netdev - Re: 200 millisecond timeouts in TCP

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Date:	Thu, 3 Jun 2010 18:13:19 -0700
From:	Mitchell Erblich <erblichs@...thlink.net>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	Ivan Novick <novickivan@...il.com>, netdev@...r.kernel.org
Subject: Re: 200 millisecond timeouts in TCP


On Jun 3, 2010, at 4:10 PM, Hagen Paul Pfeifer wrote:

> * Ivan Novick | 2010-06-03 15:37:24 [-0700]:
> 
>> Using tcpdump and systemtap I am seeing that sometimes retransmission
>> of data is sent after waiting 200 milliseconds.  However sometimes
>> retransmissions happen quicker.
> 
> Quicker as 200ms? Conservatively the minimum TCP RTO should be 1s (rfc2988),
> Linux differs from this default and define the minimum RTO to 200ms:
> 
> #define TCP_RTO_MIN	((unsigned)(HZ/5))
> 
> Can you post the tcpdump traces where the relevant retransmission is recognizable?
> 
>> Also do you know if the timeout numbers for TCP are configurable parameters?
> 
> Some values are documented in Documentation/networking/ip-sysctl.txt, you can
> find the relevant timer implementation in ipv4/tcp_input.c and the definition
> of TCP_RTO_MIN in include/net/tcp.h.
> 
> 
> Hagen Paul Pfeifer
> 
> 
> -- 
> Hagen Paul Pfeifer <hagen@...u.net>  ||  http://jauu.net/
> Telephone: +49 174 5455209           ||  Key Id: 0x98350C22
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Group, from page 7 of the Linux TCP Congestion Control doc.

"Linux TCP deviates from the IETF specification by al- lowing a minimum limit of 200 ms for the RTO."

Mitchell Erblich--
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