netdev - Re: [PATCH v2 net-next 04/14] tcp: L4S ECT(1) identifier and NEEDS

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Message-ID: <476c5c79-bc37-4c41-865d-d04d1d6974c4@redhat.com>
Date: Tue, 23 Sep 2025 12:04:56 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: 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,
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 ncardwell@...gle.com, koen.de_schepper@...ia-bell-labs.com,
 g.white@...lelabs.com, ingemar.s.johansson@...csson.com,
 mirja.kuehlewind@...csson.com, cheshire@...le.com, rs.ietf@....at,
 Jason_Livingood@...cast.com, vidhi_goel@...le.com
Cc: Olivier Tilmans <olivier.tilmans@...ia.com>
Subject: Re: [PATCH v2 net-next 04/14] tcp: L4S ECT(1) identifier and
 NEEDS_ACCECN for CC modules

On 9/18/25 6:21 PM, chia-yu.chang@...ia-bell-labs.com wrote:
> From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> 
> Two CA module flags are added in this patch. First, a new CA module
> flag (TCP_CONG_NEEDS_ACCECN) defines that the CA expects to negotiate
> AccECN functionality using the ECE, CWR and AE flags in the TCP header.
> The detailed AccECN negotiaotn during the 3WHS can be found in the
> AccECN spec:
>   https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt
> 
> Second, when ECN is negociated for a TCP flow, it defaults to use
> ECT(0) in the IP header. L4S service, however, needs to se ECT(1).
> This patch enables CA to control whether ECT(0) or ECT(1) should
> be used on a per-segment basis. A new flag (TCP_CONG_WANTS_ECT_1)

I find this description confusing/contradictory with the implementation
where TCP_CONG_WANTS_ECT_1 is actually a mask.


> @@ -1322,6 +1328,18 @@ static inline bool tcp_ca_needs_ecn(const struct sock *sk)
>  	return icsk->icsk_ca_ops->flags & TCP_CONG_NEEDS_ECN;
>  }
>  
> +static inline bool tcp_ca_needs_accecn(const struct sock *sk)
> +{
> +	const struct inet_connection_sock *icsk = inet_csk(sk);
> +
> +	return icsk->icsk_ca_ops->flags & TCP_CONG_NEEDS_ACCECN;
> +}
> +
> +static inline bool tcp_ca_wants_ect_1(const struct sock *sk)
> +{
> +	return inet_csk(sk)->icsk_ca_ops->flags & TCP_CONG_WANTS_ECT_1;

Should the above tests be:

	(inet_csk(sk)->icsk_ca_ops->flags & TCP_CONG_WANTS_ECT_1) ==
TCP_CONG_WANTS_ECT_1

?

Otherwise existing CC with TCP_CONG_NEEDS_ECN will unexpectedly switch
to ECT_1 usage.

[...]
> diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
> index df758adbb445..f9efbcf1d856 100644
> --- a/net/ipv4/tcp_cong.c
> +++ b/net/ipv4/tcp_cong.c
> @@ -227,7 +227,7 @@ void tcp_assign_congestion_control(struct sock *sk)
>  
>  	memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));
>  	if (ca->flags & TCP_CONG_NEEDS_ECN)
> -		INET_ECN_xmit(sk);
> +		__INET_ECN_xmit(sk, tcp_ca_wants_ect_1(sk));

Possibly a new helper for the above statement could be useful

/P