From 1b92780ae4fc3be64f05d97ed8833c445e73b0cf Mon Sep 17 00:00:00 2001 From: Jeff Jo Date: Thu, 24 Sep 2026 15:44:58 -0700 Subject: [PATCH 1/2] tcp: refresh TS.Recent for accepted old ACKs A TCP packet can carry new data while acknowledging traffic in the opposite direction. With overlapping traffic in both directions, a delayed packet's acknowledgment can be older than one Linux has already accepted, even when that packet fills a gap in the received data. Linux accepts the data, but tcp_ack() takes the old_ack path and skips updating TS.Recent, the timestamp saved for outgoing acknowledgments. The reply therefore echoes an older timestamp. If the sender uses this echo to measure round-trip time after a long idle period, its estimate includes the idle time and can reduce its sending rate. Update TS.Recent in old_ack using tcp_replace_ts_recent(), before SACK processing can trigger a transmission. This reuses the existing timestamp and sequence checks, including PAWS protection against old duplicate packets. ACK validation already rejects old ACKs in SYN_RECV before this path, so no additional state check is needed. Echoing the timestamp of the packet that fills the receive gap follows RFC 7323 section 4.3. In a socket reproduction with 300 seconds idle, controlled reordering and retransmission to exercise timestamp-based RTT sampling, the sender's smoothed round-trip time was 37.5 seconds without the fix and 15.5 ms with it. Fixes: 12fb3dd9dc3c ("tcp: call tcp_replace_ts_recent() from tcp_ack()") Assisted-by: LLM sparse Signed-off-by: Jeff Jo Reviewed-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 92bc60716f33..99baf14afdfd 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -4465,6 +4465,12 @@ no_queue: return 1; old_ack: + /* An old ACK can carry new data. Update TS.Recent before SACK + * processing can trigger a retransmission. + */ + if (flag & FLAG_UPDATE_TS_RECENT) + tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq); + /* If data was SACKed, tag it and see if we should send more data. * If data was DSACKed, see if we can undo a cwnd reduction. */ From 81fe707ae58e67e5a029a9778c4b0c1b478bcb97 Mon Sep 17 00:00:00 2001 From: Jeff Jo Date: Thu, 24 Sep 2026 15:44:59 -0700 Subject: [PATCH 2/2] selftests: net: check timestamp echo after an old ACK Add a regression test for a gap-filling packet whose acknowledgment has become old. Linux first sends data to the peer. Deliver two peer data packets out of order: the later packet acknowledges Linux's data, while the delayed packet still carries the earlier acknowledgment. Require the ACK that closes the receive gap to echo the delayed packet's timestamp, 301000. Without the fix, Linux accepts the data but still echoes the previously saved timestamp, 1000. Also check that the application can read all 34 bytes. Use a large jump in peer timestamps to represent the idle interval, with no real wait, loss or retransmission. The test directly checks the outgoing timestamp echo. It requires packetdrill's merged TSecr verification fix (linked below); older tools incorrectly pass on an unfixed kernel. Use the existing packetdrill selftest runner for IPv4, IPv6 and IPv4-mapped IPv6. Link: https://github.com/google/packetdrill/commit/83f72d3f9085d0e26eb4d206fe4d7cfab5b6d872 Assisted-by: LLM Signed-off-by: Jeff Jo Reviewed-by: Eric Dumazet Signed-off-by: David S. Miller --- .../net/packetdrill/tcp_old_ack_ts.pkt | 22 +++++++++++++++++++ 1 file changed, 22 insertions(+) create mode 100644 tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt diff --git a/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt b/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt new file mode 100644 index 000000000000..f072e5e4c449 --- /dev/null +++ b/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt @@ -0,0 +1,22 @@ +// SPDX-License-Identifier: GPL-2.0 +// An out-of-order segment advances SND.UNA. The gap filler still carries +// its original, older ACK. The reply must echo the gap filler's timestamp. +--tcp_ts_tick_usecs=1000 + +`./defaults.sh` + +0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 ++0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 ++0 bind(3, ..., ...) = 0 ++0 listen(3, 1) = 0 +0.100 < S 0:0(0) win 20000 ++0 > S. 0:0(0) ack 1 +0.200 < . 1:1(0) ack 1 win 20000 ++0 accept(3, ..., ...) = 4 +0.300 write(4, ..., 17) = 17 ++0 > P. 1:18(17) ack 1 +0.310 < P. 18:35(17) ack 18 win 20000 ++0 > . 18:18(0) ack 1 +0.320 < P. 1:18(17) ack 1 win 20000 ++0 > . 18:18(0) ack 35 ++0 read(4, ..., 34) = 34