incentivized relays design doc

[hamdiallam]

framework for onchain incentives that can drive relays in the syperchain

[hamdiallam]

Draft notion doc on a rough design: https://www.notion.so/oplabs/Incentivized-Message-Relays-1b2f153ee162800e9f36ceab34efa7d9?pvs=4

TLDR: async settlement framework based on gas receipts emitted by the L2ToL2CDM

[hamdiallam]

Design doc on the messenger changes: ethereum-optimism/design-docs#266