Holocene Hardfork: Block Derivation & Frame Queue

• Holocene hardfork introduces changes to block derivation rules, making the derivation pipeline stricter and simpler.
• It improves worst-case scenarios for Fault Proofs and Interop.
• The impact of an invalid batch is contained to the block number, instead of propagating forwards or backwards in the safe chain.
• Invalid payloads are contained at the engine stage, not propagating backwards in the derivation pipeline.

Frame Queue Changes

• The frame queue retains its function and queues all frames which weren't assembled into a channel yet.
• Holocene still allows multiple frames per batcher transaction, possibly from different channels.
• Strict Batch Ordering leads to additional checks and rules to the frame queue.
• The rules guarantee that the frame queue always holds frames whose indices are ordered, contiguous and include the first frame, per channel.
• A first frame of a channel is either the first frame in the queue, or is preceded by a closing frame of a previous channel.
• Pre-Holocene, frame validity checks were only done at the Channel Bank stage.
• Checks are performed at the Frame Queue stage, leading to faster discarding of invalid frames.

Channel Bank Changes

• Because channel frames have to arrive in order, the Channel Bank becomes much simpler and only holds at most a single channel at a time.
• Pruning is vastly simplified as there is at most only one open channel in the channel bank.
• The channel bank's queue becomes effectively a staging slot for a single channel, the staging channel.
• The MAX_CHANNEL_BANK_SIZE parameter is no longer used, and the compressed size of the staging channel is required to be at most MAX_RLP_BYTES_PER_CHANNEL.
• The timeout is applied as before, just only to the single staging channel.
• The frame queue is guaranteed to hold ordered and contiguous frames, per channel.

Partial Span Batch Validity

• Treats a span batch as an optional stage in the derivation pipeline that sits before the batch queue.
• When encountering an invalid singular batch, it is dropped, as is the remaining span batch for consistency reasons (forwards-invalidation).
• Doesn't backwards-invalidate previous valid batches that came from the same span batch.
• When a batch derived from the current staging channel is a singular batch, it is directly forwarded to the batch queue.
• Otherwise, it is set as the current span batch in the span batch stage.
• Span batch validity checks are performed before singular batches are derived from it.
• If any checks invalidate the span batch, it is dropped and the remaining channel from which the span batch was derived is also immediately dropped (Fast Channel Invalidation).
• A past span batch is only dropped, without dropping the remaining channel.

Batch Queue Changes

• Batches are required to arrive strictly ordered, and any batches that violate the ordering requirements are immediately dropped, instead of buffered.
• Changes to batch validity rules also activate by the L1 inclusion block timestamp of a batch, not with the batch timestamp.
• When the L1 origin of the batch queue moves forward, it is guaranteed that it is empty, because future batches aren't buffered any more.
• Batch queue effectively becomes a simpler batch stage that holds at most one span batch from which singular batches are read from, and doesn't buffer singular batches itself in a queue anymore.
• A valid batch is directly forwarded to the next stage.
• Upon finding an invalid batch, the remaining channel it got derived from is also discarded.

Payload Attributes

• Starting after the fork activation block, the PayloadAttributes produced by the attributes builder will include the eip1559Params field.
• On the fork activation block, the attributes builder will include a 0'd out eip1559Params, as to instruct the engine to use the canyon base fee parameter constants.
• After the first Holocene payload has been processed, future payloads should use the SystemConfig's EIP-1559 denominator and elasticity parameter as the eip1559Params field's value.
• When the pipeline encounters a UpdateType. EIP_1559_PARAMS, ConfigUpdate event, the pipeline's system config will be synchronized with the SystemConfig contract's.

Holocene Activation

• The new batch rules activate when the L1 inclusion block timestamp is greater or equal to the Holocene activation timestamp.
• When the L1 traversal stage of the derivation pipeline moves its origin to L1 block, the derivation pipeline's state is mostly reset by discarding.
• The three stages are then replaced by the new Holocene frame queue, channel bank and batch queue.
• Batcher implementations must be aware of this activation behavior, so any frames of a partially submitted channel that were included pre-Holocene must be sent again.

Advantages of Holocene

• Strict Frame and Batch Ordering simplifies implementations of the derivation pipeline.
• Leads to better worst-case cached data usage.
• Partial Span Batch Validity guarantees that a valid singular batch derived from a span batch can immediately be processed as valid.
• The new Fast Channel Invalidation rule is a consistency implication of the Strict Ordering Rules.
• Steady Block Derivation changes the derivation rules for invalid payload attributes, replacing an invalid payload by a deposit-only/empty payload.
• Stricter derivation rules lead to a less defensive protocol.
• Shifts some complexity from derivation to the batching phase.

Batcher Implementation

• Simpler and stricter derivation rules need to be met by a more complex batcher implementation.
• Batcher must be hardened to guarantee the strict ordering requirements.
• Batcher implementations are suggested to follow a fixed nonce to block-range assignment, once the first batcher transaction starts being submitted.
• Batcher implementations need to be made aware of the Steady Block Derivation rules, namely that invalid payloads will be derived as deposit-only blocks.
• The sync-status should repeatedly be queried and matched against the expected safe chain, and halt operations until derivation catches up.

Sync Start Algorithm

• Thanks to the new strict frame and batch ordering rules, the sync start algorithm can be simplified in the average case.
• The sync start algorithm can optimistically select the last L2 unsafe, safe and finalized heads from the engine.