof. If missing, indicates that the remote couldn't answer, for example because
+@@ -65,3 +81,8 @@ message RemoteReadChildRequest {
+        // Storage keys.
+        repeated bytes keys = 6;
+ }
++
++message Key {
++       required bytes key = 1;
++       optional bool skipValue = 2; // Defaults to `false` if missing
++       optional bool includeDescendants = 3; // Defaults to `false` if missing
++}
+

Note that the field names aren't very important as they are not sent over the wire. They can be changed at any time without any consequence. I would invite people to not discuss these field names as they are implementation details.

This diff adds a new type of request (RemoteReadRequestV2).

The new child_trie_info field in the request makes it possible to specify which trie is concerned by the request. The current networking protocol uses two different structs (RemoteReadRequest and RemoteReadChildRequest) for main trie and child trie queries, while this new request would make it possible to query either. This change doesn't fix any of the issues mentioned in the previous section, but is a side change that has been done for simplicity. +An alternative could have been to specify the child_trie_info for each individual Key. However this would make it necessary to send the child trie hash many times over the network, which leads to a waste of bandwidth, and in my opinion makes things more complicated for no actual gain. If a querier would like to access more than one trie at the same time, it is always possible to send one query per trie.

If skipValue is true for a Key, then the value associated with this key isn't important to the querier, and the replier is encouraged to replace the value with its hash provided that the storage item has a state_version equal to 1. If the storage value has a state_version equal to 0, then the optimization isn't possible and the replier should behave as if skipValue was false.

If includeDescendants is true for a Key, then the replier must also include in the proof all keys that are descendant of the given key (in other words, its children, children of children, children of children of children, etc.). It must do so even if key itself doesn't have any storage value associated to it. The values of all of these descendants are replaced with their hashes if skipValue is true, similarly to key itself.

The optional onlyKeysAfter and onlyKeysAfterIgnoreLastNibble fields can provide a lower bound for the keys contained in the proof. The responder must not include in its proof any node whose key is strictly inferior to the value in onlyKeysAfter. If onlyKeysAfterIgnoreLastNibble is provided, then the last 4 bits for onlyKeysAfter must be ignored. This makes it possible to represent a trie branch node that doesn't have an even number of nibbles. If no onlyKeysAfter is provided, it is equivalent to being empty, meaning that the response must start with the root node of the trie.

If onlyKeysAfterIgnoreLastNibble is missing, it is equivalent to false. If onlyKeysAfterIgnoreLastNibble is true and onlyKeysAfter is missing or empty, then the request is invalid.

For the purpose of this networking protocol, it should be considered as if the main trie contained an entry for each default child trie whose key is concat(":child_storage:default:", child_trie_hash) and whose value is equal to the trie root hash of that default child trie. This behavior is consistent with what the host functions observe when querying the storage. This behavior is present in the existing networking protocol, in other words this proposal doesn't change anything to the situation, but it is worth mentioning. +Also note that child tries aren't considered as descendants of the main trie when it comes to the includeDescendants flag. In other words, if the request concerns the main trie, no content coming from child tries is ever sent back.

This protocol keeps the same maximum response size limit as currently exists (16 MiB). It is not possible for the querier to know in advance whether its query will lead to a reply that exceeds the maximum size. If the reply is too large, the replier should send back only a limited number (but at least one) of requested items in the proof. The querier should then send additional requests for the rest of the items. A response containing none of the requested items is invalid.

The server is allowed to silently discard some keys of the request if it judges that the number of requested keys is too high. This is in line with the fact that the server might truncate the response.

Drawbacks

This proposal doesn't handle one specific situation: what if a proof containing a single specific item would exceed the response size limit? For example, if the response size limit was 1 MiB, querying the runtime code (which is typically 1.0 to 1.5 MiB) would be impossible as it's impossible to generate a proof less than 1 MiB. The response size limit is currently 16 MiB, meaning that no single storage item must exceed 16 MiB.

Unfortunately, because it's impossible to verify a Merkle proof before having received it entirely, parsing the proof in a streaming way is also not possible.

A way to solve this issue would be to Merkle-ize large storage items, so that a proof could include only a portion of a large storage item. Since this would require a change to the trie format, it is not realistically feasible in a short time frame.

Testing, Security, and Privacy

The main security consideration concerns the size of replies and the resources necessary to generate them. It is for example easily possible to ask for all keys and values of the chain, which would take a very long time to generate. Since responses to this networking protocol have a maximum size, the replier should truncate proofs that would lead to the response being too large. Note that it is already possible to send a query that would lead to a very large reply with the existing network protocol. The only thing that this proposal changes is that it would make it less complicated to perform such an attack.

Implementers of the replier side should be careful to detect early on when a reply would exceed the maximum reply size, rather than inconditionally generate a reply, as this could take a very large amount of CPU, disk I/O, and memory. Existing implementations might currently be accidentally protected from such an attack thanks to the fact that requests have a maximum size, and thus that the list of keys in the query was bounded. After this proposal, this accidental protection would no longer exist.

Malicious server nodes might truncate Merkle proofs even when they don't strictly need to, and it is not possible for the client to (easily) detect this situation. However, malicious server nodes can already do undesirable things such as throttle down their upload bandwidth or simply not respond. There is no need to handle unnecessarily truncated Merkle proofs any differently than a server simply not answering the request.

Performance, Ergonomics, and Compatibility

Performance

It is unclear to the author of the RFC what the performance implications are. Servers are supposed to have limits to the amount of resources they use to respond to requests, and as such the worst that can happen is that light client requests become a bit slower than they currently are.

Ergonomics

Irrelevant.

Compatibility

The prior networking protocol is maintained for now. The older version of this protocol could get removed in a long time.

Prior Art and References

None. This RFC is a clean-up of an existing mechanism.

Unresolved Questions

None

+ +

The current networking protocol could be deprecated in a long time. Additionally, the current "state requests" protocol (used for warp syncing) could also be deprecated in favor of this one.

(source)

Table of Contents

RFC-0000: Lowering NFT Deposits on Polkadot and Kusama Asset Hubs +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0000: Lowering NFT Deposits on Polkadot and Kusama Asset Hubs

+ + + +


Start Date	2 November 2023
Description	A proposal to reduce the minimum deposit required for collection creation on the Polkadot and Kusama Asset Hub, making it more accessible and affordable for artists.
Authors	Aurora Poppyseed, Just_Luuuu, VikiiVal

Summary

This RFC proposes a change to the current deposit requirements on the Polkadot and Kusama Asset Hub for creating NFT collections. The objective is to lower the barrier to entry for artists, fostering a more inclusive and vibrant ecosystem, while maintaining network integrity and preventing spam.

Motivation

The current deposit of 10 DOT for collection creation on the Polkadot Asset Hub presents a significant financial barrier for many artists. By lowering the deposit requirements, we aim to encourage more artists to participate in the Polkadot NFT ecosystem, thereby enriching the diversity and vibrancy of the community and its offerings.

Stakeholders

NFT Creators: Primary beneficiaries of the proposed change, particularly those who found the current deposit requirements prohibitive.
NFT Platforms: As the facilitator of artists' relations, KodaDot has a vested interest in making the platform more accessible.
Polkadot Community: Stands to benefit from an influx of artists, creators and diverse NFT collections, enhancing the overall ecosystem.

Previous discussions have been held within the KodaDot community, as well as with artists expressing their concerns about the deposit amounts. Referencing to Polkadot Forum conversation.

Explanation

This RFC suggests modifying the smart contract governing collection creation on the Polkadot Asset Hub to require a lower deposit. The exact amount of the reduced deposit is yet to be determined and should be discussed and agreed upon by the stakeholders. The implementation of this change requires careful consideration of the network's integrity and the prevention of spam, possibly through alternative means such as rate limiting or account verification.

Prices and Proposed Prices on Polkadot Asset Hub: +Scroll right

| **Name**                  | **Current price implementation** | **Price if DOT = 5$**  | **Price if DOT goes to 50$**  | **Proposed Price in DOT** | **Proposed Price if DOT = 5$**   | **Proposed Price if DOT goes to 50$**|
+|---------------------------|----------------------------------|------------------------|-------------------------------|---------------------------|----------------------------------|--------------------------------------|
+| collectionDeposit         | 10 DOT                           | 50 $                   | 500 $                         | 0.1 DOT                   | 0.5 $                            | 5$                                   |
+| itemDeposit               | 0.01 DOT                         | 0.05 $                 | 0.5 $                         | 0.001 DOT                 | 0.005 $                          | 0.05$                                |
+| metadataDepositBase       | 0.20129 DOT                      | 1.00645 $              | 10.0645 $                     | 0.0020129 DOT             | 0.0100645 $                      | 0.100645$                            |
+| attributeDepositBase      | 0.2 DOT                          | 1 $                    | 10 $                          | 0.002 DOT                 | 0.01 $                           | 0.1$                                 |
+

Prices and Proposed Prices on Kusama Asset Hub: +Scroll right

| **Name**                  | **Current price implementation** | **Price if KSM = 23$** | **Price if KSM goes to 500$** | **Proposed Price in KSM** | **Proposed Price if KSM = 23$**  | **Proposed Price if KSM goes to 500$** |
+|---------------------------|----------------------------------|------------------------|-------------------------------|---------------------------|----------------------------------|----------------------------------------|
+| collectionDeposit         | 0.1 KSM                          | 2.3 $                  | 50 $                          | 0.01 KSM                  | 0.23 $                           | 5 $                                    |
+| itemDeposit               | 0.001 KSM                        | 0.023 $                | 0.5 $                         | 0.0001 KSM                | 0.0023 $                         | 0.05 $                                 |
+| metadataDepositBase       | 0.006709666617 KSM               | 0.15432183319 $        | 3.3548333085 $                | 0.0006709666617 KSM       | 0.015432183319 $                 | 0.33548333085 $                        |
+| attributeDepositBase      | 0.00666666666 KSM                | 0.15333333318 $        | 3.333333333 $                 | 0.000666666666 KSM        | 0.015333333318 $                 | 0.3333333333 $                         |
+
+

+
Note: This is only a proposal for change and it can be modified upon additional conversation.
+

Drawbacks

The contemplation of modifying deposit requirements necessitates a balanced assessment of the potential drawbacks. Highlighted below are cogent points extracted from the discourse on the Polkadot Forum conversation, which provide critical perspectives on the implications of such changes:

+
But NFT deposits were chosen somewhat arbitrarily at genesis and it’s a good exercise to re-evaluate them and adapt if they are causing pain and if lowering them has little or no negative side effect (or if the trade-off is worth it). +-> joepetrowski
+

+
Underestimates mean that state grows faster, although not unbounded - effectively an economic subsidy on activity. Overestimates mean that the state grows slower - effectively an economic depressant on activity. +-> rphmeier
+

+
Technical: We want to prevent state bloat, therefore using state should have a cost associated with it. +-> joepetrowski
+

Testing, Security, and Privacy

Performance, Ergonomics, and Compatibility

Performance

This change is not expected to have a significant impact on the overall performance of the Polkadot Asset Hub. However, it is crucial to monitor the network closely, especially in the initial stages after implementation, to identify and mitigate any potential issues.

