FOCIL Breakout #16, July 29th, 2025, 14:00 UTC

[soispoke]

FOCIL Breakout #16, July 29th, 2025, 14:00 UTC

Agenda

• ePBS and FOCIL compatibility presentation
• Glamsterdam temperature check

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• Duration in minutes : 60 mins
• Recurring meeting : true
• Call series : FOCIL Breakout
• Occurrence rate : bi-weekly
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• Need YouTube stream links : false
• display zoom link in invite : true

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[akashkshirsagar31]

X Stream: https://x.com/i/broadcasts/1ypKdZpNznrJW

[poojaranjan]

Recording: https://www.youtube.com/watch?v=gjWReOSaT-o

[jihoonsong]

We have three candidates for the Glasmterdam headliner: ePBS, FOCIL, and Six Second Slots. Stay tuned for upcoming ACD calls to check for the headliner decision!

Research

FOCIL and ePBS

• We revisited the compatibility and synergies between FOCIL and ePBS. The focus was on the slot pipelining aspect of ePBS, where the two interact the most.

• One of the synergies between FOCIL and ePBS comes from the separation of the beacon block and the execution payload. With ePBS, if the execution payload is not IL compliant, only the execution payload is reorged without affecting beacon chain stability.

• The major changes are the IL bitlist and the IL inclusivity check. When a proposer accepts a bid in ePBS, they need to know whether the bid would satisfy the IL constraints. For this reason, builders include the IL bitlist in their bids, indicating which ILs they’ve considered. The proposer compares this against its subjective IL view and selects a bid whose IL bitlist is a superset.

• Validators also compare the IL bitlist contained in the beacon block against their local IL view. One thing to note here is that they only check whether the IL bitlist matches their IL view. Even if they’ve received the execution payload, they don’t validate whether the execution payload satisfies the IL constraints during the IL inclusivity check. The IL constraints are enforced by attesters for the next slot.

• Either attesters or PTC can perform this check, with different trade-offs. When attesters take this into account during attestation, we leverage the full beacon committee, but failing the IL inclusivity check results in a beacon block reorg, affecting beacon chain stability. On the other hand, the PTC approach can preserve the beacon block, although it involves only a small set of validators.

• This check, whether performed by attesters or PTC, provides a signal that allows proposers to see if their IL view aligns with others’. This is another synergy that we don’t have in vanilla FOCIL.

• The other important interaction is timing. The IL is a list of transactions and has no other strict requirements. Since it doesn’t have to be built on the head, the execution payload release and IL building can happen independently. Having the payload released before IL building would yield fresher ILs, but it’s not necessary.

Q: Do you need the post-state of the execution payload to build the ILs?
A: It’s nice to have for a fresher IL, but not necessary.

Q: If we go with PTC for the IL inclusivity check, can a proposer for the next slot neutralize it by extending the non-IL-compliant execution payload?
A: No, it’s the attesters who enforce the IL constraints. They will vote against it, preventing it from becoming part of the canonical chain.

Q: The builder IL view freeze deadline is 11s into the slot. The 1 second buffer is enough for builders to make sure their payload satisfy the IL constraints and propagate it over the P2P network. But this might not hold if we reduce slot time or have larger payloads after increasing the gas limit.
A: It’s something we need to research and benchmark further.

Q: FOCIL has several deadlines (IL submission deadline and IL view freeze deadlines for attesters and builders). How far can we push these deadlines toward the end of the slot?
A: Quite far, provided sufficient propagation time remains. Building the IL is an unsophisticated task.

Q: Are the IL constraints validated based on the IL bitlist in the beacon block or the attester’s local IL view?
A: The attester’s IL view.

Links

• ePBS <> FOCIL Compatibility Article
• ePBS <> FOCIL Compatibility Slides