netdev - Re: TCP Delayed ACK in FIN/ACK

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Message-ID: <CAFLxGvxqj+-rf5twsMPk-pRQqzqsK8-MSsKaez9cwarPmyEqzw@mail.gmail.com>
Date:	Sat, 4 Aug 2012 16:51:24 +0200
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	Sławek Janecki <janecki@...il.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: TCP Delayed ACK in FIN/ACK

On Sat, Aug 4, 2012 at 4:45 PM, Sławek Janecki <janecki@...il.com> wrote:
> I have a node.js client (10.177.62.7) requesting some data from http
> rest service from server (10.177.0.1).
> Client is simply using nodejs http.request() method (agent=false).
> Client is on Ubuntu 11.10 box.
> Why client sends FIN ACK after 475ms? Why so slow? He should send FIN
> ACK immediately.
> I have many situations like this. About 1% of whole traffic is request
> with delayed FIN ACK.
> Cpu idle on the client is about 99%, so nothing is draining CPU.
> How to debug this? What could it be? Is there any sysctl option I need to tune?
> I think this behaviour is the Delayed ACK feature of RFC1122 TCP stack.
>
> Link to tcpdump picture (done on a client machine) :
> http://i48.tinypic.com/35cpogx.png
>
> Can you tell why kernel delayed that FIN/ACK.
> In tcpflow data there is exacly one ACK per packet comming from server.
> Why kernel delayed client FIN/ACK.
> It could avoid sending ACK every 'data' packet.
> But it choose to delay FIN/ACK?
> Is this possible? Is this a bug?
>
> I've also posted question on stackexchange:
> http://stackoverflow.com/questions/11711218/client-sends-delayed-fin-ack-500ms-to-server
>
> Please help.

CC'ing netdev.

-- 
Thanks,
//richard
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