Additionally, a supplementary proposal aims to augment the network's adaptability:

+
Just from a technical perspective; I think the best we can do is to use a weak governance origin that is controlled by some consortium (ie. System Collective). +This origin could then update the NFT deposits any time the market conditions warrant it - obviously while honoring the storage deposit requirements. +To implement this, we need RFC#12 and the Parameters pallet from @xlc. +-> OliverTY
+

This dynamic governance approach would facilitate a responsive and agile economic model for deposit management, ensuring that the network remains both accessible and robust in the face of market volatility.

Ergonomics

The proposed change aims to enhance the user experience for artists, making Polkadot more accessible and user-friendly.

Compatibility

The change doesn't impact compatibility.

Unresolved Questions

Determining the optimal new deposit amount that reduces the barrier to entry while preventing state bloat remains a subject for debate. What is the balance that allows for inclusivity without compromising the network's efficiency and security?

+ +

If accepted, this RFC could pave the way for further discussions and proposals aimed at enhancing the inclusivity and accessibility of the Polkadot ecosystem. Future work could also explore having a weak governance origin for deposits as proposed by Oliver.

(source)

Table of Contents

RFC-0010: Burn Coretime Revenue +
- Summary
- Motivation
- Stakeholders
- Explanation
+

RFC-0010: Burn Coretime Revenue

+ + + +


Start Date	19.07.2023
Description	Revenue from Coretime sales should be burned
Authors	Jonas Gehrlein

Summary

The Polkadot UC will generate revenue from the sale of available Coretime. The question then arises: how should we handle these revenues? Broadly, there are two reasonable paths – burning the revenue and thereby removing it from total issuance or divert it to the Treasury. This Request for Comment (RFC) presents arguments favoring burning as the preferred mechanism for handling revenues from Coretime sales.

Motivation

How to handle the revenue accrued from Coretime sales is an important economic question that influences the value of DOT and should be properly discussed before deciding for either of the options. Now is the best time to start this discussion.

Stakeholders

Polkadot DOT token holders.

Explanation

This RFC discusses potential benefits of burning the revenue accrued from Coretime sales instead of diverting them to Treasury. Here are the following arguments for it.

It's in the interest of the Polkadot community to have a consistent and predictable Treasury income, because volatility in the inflow can be damaging, especially in situations when it is insufficient. As such, this RFC operates under the presumption of a steady and sustainable Treasury income flow, which is crucial for the Polkadot community's stability. The assurance of a predictable Treasury income, as outlined in a prior discussion here, or through other equally effective measures, serves as a baseline assumption for this argument.

Consequently, we need not concern ourselves with this particular issue here. This naturally begs the question - why should we introduce additional volatility to the Treasury by aligning it with the variable Coretime sales? It's worth noting that Coretime revenues often exhibit an inverse relationship with periods when Treasury spending should ideally be ramped up. During periods of low Coretime utilization (indicated by lower revenue), Treasury should spend more on projects and endeavours to increase the demand for Coretime. This pattern underscores that Coretime sales, by their very nature, are an inconsistent and unpredictable source of funding for the Treasury. Given the importance of maintaining a steady and predictable inflow, it's unnecessary to rely on another volatile mechanism. Some might argue that we could have both: a steady inflow (from inflation) and some added bonus from Coretime sales, but burning the revenue would offer further benefits as described below.

+
Balancing Inflation: While DOT as a utility token inherently profits from a (reasonable) net inflation, it also benefits from a deflationary force that functions as a counterbalance to the overall inflation. Right now, the only mechanism on Polkadot that burns fees is the one for underutilized DOT in the Treasury. Finding other, more direct target for burns makes sense and the Coretime market is a good option.
+
+
Clear incentives: By burning the revenue accrued on Coretime sales, prices paid by buyers are clearly costs. This removes distortion from the market that might arise when the paid tokens occur on some other places within the network. In that case, some actors might have secondary motives of influencing the price of Coretime sales, because they benefit down the line. For example, actors that actively participate in the Coretime sales are likely to also benefit from a higher Treasury balance, because they might frequently request funds for their projects. While those effects might appear far-fetched, they could accumulate. Burning the revenues makes sure that the prices paid are clearly costs to the actors themselves.
+
+
Collective Value Accrual: Following the previous argument, burning the revenue also generates some externality, because it reduces the overall issuance of DOT and thereby increases the value of each remaining token. In contrast to the aforementioned argument, this benefits all token holders collectively and equally. Therefore, I'd consider this as the preferrable option, because burns lets all token holders participate at Polkadot's success as Coretime usage increases.
+

(source)

Table of Contents

RFC-0011: Add New Path to Account Creation on Asset Hubs +
- Summary
- Motivation +
  - Requirements
  +
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0011: Add New Path to Account Creation on Asset Hubs

+ + + +


Start Date	19 July 2023
Description	Proposal for a new secure means of creating an account on Asset Hub.
Authors	Joe Petrowski

Summary

The Assets pallet includes a notion of asset "sufficiency". Sufficient assets, when transferred to +a non-existent account, will provide a sufficient reference that creates the account. That is, the +asset is sufficient to justify an account's existence, even in lieu of the existential deposit of +DOT.

While convenient for sufficient assets, the vast majority of assets are not sufficient. This RFC +proposes an opt-in means for users to create accounts from non-sufficient assets by swapping a +portion of the first transfer to acquire the existential deposit of DOT.

Motivation

The network can make an asset "sufficient" via governance call. However, the network is still +placing trust in the asset's administrator (which may be a third-party account or a protocol). The +asset's administrator could mint the asset and create many accounts without paying an adequate +storage deposit. For this reason, governance has been extremely strict in granting sufficiency, so +far only doing so to one asset (USDT).

With the introduction of the Asset Conversion pallet, the Asset Hub can offer a new path to account +creation. The current paths are:

An account can have the existential deposit of DOT;
An account can have the minimum balance of a sufficient asset;
Someone else can create an account in the context of an asset class by placing a deposit in DOT. +This path is only available to the asset class's Admin or Freezer.

This RFC proposes a fourth path that does not introduce prior steps for either the sender or +receiver of the asset.

Requirements

The system MUST be secure against economic attacks that allow an attacker to create a virtually +unlimited number of accounts.
The system SHOULD allow users to hold and transact in any asset without separately and priorly +acquiring DOT.

Stakeholders

Polkadot users
Wallet and UI/UX developers

Explanation

By using the Asset Conversion protocol, the system can convert any asset to DOT as long as there is +a path from that asset to DOT. As such, we can rely on the economic security provided by the +existential deposit of DOT by simply converting some amount of the asset being transferred to the +existential deposit.

This conversion only need happen when the account does not yet exist. When the destination account +does exist, the full amount of the asset can be transferred. This would mean that only the first +asset transfer to an account has some amount debited to acquire the DOT to create the account, but +subsequent transfers would always be in full.

The main benefit of this approach is that it removes the sender's need to know about the +desination's existence and the recipient's need to "prepare" an account by endowing it.

The primary tradeoff, of course, is that transactions like "send 10 USDT" could result in fewer +than 10 USDT arriving in the destination account. This can be solved by having the conversion be +opt-in for the sender.

Because the existential deposit is small (0.1 DOT on Asset Hub), and the user need not interact +with the DOT in any way -- because transaction fee payment can also be handled via Asset Conversion +-- many users may find this path convenient in avoiding transfer errors due to non-existent +accounts or asset insufficiency.

Stripping out all other asset transfer-associated logic, this RFC proposes the following logic:

#![allow(unused)]
+fn main() {
+fn transfer(
+    origin: OriginFor,
+    asset: AssetId,
+    destination: AccountId,
+    amount: Balance,
+    create_destination: bool,
+    ..
+) -> DispatchResult {
+    let from = ensure_signed(origin)?;
+    let details = Asset::<T, I>::get(&id).ok_or(Error::<T, I>::Unknown)?;
+    if destination.exists() || !create_destination || details.sufficient {
+        // Either the destination already exists (holds ED of DOT), the user does not want to create
+        // the destination account, or the asset class is sufficient. We can just transfer the
+        // asset as normal.
+        Self::do_transfer(asset, from, destination, amount, ..)?;
+    } else {
+        // The destination does not exist and the user has opted in to create it via a swap.
+        //
+        // We will try to swap the asset provided for the existential deposit, depositing the ED in
+        // the destination account. If the asset does not have an Asset Conversion pair with DOT or
+        // the asset amount isn't enough to acquire the existential deposit, this will fail. But we
+        // generally think (a) pairs will exist, and (b) the ED is small and UIs can easily verify
+        // that this should succeed, so failures should be rare.
+        //
+        // The swap returns the amount of the asset consumed to acquire the ED.
+        let consumed = Swap::swap_tokens_for_exact_tokens(
+            from,                // sender
+            vec![asset, dot],    // path, where `dot` is Multilocation {parents: 1, interior: Here}
+            existential_deposit, // amount_out, we need the ED for the account
+            destination,         // send_to
+            ..
+        )?;
+        // We used some asset for the swap, so we have to subtract that from the amount.
+        let remaining_asset_amount = amount.saturating_sub(consumed);
+        // Now we transfer whatever amount is left, knowing that the destination account exists.
+        // This could still fail if the remaining amount is less than the minimum balance required
+        // by the asset class.
+        Self::do_transfer(asset, from, destination, remaining_asset_amount, ..)?;
+    }
+}
+}

Drawbacks

This solution would automatically convert some amount of another asset to DOT when acquiring DOT +was perhaps not the recipient's intent. However, this is opt-in.

