[PATCH v5 net-next 10/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK
From: <hidden>
Date: 2025-10-29 08:06:42
Also in:
bpf, linux-doc, linux-kselftest
Subsystem:
networking [general], networking [tcp], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Neal Cardwell, Linus Torvalds
From: Chia-Yu Chang <redacted> 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 <redacted> --- 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); }
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2.34.1