# Bitfield Distribution

Validators vote on the availability of a backed candidate by issuing signed bitfields, where each bit corresponds to a
single candidate. These bitfields can be used to compactly determine which backed candidates are available or not based
on a 2/3+ quorum.

## Protocol

`PeerSet`: `Validation`

Input: [`BitfieldDistributionMessage`](../../types/overseer-protocol.md#bitfield-distribution-message) which are
gossiped to all peers, no matter if validator or not.

Output:

- `NetworkBridge::SendValidationMessage([PeerId], message)` gossip a verified incoming bitfield on to interested
  subsystems within this validator node.
- `NetworkBridge::ReportPeer(PeerId, cost_or_benefit)` improve or penalize the reputation of peers based on the messages
  that are received relative to the current view.
- `ProvisionerMessage::ProvisionableData(ProvisionableData::Bitfield(relay_parent, SignedAvailabilityBitfield))` pass on
  the bitfield to the other submodules via the overseer.

## Functionality

This is implemented as a gossip system.

It is necessary to track peer connection, view change, and disconnection events, in order to maintain an index of which
peers are interested in which relay parent bitfields.


Before gossiping incoming bitfields, they must be checked to be signed by one of the validators of the validator set
relevant to the current relay parent. Only accept bitfields relevant to our current view and only distribute bitfields
to other peers when relevant to their most recent view. Accept and distribute only one bitfield per validator.


When receiving a bitfield either from the network or from a `DistributeBitfield` message, forward it along to the block
authorship (provisioning) subsystem for potential inclusion in a block.

Peers connecting after a set of valid bitfield gossip messages was received, those messages must be cached and sent upon
connection of new peers or re-connecting peers.