Testing, Security, and Privacy

An attacker that wanted to bloat state by sending worthless assets to many new accounts would need +to put the DOT into an Asset Conversion pool with the asset (thereby making the asset not +worthless with respect to DOT). This would provide the same cost and economic security as just +sending the existential deposit of DOT to all the new accounts. This approach is no less secure +than the DOT-only existential deposit system.

This proposal introduces no privacy enhancements or reductions.

Performance, Ergonomics, and Compatibility

Performance

The function to transfer assets will need to charge a larger weight at dispatch to account for the +possibility of needing to perform a swap for DOT. It could return any unused weight.

The implementation could also include witness data as to the destination account's existence so +that the block builder can appropriately budget for the weight.

Ergonomics

This proposal would benefit the ergonomics of the system for end users by allowing all assets to +create destination accounts when needed.

Compatibility

This change would require changes to the Assets pallet to add the new account creation path.

Prior Art and References

Discussions with:

SR Labs auditors, in particular Jakob Lell and Louis Merlin
The monthly Asset Conversion ecosystem call, particular inspiration from Jakub Gregus

Unresolved Questions

None at this time.

+ +

Not applicable.

(source)

Table of Contents

RFC-0013: Prepare BlockBuilder and Core runtime APIs for MBMs +
- Summary
- Motivation
- Stakeholders
- Explanation +
  +
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0013: Prepare `BlockBuilder` and `Core` runtime APIs for MBMs

+ + + +


Start Date	July 24, 2023
Description	Prepare the `BlockBuilder` and `Core` Runtime APIs for Multi-Block-Migrations.
Authors	Oliver Tale-Yazdi

Summary

Introduces breaking changes to the BlockBuilder and Core runtime APIs.
+A new function BlockBuilder::last_inherent is introduced and the return value of Core::initialize_block is changed to an enum.
+The versions of both APIs are bumped; BlockBuilder to 7 and Core to 5.

Motivation

There are three main features that motivate for this RFC:

Multi-Block-Migrations: These make it possible to split a migration over multiple blocks.
Pallet poll hook: Can be used to gradually replace on_initialize/on_finalize in places where the code does not need to run by a hard deadline, since it is not guaranteed to execute each block.
New callback System::PostInherents: Can replace on_initialize/on_finalize where a hard deadline is required (complements poll). It is guaranteed to execute each block.

These three features can be implemented when fulfilling these two requirements:

The runtime can tell the block author to not include any transactions in the block.
The runtime can execute logic right after all pallet-provided inherents have been applied.

Stakeholders

Substrate Maintainers: They have to implement this, including tests, audit and +maintenance burden.
Polkadot Runtime developers: They will have to adapt the runtime files to this breaking change.
Polkadot Parachain Teams: They also have to adapt to the breaking changes but then eventually have +multi-block migrations available.

Explanation

`Core::initialize_block`

This runtime API function is changed from returning () to ExtrinsicInclusionMode:

#![allow(unused)]
+fn main() {
+enum ExtrinsicInclusionMode {
+  /// All extrinsics are allowed in this block.
+  AllExtrinsics,
+  /// Only inherents are allowed in this block.
+  OnlyInherents,
+}
+}

A block author MUST respect the ExtrinsicInclusionMode that is returned by initialize_block. The runtime MUST reject blocks that do have forbidden extrinsics in them.

`BlockBuilder::last_inherent`

A block author MUST always invoke last_inherent directly after applying all runtime-provided inherents. The runtime MUST reject blocks that violate this requirement.

Combined

Coming back to the three main features and how they can be implemented with these runtime APIs changes:

1. Multi-Block-Migrations: The runtime is being put into lock-down mode for the duration of the migration process by returning OnlyInherents from initialize_block. This ensures that no user provided transaction can interfere with the migration process. It is absolutely necessary to ensure this, since otherwise a transaction could call into un-migrated storage and violate storage invariants. The entry-point for the MBM logic is last_inherent. This is a good spot, because any data that is touched in inherents, is not MBM-migratable anyway. It could also be done before all other inherents or at the end of the block in finalize_block, but there is no downside from doing it in last_inherent and the other two features are in favour of this.

2. poll becomes possible by using last_inherent as entry-point. It would not be possible to use a pallet inherent like System::last_inherent to achieve this for two reasons. First is that pallets do not have access to AllPalletsWithSystem that is required to invoke the poll hook on all pallets. Second is that the runtime does currently not enforce an order of inherents.

3. System::PostInherents can be done in the same manner as poll.

Drawbacks

As noted in the review comments: this cements some assumptions about the order of inherents into the BlockBuilder traits. It was criticized for being to rigid in its assumptions.

Testing, Security, and Privacy

Compliance of a block author can be tested by adding specific code to the last_inherent hook and +checking that it always executes. The new logic of initialize_block can be tested by checking that +the block-builder will skip transactions and optional hooks when OnlyInherents is returned.

Security: n/a

Privacy: n/a

Performance, Ergonomics, and Compatibility

Performance

The performance overhead is minimal in the sense that no clutter was added after fulfilling the +requirements. A slight performance penalty is expected from invoking +last_inherent once per block.

Ergonomics

The new interface allows for more extensible runtime logic. In the future, this will be utilized for +multi-block-migrations which should be a huge ergonomic advantage for parachain developers.

Compatibility

The advice here is OPTIONAL and outside of the RFC. To not degrade +user experience, it is recommended to ensure that an updated node can still import historic blocks.

Prior Art and References

The RFC is currently being implemented in polkadot-sdk#1781. Related issues and merge +requests:

Unresolved Questions

Please suggest a better name for BlockExecutiveMode. We already tried: RuntimeExecutiveMode, +ExtrinsicInclusionMode. The names of the modes Normal and Minimal were also called +AllExtrinsics and OnlyInherents, so if you have naming preferences; please post them.
+=> renamed to ExtrinsicInclusionMode

~~Is post_inherents more consistent instead of last_inherent? Then we should change it.~~
+=> renamed to last_inherent

+ +

The long-term future here is to move the block building logic into the runtime. Currently there is a tight dance between the block author and the runtime; the author has to call into different runtime functions in quick succession and exact order. Any misstep causes the built block to be invalid.
+This can be unified and simplified by moving both parts of the logic into the runtime.

(source)

Table of Contents

RFC-0015: Market Design Revisit +
- Summary
- Motivation
- Stakeholders
- Explanation +
  +
- Drawbacks
- Prior Art and References
- Unresolved Questions
+

RFC-0015: Market Design Revisit

+ + + +


Start Date	05.08.2023
Description	This RFC refines the previously proposed mechanisms involving the various Coretime markets and presents an integrated framework for harmonious interaction between all markets.
Authors	Jonas Gehrlein

Summary

This document is a proposal for restructuring the bulk markets in the Polkadot UC's coretime allocation system to improve efficiency and fairness. The proposal suggests separating the BULK_PERIOD into MARKET_PERIOD and RENEWAL_PERIOD, allowing for a market-driven price discovery through a clearing price Dutch auction during the MARKET_PERIOD followed by renewal offers at the MARKET_PRICE during the RENEWAL_PERIOD. The new system ensures synchronicity between renewal and market prices, fairness among all current tenants, and efficient price discovery, while preserving price caps to provide security for current tenants. It seeks to start a discussion about the possibility of long-term leases.

Motivation

While the initial RFC-1 has provided a robust framework for Coretime allocation within the Polkadot UC, this proposal builds upon its strengths and uses many provided building blocks to address some areas that could be further improved.

In particular, this proposal introduces the following changes:

It introduces a RESERVE_PRICE that anchors all markets, promoting price synchronicity within the Bulk markets (flexible + renewals). +
- This reduces complexity.
- This makes sure all consumers pay a closely correlated price for coretime within a BULK_PERIOD.
+
It reverses the order of the market and renewal phase. +
- This allows to fine-tune the price through market forces.
+
It exposes the renewal prices, while still being beneficial for longterm tenants, more to market forces.
It removes the LeadIn period and introduces a (from the perspective of the coretime systemchain) passive Settlement Phase, that allows the secondary market to exert it's force.

The premise of this proposal is to reduce complexity by introducing a common price (that develops releative to capacity consumption of Polkadot UC), while still allowing for market forces to add efficiency. Longterm lease owners still receive priority IF they can pay (close to) the market price. This prevents a situation where the renewal price significantly diverges from renewal prices which allows for core captures. While maximum price increase certainty might seem contradictory to efficient price discovery, the proposed model aims to balance these elements, utilizing market forces to determine the price and allocate cores effectively within certain bounds. It must be stated, that potential price increases remain predictable (in the worst-case) but could be higher than in the originally proposed design. The argument remains, however, that we need to allow market forces to affect all prices for an efficient Coretime pricing and allocation.

Ultimately, this the framework proposed here adheres to all requirements stated in RFC-1.

Stakeholders

Primary stakeholder sets are:

Protocol researchers and developers, largely represented by the Polkadot Fellowship and Parity Technologies' Engineering division.
Polkadot Parachain teams both present and future, and their users.
Polkadot DOT token holders.

Explanation

Bulk Markets

The BULK_PERIOD has been restructured into two primary segments: the MARKET_PERIOD and RENEWAL_PERIOD, along with an auxiliary SETTLEMENT_PERIOD. This latter period doesn't necessitate any actions from the coretime system chain, but it facilitates a more efficient allocation of coretime in secondary markets. A significant departure from the original proposal lies in the timing of renewals, which now occur post-market phase. This adjustment aims to harmonize renewal prices with their market counterparts, ensuring a more consistent and equitable pricing model.

Market Period (14 days)

During the market period, core sales are conducted through a well-established clearing price Dutch auction that features a RESERVE_PRICE. The price initiates at a premium, designated as PRICE_PREMIUM (for instance, 30%) and descends linearly to the RESERVE_PRICE throughout the duration of the MARKET_PERIOD. Each bidder is expected to submit both their desired price and the quantity (that is, the amount of Coretime) they wish to purchase. To secure these acquisitions, bidders must make a deposit equivalent to their bid multiplied by the chosen quantity, in DOT.

The market achieves resolution once all quantities have been sold, or the RESERVE_PRICE has been reached. This situation leads to determining the MARKET_PRICE either by the lowest bid that was successful in clearing the entire market or by the RESERVE_PRICE. This mechanism yields a uniform price, shaped by market forces (refer to the following discussion for an explanation of its benefits). In other words, all buyers pay the same price (per unit of Coretime). Further down the benefits of this variant of a Dutch auction is discussed.

