netdev - [PATCH 5/6] tcp: fastopen: retransmit data when only the SYN of a synack-with-data is acked

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Message-ID: <6201e4cb8374df23e86bf3af0fd6794005f77b4e.1747409911.git.jgh@exim.org>
Date: Fri, 16 May 2025 16:55:03 +0100
From: Jeremy Harris <jgh@...m.org>
To: netdev@...r.kernel.org
Cc: linux-api@...r.kernel.org,
	edumazet@...gle.com,
	ncardwell@...gle.com,
	Jeremy Harris <jgh@...m.org>
Subject: [PATCH 5/6] tcp: fastopen: retransmit data when only the SYN of a synack-with-data is acked

A corner-case for the 3rd-ack after a data-on-synack is for only
the SYN to be acked. Handle this by, in ack processing, when in
SYN_RECV state (the state is not yet updated to ESTABLISHED)
marking the retransmit-queue sk_buff as having been lost.

Signed-off-by: Jeremy Harris <jgh@...m.org>
---
 net/ipv4/tcp_input.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 345a08baaf02..a53021edddd5 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4069,6 +4069,18 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 				      &rexmit);
 	}
 
+	/* On receiving a 3rd-ack, if we never sent a packet via
+	 * the normal means (which counts them), yet there is data
+	 * remaining for retransmit, it was data-on-synack not acked;
+	 * mark the skb for retransmission.
+	 */
+	if (sk->sk_state == TCP_SYN_RECV && tp->segs_out == 0) {
+		struct sk_buff *skb = tcp_rtx_queue_head(sk);
+
+		if (skb)
+			tcp_mark_skb_lost(sk, skb);
+	}
+
 	/* If needed, reset TLP/RTO timer when RACK doesn't set. */
 	if (flag & FLAG_SET_XMIT_TIMER)
 		tcp_set_xmit_timer(sk);
-- 
2.49.0