netdev - Re: TCP throughput drops sharply around MTU of 180 bytes

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Message-ID: <4AFE6FF0.3030101@gmail.com>
Date:	Sat, 14 Nov 2009 09:53:04 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ang Way Chuang <wcang79@...il.com>
CC:	netdev@...r.kernel.org
Subject: Re: TCP throughput drops sharply around MTU of 180 bytes

Ang Way Chuang a écrit :
> Hi kernel networking gurus,
> 
>       I came across a situation where I need to evaluate the packing
> efficiency of
> small packets. Because TCP is byte oriented protocol, there is no way
> to force it
> to generate small packets, so I forced it by adjusting MTU.
> 
>      However, through the experiment, I found that measured throughput
> of TCP stream dropped significantly if MTU is set to value less than 180 bytes.
> At first, I thought it must be some bugs on my code. So, I decided to repeat
> the test over a dedicated 10Mbps Ethernet link. The results that I measured
> over Ethernet is shown below:
> 
> 
> MTU                 Throughput (Mbps)
> -----                  --------------
> 181                  4.50
> 180                  4.39
> 179                  3.05
> 178                  3.04
> 
>      I used iperf throughout the tests. Can someone enlighten me on this matter?
> Why the throughput drops sharply if MTU is less than 180 bytes? Is there some
> special meaning associated with 180 that I don't know of?
> 
>    Some basic information about my system:
> kernel version: 2.6.27.5-117.fc10.x86_64
> distro: fedora 10
> CPU: Pentium 4 3.00 GHz
> 
>     Should you need more information, I shall gladly cooperate. Please keep me
> in the loop because it is hard for me to cop with the volume of emails on netdev
> mailing list.
> 

Oh well, this reminds me TCP_MAXSEG doesnt work as expected...
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