Note: In cases where some cores remain unsold in the market, all buyers are obligated to pay the RESERVE_PRICE.

Renewal Period (7 days)

As the RENEWAL_PERIOD commences, all current tenants are granted the opportunity to renew their cores at a slight discount of MARKET_PRICE * RENEWAL_DISCOUNT (for instance, 10%). This provision affords marginal benefits to existing tenants, balancing out the non-transferability aspect of renewals.

At the end of the period, all available cores are allocated to the current tenants who have opted for renewal and the participants who placed bids during the market period. If the demand for cores exceeds supply, the cores left unclaimed from renewals may be awarded to bidders who placed their bids early in the auction, thereby subtly incentivizing early participation. If the supply exceeds the demand, all unsold cores are transferred to the Instantanous Market.

Reserve Price Adjustment

After all cores are allocated, the RESERVE_PRICE is adjusted following the process described in RFC-1 and serves as baseline price in the next BULK_PERIOD.

Note: The particular price curve is outside the scope of the proposal. The MARKET_PRICE (as a function of RESERVE_PRICE), however, is able to capture higher demand very well while being capped downwards. That means, the curve that adjusts the RESERVE_PRICE should be more sensitive to undercapacity.

Price Predictability

Tasks that are in the "renewal-pipeline" can determine the upper bound for the price they will pay in any future period. The main driver of any price increase over time is the adjustment of the RESERVE_PRICE, that occurs at the end of each BULK_PERIOD after determining the capacity fillment of Polkadot UC. To calculate the maximum price in some future period, a task could assume maximum capacity in all upcoming periods and track the resulting price increase of RESERVE_PRICE. In the final period, that price can get a maximum premium of PRICE_PREMIUM and after deducting a potential RENEWAL_DISCOUNT, the maximum price can be determined.

Settlement Period (7 days)

During the settlement period, participants have ample time to trade Coretime on secondary markets before the onset of the next BULK_PERIOD. This allows for trading with full Coretime availability. Trading transferrable Coretime naturally continues during each BULK_PERIOD, albeit with cores already in use.

Benefits of this system

The introduction of a single price, the RESERVE_PRICE, provides an anchor for all Coretime markets. This is a preventative measure against the possible divergence and mismatch of prices, which could inadvertently lead to a situation where existing tenants secure cores at significantly below-market rates.
With a more market-responsive pricing system, we can achieve a more efficient price discovery process. Any price increases will be less arbitrary and more dynamic.
The ideal strategy for existing tenants is to maintain passivity, i.e., refrain from active market participation and simply accept the offer presented to them during the renewal phase. This approach lessens the organizational overhead for long-term projects.
In the two-week market phase, the maximum price increase is known well in advance, providing ample time for tenants to secure necessary funds to meet the potential price escalation.
All existing tenants pay an equal amount for Coretime, reflecting our intent to price the Coretime itself and not the relative timing of individual projects.

Discussion: Clearing Price Dutch Auctions

Having all bidders pay the market clearing price offers some benefits and disadvantages.

Advantages: +
- Fairness: All bidders pay the same price.
- Active participation: Because bidders are protected from overbidding (winner's curse), they are more likely to engage and reveal their true valuations.
- Simplicity: A single price is easier to work with for pricing renewals later.
- Truthfulness: There is no need to try to game the market by waiting with bidding. Bidders can just bid their valuations.
+
Disadvantages: +
- (Potentially) Lower Revenue: While the theory predicts revenue-equivalence between a uniform price and pay-as-bid type of auction, slightly lower revenue for the former type is observed empirically. Arguably, revenue maximization (i.e., squeezing out the maximum willingness to pay from bidders) is not the priority for Polkadot UC. Instead, it is interested in efficient allocation and the other benefits illustrated above.
- (Technical) Complexity: Instead of making a final purchase within the auction, the bid is only a deposit. Some refunds might happen after the auction is finished. This might pose additional challenges from the technical side (e.g., storage requirements).
+

Further Discussion Points

Long-term Coretime: The Polkadot UC is undergoing a transition from two-year leases without an instantaneous market to a model encompassing instantaneous and one-month leases. This shift seems to pivot from one extreme to another. While the introduction of short-term leases, both instantaneous and for one month, is a constructive move to lower barriers to entry and promote experimentation, it seems to be the case that established projects might benefit from more extended lease options. We could consider offering another product, such as a six-month Coretime lease, using the same mechanism described herein. Although the majority of leases would still be sold on a one-month basis, the addition of this option would enhance market efficiency as it would strengthen the impact of a secondary market.

Drawbacks

There are trade-offs that arise from this proposal, compared to the initial model. The most notable one is that here, I prioritize requirement 6 over requirement 2. The price, in the very "worst-case" (meaning a huge explosion in demand for coretime) could lead to a much larger increase of prices in Coretime. From an economic perspective, this (rare edgecase) would also mean that we'd vastly underprice Coretime in the original model, leading to highly inefficient allocations.

Prior Art and References

This RFC builds extensively on the available ideas put forward in RFC-1.

Additionally, I want to express a special thanks to Samuel Haefner and Shahar Dobzinski for fruitful discussions and helping me structure my thoughts.

Unresolved Questions

The technical feasability needs to be assessed.

(source)

Table of Contents

RFC-0020: Treasurer Track Confirmation Period Duration Modification +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics & Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0020: Treasurer Track Confirmation Period Duration Modification

+ + + +


Start Date	August 10, 2023
Description	Treasurer Track Confirmation Period Duration Modification
Authors	ChaosDAO

Summary

This RFC proposes a change to the duration of the confirmation period for the treasurer track from 3 hours to at least 48 hours.

Motivation

Track parameters for Polkadot OpenGov should be configured in a way that their "difficulty" increases relative to the power associated with their respective origin. When we look at the confirmation periods for treasury based tracks, we can see that this is clearly the case - with the one notable exception to the trend being the treasurer track:

+ + + + + + +

Track Description	Confirmation Period Duration
Small Tipper	10 Min
Big Tipper	1 Hour
Small Spender	12 Hours
Medium Spender	24 Hours
Big Spender	48 Hours
Treasurer	3 Hours

The confirmation period is one of the last lines of defence for the collective Polkadot stakeholders to react to a potentially bad referendum and vote NAY in order for its confirmation period to be aborted.

Since the power / privilege level of the treasurer track is greater than that of the the big spender track – their confirmation period should be either equal, or the treasurer track's should be higher (note: currently the big spender track has a longer confirmation period than even the root track).

Stakeholders

The primary stakeholders of this RFC are:

DOT token holders – as this affects the protocol's treasury
Entities wishing to submit a referendum via the treasurer track - as this affects the referendum timeline
Projects with governance app integrations - see Performance, Ergonomics, and Compatibility section below.
lolmcshizz - expressed interest to change this parameter
Leemo - expressed interest to change this parameter
Paradox - expressed interest to change this parameter

Explanation

This RFC proposes to change the duration of the confirmation period for the treasurer track. In order to achieve that, the confirm_period parameter for the treasurer track in runtime/polkadot/src/governance/tracks.rs must be changed.

Currently it is set to confirm_period: 3 * HOURS

It should be changed to confirm_period: 48 * HOURS as a minimum.

It may make sense for it to be changed to a value greater than 48 hours since the treasurer track has more power than the big spender track (48 hour confirmation period); however, the root track's confirmation period is 24 hours. 48 hours may be on the upper bounds of a trade-off between security and flexibility.

Drawbacks

The drawback of changing the treasurer track's confirmation period would be that the lifecycle of a referendum submitted on the treasurer track would ultimately be longer. However, the security of the protocol and its treasury should take priority here.

Testing, Security, and Privacy

This change will enhance / improve the security of the protocol as it relates to its treasury. The confirmation period is one of the last lines of defence for the collective Polkadot stakeholders to react to a potentially bad referendum and vote NAY in order for its confirmation period to be aborted. It makes sense for the treasurer track's confirmation period duration to be either equal to, or higher than, the big spender track confirmation period.

Performance, Ergonomics, and Compatibility

Performance

This is a simple change (code wise) which should not affect the performance of the Polkadot protocol, outside of increasing the duration of the confirmation period on the treasurer track.

Ergonomics & Compatibility

If the proposal alters exposed interfaces to developers or end-users, which types of usage patterns have been optimized for?

I have confirmed with the following projects that this is not a breaking change for their governance apps:

Nova Wallet - directly uses on-chain data, and change will be automatically reflected.
Polkassembly - directly uses on-chain data via rpc to fetch trackInfo so the change will be automatically reflected.
SubSquare - scan script will update their app to the latest parameters and it will be automatically reflected in their app.

Prior Art and References

N/A

Unresolved Questions

The proposed change to the confirmation period duration for the treasurer track is to set it to 48 hours. This is equal to the current confirmation period for the big spender track.

Typically it seems that track parameters increase in difficulty (duration, etc.) based on the power level of their associated origin.

The longest confirmation period is that of the big spender, at 48 hours. There may be value in discussing whether or not the treasurer track confirmation period should be longer than 48 hours – a discussion of the trade-offs between security vs flexibility/agility.

As a side note, the root track confirmation period is 24 hours.

+ +

This RFC hopefully reminds the greater Polkadot community that it is possible to submit changes to the parameters of Polkadot OpenGov, and the greater protocol as a whole through the RFC process.

(source)

Table of Contents

RFC-0035: Conviction Voting Delegation Modifications +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics & Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0035: Conviction Voting Delegation Modifications

+ + + +


October 10, 2023
Conviction Voting Delegation Modifications
ChaosDAO

Summary

This RFC proposes to make modifications to voting power delegations as part of the Conviction Voting pallet. The changes being proposed include:

Allow a Delegator to vote independently of their Delegate if they so desire.
Allow nested delegations – for example Charlie delegates to Bob who delegates to Alice – when Alice votes then both Bob and Charlie vote alongside Alice (in the current implementation Charlie will not vote when Alice votes).
Make a change so that when a delegate votes abstain their delegated votes also vote abstain.
Allow a Delegator to delegate/ undelegate their votes for all tracks with a single call.

