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Table of Contents

• RFC-0103: Introduce a CoreIndex commitment and a SessionIndex field in candidate receipts
  • Summary
  • Motivation
  • Stakeholders
  • Explanation
    • Reclaiming unused space in the descriptor
    • UMP transport
    • Polkadot Primitive changes
    • Parachain block validation
    • Backwards compatibility
  • Drawbacks
  • Testing, Security, and Privacy
  • Performance
  • Ergonomics
  • Compatibility
    • Relay chain runtime
    • Validators
    • Parachains
    • Tooling
  • Prior Art and References
  • Unresolved Questions
  • Future Directions and Related Material

RFC-0103: Introduce a CoreIndex commitment and a SessionIndex field in candidate receipts

Summary

Elastic scaling is not resilient against griefing attacks without a way for a PoV (Proof of Validity) to commit to the particular core index it was intended for. This RFC proposes a way to include core index information in the candidate commitments and the CandidateDescriptor data structure in a backwards compatible way. Additionally it proposes the addition of a SessionIndex field in the CandidateDescriptor to make dispute resolution more secure and robust.

Motivation

This RFC proposes a way to solve two different problems:

1. For Elastic Scaling, it prevents anyone who has acquired a valid collation to DoS the parachain by providing the same collation to all backing groups assigned to the parachain. This can happen before the next valid parachain block is authored and will prevent the chain of candidates to be formed, reducing the throughput of the parachain to a single core.
2. The dispute protocol relies on validators trusting the session index provided by other valdiators when initiating and participating in disputes. It is used to lookup validator keys and check dispute vote signatures. By adding a SessionIndex in the CandidateDescriptor, validators no longer have to trust the Sessionindex provided by the validator raising a dispute. It can happen that the dispute concerns a relay chain block not yet imported by a validator. In this case validators can safely assume the session index refers to the session the candidate has appeared in, otherwise the chain would have rejected candidate.

Stakeholders

• Polkadot core developers.
• Cumulus node developers.
• Tooling, block explorer developers.

This approach and alternatives have been considered and discussed in this issue.

Explanation

The approach proposed below was chosen primarly because it minimizes the number of breaking changes, the complexity and takes less implementation and testing time. The proposal is to change the existing primitives while keeping binary compatibility with the older versions. We repurpose unused fields to introduce core index and a session index information in the CandidateDescriptor and extend the UMP for transporting non-XCM messages.

Reclaiming unused space in the descriptor

The CandidateDescriptor currently includes collator and signature fields. The collator includes a signature on the following descriptor fields: parachain id, relay parent, validation data hash, validation code hash and the PoV hash.

However, in practice, having a collator signature in the receipt on the relay chain does not provide any benefits as there is no mechanism to punish or reward collators that have provided bad parachain blocks.

This proposal aims to remove the collator signature and all the logic that checks the collator signatures of candidate receipts. We use the first 7 reclaimed bytes to represent version, the core and session index, and fill the rest with zeroes. So, there is no change in the layout and length of the receipt. The new primitive is binary compatible with the old one.

UMP transport

CandidateCommitments remains unchanged as we will store scale encoded UMPSignal messages directly in the parachain UMP queue by outputing them in upward_messages.

The UMP queue layout is changed to allow the relay chain to receive both the XCM messages and UMPSignal messages. An empty message (empty Vec<u8>) is used to mark the end XCM messages and the start of UMPSignal messages.

This way of representing the new messages has been chosen over introducing an enum wrapper to minimize breaking changes of XCM message decoding in tools like Subscan for example.

Example:

#![allow(unused)]
fn main() {
[ XCM message1, XCM message2, ..., EMPTY message, UMPSignal::SelectCore ]
}

UMPSignal messages

#![allow(unused)]
fn main() {
/// An `u8` wrap around sequence number. Typically this would be the least significant byte of the parachain block number.
pub struct CoreSelector(pub u8);

/// An offset in the relay chain claim queue.
pub struct ClaimQueueOffset(pub u8);

/// Default claim queue offset
pub const DEFAULT_CLAIM_QUEUE_OFFSET: ClaimQueueOffset = ClaimQueueOffset(1);

pub enum UMPSignal {
	/// A message sent by a parachain to select the core the candidate is commited to.
	/// Relay chain validators, in particular backers, use the `CoreSelector` and `ClaimQueueOffset`
	/// to compute the index of the core the candidate has commited to.
	///
	SelectCore(CoreSelector, ClaimQueueOffset),
}
}

The parachain runtime is not concerned with the actual CoreIndex the candidate is intended for, but must provide enough information for the validators and collators to compute it. CoreSelector and ClaimQueueOffset use only 2 bytes and fullfil the requirement.

Example:

cq_offset = 1 and core_selector = 3

The table below represents a snapshot of the claim queue:

| | offset = 0 | offset = 1 | offset = 2 | | :--: | :--: | :--: | :--: | | Core 1 | Para A | Para A | Para A | | Core 2 | Para A | Para B | Para A | | Core 3 | Para B | Para A | Para A |

The purpose of ClaimQueueOffset is to select the column from the above table. For cq_offset = 1 we get [ Para A, Para B, Para A] and use as input to create a sorted vec with the cores A is assigned to: [ Core 1, Core 3] and call it para_assigned_cores. We use core_selector and determine the commited core index is Core 3 like this:

#![allow(unused)]
fn main() {
let committed_core_index = para_assigned_cores[core_selector % para_assigned_cores.len()];
}

Parachains should prefer to have a static ClaimQueueOffset value that makes sense for their usecase which can be changed by governance at some future point. Changing the value dynamically can be a friction point. It will work out fine to decrease the value to build more into the present. But if the value is increased to build more into the future, a relay chain block will be skipped.

