netdev - Re: Question about TCP delayed-ack and PUSH flag behavior

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Message-ID: <20250214171219.GA8209@1wt.eu>
Date: Fri, 14 Feb 2025 18:12:19 +0100
From: Willy Tarreau <w@....eu>
To: ??? <jaymaacton@...il.com>
Cc: netdev@...r.kernel.org
Subject: Re: Question about TCP delayed-ack and PUSH flag behavior

On Fri, Feb 14, 2025 at 12:03:17PM -0500, ??? wrote:
> Hi netdev,
> 
> When tcp stack receives a segment with PUSH flag set, does the stack
> immediately send out for the corresponding ACK with ignoring delay-ack
> timer?
> Or regardless of the PUSH flag, delay-ack is always enforced?

It depends, it's possible for the application to force a delayed ack
by setting TCP_QUICKACK to zero. This is convenient for web servers
that know they respond quickly and can merge this ack with the
response to save one packet.

Willy