For a working L1 resolution we need to transmit data about the filler of a given request. In general rollups provide mechanisms for data exchange which are often used to implement their own L1/L2 bridges. Optimism has some documentation here.
Given the possibility to transmit information, there are two problems that need to be solved: how to route the information to the correct source rollup and how to make this information trustworthy.
The proof of request fulfillment will always be written by the
contract on the target rollup and, in case a L1 resolution is triggered, must be
submitted to the
RequestManager contract on the source rollup.
This process is started by the liquidity provider who fills a request. This writes a proof on that rollup. This proof includes information about the filler, the chain id of the source rollup and the request id. The proof is basically an encoded transaction to the resolver, which can be executed on L1.
The central role for routing this information correctly has the
contract, which is deployed on the shared L1 chain of both rollups. The resolver
holds a mapping of chain ids to the contract address of the
on that chain.
When a proof transaction sent by the target rollup is executed, the resolver can find the correct request manager in the mapping and forward the information about the filler. This again happens in form of a transaction, that can be executed on the source rollup.
To make sure that the resolved information is correct, only trusted contracts must be allowed to take part in the message forwarding scheme. Otherwise, bad actors could freely write invalid proofs on the target rollup or even on the L1 chain.
Avoiding this requires whitelisting valid senders in all contracts on the path of information.
On the target rollup, the
Messengermust only be callable by the
On L1, the resolver must only accept messages send by a whitelisted L2 messenger for the given chain id.
On the source rollup the request manager must only accept transactions that have been sent by the L1 messenger.