netdev - Re: TCP not triggering a fast retransmit?

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Message-ID: <AANLkTimL7_h4J46-WyUYfXYX87t6tQzIn1YRrvJletWB@mail.gmail.com>
Date:	Wed, 30 Jun 2010 17:27:28 -0700
From:	Ivan Novick <novickivan@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	netdev@...r.kernel.org, jmatthews@...enplum.com,
	Tim Heath <theath@...enplum.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: TCP not triggering a fast retransmit?

On Wed, Jun 30, 2010 at 2:03 PM, Ben Hutchings
<bhutchings@...arflare.com> wrote:
> On Wed, 2010-06-30 at 11:04 -0700, Ivan Novick wrote:
>> Hello all,
>>
>> Attached is a packet capture from my application that is running on
>> RedHat Enterprise Linux 5.4
>>
>> I am seeing a Retransmission timeout but I was hoping this case would
>> go into fast retransmit and not RTO.
>>
>> I am wondering why did the sender not send more data?  If the sender
>> was to send more data and extend the window then it would seem the
>> duplicate acks or SACKS should trigger fast retransmit.
> [...]
>
> In that packet capture I see TCP payload lengths which are 2, 3 and 4
> times the usual MSS of 1448 bytes, which implies that GRO or LRO is in
> use.  In RHEL 5.4 the TCP stack does not ACK often enough in this case
> because it is missing this change:
>
> commit ff9b5e0f08cb650d113eef0c654f931c0a7ae730
> Author: Herbert Xu <herbert@...dor.apana.org.au>
> Date:   Thu Aug 31 15:11:02 2006 -0700
>
>    [TCP]: Fix rcv mss estimate for LRO

Wow, Thanks! Is this the patch you are talking about:
http://www.mail-archive.com/netdev@vger.kernel.org/msg21151.html  ?

It looks like a one liner.   I could apply it and rebuild my RHEL 5.4
to see if it helps.

Cheers,
Ivan Novick
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