Motivation

It has become clear since the launch of OpenGov that there are a few common tropes which pop up time and time again:

The frequency of referenda is often too high for network participants to have sufficient time to review, comprehend, and ultimately vote on each individual referendum. This means that these network participants end up being inactive in on-chain governance.
There are active network participants who are reviewing every referendum and are providing feedback in an attempt to help make the network thrive – but often time these participants do not control enough voting power to influence the network with their positive efforts.
Delegating votes for all tracks currently requires long batched calls which result in high fees for the Delegator - resulting in a reluctance from many to delegate their votes.

We believe (based on feedback from token holders with a larger stake in the network) that if there were some changes made to delegation mechanics, these larger stake holders would be more likely to delegate their voting power to active network participants – thus greatly increasing the support turnout.

Stakeholders

The primary stakeholders of this RFC are:

The Polkadot Technical Fellowship who will have to research and implement the technical aspects of this RFC
DOT token holders in general

Explanation

This RFC proposes to make 4 changes to the convictionVoting pallet logic in order to improve the user experience of those delegating their voting power to another account.

+
Allow a Delegator to vote independently of their Delegate if they so desire – this would empower network participants to more actively delegate their voting power to active voters, removing the tedious steps of having to undelegate across an entire track every time they do not agree with their delegate's voting direction for a particular referendum.
+
+
Allow nested delegations – for example Charlie delegates to Bob who delegates to Alice – when Alice votes then both Bob and Charlie vote alongside Alice (in the current runtime Charlie will not vote when Alice votes) – This would allow network participants who control multiple (possibly derived) accounts to be able to delegate all of their voting power to a single account under their control, which would in turn delegate to a more active voting participant. Then if the delegator wishes to vote independently of their delegate they can control all of their voting power from a single account, which again removes the pain point of having to issue multiple undelegate extrinsics in the event that they disagree with their delegate.
+
+
Have delegated votes follow their delegates abstain votes – there are times where delegates may vote abstain on a particular referendum and adding this functionality will increase the support of a particular referendum. It has a secondary benefit of meaning that Validators who are delegating their voting power do not lose points in the 1KV program in the event that their delegate votes abstain (another pain point which may be preventing those network participants from delegating).
+
+
Allow a Delegator to delegate/ undelegate their votes for all tracks with a single call - in order to delegate votes across all tracks, a user must batch 15 calls - resulting in high costs for delegation. A single call for delegate_all/ undelegate_all would reduce the complexity and therefore costs of delegations considerably for prospective Delegators.
+

Drawbacks

We do not foresee any drawbacks by implementing these changes. If anything we believe that this should help to increase overall voter turnout (via the means of delegation) which we see as a net positive.

Testing, Security, and Privacy

We feel that the Polkadot Technical Fellowship would be the most competent collective to identify the testing requirements for the ideas presented in this RFC.

Performance, Ergonomics, and Compatibility

Performance

This change may add extra chain storage requirements on Polkadot, especially with respect to nested delegations.

Ergonomics & Compatibility

The change to add nested delegations may affect governance interfaces such as Nova Wallet who will have to apply changes to their indexers to support nested delegations. It may also affect the Polkadot Delegation Dashboard as well as Polkassembly & SubSquare.

We want to highlight the importance for ecosystem builders to create a mechanism for indexers and wallets to be able to understand that changes have occurred such as increasing the pallet version, etc.

Prior Art and References

N/A

Unresolved Questions

N/A

+ +

Additionally we would like to re-open the conversation about the potential for there to be free delegations. This was discussed by Dr Gavin Wood at Sub0 2022 and we feel like this would go a great way towards increasing the amount of network participants that are delegating: https://youtu.be/hSoSA6laK3Q?t=526

Overall, we strongly feel that delegations are a great way to increase voter turnout, and the ideas presented in this RFC would hopefully help in that aspect.

(source)

Table of Contents

RFC-0048: Generate ownership proof for SessionKeys +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0048: Generate ownership proof for `SessionKeys`

+ + + +


Start Date	13 November 2023
Description	Change `SessionKeys` runtime api to also create a proof of ownership for on chain registration.
Authors	Bastian Köcher

Summary

When rotating/generating the SessionKeys of a node, the node calls into the runtime using the +SessionKeys::generate_session_keys runtime api. This runtime api function needs to be changed +to add an extra parameter owner and to change the return value to also include the proof of +ownership. The owner should be the account id of the account setting the SessionKeys on chain +to allow the on chain logic the verification of the proof. The on chain logic is then able to proof +the possession of the private keys of the SessionKeys using the proof.

Motivation

When a user sets new SessionKeys on chain the chain can currently not ensure that the user +actually has control over the private keys of the SessionKeys. With the RFC applied the chain is able +to ensure that the user actually is in possession of the private keys.

Stakeholders

Polkadot runtime implementors
Polkadot node implementors
Validator operators

Explanation

We are first going to explain the proof format being used:

#![allow(unused)]
+fn main() {
+type Proof = (Signature, Signature, ..);
+}

The proof being a SCALE encoded tuple over all signatures of each private session +key signing the owner. The actual type of each signature depends on the +corresponding session key cryptographic algorithm. The order of the signatures in +the proof is the same as the order of the session keys in the SessionKeys type.

The version of the SessionKeys needs to be bumped to 1 to reflect the changes to the +signature of SessionKeys_generate_session_keys:

#![allow(unused)]
+fn main() {
+pub struct OpaqueGeneratedSessionKeys {
+	pub keys: Vec<u8>,
+	pub proof: Vec<u8>,
+}
+
+fn SessionKeys_generate_session_keys(owner: Vec<u8>, seed: Option<Vec<u8>>) -> OpaqueGeneratedSessionKeys;
+}

The default calling convention for runtime apis is applied, meaning the parameters +passed as SCALE encoded array and the length of the encoded array. The return value +being the SCALE encoded return value as u64 (array_ptr | length << 32). So, the +actual exported function signature looks like:

#![allow(unused)]
+fn main() {
+fn SessionKeys_generate_session_keys(array: *const u8, len: usize) -> u64;
+}

The on chain logic for setting the SessionKeys needs to be changed as well. It +already gets the proof passed as Vec<u8>. This proof needs to be decoded to +the actual Proof type as explained above. The proof and the SCALE encoded +account_id of the sender are used to verify the ownership of the SessionKeys.

Drawbacks

Validator operators need to pass the their account id when rotating their session keys in a node. +This will require updating some high level docs and making users familiar with the slightly changed ergonomics.

Testing, Security, and Privacy

Testing of the new changes is quite easy as it only requires passing an appropriate owner +for the current testing context. The changes to the proof generation and verification got +audited to ensure they are correct.

Performance, Ergonomics, and Compatibility

Performance

Does not have any impact on the overall performance, only setting SessionKeys will require more weight.

Ergonomics

If the proposal alters exposed interfaces to developers or end-users, which types of usage patterns have been optimized for?

Compatibility

Introduces a new version of the SessionKeys runtime api. Thus, nodes should be updated before +a runtime is enacted that contains these changes otherwise they will fail to generate session keys.

Prior Art and References

None.

Unresolved Questions

None.

+ +

Substrate implementation of the RFC.

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Table of Contents

RFC-0054: Remove the concept of "heap pages" from the client +
- Summary
- Motivation
- Stakeholders
- Explanation
- Drawbacks
- Testing, Security, and Privacy
- Performance, Ergonomics, and Compatibility +
  - Performance
  - Ergonomics
  - Compatibility
  +
- Prior Art and References
- Unresolved Questions
- Future Directions and Related Material
+

RFC-0054: Remove the concept of "heap pages" from the client

+ + + +


Start Date	2023-11-24
Description	Remove the concept of heap pages from the client and move it to the runtime.
Authors	Pierre Krieger

Summary

Rather than enforce a limit to the total memory consumption on the client side by loading the value at :heappages, enforce that limit on the runtime side.

Motivation

From the early days of Substrate up until recently, the runtime was present in two forms: the wasm runtime (wasm bytecode passed through an interpreter) and the native runtime (native code directly run by the client).

Since the wasm runtime has a lower amount of available memory (4 GiB maximum) compared to the native runtime, and in order to ensure sure that the wasm and native runtimes always produce the same outcome, it was necessary to clamp the amount of memory available to both runtimes to the same value.

In order to achieve this, a special storage key (a "well-known" key) :heappages was introduced and represents the number of "wasm pages" (one page equals 64kiB) of memory that are available to the memory allocator of the runtimes. If this storage key is absent, it defaults to 2048, which is 128 MiB.

The native runtime has since then been disappeared, but the concept of "heap pages" still exists. This RFC proposes a simplification to the design of Polkadot by removing the concept of "heap pages" as is currently known, and proposes alternative ways to achieve the goal of limiting the amount of memory available.

Stakeholders

Client implementers and low-level runtime developers.

Explanation

This RFC proposes the following changes to the client:

The client no longer considers :heappages as special.
The memory allocator of the runtime is no longer bounded by the value of :heappages.

With these changes, the memory available to the runtime is now only bounded by the available memory space (4 GiB), and optionally by the maximum amount of memory specified in the Wasm binary (see https://webassembly.github.io/spec/core/bikeshed/#memories%E2%91%A0). In Rust, the latter can be controlled during compilation with the flag -Clink-arg=--max-memory=....

Since the client-side change is strictly more tolerant than before, we can perform the change immediately after the runtime has been updated, and without having to worry about backwards compatibility.

This RFC proposes three alternative paths (different chains might choose to follow different paths):

+
Path A: add back the same memory limit to the runtime, like so:
+
- At initialization, the runtime loads the value of :heappages from the storage (using ext_storage_get or similar), and sets a global variable to the decoded value.
- The runtime tracks the total amount of memory that it has allocated using its instance of #[global_allocator] (https://github.com/paritytech/polkadot-sdk/blob/e3242d2c1e2018395c218357046cc88caaed78f3/substrate/primitives/io/src/lib.rs#L1748-L1762). This tracking should also be added around the host functions that perform allocations.
- If an allocation is attempted that would go over the value in the global variable, the memory allocation fails.
+
+
Path B: define the memory limit using the -Clink-arg=--max-memory=... flag.
+
+
Path C: don't add anything to the runtime. This is effectively the same as setting the memory limit to ~4 GiB (compared to the current default limit of 128 MiB). This solution is viable only because we're compiling for 32bits wasm rather than for example 64bits wasm. If we ever compile for 64bits wasm, this would need to be revisited.
+

Each parachain can choose the option that they prefer, but the author of this RFC strongly suggests either option C or B.

