feat: sequencer-trusting Following Mode

Author: clabby

protocol/cross-safe-gossip.md

@@ -0,0 +1,93 @@
+# [interop]: Following Mode
+
+|                    |                                                    |
+| ------------------ | -------------------------------------------------- |
+| Author             | _@clabby_                                          |
+| Created at         | _2025-06-24_                                       |
+| Initial Reviewers  | _Protolambda, Adrian Sutton_                       |
+| Need Approval From | _Mark Tyneway_                                     |
+| Status             | _Draft / In Review / Implementing Actions / Final_ |
+
+## Purpose
+
+The purpose of this design doc is to specify a potential new "mode" for the [rollup node][rollup-node] to operate in a
+post-interop world, that alleviates the need to reach out to an `op-supervisor` to determine [cross-safety][cross-safe].
+
+## Summary
+
+This design doc introduces an alternative design for "following mode," where a [rollup node][rollup-node] operator
+trusts a sequencer's determination of
+[message validity](https://specs.optimism.io/interop/messaging.html#invalid-messages) in order to advance its
+[`cross-safe`][cross-safe] chain.
+
+## Problem Statement + Context
+
+For the vast majority of [Rollup Node][rollup-node] operators, operating a [supervisor][supervisor] is costly and
+undesirable. The benefit of operating one's own [supervisor][supervisor] is primarily to _ensure_ that the messages
+included in blocks by the sequencer are indeed valid. While this is a necessary step to _ensure_ an L2 chain's integrity
+post-interop, there are valid user stories that don't involve the costly validation of message validity, beyond trusting
+an _attestation_ from the sequencer that it has already been done.
+
+For operators that are willing to trust the sequencer's attestations, an alternative mode can be introduced which
+bypasses the need for a [rollup node][rollup-node] to directly consult a [supervisor][supervisor] when determining
+[cross-safety][cross-safe].
+
+## Proposed Solution
+
+"Following Mode," at a high level, consists of [rollup nodes][rollup-node] subscribing to a new gossip topic that
+receives _signed [`cross-safe`][cross-safe] blocks_.
+
+1. When the sequencing node consults with its own [supervisor][supervisor] and determines that a block can be promoted
+to [`cross-safe`][cross-safe], it will sign the payload hash of the new [cross-safe][cross-safe] block as specified in
+["Rollup Node P2P"](https://specs.optimism.io/protocol/rollup-node-p2p.html#block-signatures) and gossip it to peers
+over the [`cross-safe-blocksv1` topic](#new-gossip-topic).
+1. Peers observing the topic that are operating in "Following Mode" will then validate the block as specified in
+["Rollup Node P2P"](https://specs.optimism.io/protocol/rollup-node-p2p.html#block-validation), and if validation succeeds:
+    1. Check that the block has been derived as `local-safe`. If not, ignore the block.
+    1. Perform [L1 consolidation](https://specs.optimism.io/protocol/derivation.html#l1-consolidation-payload-attributes-matching)
+    on the derived attributes, relative to the received block.
+        1. If L1 consolidation fails, reduce the `transactions` array in the derived attributes to only deposits
+        following the [interop block replacement rules](https://specs.optimism.io/interop/derivation.html#replacing-invalid-blocks)
+        , and try again.
+            1. If finally, L1 consolidation fails after reducing the attributes to deposits-only, discard the block.
+
+### New Gossip Topic
+
+The new gossip topic that is introduced is `cross-safe-blocksv1`, broadcasted on
+`/optimism/<chainId>/0/cross-safe-blocks`. Block encoding for the `cross-safe-blocksv1` topic is as specified within
+["Rollup Node P2P - blocksv4"](https://specs.optimism.io/protocol/rollup-node-p2p.html#block-encoding).
+
+Peers that broadcast invalid blocks on this topic, per the block validity rules mentioned above, should be downscored
+to mitigate possible DoS.
+
+### Resource Usage
+
+Resource utilization for this new mode should be minimal. A new gossip topic will need to be subscribed to among
+rollup node participants operating as a sequencer or in "Following Mode," and the sequencer will need to both sign
+and broadcast blocks as it promotes them to [`cross-safe`][cross-safe].
+
+### Single Point of Failure and Multi Client Considerations
+
+This change affects only the [rollup node][rollup-node], and will need to be implemented in both
+[`op-node`](https://github.com/ethereum-optimism/optimism/tree/develop/op-node) and
+[`kona-node`](https://github.com/op-rs/kona). It is invisible to the execution layer.
+
+## Failure Mode Analysis
+
+<!-- Link to the failure mode analysis document, created from the fma-template.md file. -->
+
+## Impact on Developer Experience
+
+n/a - change affects node operators.
+
+## Alternatives Considered
+
+_TODO_
+
+## Risks & Uncertainties
+
+_TODO_
+
+[cross-safe]: https://specs.optimism.io/interop/verifier.html#safe-inputs
+[supervisor]: https://specs.optimism.io/interop/supervisor.html#supervisor
+[rollup-node]: https://specs.optimism.io/protocol/rollup-node.html