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Mon, 21 Sep 2026 15:26:17 -0700 (PDT) Received: from com-94485.corp.openai.org ([199.47.143.14]) by smtp.gmail.com with ESMTPSA id af79cd13be357-93c19bc6f4bsm30675085a.44.2026.09.21.15.26.16 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Mon, 21 Sep 2026 15:26:17 -0700 (PDT) From: Jeff Jo To: netdev@vger.kernel.org Cc: edumazet@google.com, ncardwell@google.com, kuniyu@google.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, shuah@kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: [PATCH net 1/2] tcp: refresh TS.Recent for accepted old ACKs Date: Mon, 21 Sep 2026 15:26:11 -0700 Message-ID: <20260921222609.50824-5-jeffjo@openai.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260921222609.50824-4-jeffjo@openai.com> References: <20260921222609.50824-4-jeffjo@openai.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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. Leave SYN_RECV unchanged because its caller rejects old ACKs; established and closing connections accept them. 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 --- net/ipv4/tcp_input.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 92bc60716f33..f3f4c905ce00 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -4465,6 +4465,13 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag) return 1; old_ack: + /* Closing connections also accept old ACKs. SYN_RECV rejects them in + * the caller, so leave its timestamp unchanged. Update before SACK + * processing can trigger a retransmission. + */ + if (sk->sk_state != TCP_SYN_RECV && (flag & FLAG_UPDATE_TS_RECENT)) + flag |= 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. */ -- 2.55.0