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

-----Original Message-----
From: Eric Dumazet <edumazet@google.com> 
Sent: Tuesday, January 20, 2026 12:05 PM
To: Chia-Yu Chang (Nokia) <redacted>
Cc: pabeni@redhat.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) <redacted>; g.white@cablelabs.com; ingemar.s.johansson@ericsson.com; mirja.kuehlewind@ericsson.com; cheshire <redacted>; rs.ietf@gmx.at; Jason_Livingood@comcast.com; Vidhi Goel <redacted>
Subject: Re: [PATCH v9 net-next 10/15] tcp: accecn: unset ECT if receive or send ACE=0 in AccECN negotiaion


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 Mon, Jan 19, 2026 at 7:59 PM [off-list ref] wrote:

quoted

From: Chia-Yu Chang <redacted>

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 <redacted>

---
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.

Hi Eric,

Thanks for the feedback.
Do you mean sk_listener here is read-only despite there is no const here?

Then, could you help to suggest the way please?
Beacuse for AccECN, here we need to set fail flag after retransmitting SYN/ACK > 1 time.
And this was done within tcp_make_synack(), but now move to every place where could retransmit SYN/ACK.

Thanks.
Chia-Yu