netdev - Re: TCP packet size and delivery packet decisions

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Message-ID: <1283834219.2585.455.camel@edumazet-laptop>
Date:	Tue, 07 Sep 2010 06:36:59 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	ツ Leandro Melo de Sales <leandroal@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: TCP packet size and delivery packet decisions

Le mardi 07 septembre 2010 à 01:20 -0300, ツ Leandro Melo de Sales a
écrit :
> Hi all,
>    We are facing a not expected behavior (at least for us) from the
> TCP stack under Linux. We need to write 78 bytes per writing using
> write() function. But instead of sending 78 bytes TCP implementation
> in Linux sends 48 bytes and later 30 bytes. We then started to debug
> the implementation, but although we got some progress on this, I have
> decided to ask in this list if someone can anticipate provide me more
> information why TCP is not sending 78 at once, in addition to have any
> clarification about if it is possible to force TCP sends 78 bytes in
> each write() function call.
>    I have tried many alternatives: set TCP_CORK; used socket API to
> write a fixed 78 bytes per write() function call; and so forth). Also,
> I know that TCP is a byte-stream protocol and this decision is made by
> many different factors, such as cwd size, mss and so forth, but 78
> bytes is too small to split it in two different packets. Our
> application works as expected under Windows, it sends 78 bytes at
> once.
> 
>    In this way, any comment from the list regarding this issue will be
> kindly accepted and helpful.


Could you send a tcpdump of such session, with say first 10 packets ?



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