Drawbacks

In case of path A, there is one situation where the behaviour pre-RFC is not equivalent to the one post-RFC: when a host function that performs an allocation (for example ext_storage_get) is called, without this RFC this allocation might fail due to reaching the maximum heap pages, while after this RFC this will always succeed. +This is most likely not a problem, as storage values aren't supposed to be larger than a few megabytes at the very maximum.

In the unfortunate event where the runtime runs out of memory, path B would make it more difficult to relax the memory limit, as we would need to re-upload the entire Wasm, compared to updating only :heappages in path A or before this RFC. +In the case where the runtime runs out of memory only in the specific event where the Wasm runtime is modified, this could brick the chain. However, this situation is no different than the thousands of other ways that a bug in the runtime can brick a chain, and there's no reason to be particularily worried about this situation in particular.

Testing, Security, and Privacy

This RFC would reduce the chance of a consensus issue between clients. +The :heappages are a rather obscure feature, and it is not clear what happens in some corner cases such as the value being too large (error? clamp?) or malformed. This RFC would completely erase these questions.

Performance, Ergonomics, and Compatibility

Performance

In case of path A, it is unclear how performances would be affected. Path A consists in moving client-side operations to the runtime without changing these operations, and as such performance differences are expected to be minimal. Overall, we're talking about one addition/subtraction per malloc and per free, so this is more than likely completely negligible.

In case of path B and C, the performance gain would be a net positive, as this RFC strictly removes things.

Ergonomics

This RFC would isolate the client and runtime more from each other, making it a bit easier to reason about the client or the runtime in isolation.

Compatibility

Not a breaking change. The runtime-side changes can be applied immediately (without even having to wait for changes in the client), then as soon as the runtime is updated, the client can be updated without any transition period. One can even consider updating the client before the runtime, as it corresponds to path C.

Prior Art and References

None.

Unresolved Questions

None.

+ +

This RFC follows the same path as https://github.com/polkadot-fellows/RFCs/pull/4 by scoping everything related to memory allocations to the runtime.

+ +

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Table of Contents

RFC-0026: Sassafras Consensus Protocol +
+

RFC-0026: Sassafras Consensus Protocol

+ + + +


Start Date	September 06, 2023
Description	Sassafras consensus protocol description and structures
Authors	Davide Galassi

Abstract

Sassafras is a novel consensus protocol designed to address the recurring +fork-related challenges encountered in other lottery-based protocols.

Sassafras aims to establish a unique association between each epoch's slots and +the validators, ensuring that there is one and only one validator per slot.

The protocol ensures the anonymity of the validator associated to a slot until +the slot is not claimed at block production time.

1. Motivation

Sassafras Protocol has been extensively documented in a comprehensive research +paper from the Web3 foundation +research team.

This RFC serves the purpose of conveying most of the essential implementation +details that are crucial for interoperability and clarifying aspects left open +for implementation discretion.

1.1. Relevance to Implementors

This RFC focuses on providing implementors with the necessary insights into the +protocol's operation.

To avoid ambiguities and interoperability issues, this document takes precedence +over the research paper in cases where discrepancies arise between the two.

1.2. Supporting Sassafras for Polkadot

2. Stakeholders

2.1. Developers of Blockchains

Developers responsible for creating blockchains who intend to leverage the +benefits offered by the Sassafras Protocol.

2.2. Contributors to the Polkadot Ecosystem

Developers contributing to the Polkadot ecosystem, both relay-chain and para-chains. +The protocol will have a central role in the next generation Polkadot relay chain +block authoring system.

3. Notation and Convention

This section outlines the notation and conventions used throughout the document +to ensure clarity and consistency.

3.1. Data Structures Definitions and Encoding

Data structures are primarily defined using ASN.1, +with a few exceptions:

Integer types are not explicitly defined in ASN.1 and in the context of +this document U<n> should be interpreted as a n-bit unsigned integers

If no context-specific instructions are given, all types must be serialized +using SCALE codec.

To ensure interoperability of serialized structures, the order of the single +fields is required to match the structures definitions found in this document.

3.2. Pseudo-Code

Through this document it is advantageous to make use of code snippets as part +of the comprehensive description. These snippets shall adhere to the subsequent +conventions:

+
For simplicity, code snippets are presented in a Rust-like pseudocode format.
+
+
The function BYTES(x: T) returns an OCTET_STRING representing the raw +byte array representation of the object x with type T.
+
- If T is VisibleString (i.e. an ASCII string): it returns the sequence +of octets of its ASCII representation.
- If T is U<n>: it returns the little-endian encoding of the integer +U<n> as n/8 octets.
+
+
The function U<n>(x: OCTET_STRING) returns a U<n> interpreting x as +the little-endian encoding of a n bits unsigned integer.
+
+
The function SCALE(x: T) returns an OCTET_STRING representing the +SCALE encoding of +x with type T.
+
+
The function BLAKE2(n: U32, x: OCTET_STRING) returns n bytes of the +standard blake2b hash of x as an OCTET_STRING.
+
+
The function CONCAT(x₀: OCTET_STRING, ..., xₖ: OCTET_STRING) returns the +concatenation of the inputs as an OCTET_STRING.
+
+
The function LENGTH(x: OCTET_STRING) returns a U32 representing the +number of octets in x.
+

3.3. Incremental Introduction of Types and Functions

Types and helper functions will be introduced incrementally as they become +relevant within the document's context.

We find this approach more agile, especially given that the set of types used is +not extensive or overly complex.

This incremental presentation enhances readability and comprehension.

4. Protocol Introduction

Timeline is partitioned in epochs, epochs are partitioned in slots.

The Sassafras protocol employs a binding mechanism between validators and slots +through the use of a ticketing system.

The protocol can be divided into five discrete and asynchronous phases:

4.1. Submission of Candidate Tickets

Validators generate and submit their candidate tickets to the blockchain. Each +ticket comes with an anonymous validity proof.

4.2. Validation of Candidate Tickets

Each candidate tickets undergo a validation process for the associated validity +proof and compliance with other protocol-specific constraints.

4.3. Tickets and Slots Binding

After collecting all candidate tickets, a deterministic method is employed to +uniquely associate a subset of these tickets to the next epoch slots.

4.4. Claim of Ticket Ownership

Validators prove ownership of tickets during the block production phase. This +step establishes a secure binding between validators and their respective slots.

4.5. Validation of Ticket Ownership

During block verification, the claims of ticket ownership are validated to +uphold the protocol's integrity.

5. Bandersnatch VRFs Cryptographic Primitives

This chapter provides a high-level overview of the Bandersnatch VRF primitive as +it relates to the Sassafras protocol.

For a more detailed understanding we recommend referring to the Ring-VRF +research paper from W3F.

5.1. VRF Input

The VRF Input, denoted as VrfInput, is constructed by combining a domain identifier +with arbitrary data using the vrf_input function:

#![allow(unused)]
+fn main() {
+    fn vrf_input(domain: OCTET_STRING, buf: OCTET_STRING) -> VrfInput;
+}

The specific implementation details of this function are intentionally omitted +here, you can find a complete reference implementation in the +bandersnatch_vrfs +project.

Helper function to construct a VrfInput from a sequence of data items:

#![allow(unused)]
+fn main() {
+    fn vrf_input_from_items(domain: OCTET_STRING, data: SEQUENCE_OF OCTET_STRING) -> VrfInput {
+        buf = OCTET_STRING(SIZE(0));
+        for item in data {
+            buf.append(item);
+            buf.append(LENGTH(item) as U8);
+        }
+        return vrf_input(domain, buf);
+    }
+}

Note that we cast the length of each item to a U8. In the context of the +protocol we never have to append strings longer than 255. The function is +internal and not designed to be generic.

+ +Or we should provide a generic one in bandersnatch primitive wrapper to be +used in other contexts? + +

5.2. VRF Output

A VrfOutput in this context is computed in function of a VrfInput and a +BandersnatchSecretKey.

In practice, the VrfOutput is a verifiable seed to produce a variable number +of pseudo-random bytes. These bytes are considered valid when VrfOutput is +accompanied by a valid signature.

To facilitate the construction of VrfOutput from a secret key and VrfInput, +the following helper function is provided:

#![allow(unused)]
+fn main() {
+    fn vrf_output(secret: BandernatchSecretKey, input: VrfInput) -> VrfOutput;
+}

Additionally, a helper function is provided for producing len bytes from +VrfInput and VrfOutput:

#![allow(unused)]
+fn main() {
+    fn vrf_bytes(len: U32, input: VrfInput, output: VrfOuput) -> OCTET_STRING;
+}

vrf_output
vrf_bytes

5.3. VRF Signature Data

This section defines the data to be signed using the VRF primitive:

#![allow(unused)]
+fn main() {
+    VrfSignatureData ::= SEQUENCE {
+        transcript: Transcript,
+        inputs: SEQUENCE_OF VrfInput
+    }
+}

transcript: an ark-transcript +object. In practice, this is a special hash of some protocol-specific data +to sign which should not influence the VrfOutput.
inputs: sequence of VrfInputs to be signed.

To simplify the construction of a VrfSignatureData object, a helper function is provided:

#![allow(unused)]
+fn main() {
+    fn vrf_signature_data(
+        transcript_label: OCTET_STRING,
+        transcript_data: SEQUENCE_OF OCTET_STRING,
+        inputs: SEQUENCE_OF VrfInput
+    ) -> VrfSignatureData {
+        let mut transcript = Transcript::new_labeled(transcript_label);
+        for data in transcript_data {
+            transcript.append(data);
+        }
+        VrfSignatureData { transcript, inputs }
+    }
+}

5.4. VRF Signature

Bandersnatch VRF offers two signature flavors:

plain signature, which is much like a traditional Schnorr signature,
ring signature which leverages a zk-SNARK to allows for anonymous signatures +using a key from a predefined set of enabled keys, known as the ring.

5.4.1. Plain VRF Signature

This section describes the signature process for VrfSignatureData using the +plain Bandersnatch signature flavor.

#![allow(unused)]
+fn main() {
+    PlainSignature ::= OCTET_STRING;
+
+    VrfSignature ::= SEQUENCE {
+        signature: PlainSignature,
+        outputs: SEQUENCE-OF VrfOutput
+    }
+}

signature: the actual signature.
outputs: a sequence of VrfOutputs corresponding to the VrfInputs values.

