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0e24d17bd9
By default, the Linux TCP implementation does not shrink the advertised window (RFC 7323 calls this "window retraction") with the following exceptions: - When an incoming segment cannot be added due to the receive buffer running out of memory. Since commit8c670bdfa5("tcp: correct handling of extreme memory squeeze") a zero window will be advertised in this case. It turns out that reaching the required memory pressure is easy when window scaling is in use. In the simplest case, sending a sufficient number of segments smaller than the scale factor to a receiver that does not read data is enough. - Commitb650d953cd("tcp: enforce receive buffer memory limits by allowing the tcp window to shrink") addressed the "eating memory" problem by introducing a sysctl knob that allows shrinking the window before running out of memory. However, RFC 7323 does not only state that shrinking the window is necessary in some cases, it also formulates requirements for TCP implementations when doing so (Section 2.4). This commit addresses the receiver-side requirements: After retracting the window, the peer may have a snd_nxt that lies within a previously advertised window but is now beyond the retracted window. This means that all incoming segments (including pure ACKs) will be rejected until the application happens to read enough data to let the peer's snd_nxt be in window again (which may be never). To comply with RFC 7323, the receiver MUST honor any segment that would have been in window for any ACK sent by the receiver and, when window scaling is in effect, SHOULD track the maximum window sequence number it has advertised. This patch tracks that maximum window sequence number rcv_mwnd_seq throughout the connection and uses it in tcp_sequence() when deciding whether a segment is acceptable. rcv_mwnd_seq is updated together with rcv_wup and rcv_wnd in tcp_select_window(). If we count tcp_sequence() as fast path, it is read in the fast path. Therefore, rcv_mwnd_seq is put into rcv_wnd's cacheline group. The logic for handling received data in tcp_data_queue() is already sufficient and does not need to be updated. Signed-off-by: Simon Baatz <gmbnomis@gmail.com> Reviewed-by: Eric Dumazet <edumazet@google.com> Link: https://patch.msgid.link/20260309-tcp_rfc7323_retract_wnd_rfc-v3-1-4c7f96b1ec69@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>