Bitfield Signing

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

Input:

There is no dedicated input mechanism for bitfield signing. Instead, Bitfield Signing produces a bitfield representing the current state of availability on StartWork.

Output:

• BitfieldDistribution::DistributeBitfield: distribute a locally signed bitfield
• AvailabilityStore::QueryChunk(CandidateHash, validator_index, response_channel)

Functionality

Upon receipt of an ActiveLeavesUpdate, launch bitfield signing job for each activated head referring to a fresh leaf. Stop the job for each deactivated head.

Bitfield Signing Job

Localized to a specific relay-parent r If not running as a validator, do nothing.

• For each fresh leaf, begin by waiting a fixed period of time so availability distribution has the chance to make candidates available.
• Determine our validator index i, the set of backed candidates pending availability in r, and which bit of the bitfield each corresponds to.
• Start with an empty bitfield. For each bit in the bitfield, if there is a candidate pending availability, query the Availability Store for whether we have the availability chunk for our validator index. The OccupiedCore struct contains the candidate hash so the full candidate does not need to be fetched from runtime.
• For all chunks we have, set the corresponding bit in the bitfield.
• Sign the bitfield and dispatch a BitfieldDistribution::DistributeBitfield message.