From: Simon Baatz Date: Mon, 9 Mar 2026 08:02:26 +0000 (+0100) Subject: tcp: implement RFC 7323 window retraction receiver requirements X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=0e24d17bd9668f9dad78ede6a0e8f13dab176682;p=thirdparty%2Flinux.git tcp: implement RFC 7323 window retraction receiver requirements 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 commit 8c670bdfa58e ("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. - Commit b650d953cd39 ("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 Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260309-tcp_rfc7323_retract_wnd_rfc-v3-1-4c7f96b1ec69@gmail.com Signed-off-by: Jakub Kicinski --- diff --git a/Documentation/networking/net_cachelines/tcp_sock.rst b/Documentation/networking/net_cachelines/tcp_sock.rst index 563daea10d6c5..fecf61166a54e 100644 --- a/Documentation/networking/net_cachelines/tcp_sock.rst +++ b/Documentation/networking/net_cachelines/tcp_sock.rst @@ -121,6 +121,7 @@ u64 delivered_mstamp read_write u32 rate_delivered read_mostly tcp_rate_gen u32 rate_interval_us read_mostly rate_delivered,rate_app_limited u32 rcv_wnd read_write read_mostly tcp_select_window,tcp_receive_window,tcp_fast_path_check +u32 rcv_mwnd_seq read_write tcp_select_window u32 write_seq read_write tcp_rate_check_app_limited,tcp_write_queue_empty,tcp_skb_entail,forced_push,tcp_mark_push u32 notsent_lowat read_mostly tcp_stream_memory_free u32 pushed_seq read_write tcp_mark_push,forced_push diff --git a/include/linux/tcp.h b/include/linux/tcp.h index bcebc4f07532f..6982f10e826b4 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -316,6 +316,9 @@ struct tcp_sock { */ u32 app_limited; /* limited until "delivered" reaches this val */ u32 rcv_wnd; /* Current receiver window */ + u32 rcv_mwnd_seq; /* Maximum window sequence number (RFC 7323, + * section 2.4, receiver requirements) + */ u32 rcv_tstamp; /* timestamp of last received ACK (for keepalives) */ /* * Options received (usually on last packet, some only on SYN packets). diff --git a/include/net/tcp.h b/include/net/tcp.h index 48dffcca0a71b..f87bdacb5a699 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -934,6 +934,28 @@ static inline u32 tcp_receive_window(const struct tcp_sock *tp) return (u32) win; } +/* Compute the maximum receive window we ever advertised. + * Rcv_nxt can be after the window if our peer push more data + * than the offered window. + */ +static inline u32 tcp_max_receive_window(const struct tcp_sock *tp) +{ + s32 win = tp->rcv_mwnd_seq - tp->rcv_nxt; + + if (win < 0) + win = 0; + return (u32) win; +} + +/* Check if we need to update the maximum receive window sequence number */ +static inline void tcp_update_max_rcv_wnd_seq(struct tcp_sock *tp) +{ + u32 wre = tp->rcv_wup + tp->rcv_wnd; + + if (after(wre, tp->rcv_mwnd_seq)) + tp->rcv_mwnd_seq = wre; +} + /* Choose a new window, without checks for shrinking, and without * scaling applied to the result. The caller does these things * if necessary. This is a "raw" window selection. diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index ed6f6712f0607..516087c622ade 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -3561,6 +3561,7 @@ static int tcp_repair_set_window(struct tcp_sock *tp, sockptr_t optbuf, int len) tp->rcv_wnd = opt.rcv_wnd; tp->rcv_wup = opt.rcv_wup; + tp->rcv_mwnd_seq = opt.rcv_wup + opt.rcv_wnd; return 0; } @@ -5275,6 +5276,7 @@ static void __init tcp_struct_check(void) CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_txrx, received_ecn_bytes); CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_txrx, app_limited); CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_txrx, rcv_wnd); + CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_txrx, rcv_mwnd_seq); CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_txrx, rcv_tstamp); CACHELINE_ASSERT_GROUP_MEMBER(struct tcp_sock, tcp_sock_write_txrx, rx_opt); diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c