[cpp-RCP-N2-03] E2E CRC coverage omits ACF_GBB message_timestamp, mismatching the transmitted bytes #66

Location: include/rcp/e2e.hpp — coverage_buffer (~lines 205-221) and its callers compute_crc/verify_crc; include/rcp/acf.hpp — encode_acf_gbb (~lines 198-207).

Finding: For an ACF_GBB message, encode_acf_gbb places a 64-bit message_timestamp on the wire immediately after the 10-byte shared ACF header and before the payload. coverage_buffer — the function that assembles the exact byte sequence the end-to-end CRC is computed over — has no parameter for message_timestamp at all: it serializes stream_id + avtp_timestamp + the 10-byte common header + payload, with nothing in between the header and payload. Two consequences follow: (1) for every ACF_GBB frame, the message_timestamp field is completely outside CRC coverage, so a corrupted timestamp on that specific message type is undetectable by the CRC — undermining the integrity guarantee the CRC exists to provide for exactly this variant; and (2) even setting aside the timestamp's own protection, the coverage buffer's byte layout does not match the actually-transmitted byte layout for ACF_GBB frames (the payload is folded in eight bytes early), so the CRC computed by compute_crc/verified by verify_crc is not actually a CRC of the real wire bytes for GBB messages.

Recommendation: Give coverage_buffer (and compute_crc/verify_crc) an optional message_timestamp parameter, and when the message type is ACF_GBB, insert the same 8-byte big-endian timestamp between the shared header and payload that encode_acf_gbb uses — so the coverage buffer's byte layout matches what's actually sent on the wire.

Verification: Independently re-read against current HEAD (commit 4b9449a, 2026-07-29). Confirmed coverage_buffer's signature takes stream_id, avtp_timestamp, info, and payload only — no timestamp parameter — while encode_acf_gbb in acf.hpp writes the 8-byte message_timestamp between the header and payload on the wire.

Filed from the 2026-07-29 ecosystem audit register; independently re-verified against current HEAD before filing.