Polkadot Primitive changes

New CandidateDescriptor

• reclaim 32 bytes from collator: CollatorId and 64 bytes from signature: CollatorSignature and rename to reserved1 and reserved2 fields.
• take 1 bytes from reserved1 for a new version: u8 field.
• take 2 bytes from reserved1 for a new core_index: u16 field.
• take 4 bytes from reserved1 for a new session_index: u32 field.
• the remaining reserved1 and reserved2 fields are zeroed

Thew new primitive will look like this:

#![allow(unused)]
fn main() {
pub struct CandidateDescriptorV2<H = Hash> {
	/// The ID of the para this is a candidate for.
	para_id: ParaId,
	/// The hash of the relay-chain block this is executed in the context of.
	relay_parent: H,
	/// Version field. The raw value here is not exposed, instead it is used
	/// to determine the `CandidateDescriptorVersion`
	version: InternalVersion,
	/// The core index where the candidate is backed.
	core_index: u16,
	/// The session index of the candidate relay parent.
	session_index: SessionIndex,
	/// Reserved bytes.
	reserved1: [u8; 25],
	/// The blake2-256 hash of the persisted validation data. This is extra data derived from
	/// relay-chain state which may vary based on bitfields included before the candidate.
	/// Thus it cannot be derived entirely from the relay-parent.
	persisted_validation_data_hash: Hash,
	/// The blake2-256 hash of the PoV.
	pov_hash: Hash,
	/// The root of a block's erasure encoding Merkle tree.
	erasure_root: Hash,
	/// Reserved bytes.
	reserved2: [u8; 64],
	/// Hash of the para header that is being generated by this candidate.
	para_head: Hash,
	/// The blake2-256 hash of the validation code bytes.
	validation_code_hash: ValidationCodeHash,
}
}

In future format versions, parts of the reserved1 and reserved2 bytes can be used to include additional information in the descriptor.

Parachain block validation

If the candidate descriptor is version 1, there are no changes.

For version 2, backers and the runtime must check the validity of core_index and session_index fields. A candidate must not be backed if any of the following are true:

• the core_index in the descriptor does not match the core the backing group is assigned to
• the session_index is not equal to the session of the relay_parent in the descriptor
• the core_index in descriptor does not match the one determined by the UMPSignal::SelectCore message

Backwards compatibility

There are two flavors of candidate receipts which are used in network protocols, runtime and node implementation:

• CommittedCandidateReceipt which includes the CanidateDescriptor and the CandidateCommitments
• CandidateReceipt which includes the CanidateDescriptor and just a hash of the commitments

We want to support both the old and new versions in the runtime and node. The implementation must be able to detect the version of a given candidate receipt.

The version of the descriptor is detected by checking if the version field is 0 and the reserved fields are zerored. If this is true it means the descriptor is version 2, otherwise we consider it is version 1.

Drawbacks

The only drawback is that further additions to the descriptor are limited to the amount of remaining unused space.

Testing, Security, and Privacy

Standard testing (unit tests, CI zombienet tests) for functionality and mandatory secuirty audit to ensure the implementation does not introduce any new security issues.

Backwards compatibility of the implementation will be tested on testnets (Versi and Westend).

There is no impact on privacy.

Performance

The expectation is that performance impact is negligible for sending and processing the UMP message has negligible performance impact in the runtime as well as on the node side.

Ergonomics

It is mandatory for elastic parachains to switch to the new receipt format. It is optional but desired that all parachains switch to the new receipts for providing the session index for disputes.

The implementation of this RFC itself must not introduce any breaking changes for the parachain runtime or collator nodes.

Compatibility

The proposed changes are backwards compatible in general, but additional care must be taken by waiting for at least 2/3 + 1 validators to upgrade. Validators that have not upgraded will not back candidates using the new descriptor format and will also initiate disputes against.

Relay chain runtime

The first step is to remove collator signature checking logic in the runtime, but keep the node side collator signature checks.

The runtime must be upgraded to the new primitives before any collator or node are allowed to use the new candidate receipts format.

Validators

To ensure a smooth launch, a new node feature is required. The feature acts as a signal for supporting the new candidate receipts on the node side and can only be safely enabled if at least 2/3 + 1 of the validators are upgraded.

Once enabled, the validators will skip checking the collator signature when processing the candidate receipts and verify the CoreIndex and SessionIndex fields if present in the receipt.

No new implementation of networking protocol versions for collation and validation are required.

Parachains

The implementation of this RFC will supersede the Elastic MVP feature that relies on injecting a core index in the validator_indices fields of the BackedCandidate primitive. Elastic parachains must upgrade to use the new receipt format.

Tooling

Any tooling that decodes UMP XCM messages needs an update to support or ignore the new UMP messages, but they should be fine to decode the regular XCM messages that come before the separator.

Prior Art and References

Forum discussion about a new CandidateReceipt format: https://forum.polkadot.network/t/pre-rfc-discussion-candidate-receipt-format-v2/3738

Unresolved Questions

N/A

Future Directions and Related Material

The implementation is extensible and future proof to some extent. With minimal or no breaking changes, additional fields can be added in the candidate descriptor until the reserved space is exhausted

At this point there is a simple way to determine the version of the receipt, by testing for zeroed reserved bytes in the descriptor. Future versions of the receipt can be implemented and identified by using the version field of the descirptor introduced in this RFC.