netdev - Re: [PATCH v9 net-next 10/15] tcp: accecn: unset ECT if receive or send ACE=0 in AccECN negotiaion

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Message-ID: <CANn89i+NEyZ+1R1pouUcroarCfNrQEN01azsEhOuZoeR0Y3mhA@mail.gmail.com>
Date: Tue, 20 Jan 2026 12:04:47 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: chia-yu.chang@...ia-bell-labs.com
Cc: pabeni@...hat.com, parav@...dia.com, linux-doc@...r.kernel.org, 
	corbet@....net, horms@...nel.org, dsahern@...nel.org, kuniyu@...gle.com, 
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	mirja.kuehlewind@...csson.com, cheshire@...le.com, rs.ietf@....at, 
	Jason_Livingood@...cast.com, vidhi_goel@...le.com
Subject: Re: [PATCH v9 net-next 10/15] tcp: accecn: unset ECT if receive or
 send ACE=0 in AccECN negotiaion

On Mon, Jan 19, 2026 at 7:59 PM <chia-yu.chang@...ia-bell-labs.com> wrote:
>
> From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
>
> Based on specification:
>   https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt
>
> Based on Section 3.1.5 of AccECN spec (RFC9768), a TCP Server in
> AccECN mode MUST NOT set ECT on any packet for the rest of the connection,
> if it has received or sent at least one valid SYN or Acceptable SYN/ACK
> with (AE,CWR,ECE) = (0,0,0) during the handshake.
>
> In addition, a host in AccECN mode that is feeding back the IP-ECN
> field on a SYN or SYN/ACK MUST feed back the IP-ECN field on the
> latest valid SYN or acceptable SYN/ACK to arrive.
>
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
>
> ---
> v8:
> - Add new helper function tcp_accecn_ace_fail_send_set_retrans()
>
> v6:
> - Do not cast const struct request_sock into struct request_sock
> - Set tcp_accecn_fail_mode after calling tcp_rtx_synack().
> ---
>  include/net/tcp_ecn.h           |  7 +++++++
>  net/ipv4/inet_connection_sock.c |  3 +++
>  net/ipv4/tcp_input.c            |  2 ++
>  net/ipv4/tcp_minisocks.c        | 36 ++++++++++++++++++++++++---------
>  net/ipv4/tcp_output.c           |  3 ++-
>  net/ipv4/tcp_timer.c            |  2 ++
>  6 files changed, 42 insertions(+), 11 deletions(-)
>
> diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
> index 796c613b5ef3..f5e1f6b1bec3 100644
> --- a/include/net/tcp_ecn.h
> +++ b/include/net/tcp_ecn.h
> @@ -97,6 +97,13 @@ static inline void tcp_accecn_fail_mode_set(struct tcp_sock *tp, u8 mode)
>         tp->accecn_fail_mode |= mode;
>  }
>
> +static inline void tcp_accecn_ace_fail_send_set_retrans(struct request_sock *req,
> +                                                       struct tcp_sock *tp)
> +{
> +       if (req->num_retrans > 1 && tcp_rsk(req)->accecn_ok)
> +               tcp_accecn_fail_mode_set(tp, TCP_ACCECN_ACE_FAIL_SEND);
> +}
> +
>  #define TCP_ACCECN_OPT_NOT_SEEN                0x0
>  #define TCP_ACCECN_OPT_EMPTY_SEEN      0x1
>  #define TCP_ACCECN_OPT_COUNTER_SEEN    0x2
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index 97d57c52b9ad..9d16cb9c3db4 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -20,6 +20,7 @@
>  #include <net/tcp_states.h>
>  #include <net/xfrm.h>
>  #include <net/tcp.h>
> +#include <net/tcp_ecn.h>
>  #include <net/sock_reuseport.h>
>  #include <net/addrconf.h>
>
> @@ -1103,6 +1104,8 @@ static void reqsk_timer_handler(struct timer_list *t)
>             (!resend ||
>              !tcp_rtx_synack(sk_listener, req) ||
>              inet_rsk(req)->acked)) {
> +               tcp_accecn_ace_fail_send_set_retrans(req,
> +                                                    tcp_sk(sk_listener));


Ouch.

I think you missed the fact that a listener is shared by many SYN_RECV requests.

Consider it as read-only here.