From 3ae62b8b4a4848f745d5ed12f10056cb4e4c81f0 Mon Sep 17 00:00:00 2001 From: Chia-Yu Chang Date: Sat, 31 Jan 2026 23:25:07 +0100 Subject: [PATCH] tcp: accecn: retransmit downgraded SYN in AccECN negotiation Based on AccECN spec (RFC9768) Section 3.1.4.1, 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 Acked-by: Paolo Abeni Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260131222515.8485-8-chia-yu.chang@nokia-bell-labs.com Signed-off-by: Paolo Abeni --- 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 597e888af36d8..b28596655d73d 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -3606,12 +3606,15 @@ start: 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); -- 2.47.3