netdev - [PATCH v3 net-next 06/12] tcp: accecn: retransmit downgraded SYN in AccECN negotiation

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Message-Id: <20250927084803.17784-7-chia-yu.chang@nokia-bell-labs.com>
Date: Sat, 27 Sep 2025 10:47:57 +0200
From: chia-yu.chang@...ia-bell-labs.com
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Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v3 net-next 06/12] tcp: accecn: retransmit downgraded SYN in AccECN negotiation

From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

Based on AccECN spec (RFC9768), if the sender of an AccECN SYN
(the TCP Client) times out before receiving the SYN/ACK, it SHOULD
attempt to negotiate the use of AccECN at least one more time by
continuing to set all three TCP ECN flags (AE,CWR,ECE) = (1,1,1) on
the first retransmitted SYN (using the usual retransmission time-outs).

If this first retransmission also fails to be acknowledged, in
deployment scenarios where AccECN path traversal might be problematic,
the TCP Client SHOULD send subsequent retransmissions of the SYN with
the three TCP-ECN flags cleared (AE,CWR,ECE) = (0,0,0).

Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 net/ipv4/tcp_output.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c7b884f173d0..6e6fea8f295b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3563,12 +3563,15 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 			tcp_retrans_try_collapse(sk, skb, avail_wnd);
 	}
 
-	/* RFC3168, section 6.1.1.1. ECN fallback
-	 * As AccECN uses the same SYN flags (+ AE), this check covers both
-	 * cases.
-	 */
-	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
-		tcp_ecn_clear_syn(sk, skb);
+	if (!tcp_ecn_mode_pending(tp) || icsk->icsk_retransmits > 1) {
+		/* RFC3168, section 6.1.1.1. ECN fallback
+		 * As AccECN uses the same SYN flags (+ AE), this check
+		 * covers both cases.
+		 */
+		if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) ==
+		    TCPHDR_SYN_ECN)
+			tcp_ecn_clear_syn(sk, skb);
+	}
 
 	/* Update global and local TCP statistics. */
 	segs = tcp_skb_pcount(skb);
-- 
2.34.1