netdev - Re: TCP fast retransmit

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Message-ID: <CAK6E8=dcG3mw8r4FXt62G-0uoRNa9tcyQL3DS62MYe8F=9R6Nw@mail.gmail.com>
Date:	Wed, 14 Dec 2011 11:00:56 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Esztermann, Ansgar" <Ansgar.Esztermann@...-bpc.mpg.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: TCP fast retransmit

On Fri, Dec 9, 2011 at 6:43 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le vendredi 09 décembre 2011 à 14:34 +0100, Esztermann, Ansgar a écrit :
>> On Nov 25, 2011, at 17:36 , Eric Dumazet wrote:
>>
>> > Could you send a sample pcap of such problem, but please include full
>> > tcp sesssion, from the first SYN packet, up to packets following
>> > restransmits.
>>
>> OK, I've got a dump now. It is rather large (>300MB), so it's probably not a good idea to send it to the list. Instead, you can find it here:
>> http://wwwuser.gwdg.de/~aeszter/tcpstream.pcap
>>
>> The capture has been taken in the sender, 10.208.9.87, with a capture filter on the receiver's IP address. OS is:
>> % uname -a
>> Linux mwolf 2.6.37.6-0.9-default #1 SMP 2011-10-19 22:33:27 +0200 x86_64 x86_64 x86_64 GNU/Linux
>>
>> The first "strange" retransmission is in frame 166859, following ACKs in frames 166849 .. 166858.
>>
>> If I can do anything to reduce the amount of data, I will of course do so.
>>
>> > A diff of "netstat -s" taken before your session and after your session
>> > on receiver would help too, if receiver is not a loaded machine of
>> > course.
>>
>> Attached.
>>
>> Thanks a lot,
>>
>> A.
>>
>
> It seems you have a lot of packet reorders.
>
I use tcptrace to check the time sequence and I am puzzled:
I see a lot of OOO packets too but how can this happen at a sender-side trace?
unless the trace is taken close to but not exactly at the sender.
I expect on seeing in-sequence packets but a lots of SACKs plus some
spurious retransmists.
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