Helper function to create a VrfPlainSignature from VrfSignatureData:

#![allow(unused)]
+fn main() {
+    BandersnatchSecretKey ::= OCTET_STRING;
+
+    fn vrf_sign(
+        secret: BandernatchSecretKey,
+        signature_data: VrfSignatureData
+    ) -> VrfSignature
+}

Helper function for validating the signature and returning a BOOLEAN value +indicating the validity of the signature.

#![allow(unused)]
+fn main() {
+    BandersnatchPublicKey ::= OCTET_STRING;
+
+    fn vrf_verify(
+        public: BandersnatchPublicKey,
+        signature: VrfSignature
+    ) -> BOOLEAN;
+}

5.4.2. Ring VRF Signature

This section deals with the signature process for VrfSignatureData using the +Bandersnatch ring signature flavor.

#![allow(unused)]
+fn main() {
+    RingSignature ::= OCTET_STRING;
+
+    RingVrfSignature ::= SEQUENCE {
+        signature: RingSignature,
+        outputs: SEQUENCE_OF VrfOutput
+    }
+}

signature: the actual signature.
outputs: sequence of VrfOutput objects corresponding to the VrfInput values.

Helper function to create a RingVrfSignature from VrfSignatureData:

#![allow(unused)]
+fn main() {
+    BandersnatchRingProverKey ::= OCTET_STRING;
+    
+    fn ring_vrf_sign(
+        secret: BandersnatchRingProverKey,
+        signature_data: VrfSignatureData,
+    ) -> RingVrfSignature;
+}

#![allow(unused)]
+fn main() {
+    BandersnatchRingVerifierKey ::= OCTET_STRING;
+
+    fn ring_vrf_verify(
+        verifier: BandersnatchRingVerifierKey,
+        signature: RingVrfSignature,
+    ) -> BOOLEAN;
+}

6. Sassafras Protocol

6.1. Epoch's First Block

The first block produced for epoch N is required to include the descriptor for +the next epoch N+1.

The descriptor for next epoch is NextEpochDescriptor.

#![allow(unused)]
+fn main() {
+    AuthorityId ::= BandersnatchPublicKey;
+
+    Randomness ::= OCTET_STRING(SIZE(32));
+
+    NextEpochDescriptor ::= SEQUENCE {
+        randomness: Randomness,
+        authorities: SEQUENCE_OF AuthorityId,
+        configuration: ProtocolConfiguration OPTIONAL
+    }
+}

randomness: randomness value.
authorities: list of authorities.
configuration: optional protocol configuration.

The NextEpochDescriptor must be SCALE encoded and embedded in the block +header digest log.

The identifier for the digest element is BYTES("SASS").

6.1.1. Epoch Randomness

Each block ships with some entropy source in the form of bandersnatch +VrfOutput. Per block randomness is accumulated in the protocol's on-chain +randomness accumulator after block import.

The exact procedure to accumulate per-block randomness is described in detail +later, in the randomness accumulator paragraph (6.7).

Next epoch randomness is computed as:

#![allow(unused)]
+fn main() {
+    next_epoch_randomness = BLAKE2(32, CONCAT(randomness_accumulator, BYTES(next_epoch_index)));
+}

6.1.2. Protocol Configuration

The ProtocolConfiguration primarily influences certain checks carried out +during tickets validation. It is defined as follows:

#![allow(unused)]
+fn main() {
+    ProtocolConfiguration ::= SEQUENCE {
+        attempts_number: U32,
+        redundancy_factor: U32
+    }
+}

attempts_number: max number of tickets that can be submitted by each +next epoch authority.
redundancy_factor: controls the expected number of extra tickets produced +beyond epoch_length.

We do not mind max_attempts < epoch_length though because this loss of +anonymity already becomes small when attempts_number = 64 or 128 and larger +values requires more computation.

Details about how exactly these parameters drives the ticket validity +probability can be found in the section dedicated to candidate ticket validation +(6.2.2).

6.1.3. Startup Parameters

Some parameters for first epoch (index = 0) are configurable via genesis configuration.

#![allow(unused)]
+fn main() {
+    GenesisConfig ::= SEQUENCE {
+        authorities: SEQUENCE_OF AuthorityId,
+        configuration: ProtocolConfiguration OPTIONAL
+    }
+}

Randomness for first epoch is set to all zeros.

As block #0 is locally produced by every node by processing the genesis configuration, +the first block explicitly produced by a validator for the first epoch is block #1.

Block #1 must embed the NextEpochDescriptor for next epoch. This is +constructed re-using the same values used for the first epoch.

6.2. Creation and Submission of Candidate Tickets

As a shorthand notation, in this section we refer to one of the next epoch +validators as 'the validator'.

Each validator is allowed to submit a maximum number of tickets whose value is +found in the next epoch ProtocolConfiguration attempts_number field.

The expected ratio between the attempts and the number of tickets which are +assigned to the next epoch slots is driven by the ticket threshold +(6.2.2).

Each ticket has an associated unique identifier, denoted as TicketId.

#![allow(unused)]
+fn main() {
+    TicketId ::= U128
+}

6.2.1. Ticket Identifier Value

The value of the TicketId is determined by the output of the Bandersnatch VRF +when using the following inputs:

Next epoch randomness: Randomness obtained from the NextEpochDescriptor.
Next epoch index: U64 computed as epoch start slot divided epoch duration.
Attempt index: U32 value going from 0 to attempts_number.

Let next_epoch be an object with the information associated to the next epoch.

#![allow(unused)]
+fn main() {
+    ticket_id_vrf_input = vrf_input_from_items(
+        BYTES("sassafras-ticket-v1.0"),
+        [ 
+            next_epoch.randomness,
+            BYTES(next_epoch.epoch_index),
+            BYTES(attempt_index)
+        ]
+    );
+
+    ticket_id_vrf_output = vrf_output(AUTHORITY_SECRET_KEY, ticket_id_vrf_input);
+
+    ticket_bytes = vrf_bytes(16, ticket_id_vrf_input, ticket_id_vrf_output);
+    ticket_id = U128(ticket_bytes);
+}

6.2.2. Tickets Threshold

A TicketId value is valid if its value is less than the ticket threshold.

T = (r·s)/(a·v)
+

Where:

v: the number of authorities (aka validators) in the epoch
s: number of slots in the epoch
r: the redundancy factor
a: number of attempts
T: ticket threshold value (0 ≤ T ≤ 1)

6.2.2.1 Formula Derivation

For an epoch of s slots we want to have a number of tickets in expectation for +block production equal to the r·s.

We need that there is a very small probability of their being less than s +winning tickets, even if up to 1/3 of authorities are offline.

First we set the probability of a ticket winning as T = (r·s)/(a·v).

Let n be the number of validators who actually participate and so v·2/3 ≤ n ≤ v.

These n validators make a attempts each, for a total of a·n attempts.

Let X be the random variable associated to the number of winning tickets, then +its expected value is:

E[X] = T·a·n = (r·s·n)/v
+

By setting r = 2, we get

s·4/3 ≤ E[X] ≤ s·2
+

Using Bernestein's inequality we get Pr[X < s] ≤ exp(-s/21).

For s = 600 this gives Pr[X < s] < 4·10⁻¹³, and thus we end up with a great +tolerance over offline nodes and we end-up filling all the slots with tickets +with high probability.

For more details about threshold formula please refer to the +probabilities and parameters +paragraph of the w3f description of the protocol.

6.2.3. Ticket Body

Every candidate ticket identifier has an associated body.

#![allow(unused)]
+fn main() {
+    TicketBody ::= SEQUENCE {
+        attempt_index: U32,
+        erased_pub: Ed25519PublicKey,
+        revealed_pub: Ed25519PublicKey
+    }
+}

attempt_index: attempt index used to generate the associated TicketId.
erased_pub: Ed25519 ephemeral public key which gets erased as soon as the +ticket is claimed.
revealed_pub: Ed25519 ephemeral public key which gets exposed as soon as the +ticket is claimed.

The process of generating an erased key pair is intentionally left undefined, +allowing the implementor the freedom to choose the most suitable strategy.

Revealed key pair is generated using bytes produced by the VRF with input +parameters equal to those employed in TicketId generation, only the label +is different.

Let next_epoch be an object with the information associated to the next epoch:

#![allow(unused)]
+fn main() {
+    revealed_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-revealed-v1.0"),
+        data: [ 
+            next_epoch.randomness,
+            BYTES(next_epoch.epoch_index),
+            BYTES(attempt_index)
+        ]
+    );
+
+    revealed_vrf_output = vrf_output(AUTHORITY_SECRET_KEY, revealed_vrf_input);
+
+    revealed_seed = vrf_bytes(32, revealed_vrf_input, revealed_vrf_output);
+    revealed_pub = ed25519_secret_from_seed(revealed_seed).public();
+}

The usage of the ephemeral public keys will be clarified in the ticket claiming +section (6.5).

6.2.4. Ring Signature Production

TicketBody must be signed using the Bandersnatch ring VRF flavor (5.4.2).

#![allow(unused)]
+fn main() {
+    sign_data = vrf_signature_data(
+        transcript_label: BYTES("sassafras-ticket-body-v1.0"),
+        transcript_data: [
+            SCALE(ticket_body)
+        ],
+        inputs: [
+            ticket_id_vrf_input
+        ]
+    )
+  
+    ring_signature = ring_vrf_sign(RING_PROVER_KEY, sign_data)
+}

The body and the ring signature are combined in the TicketEnvelope:

#![allow(unused)]
+fn main() {
+    TicketEnvelope ::= SEQUENCE {
+        ticket_body: TicketBody,
+        ring_signature: RingVrfSignature
+    }   
+}

All the ticket envelopes corresponding to valid tickets are submitted on-chain +via a dedicated unsigned extrinsic.

6.3. Validation of candidate tickets

All the actions in the steps described by this paragraph are executed by +on-chain code.

The tickets are received via a dedicated extrinsic call.

Generic validation rules:

Tickets submissions must occur within the first half of the epoch.
For unsigned extrinsics, it must be submitted by one of the current session +validators.

Ticket specific validation rules:

Ring signature is verified using the on-chain BandersnatchRingVerifierKey.
Ticket identifier is locally computed from the VrfOutput contained in the +RingVrfSignature and its value is checked to be less than the ticket-threshold.

