Re: [PATCH v2 net-next 13/14] tcp: accecn: stop sending AccECN opt when loss ACK w/ option

[Paolo Abeni <pabeni@redhat.com>]

On 9/18/25 6:21 PM, chia-yu.chang@nokia-bell-labs.com wrote:
> From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> 
> 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).

Is this really useful/triggers in practice?

AFAICS it will take effect only it the retransmission happens just after
an egress AccECN packet, i.e. will not trigger if the there are more
later non AccECN packets pending.
> 
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> ---
>  include/linux/tcp.h   | 3 ++-
>  include/net/tcp_ecn.h | 1 +
>  net/ipv4/tcp_input.c  | 9 +++++++++
>  net/ipv4/tcp_output.c | 3 +++
>  4 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 53a4a0da35cc..f3b72fb6fa76 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -295,7 +295,8 @@ 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 8317c3f279c9..3eb450b4b648 100644
> --- a/include/net/tcp_ecn.h
> +++ b/include/net/tcp_ecn.h
> @@ -401,6 +401,7 @@ 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/tcp_input.c b/net/ipv4/tcp_input.c
> index f250b4912a0b..6083260133e9 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4804,6 +4804,7 @@ 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);

Minor nit: please leave an empty line after variable declaration.

/P