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tcp: accecn: retransmit downgraded SYN in AccECN negotiation

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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 <chia-yu.chang@nokia-bell-labs.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260131222515.8485-8-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Chia-Yu Chang 2026-01-31 23:25:07 +01:00 committed by Paolo Abeni
parent c5ff6b8371
commit 3ae62b8b4a
1 changed files with 9 additions and 6 deletions
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15
net/ipv4/tcp_output.c
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@ -3606,12 +3606,15 @@ 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);