# L2 Interop Working Group - Call #15
**Oct 1, 2025**
- [Recording](https://youtu.be/ySiKDAdyVMw)
- [Calendar invite for future calls](https://docs.google.com/forms/d/e/1FAIpQLSfMFEJmyVgjLuiipgxprEkiQXwwK3F_PfGbWvU8ZmV6e_ka0A/viewform)
### [Agenda](https://github.com/ethereum/pm/issues/1741)
1. Fast Synchronous Finality (FSF) [EF Protocol Consensus team]
1. ENS-based chain registry [Unruggable + Wonderland]
1. Sync Composability UX [ssv.network]
1. L2 withdrawal windows / reducing 7 day challenge periods
## Call Notes
*condensed notes below – watch the recording (above) for full discussion*
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### 1. [Fast Synchronous Finality](https://docs.google.com/presentation/d/1kbl-yvLNaplca_PUecfUOO_KAI-SRyeVEMpx8JsHemw/edit?usp=sharing) (FSF) Development Update
- Finality rule providing single slot finality under synchronous network conditions
- [slides here](https://docs.google.com/presentation/d/1kbl-yvLNaplca_PUecfUOO_KAI-SRyeVEMpx8JsHemw/edit?usp=sharing)
- 98.75% improvement over economic finality (12 seconds vs 16 minutes)
- Accepts lower adversarial threshold (20-25% vs 33%)
- No economic security guarantees (vs 33% stake requirement)
- Requires synchronous network
- Performance measurements show strong results:
- 95% of blocks finalized in <20 seconds
- 99% finalized in <2 slots
- 100% finalized in 7-8 slots
- Long tail performance issues due to late blocks/empty slots being addressed
#### Technical Implementation & Trust Assumptions
- Algorithm analyzes attestations with strong conditions to ensure canonical chain inclusion
- Assumes maximum fraction β (20-25%, configurable) of committee stake is dishonest
- Works by checking block support against threshold including adversarial weight considerations
- Concerns about assumption strength vs economic finality
- FSF doesn’t replace full finality for high-security use cases
- Stronger than current block confirmation patterns for many applications
- Highlighted need for L1 origin verification for L2 interop protocols
#### Development Status & Next Steps
- Current state:
- Draft specifications and proof of correctness complete
- Proof-of-concept in Teku and Lighthouse tracking latest spec
- Proof-of-concept in Prysm implementing partial specs
- Working on empty slot case improvements
- Next priorities:
- Finalize specifications and proofs
- Write spec tests
- Get more real-life measurements
- Move proofs-of-concept to production
- Implement FSF in any client
- No hard fork required - algorithm applies by analyzing existing attestations
- Ellie to help lead work on FSF at EF, will reach out for feedback
### 2. ENS-Based Chain Registry & Synchronous Composability
- Unruggable/Wonderland presenting ENS resolver for chain name to ERC-7930 identifier resolution
- Three contracts: chain registry (source of truth), chain resolver, reverse resolver
- Migration script from GitHub to onchain registry
- Multisig governance initially, ownership model to be refined
### 3. SSV Network introduced “Compose” project for sync composability
- Latest work from ssv.network called Compose: shared publisher for synchronous crosschain composability
- Uses shared publisher with independent sequencers vs shared sequencer model
- see https://www.compose.network/shared-publisher
- and https://github.com/compose-network
- Custom RPC calls for cross-chain transaction coordination
- Trust assumptions: shared publisher based on Ethereum validators, produces ZK proofs
### 4. L2 withdrawal windows discussion flagged for reducing 7-day challenge period to 1 day
- Exploring secure designs while maintaining strong censorship protection
- See old [ethresearch post](https://ethresear.ch/t/optimistic-rollups-the-challenge-period-and-strong-censorship-attacks/21721) from Donnoh (l2beat) for more context