On 11/14/25 8:13 AM, 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
v6:
- Use new synack_type TCP_SYNACK_RETRANS and num_retrans.
include/net/tcp_ecn.h | 20 ++++++++++++++------ net/ipv4/tcp_output.c | 4 ++-- 2 files changed, 16 insertions(+), 8 deletions(-)
diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h index a709fb1756eb..57841dfa6705 100644 --- a/include/net/tcp_ecn.h +++ b/include/net/tcp_ecn.h @@ -649,12 +649,20 @@ 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;+tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th,
enum tcp_synack_type synack_type)+{
- // num_retrans will be incresaed after tcp_ecn_make_synack()
Please use /* */ for comments
- if (!req->num_retrans) {
It's unclear you this function receives a `synack_type` argument and don't use it. Should the above be
if (synack_type != TCP_SYNACK_RETRANS) {
?
Or just remove such argument.
/P