index 9fdc19accafd2..4e389d609f919 100644 --- a/net/ipv4/tcp_fastopen.c +++ b/net/ipv4/tcp_fastopen.c @@ -377,6 +377,7 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk, tcp_rsk(req)->rcv_nxt = tp->rcv_nxt; tp->rcv_wup = tp->rcv_nxt; + tp->rcv_mwnd_seq = tp->rcv_wup + tp->rcv_wnd; /* tcp_conn_request() is sending the SYNACK, * and queues the child into listener accept queue. */ diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 71ac69b7b75e4..2e1b237608150 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -4808,20 +4808,18 @@ static enum skb_drop_reason tcp_sequence(const struct sock *sk, const struct tcphdr *th) { const struct tcp_sock *tp = tcp_sk(sk); - u32 seq_limit; if (before(end_seq, tp->rcv_wup)) return SKB_DROP_REASON_TCP_OLD_SEQUENCE; - seq_limit = tp->rcv_nxt + tcp_receive_window(tp); - if (unlikely(after(end_seq, seq_limit))) { + if (unlikely(after(end_seq, tp->rcv_nxt + tcp_max_receive_window(tp)))) { /* Some stacks are known to handle FIN incorrectly; allow the * FIN to extend beyond the window and check it in detail later. */ - if (!after(end_seq - th->fin, seq_limit)) + if (!after(end_seq - th->fin, tp->rcv_nxt + tcp_receive_window(tp))) return SKB_NOT_DROPPED_YET; - if (after(seq, seq_limit)) + if (after(seq, tp->rcv_nxt + tcp_max_receive_window(tp))) return SKB_DROP_REASON_TCP_INVALID_SEQUENCE; /* Only accept this packet if receive queue is empty. */ @@ -6903,6 +6901,7 @@ consume: */ WRITE_ONCE(tp->rcv_nxt, TCP_SKB_CB(skb)->seq + 1); tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1; + tp->rcv_mwnd_seq = tp->rcv_wup + tp->rcv_wnd; /* RFC1323: The window in SYN & SYN/ACK segments is * never scaled. @@ -7015,6 +7014,7 @@ consume: WRITE_ONCE(tp->rcv_nxt, TCP_SKB_CB(skb)->seq + 1); WRITE_ONCE(tp->copied_seq, tp->rcv_nxt); tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1; + tp->rcv_mwnd_seq = tp->rcv_wup + tp->rcv_wnd; /* RFC1323: The window in SYN & SYN/ACK segments is * never scaled. diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c index dafb63b923d0d..d350d794a9597 100644 --- a/net/ipv4/tcp_minisocks.c +++ b/net/ipv4/tcp_minisocks.c @@ -604,6 +604,7 @@ struct sock *tcp_create_openreq_child(const struct sock *sk, newtp->window_clamp = req->rsk_window_clamp; newtp->rcv_ssthresh = req->rsk_rcv_wnd; newtp->rcv_wnd = req->rsk_rcv_wnd; + newtp->rcv_mwnd_seq = newtp->rcv_wup + req->rsk_rcv_wnd; newtp->rx_opt.wscale_ok = ireq->wscale_ok; if (newtp->rx_opt.wscale_ok) { newtp->rx_opt.snd_wscale = ireq->snd_wscale; diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 34a25ef610060..35c3b0ab5a0cb 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -293,6 +293,7 @@ static u16 tcp_select_window(struct sock *sk) tp->pred_flags = 0; tp->rcv_wnd = 0; tp->rcv_wup = tp->rcv_nxt; + tcp_update_max_rcv_wnd_seq(tp); return 0; } @@ -316,6 +317,7 @@ static u16 tcp_select_window(struct sock *sk) tp->rcv_wnd = new_win; tp->rcv_wup = tp->rcv_nxt; + tcp_update_max_rcv_wnd_seq(tp); /* Make sure we do not exceed the maximum possible * scaled window. @@ -4165,6 +4167,7 @@ static void tcp_connect_init(struct sock *sk) else tp->rcv_tstamp = tcp_jiffies32; tp->rcv_wup = tp->rcv_nxt; + tp->rcv_mwnd_seq = tp->rcv_nxt + tp->rcv_wnd; WRITE_ONCE(tp->copied_seq, tp->rcv_nxt); inet_csk(sk)->icsk_rto = tcp_timeout_init(sk); diff --git a/tools/testing/selftests/net/packetdrill/tcp_rcv_big_endseq.pkt b/tools/testing/selftests/net/packetdrill/tcp_rcv_big_endseq.pkt index 6c0f32c40f19b..12882be10f2e0 100644 --- a/tools/testing/selftests/net/packetdrill/tcp_rcv_big_endseq.pkt +++ b/tools/testing/selftests/net/packetdrill/tcp_rcv_big_endseq.pkt @@ -36,7 +36,7 @@ +0 read(4, ..., 100000) = 4000 -// If queue is empty, accept a packet even if its end_seq is above wup + rcv_wnd +// If queue is empty, accept a packet even if its end_seq is above rcv_mwnd_seq +0 < P. 4001:54001(50000) ack 1 win 257 * > . 1:1(0) ack 54001 win 0