Consensus - Proof of Block Inclusion

High-Level DescriptionPermalink

The high level goals of the POBI protocol are:

1. Each round, distribute the sequencer function fairly among all the active registered Aggregators.

2. To synchronise the L2 round duration to L1 rounds. Because the L1 is the source of truth, the finality of the L2 transactions is dependent on the finality of the L1 rollup transaction that includes them, which means there is no advantage in publishing multiple rollups in a single L1 block. It is impossible to decrease the finality time below that of the L1, and, on the other hand, publishing L2 rollups less frequently means that L2 finality is unnecessarily long. The optimum frequency is to publish one rollup per L1 block.

To achieve fairness, the POBI protocol states that the TEE can generate one random nonce each round, and the winner of a round is the Aggregator whose TEE generates the lowest random number from the group. The TEEs generate these numbers independently and then gossip them. The Aggregators who do not win the round, similar to L1 miners, respect this decision because it is rational to do based on the incentive mechanism. If they choose to not respect the protocol, they are free to submit a losing rollup to the L1, which is ignored by all compliant Aggregators, meaning such an Aggregator has to pay L1 gas and not get any useful reward.

The second goal is achieved by linking the random nonce generation, which terminates a round, to the Merkle proof of inclusion of the parent rollup (which exists as a transaction in the L1 transaction Patricia trie) in an L1 block. This property is what gives the name of the protocol. This means that an Aggregator can obtain a signed rollup from the TEE only if it can show the rollup is based on a published rollup in a prior L1 block. Furthermore, this feature links the creation of L2 rollup to an L1 block, thus synchronising their cadence.

A party wishing to increase its chances of winning rounds must register multiple Aggregators and pay the stake for each. The value of the stake needs to be calculated in such a way as to achieve a right decentralisation and practicality balance.

It is straightforward for all the other Aggregators to verify which rollup is the winner by comparing the nonces and checking that the rollup signature is from an approved Aggregator.

Note that the L1 Management Contract is not checking the nonces of the submitted rollups, but it checks that the block inclusion proof is valid. The L1 contract rejects rollups generated using a proof of inclusion that is not an ancestor of the current block.