-----Original Message----- From: Paolo Abeni pabeni@redhat.com Sent: Tuesday, November 18, 2025 2:59 PM To: Chia-Yu Chang (Nokia) chia-yu.chang@nokia-bell-labs.com; edumazet@google.com; parav@nvidia.com; linux-doc@vger.kernel.org; corbet@lwn.net; horms@kernel.org; dsahern@kernel.org; kuniyu@google.com; bpf@vger.kernel.org; netdev@vger.kernel.org; dave.taht@gmail.com; jhs@mojatatu.com; kuba@kernel.org; stephen@networkplumber.org; xiyou.wangcong@gmail.com; jiri@resnulli.us; davem@davemloft.net; andrew+netdev@lunn.ch; donald.hunter@gmail.com; ast@fiberby.net; liuhangbin@gmail.com; shuah@kernel.org; linux-kselftest@vger.kernel.org; ij@kernel.org; ncardwell@google.com; Koen De Schepper (Nokia) koen.de_schepper@nokia-bell-labs.com; g.white@cablelabs.com; ingemar.s.johansson@ericsson.com; mirja.kuehlewind@ericsson.com; cheshire cheshire@apple.com; rs.ietf@gmx.at; Jason_Livingood@comcast.com; Vidhi Goel vidhi_goel@apple.com Subject: Re: [PATCH v6 net-next 10/14] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK
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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
Hi Paolo,
You are right, and I will use both "synack_type != TCP_SYNACK_RETRANS" || "!req->num_retrans".
Because this ACE field fallback will only happen from the 2nd retansmitted SYN/ACK.
Chia-Yu