Valid tickets bodies are persisted on-chain.

6.4. Ticket-Slot assignment

Before the beginning of the next epoch, the on-chain list of tickets must be +associated with the next epoch's slots.

The assignment process happens in the second half of the submission epoch.

In the end, there must be at most one ticket per slot.

Initially, the complete list of tickets is sorted based on their ticket-id, +with smaller values coming first.
In cases where there are more tickets than available slots, the list is pruned +by removing the larger value.
Tickets are then assigned to the slots using an outside-in assignment strategy.

6.4.1. Outside-In Assignment

Given an ordered sequence of tickets [t0, t1, t2, ..., tk] to be assigned to +n slots, where n ≥ k, the tickets are allocated according to the following +strategy:

    slot-index  : [  0,  1,  2, ............ , n ]
+    tickets     : [ t1, t3, t5, ... , t4, t2, t0 ]
+

Here slot-index is a relative value computed as:

slot-index = absolute_slot_index - epoch_start_slot
+

6.4.2. Fallback Assignment

In case the number of available tickets is less than the number of epoch slots, +some (orphan) slots in the middle of the epoch will remain unbounded to any +ticket.

In such situation, these unassigned slots are allocated using a fallback +assignment strategy.

The authorities registered on-chain are kept in a sorted buffer.

The index of the authority which has the privilege to claim an unbounded slot is +calculated as follows:

#![allow(unused)]
+fn main() {
+    index_bytes = BLAKE2(4, CONCAT(epoch_randomness, BYTES(slot)));
+    index = U32(index_bytes) mod authorities_number;
+}

6.5. Claim of ticket ownership during block production

With tickets bound to epoch slots, every validator acquires information about +the slots for which they are supposed to produce a block.

The procedure for block authoring varies based on whether a given slot has an +associated ticket according to the on-chain state.

6.5.1. Primary Claim Method

Follows the construction of VrfSignatureData:

#![allow(unused)]
+fn main() {
+    randomness_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-randomness-v1.0"),
+        data: [
+            curr_epoch.randomness,
+            BYTES(curr_epoch.epoch_index),
+            BYTES(slot)
+        ]
+    );
+
+    revealed_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-revealed-v1.0"),
+        data: [
+            curr_epoch.randomness,
+            BYTES(curr_epoch.epoch_index),
+            BYTES(ticket_body.attempt_index)
+        ]
+    );
+    
+    sign_data = vrf_signature_data(
+        transcript_label: BYTES("sassafras-claim-v1.0"),
+        transcript_data: [
+            SCALE(ticket_body)
+        ],
+        inputs: [
+            randomness_vrf_input,
+            revealed_vrf_input
+        ]
+    );
+}

6.5.1.1. (Optional) Ed25519 Erased Ephemeral Key Claim

As the ticket ownership can be claimed by reconstructing the revealed_pub +entry of the ticket, this step is purely optional and serves only to enforce +the claim.

+ +Is this step really necessary? +- Isn't better to keep it simple if this step doesn't offer any extra security? +- We already have a strong method to claim ticket ownership. + +

The Fiat-Shamir transform is used to obtain a 32-byte challenge associated +with the VrfSignData transcript.

Validators employ the secret key associated with erased_pub, which has been +committed in the TicketBody, to sign this challenge.

#![allow(unused)]
+fn main() {
+    challenge = sign_data.transcript.challenge();
+    erased_signature = ed25519_sign(ERASED_SECRET_KEY, challenge)
+}

6.5.2. Secondary Claim Method

If the slot doesn't have any associated ticket then the validator is the one +with index equal to the rule exposed in the fallback assignment section +(6.4.2).

Given randomness_vrf_input constructed as shown for the primary method, the +VrfSignatureData is constructed as:

#![allow(unused)]
+fn main() {
+    sign_data = vrf_signature_data(
+        transcript_label: BYTES("sassafras-slot-claim-transcript-v1.0"),
+        transcript_data: [ ],
+        inputs: [
+            randomness_vrf_input
+        ]
+    )
+}

6.5.3. Slot Claim object

To establish ownership of a slot, the block author must construct a SlotClaim object +which contains all the necessary information to assert ownership of the slot.

#![allow(unused)]
+fn main() {
+    SlotClaim ::= SEQUENCE {
+        authority_index: U32,
+        slot: U64,
+        signature: VrfSignature,
+        erased_signature: Ed25519Signature OPTIONAL
+    }
+}

+
authority_index: index of the block author in the on-chain authorities list.
+
+
slot: absolute slot number (not relative with respect to the epoch start)
+
+
signature: signature that includes one or two VrfOutputs.
+
- The first VrfOutput is always present and is used to generate per-block +randomness. This is used to claim ticket ownership.
- The second VrfOutput is included if the slot is associated with a ticket. +This is relevant to claim ticket ownership.
+
+
erased_signature: optional signature providing an additional proof of ticket +ownership (see 6.5.1.1).
+

#![allow(unused)]
+fn main() {
+    signature = vrf_sign(AUTHORITY_SECRET_KEY, sign_data);
+
+    claim = SlotClaim {
+        authority_index,
+        slot,
+        signature,
+        erased_signature
+    }
+}

The claim object is SCALE encoded and sent in the block's header digest log.

6.6. Validation of the claim during block verification

Validation of SlotClaim object found in the block's header.

The procedure depends on whether the slot has an associated ticket or not +according to the on-chain state.

If there is a ticket linked to the slot, the primary verification method will be +used; otherwise, the protocol resorts to the secondary one.

In both scenarios, the signature within the SlotClaim is verified using +a VrfSignData constructed as specified by paragraph 6.5.

Given claim an instance of SlotClaim:

#![allow(unused)]
+fn main() {
+    public_key = AUTHORITIES[claim.authority_index];
+
+    vrf_verify(public_key, sign_data, claim.signature);
+}

If signature verification fails then the claim is not legit.

6.6.1. Primary Claim Method Verification

This verification is performed to confirm ticket ownership and is performed +utilizing the second VrfOutput contained within the SlotClaim signature.

#![allow(unused)]
+fn main() {
+    revealed_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-revealed-v1.0"),
+        data: [
+            curr_epoch.randomness,
+            BYTES(curr_epoch.epoch_index),
+            BYTES(ticket_body.attempt_index)
+        ]
+    );
+
+    reveled_vrf_output = claim.signature.outputs[1];
+
+    revealed_seed = vrf_bytes(32, revealed_vrf_input, revealed_vrf_output);
+    revealed_pub = ed25519_secret_from_seed(revealed_seed).public();
+
+    assert(revealed_pub == ticket_body.revealed_pub);
+}

6.6.1.1. (Optional) Ephemeral Key Signature Check

If the erased_signature element within the SlotClaim is present the +erased_pub key is used to verify it.

The signed challenge is generated with identical steps as outlined in section +6.5.1.1.

#![allow(unused)]
+fn main() {
+    challenge = sign_data.transcript.challenge();
+    result = ed25519_verify(ticket_body.erased_pub, challenge, claim.erased_signature);
+
+    assert(result == true);
+}

6.6.2. Secondary Claim Method Verification

If the slot doesn't have any associated ticket then the validator index contained in +the claim should match the one given by the rule outlined in the fallback assignment +section (6.4.2)

6.7. Randomness Accumulator

Given claim the instance of SlotClaim within the block header, and +accumulator the current value for the current epoch randomness accumulator, +the accumulator value is updated as follows:

#![allow(unused)]
+fn main() {
+    randomness_vrf_input = vrf_input_from_items(
+        domain: BYTES("sassafras-randomness-v1.0"),
+        data: [
+            curr_epoch.randomness,
+            BYTES(curr_epoch.epoch_index),
+            BYTES(slot)
+        ]
+    );
+
+    randomness_vrf_output = claim.signature.outputs[0];
+
+    randomness = vrf_bytes(32, randomness_vrf_input, randomness_vrf_output);
+
+    randomness_accumulator = BLAKE2(32, CONCAT(randomness_accumulator, randomness));
+}

The updated accumulator value is stored on-chain.

The randomness accumulated during epoch N will be used, at the start of the +next epoch (N+1), as an input to compute the NextEpochDescriptor +randomness element (see section 6.1).

This subject is left open for a dedicated RFC.

10. Prior Art and References

Web3 Foundation research page: https://research.web3.foundation/Polkadot/protocols/block-production/SASSAFRAS
Sassafras whitepaper: https://eprint.iacr.org/2023/031.pdf
Ring-VRF whitepaper: https://eprint.iacr.org/2023/002.pdf
Sassafras reference implementation tracking issue: https://github.com/paritytech/substrate/issues/11515
Sassafras reference implementation main PR: https://github.com/paritytech/substrate/pull/11879

11. Unresolved Questions

None

+ +

While this RFC lays the groundwork and outlines the core aspects of the +protocol, several crucial topics remain to be addressed in future RFCs to ensure +the protocol's completeness and security.

These topics include:

12.1. Interactions with the Runtime

+
Outbound Interface. Interfaces exposed by the host which are required by the runtime. +These are commonly dubbed Host Functions.
+
+
Unrecorded Inbound Interfaces. Interfaces exposed by the runtime which are required by the host. +These are commonly dubbed Runtime APIs.
+
+
Transactional Inbound Interfaces. Interfaces exposed by the runtime which alter the state. +These are commonly dubbed Extrinsics and Inherents.
+

12.2. Deployment Strategies

Protocol Migration. Exploring how this protocol can seamlessly replace +an already operational instance of another protocol is essential. Future RFCs +should delve into the deployment strategy, including considerations for a smooth +transition process.

12.3. ZK-SNARK SRS Initialization Ceremony.

+
Timing and Procedure: Determining the timing and procedure for the ZK-SNARK +SRS (Structured Reference String) initialization ceremony. Future RFCs should +provide insights into whether this process should be performed before the +deployment of Sassafras and the steps involved.
+
+
Sharing with Para-chains: Considering the complexity of the ceremony, we +must understand whether the SRS is shared with para-chains or maintained +independently.
+

12.4. Anonymous Submission of Tickets.

Mixnet Integration: Submitting tickets directly can pose a risk of +potential deanonymization through traffic analysis. Subsequent RFCs should +investigate the potential for incorporating Mixnet technology or other +privacy-enhancing mechanisms to address this concern.

+ +

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