netdev - RE: [PATCH v4 net-next 08/13] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

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Message-ID:
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Date: Sat, 18 Oct 2025 16:06:58 +0000
From: "Chia-Yu Chang (Nokia)" <chia-yu.chang@...ia-bell-labs.com>
To: Paolo Abeni <pabeni@...hat.com>, "edumazet@...gle.com"
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Subject: RE: [PATCH v4 net-next 08/13] tcp: accecn: retransmit SYN/ACK without
 AccECN option or non-AccECN SYN/ACK

> -----Original Message-----
> From: Paolo Abeni <pabeni@...hat.com> 
> Sent: Thursday, October 16, 2025 11:14 AM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@...ia-bell-labs.com>; edumazet@...gle.com; linux-doc@...r.kernel.org; corbet@....net; horms@...nel.org; dsahern@...nel.org; kuniyu@...zon.com; bpf@...r.kernel.org; netdev@...r.kernel.org; dave.taht@...il.com; jhs@...atatu.com; kuba@...nel.org; stephen@...workplumber.org; xiyou.wangcong@...il.com; jiri@...nulli.us; davem@...emloft.net; andrew+netdev@...n.ch; donald.hunter@...il.com; ast@...erby.net; liuhangbin@...il.com; shuah@...nel.org; linux-kselftest@...r.kernel.org; ij@...nel.org; ncardwell@...gle.com; Koen De Schepper (Nokia) <koen.de_schepper@...ia-bell-labs.com>; g.white@...lelabs.com; ingemar.s.johansson@...csson.com; mirja.kuehlewind@...csson.com; cheshire <cheshire@...le.com>; rs.ietf@....at; Jason_Livingood@...cast.com; Vidhi Goel <vidhi_goel@...le.com>
> Subject: Re: [PATCH v4 net-next 08/13] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK
> 
> 
> CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.
> 
> 
> 
> On 10/13/25 7:03 PM, chia-yu.chang@...ia-bell-labs.com wrote:
> > From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> >
> > If the TCP Server has not received an ACK to acknowledge its SYN/ACK 
> > after the normal TCP timeout or it receives a second SYN with a 
> > request for AccECN support, then either the SYN/ACK might just have 
> > been lost, e.g. due to congestion, or a middlebox might be blocking 
> > AccECN Options. To expedite connection setup in deployment scenarios 
> > where AccECN path traversal might be problematic, the TCP Server 
> > SHOULD retransmit the SYN/ACK, but with no AccECN Option.
> >
> > If this retransmission times out, to expedite connection setup, the 
> > TCP Server SHOULD retransmit the SYN/ACK with (AE,CWR,ECE) = (0,0,0) 
> > and no AccECN Option, but it remains in AccECN feedback mode.
> >
> > This follows Section 3.2.3.2.2 of AccECN spec (RFC9768).
> >
> > Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-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..97a3a7f36aff 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 < 1 || req->num_timeout < 1) {
> 
> I think the above condition does not match the commit message. Should be:
>         if (!req->num_retrans && !req->num_timeout) {
>
> /P

Hi Paolo,

This patch includes two differetn SYN/ACK retransmissions:
In the first retransmited SYN/ACK, the retransmitted SYN/ACK will not include AccECN option.
This uses the condition of "req->num_retrans >1" in tcp_synack_options().

In the second retransmitted SYN/ACK, the retransmitted SYN/ACK will further set ACE=0.
This uses the condition of "req->num_retrans>1 && req->num_timeout>1" in tcp_ecn_make_synack().

I was thinking, in the next version, I could update the commit message to clarify it.

Chia-Yu