netdev - Re: TCP not triggering a fast retransmit?

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Date:	Wed, 30 Jun 2010 16:10:10 -0700
From:	Mitchell Erblich <erblichs@...thlink.net>
To:	David Miller <davem@...emloft.net>
Cc:	bhutchings@...arflare.com, novickivan@...il.com,
	netdev@...r.kernel.org, jmatthews@...enplum.com,
	theath@...enplum.com, herbert@...dor.apana.org.au
Subject: Re: TCP not triggering a fast retransmit?


On Jun 30, 2010, at 2:22 PM, David Miller wrote:

> From: Ben Hutchings <bhutchings@...arflare.com>
> Date: Wed, 30 Jun 2010 22:03:49 +0100
> 
>> 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
> 
> It certainly could be, I'll try make sure this gets rectified,
> thanks!
> --

Guys,

	I think you suggesting that:

__tcp_ack_snd_chk() within tcp_input.c needs: 
an ABC (Appropriate Byte Counting) Allman type check
where the frame rcv'd computed size is 2x or larger
(and no out of order queue) , then

even if NOT in quickack mode, needs to be ACKed,
with tcp_send_ack()

and if in quickack mode, needs a mss incr seq number of ACKs where
number of ACKs equals the number of the multiple of mss

Note: without incrementing the SEQ between ACKs, it would result in
a DupACK at the other end system,

Correct?

Mitchell Erblich


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