On 10/30/25 3:34 PM, chia-yu.chang@nokia-bell-labs.com wrote:
From: Chia-Yu Chang chia-yu.chang@nokia-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@nokia-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) {
Why `if (!req->num_timeout)` is not a sufficient condition here?
Simplifying the above condition will make the TCP_SYNACK_RETRANS alternative simpler, I think.
/P