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Proposer-builder separation is currently being implemented via a protocol sidecar called MEV-Boost. Validators can outsource block building to a network of builders.
Builders will construct full blocks on behalf of validators, optimizing for MEV extraction, and send blocks to relays. Relays then aggregate blocks from multiple builders and pick out the most profitable block to submit to the validators.
Validators then propagete the block received to the network for attestation and block inclusion.
The implications of the current PBS mechanism include:
- It introduces trust assumptions in builders and relays due to their centralizing tendencies and the potential to censor transactions
- It enables validator timing games, whereby validators and builders that run sophisticated low-latency node infrastructure can strategically delay the inclusion of blocks until the last moment to capture more MEV
- Builder are able to censor transactions entirely in the absence of strong anti-censorship protection mechanisms
- It enables toxic MEV, like frontrunning and sandwiching, which degrades users' experience
In the article titled Endgame, Vitalik explained that while there are many paths toward building a scalable and secure blockchain ecosystem, it's likely that block production will end up centralized. What we can do is enforce a decentralized validation mechanism to "regulate" this market and include strong anti-censorship protection mechanisms.
Inclusion lists allow validators to specify transactions that must go into subsequent blocks to be considered valid. Builders can still reorder transactions to their favor, but they must include transactions specified in the IL, thereby removing builders' ability to censor transactions entirely.