netdev - Re: 200 millisecond timeouts in TCP

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Message-ID: <20100603231002.GG6914@nuttenaction>
Date:	Fri, 4 Jun 2010 01:10:02 +0200
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Ivan Novick <novickivan@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: 200 millisecond timeouts in TCP

* 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


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