netdev - [PATCH v5 net-next 10/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

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Message-Id: <20251030143435.13003-11-chia-yu.chang@nokia-bell-labs.com>
Date: Thu, 30 Oct 2025 15:34:31 +0100
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 v5 net-next 10/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

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

For Accurate ECN, the first SYN/ACK sent by the TCP server shall set the
ACE flag (see Table 1 of RFC9768) and the AccECN option to complete the
capability negotiation. However, if the TCP server needs to retransmit such
a SYN/ACK (for example, because it did not receive an ACK acknowledging its
SYN/ACK, or received a second SYN requesting AccECN support), the TCP server
retransmits the SYN/ACK without the AccECN option. This is because the
SYN/ACK may be lost due to congestion, or a middlebox may block the AccECN
option. Furthermore, if this retransmission also times out, to expedite
connection establishment, the TCP server should retransmit the SYN/ACK with
(AE,CWR,ECE) = (0,0,0) and without the AccECN option, while maintaining
AccECN feedback mode.

This complies with Section 3.2.3.2.2 of the AccECN specification (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 c66f0d944e1c..99d095ed01b3 100644
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h
@@ -651,10 +651,16 @@ 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 || !req->num_timeout) {
+		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 90a71556b93c..37c04da4cfb1 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