netdev - Re: [PATCH v1 net-next 1/4] tcp: Make TFO client fallback behaviour consistent.

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Message-ID: <f93076da-4df7-4e02-9d57-30e9b19b3608@wizmail.org>
Date: Sat, 18 Oct 2025 19:41:45 +0100
From: Jeremy Harris <jgh@...mail.org>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: Neal Cardwell <ncardwell@...gle.com>,
 "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 Yuchung Cheng <ycheng@...gle.com>, Willem de Bruijn <willemb@...gle.com>,
 Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org,
 Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v1 net-next 1/4] tcp: Make TFO client fallback behaviour
 consistent.

On 2025/10/16 5:10 PM, Eric Dumazet wrote:
> On Wed, Oct 15, 2025 at 9:02 PM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
>>
>> In tcp_send_syn_data(), the TCP Fast Open client could give up
>> embedding payload into SYN, but the behaviour is inconsistent.
>>
>>    1. Send a bare SYN with TFO request (option w/o cookie)
>>    2. Send a bare SYN with TFO cookie
>>
>> When the client does not have a valid cookie, a bare SYN is
>> sent with the TFO option without a cookie.
>>
>> When sendmsg(MSG_FASTOPEN) is called with zero payload and the
>> client has a valid cookie, a bare SYN is sent with the TFO
>> cookie, which is confusing.

> I am unsure. Some applications could break ?
> 
> They might prime the cookie cache initiating a TCP flow with no payload,
> so that later at critical times then can save one RTT at their
> connection establishment.

In addition, a client doing this (SYN with cookie but no data) is granting
permission for the server to respond with data on the SYN,ACK (before
3rd-ACK).

-- 
Cheers,
   Jeremy