Connext

The bridge can operate in one of two modes: Optimistic or Native. In both modes, so-called routers can accelerate the bridging for users by fronting liquidity (for token transfers) or a bond (for crosschain contract calls) at the destination. The routers are reimbursed after the message has arrived at the destination through one of the two modes.

In optimistic mode the messages (bridging transactions) go through the central Connext sequencer, who reads them from the source chains, then sequences them and calculates an aggregate root offchain. This aggregate root can be proposed by a relayer at the destination triggering a disputeBlocks window where any watcher can turn the system back into native mode, invalidating the proposed root. Only the owner can set the system back into optimistic mode. Non-invalidated roots get finalized after disputeBlocks. In summary, optimistic mode skips the hub domain (Ethereum in the case of an L2-to-L2 transfer) and native arbitrary message bridges (AMBs) completely.

In native mode, messages from various spoke domains are aggregated and periodically sent to Ethereum (hub domain) using the native (non-Connext) AMBs. When delivered to the hub domain, these message roots are aggregated again into a root-of-root of messages before being delivered to their destination (spoke domains). A custom delayBlocks value can be set individually in message-receiving Connext contracts to grant a time delay in which Connext-permissioned watchers could invalidate a potentially fraudulent message from the AMBs.

In the case of a Connext router having accelerated a message by fronting liquidity, they will have to wait a certain time to get their liquidity back. In native mode, this is the time it takes to pass the message via AMBs and then verify / invalidate it during the delayBlocks period. In optimistic mode, it is the time to pass it via the offchain sequencer and finalize / dispute it during the disputeBlocks period. In both cases this reconciliation of funds for the router takes longer than the bridging for the user, while native mode has the longest delay for reconciliation.

Although the values can be different for every message-receiving contract on each chain, current examples are 120 blocks for delayBlocks and 120 blocks for disputeBlocks on the MainnetSpokeConnector on Ethereum.

For speed, users mainly depend on the bids of routers to fulfill their bridging transactions at the destination. But ultimately, after a message has been passed to the destination via the optimistic or native way and the delay period has passed for watchers to invalidate it, there is no router needed to execute at the destination. This means that the users are ultimately dependent on the correct functioning of the sequencer or the AMBs, and the watchers. Note that for Optimistic Rollups (Arbitrum, Optimism) the AMB is (during Connext native mode) only used as a transport layer, and the 7-day optimistic challenge period is ignored.