[PATCH net] tcp: Fix dup-ack detection on non-SACK bidirectional data transfer.
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HOTtoday
From: Guillaume Nault <hidden>
Date: 2026-10-22 08:49:30
Subsystem:
kernel selftest framework, networking [general], networking [tcp], the rest · Maintainers:
Shuah Khan, Shuah Khan, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Neal Cardwell, Linus Torvalds
When receiving a pure ack from a peer that also sends data, we can
have SEG.SEQ > SND.WL1, even though the segment contains no data. This
makes tcp_may_update_window() return true (because of the
"after(ack_seq, tp->snd_wl1)" condition), and tcp_ack_update_window()
then adds FLAG_WIN_UPDATE to the segment set of flags, even when
there's no window update.
Since FLAG_WIN_UPDATE prevents the segment from being considered as
dup-ack (as it's part of FLAG_NOT_DUP), this can inhibit fast
retransmit on non-SACK connections, making recovery very slow as it
now entirely relies on RTO.
Avoid this scenario by setting FLAG_WIN_UPDATE only when a window
update actually occurs. Pure acks with SEG.SEQ > SND.WL1 can now
be detected as dup-ack and trigger fast retransmission.
Add packetdrill selftest. Before this patch, the 1001:2001 segment was
retransmitted upon RTO, now it's sent right after reception of the
third duplicate ack.
Thanks to Tom Williams for the investigation help and initial
reproducer.
Reported-by: Tom Williams <redacted>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Guillaume Nault <redacted>
---
net/ipv4/tcp_input.c | 2 +-
.../tcp_bidir_fast_retransmit_nosack.pkt | 42 +++++++++++++++++++
2 files changed, 43 insertions(+), 1 deletion(-)
create mode 100644 tools/testing/selftests/net/packetdrill/tcp_bidir_fast_retransmit_nosack.pkt
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 99baf14afdfd..ef20bb5a0523 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c@@ -3959,11 +3959,11 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32 nwin <<= tp->rx_opt.snd_wscale; if (tcp_may_update_window(tp, ack, ack_seq, nwin)) { - flag |= FLAG_WIN_UPDATE; tcp_update_wl(tp, ack_seq); if (tp->snd_wnd != nwin) { tp->snd_wnd = nwin; + flag |= FLAG_WIN_UPDATE; /* Note, it is the only place, where * fast path is recovered for sending TCP.
diff --git a/tools/testing/selftests/net/packetdrill/tcp_bidir_fast_retransmit_nosack.pkt b/tools/testing/selftests/net/packetdrill/tcp_bidir_fast_retransmit_nosack.pkt
new file mode 100644
index 000000000000..5c66b7171d22
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_bidir_fast_retransmit_nosack.pkt@@ -0,0 +1,42 @@ +// SPDX-License-Identifier: GPL-2.0 +// +// Verify that non-SACK connections properly detect duplicate acks when mixed +// with other data segments from peer. + +`./defaults.sh` + + 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 4 + +0...0 connect(4, ..., ...) = 0 + + // Three-way handshake, SACK not supported by peer + +0 > S 0:0(0) <mss 1460,sackOK,TS val 0 ecr 0,nop,wscale 8> + +0 < S. 0:0(0) ack 1 win 65535 <mss 1000> + +0 > . 1:1(0) ack 1 + + // Send 1000 bytes, peer acks everything + +0 send(4, ..., 1000, 0) = 1000 + +0 > P. 1:1001(1000) ack 1 + +0 < . 1:1(0) ack 1001 win 65535 + + // Send 4000 more bytes + +0 send(4, ..., 4000, 0) = 4000 + +0 > P. 1001:5001(4000) ack 1 + + // Receive first dup-ack (considering segment 1001-2001 was lost) + +0 < . 1:1(0) ack 1001 win 65535 + + // Receive unrelated data segment from peer and ack it + +0 < P. 1:11(10) ack 1001 win 65535 + +0 > . 5001:5001(0) ack 11 + + // Receive second dup-ack + +0 < . 11:11(0) ack 1001 win 65535 + + // Receive another data segment from peer and ack it + +0 < P. 11:21(10) ack 1001 win 65535 + +0 > . 5001:5001(0) ack 21 + + // Receive third dup-ack, which should trigger fast retransmit (without + // waiting for RTO) + +0 < . 21:21(0) ack 1001 win 65535 + +0 > . 1001:2001(1000) ack 21
--
2.47.3