fix gossip and tx size limits for the merge

specs/merge/beacon-chain.md

@@ -59,7 +59,7 @@ This patch adds transaction execution to the beacon chain as part of the Merge f
 
 | Name | Value |
 | - | - |
-| `MAX_BYTES_PER_TRANSACTION` | `uint64(2**20)` (= 1,048,576) |
+| `MAX_BYTES_PER_TRANSACTION` | `uint64(2**24)` (= 16,777,216) |
 | `MAX_TRANSACTIONS_PER_PAYLOAD` | `uint64(2**14)` (= 16,384) |
 | `BYTES_PER_LOGS_BLOOM` | `uint64(2**8)` (= 256) |
 | `GAS_LIMIT_DENOMINATOR` | `uint64(2**10)` (= 1,024) |

specs/merge/p2p-interface.md

@@ -34,6 +34,14 @@ Refer to the note in the [validator guide](./validator.md) for further details.
 
 # Modifications in the Merge
 
+## Configuration
+
+This section outlines modifications constants that are used in this spec.
+
+| Name | Value | Description |
+|---|---|---|
+| `GOSSIP_MAX_SIZE_MERGE` | `10 * 2**20` (= 10485760, 10 MiB) | The maximum allowed size of uncompressed gossip messages starting at the Merge upgrade |
+
 ## The gossip domain: gossipsub
 
 Some gossip meshes are upgraded in the Merge to support upgraded types.
@@ -43,7 +51,12 @@ Some gossip meshes are upgraded in the Merge to support upgraded types.
 Topics follow the same specification as in prior upgrades.
 All topics remain stable except the beacon block topic which is updated with the modified type.
 
-The specification around the creation, validation, and dissemination of messages has not changed from the Phase 0 and Altair documents.
+The specification around the creation, validation, and dissemination of messages has not changed from the Phase 0 and Altair documents unless explicitly noted here.
+
+Starting at the Merge upgrade, each gossipsub [message](https://github.com/libp2p/go-libp2p-pubsub/blob/master/pb/rpc.proto#L17-L24)
+has a maximum size of `GOSSIP_MAX_SIZE_MERGE`.
+Clients MUST reject (fail validation) messages that are over this size limit.
+Likewise, clients MUST NOT emit or propagate messages larger than this limit.
 
 The derivation of the `message-id` remains stable.
 
@@ -77,8 +90,7 @@ Alias `block = signed_beacon_block.message`, `execution_payload = block.body.exe
   - _[REJECT]_ The execution payload block hash is not equal to the parent hash --
     i.e. `execution_payload.block_hash != execution_payload.parent_hash`.
   - _[REJECT]_ The execution payload transaction list data is within expected size limits,
-    the data MUST NOT be larger than the SSZ list-limit,
-    and a client MAY be more strict.
+    the data MUST NOT be larger than the SSZ list-limit.
 
 *Note*: Additional [gossip validations](https://github.com/ethereum/devp2p/blob/master/caps/eth.md#block-encoding-and-validity)
 (see block "data validity" conditions) that rely more heavily on execution-layer state and logic are currently under consideration.
@@ -125,3 +137,30 @@ Per `context = compute_fork_digest(fork_version, genesis_validators_root)`:
 | `GENESIS_FORK_VERSION`   | `phase0.SignedBeaconBlock` |
 | `ALTAIR_FORK_VERSION`    | `altair.SignedBeaconBlock` |
 | `MERGE_FORK_VERSION`     | `merge.SignedBeaconBlock`  |
+
+# Design decision rationale
+
+## Gossipsub
+
+### Why was the max gossip message size increased at the Merge?
+
+With the addition of `ExecutionPayload` to `BeaconBlock`s, there is a dynamic
+field -- `transactions` -- which can validly exceed the `GOSSIP_MAX_SIZE` limit (1 MiB) put in place in
+place at Phase 0. At the `GAS_LIMIT` (~30M) currently seen on mainnet in 2021, a single transaction
+filled entirely with data at a cost of 16 gas per byte can create a valid
+`ExecutionPayload` of ~2 MiB. Thus we need a size limit to at least account for
+current mainnet conditions.
+
+Geth currently has a [max gossip message size](https://github.com/ethereum/go-ethereum/blob/3ce9f6d96f38712f5d6756e97b59ccc20cc403b3/eth/protocols/eth/protocol.go#L49) of 10 MiB.
+To support backward compatibility with this previously defined network limit,
+we adopt `GOSSIP_MAX_SIZE_MERGE` of 10 MiB for maximum gossip sizes at the
+point of the Merge and beyond. Note, that clients SHOULD still reject objects
+that exceed their maximum theoretical bounds which in most cases is less than `GOSSIP_MAX_SIZE_MERGE`.
+
+Note, that due to additional size induced by the `BeaconBlock` contents (e.g.
+proposer signature, operations lists, etc) this does reduce the
+theoritical max valid `ExecutionPayload` (and `transactions` list) size as
+slightly lower than 10 MiB. Considering that `BeaconBlock` max size is on the
+order of 128 KiB in the worst case and the current gas limit (~30M) bounds max blocksize to less
+than 2 MiB today, this marginal difference in theoretical bounds will have zero
+impact on network functionality and security.