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tcp: report duplicate data below the cumulative ACK - #16622
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A sender that retransmits data already acknowledged by the receiver gets the same ACK it would get for a gap. With SACK negotiated, put the duplicate sequence range in the first SACK block of the response so the sender can recognize a spurious retransmission [1]. Report only the previously acknowledged prefix when a segment crosses RCV.NXT. Advance the cumulative ACK through any queued data that the new suffix makes contiguous. If that consumes a queued FIN, return the duplicate block in its ACK. A retransmitted final segment also reports the FIN's sequence number, as Linux tcp_send_dupack() does [2]. Keep the duplicate block out of the receiver's ordinary SACK history. [1]: https://www.rfc-editor.org/rfc/rfc2883#section-4 [2]: https://github.com/torvalds/linux/blob/830b3c68c1fb1e9176028d02ef86f3cf76aa2476/net/ipv4/tcp_input.c#L4481-L4501 FUTURE_COPYBARA_INTEGRATE_REVIEW=#16403 from tamird:fix/tcp-receiver-dsack c451fa7 PiperOrigin-RevId: 1009163261
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tcp: report duplicate data below the cumulative ACK
A sender that retransmits data already acknowledged by the receiver
gets the same ACK it would get for a gap. With SACK negotiated, put
the duplicate sequence range in the first SACK block of the response
so the sender can recognize a spurious retransmission 1.
Report only the previously acknowledged prefix when a segment crosses
RCV.NXT. Advance the cumulative ACK through any queued data that the
new suffix makes contiguous. If that consumes a queued FIN, return the
duplicate block in its ACK. A retransmitted final segment also reports
the FIN's sequence number, as Linux tcp_send_dupack() does 2. Keep
the duplicate block out of the receiver's ordinary SACK history.
FUTURE_COPYBARA_INTEGRATE_REVIEW=#16403 from tamird:fix/tcp-receiver-dsack c451fa7