RE: [PATCH v10 net-next 12/15] tcp: accecn: detect loss ACK w/ AccECN option and add TCP_ACCECN_OPTION_PERSIST

-----Original Message-----
From: Eric Dumazet <edumazet@google.com> 
Sent: Wednesday, January 21, 2026 3:54 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 v10 net-next 12/15] tcp: accecn: detect loss ACK w/ AccECN option and add TCP_ACCECN_OPTION_PERSIST


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 Wed, Jan 21, 2026 at 3:32 PM [off-list ref] wrote:

quoted

From: Chia-Yu Chang <redacted>

Detect spurious retransmission of a previously sent ACK carrying the 
AccECN option after the second retransmission. Since this might be 
caused by the middlebox dropping ACK with options it does not 
recognize, disable the sending of the AccECN option in all subsequent 
ACKs. This patch follows Section 3.2.3.2.2 of AccECN spec (RFC9768).

Also, a new AccECN option sending mode is added to tcp_ecn_option sysctl:
(TCP_ECN_OPTION_PERSIST), which ignores the AccECN fallback policy and 
persistently sends AccECN option once it fits into TCP option space.

Signed-off-by: Chia-Yu Chang <redacted>
Acked-by: Paolo Abeni <pabeni@redhat.com>

---
v5:
- Add empty line between variable declarations and code
---
 Documentation/networking/ip-sysctl.rst |  4 +++-
 include/linux/tcp.h                    |  3 ++-
 include/net/tcp_ecn.h                  |  2 ++
 net/ipv4/sysctl_net_ipv4.c             |  2 +-
 net/ipv4/tcp_input.c                   | 10 ++++++++++
 net/ipv4/tcp_output.c                  |  7 ++++++-
 6 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.rst 

b/Documentation/networking/ip-sysctl.rst
index bc9a01606daf..28c7e4f5ecf9 100644

--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst

@@ -482,7 +482,9 @@ tcp_ecn_option - INTEGER
        1 Send AccECN option sparingly according to the minimum option
          rules outlined in draft-ietf-tcpm-accurate-ecn.
        2 Send AccECN option on every packet whenever it fits into TCP
-         option space.
+         option space except when AccECN fallback is triggered.
+       3 Send AccECN option on every packet whenever it fits into TCP
+         option space even when AccECN fallback is triggered.
        = ============================================================

        Default: 2

diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 

683f38362977..32b031d09294 100644

--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h

@@ -294,7 +294,8 @@ struct tcp_sock {
        u8      nonagle     : 4,/* Disable Nagle algorithm?             */
                rate_app_limited:1;  /* rate_{delivered,interval_us} limited? */
        u8      received_ce_pending:4, /* Not yet transmit cnt of received_ce */
-               unused2:4;
+               accecn_opt_sent:1,/* Sent AccECN option in previous ACK */
+               unused2:3;
        u8      accecn_minlen:2,/* Minimum length of AccECN option sent */
                est_ecnfield:2,/* ECN field for AccECN delivered estimates */
                accecn_opt_demand:2,/* Demand AccECN option for n next 

ACKs */ diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h 
index bf7d3f9f22c7..41b593ece1dd 100644

--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h

@@ -29,6 +29,7 @@ enum tcp_accecn_option {
        TCP_ACCECN_OPTION_DISABLED = 0,
        TCP_ACCECN_OPTION_MINIMUM = 1,
        TCP_ACCECN_OPTION_FULL = 2,
+       TCP_ACCECN_OPTION_PERSIST = 3,
 };

 /* Apply either ECT(0) or ECT(1) based on TCP_CONG_ECT_1_NEGOTIATION 

flag */ @@ -413,6 +414,7 @@ static inline void tcp_accecn_init_counters(struct tcp_sock *tp)
        tp->received_ce_pending = 0;
        __tcp_accecn_init_bytes_counters(tp->received_ecn_bytes);
        __tcp_accecn_init_bytes_counters(tp->delivered_ecn_bytes);
+       tp->accecn_opt_sent = 0;
        tp->accecn_minlen = 0;
        tp->accecn_opt_demand = 0;
        tp->est_ecnfield = 0;

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c 
index a1a50a5c80dc..385b5b986d23 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c

@@ -749,7 +749,7 @@ static struct ctl_table ipv4_net_table[] = {
                .mode           = 0644,
                .proc_handler   = proc_dou8vec_minmax,
                .extra1         = SYSCTL_ZERO,
-               .extra2         = SYSCTL_TWO,
+               .extra2         = SYSCTL_THREE,
        },
        {
                .procname       = "tcp_ecn_option_beacon",

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 

8b774019a3a6..472bd57913ae 100644

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -4863,6 +4863,8 @@ static void tcp_dsack_extend(struct sock *sk, 

u32 seq, u32 end_seq)

 static void tcp_rcv_spurious_retrans(struct sock *sk, const struct 
sk_buff *skb)  {
+       struct tcp_sock *tp = tcp_sk(sk);
+
        /* When the ACK path fails or drops most ACKs, the sender would
         * timeout and spuriously retransmit the same segment repeatedly.
         * If it seems our ACKs are not reaching the other side, @@ 
-4882,6 +4884,14 @@ static void tcp_rcv_spurious_retrans(struct sock *sk, const struct sk_buff *skb)
        /* Save last flowlabel after a spurious retrans. */
        tcp_save_lrcv_flowlabel(sk, skb);  #endif
+       /* Check DSACK info to detect that the previous ACK carrying the
+        * AccECN option was lost after the second retransmision, and then
+        * stop sending AccECN option in all subsequent ACKs.
+        */
+       if (tcp_ecn_mode_accecn(tp) &&
+           TCP_SKB_CB(skb)->seq == tp->duplicate_sack[0].start_seq &&
+           tp->accecn_opt_sent)
+               tcp_accecn_fail_mode_set(tp, 
+ TCP_ACCECN_OPT_FAIL_SEND);
 }

tcp_rcv_spurious_retrans() has two callers.

tcp_send_dupack() checked dsack is enabled.

tcp_data_queue() : No such check.

So I wonder if tp->duplicate_sack[0].start_seq could contain garbage ?

Perhaps test tp->rx_opt.dsack ?

Hi Eric,

Looks like tp->rx_opt.dsack could be erased in tcp_output.c after sending out TCP options.
In v11, "accecn_opt_sent_w_dsack" is added to identify whether AccECN option is sent together with D-SACK.
And I think it can be used to justify whether duplicate_sack[0].start_seq includes ok value to be checked.

Thanks.
Chia-Yu