netdev - Re: [PATCH v12 net-next 07/15] tcp: Add wait_third_ack for ECN negotiation in simultaneous connect

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Message-ID: <eaeaf8e5-ac94-4368-b897-538c757f4e34@redhat.com>
Date: Mon, 14 Jul 2025 15:22:47 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: chia-yu.chang@...ia-bell-labs.com, edumazet@...gle.com,
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Subject: Re: [PATCH v12 net-next 07/15] tcp: Add wait_third_ack for ECN
 negotiation in simultaneous connect

On 7/4/25 10:53 AM, chia-yu.chang@...ia-bell-labs.com wrote:
> From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> 
> In simultaneous connect scenario, the connection will be established
> when SYN/ACK is received after patch 23e89e8ee7be7. However, the
> third ACK is still anticipated to complete the negotiation for either
> RFC3168 ECN and Accurate ECN. In this sense, an additional flag
> wait_third_ack is introduced to identify that the 3rd ACK is still
> anticipated to ensure ECN or AccECN negotiation will be done in the
> ESTABLISHED state.
> 
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> Co-developed-by: Ilpo Järvinen <ij@...nel.org>
> Signed-off-by: Ilpo Järvinen <ij@...nel.org>

I saw there are existing pktdrill test for ECN with simultaneous
connect, but I'm still wondering it there is a real use case behind?

AFAICS simult connect can happen only on loopback, and [Acc]ECN on
loopback looks useless.

I would simply not allow AccECN on simultaneous connect - assuming that
would basically drop all the code in this patch without no fourther
modification required.

/P