netdev - [PATCH v2 net-next 09/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

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Message-Id: <20250918162133.111922-10-chia-yu.chang@nokia-bell-labs.com>
Date: Thu, 18 Sep 2025 18:21:28 +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 v2 net-next 09/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

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

If the TCP Server has not received an ACK to acknowledge its SYN/ACK
after the normal TCP timeout or it receives a second SYN with a
request for AccECN support, then either the SYN/ACK might just have
been lost, e.g. due to congestion, or a middlebox might be blocking
AccECN Options. To expedite connection setup in deployment scenarios
where AccECN path traversal might be problematic, the TCP Server SHOULD
retransmit the SYN/ACK, but with no AccECN Option.

If this retransmission times out, to expedite connection setup, the TCP
Server SHOULD retransmit the SYN/ACK with (AE,CWR,ECE) = (0,0,0)
and no AccECN Option, but it remains in AccECN feedback mode.

This follows Section 3.2.3.2.2 of AccECN spec (RFC9768).

Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
---
 include/net/tcp_ecn.h | 14 ++++++++++----
 net/ipv4/tcp_output.c |  2 +-
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index 7ceb191893c6..f01cac9d39c9 100644
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h
@@ -644,10 +644,16 @@ static inline void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
 static inline void
 tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
 {
-	if (tcp_rsk(req)->accecn_ok)
-		tcp_accecn_echo_syn_ect(th, tcp_rsk(req)->syn_ect_rcv);
-	else if (inet_rsk(req)->ecn_ok)
-		th->ece = 1;
+	if (req->num_retrans < 1 || req->num_timeout < 1) {
+		if (tcp_rsk(req)->accecn_ok)
+			tcp_accecn_echo_syn_ect(th, tcp_rsk(req)->syn_ect_rcv);
+		else if (inet_rsk(req)->ecn_ok)
+			th->ece = 1;
+	} else if (tcp_rsk(req)->accecn_ok) {
+		th->ae  = 0;
+		th->cwr = 0;
+		th->ece = 0;
+	}
 }
 
 static inline bool tcp_accecn_option_beacon_check(const struct sock *sk)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index bf90c2b33af3..d4aae5dc2201 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1109,7 +1109,7 @@ static unsigned int tcp_synack_options(const struct sock *sk,
 
 	if (treq->accecn_ok &&
 	    READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option) &&
-	    req->num_timeout < 1 && remaining >= TCPOLEN_ACCECN_BASE) {
+	    req->num_retrans < 1 && remaining >= TCPOLEN_ACCECN_BASE) {
 		opts->use_synack_ecn_bytes = 1;
 		remaining -= tcp_options_fit_accecn(opts, 0, remaining);
 	}
-- 
2.34.1