wire: use maxprotocol message len on msg reject#2518
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Roasbeef merged 2 commits intobtcsuite:masterfrom Apr 8, 2026
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Revert MaxMessagePayload to 32MB and introduce MaxProtocolMessageLength (~4MB) for p2p network message size enforcement. This mirrors Bitcoin Core's separation between MAX_SIZE (32MB serialization bound) and MAX_PROTOCOL_MESSAGE_LENGTH (~4MB network limit) introduced in bitcoin/bitcoin#5843. The previous commit reduced MaxMessagePayload to 4MB, but that constant is also used as a serialization bound for deriving maxTxInPerMessage, maxTxOutPerMessage, and variable-length string limits in contexts beyond network messages (e.g. database deserialization via MsgTx.Deserialize). While consensus limits keep real values well below the 4MB-derived bounds, conflating the two constants is architecturally incorrect and diverges from Bitcoin Core's design. The new MaxProtocolMessageLength is now enforced in all four network read/write paths: WriteMessageN, WriteMessageWithEncodingN, ReadMessageWithEncodingN, and ReadV2MessageN (which previously had no overall message size check).
As we introduced a new MaxProtocolMessageLength, MsgReject max size should also be lowered to MaxProtocolMessageLength. If this isn't lowered and if we ever have a MsgReject message that is bigger than MaxProtocolMessageLength, then the MaxPayloadLength() check on the message will pass but it won't serialize in functions WriteMessageWithEncodingN() and WriteV2MessageN() as both of these functions have a separate check that each message isn't greater than MaxProtocolMessageLength.
erickcestari
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Apr 6, 2026
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erickcestari
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LGTM!
By the way, is it worth keeping the reject message? The Core team removed support for it in 2019 (bitcoin/bitcoin#15437). It looks like it adds a lot of complexity, and other nodes don't listen for it either. So we may remove it later.
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It's worth keeping for light clients IMO. As otherwise, you have no idea why your transaction wasn't accepted. Light clients don't have a mempool, and mempool policy is somewhat fast moving lately. |
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Change Description
As we introduced a new MaxProtocolMessageLength, MsgReject max size
should also be lowered to MaxProtocolMessageLength.
If this isn't lowered and if we ever have a MsgReject message that is
bigger than MaxProtocolMessageLength, then the MaxPayloadLength() check
on the message will pass but it won't serialize in functions
WriteMessageWithEncodingN() and WriteV2MessageN() as both of these
functions have a separate check that each message isn't greater than
MaxProtocolMessageLength.
I really doubt that we'll ever trigger this but the code is technically incorrect and we should change this.
Follow up to #2504
Steps to Test
cd wire; go test
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