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Date:	Thu, 10 Apr 2014 18:58:45 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	David Miller <davem@...emloft.net>,
	Francois WELLENREITER <f.wellenreiter@...il.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH] ipv6: Limit mtu to 65572 bytes

On Thu, 2014-04-10 at 18:42 -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> Francois reported that setting big mtu on loopback device could prevent
> tcp sessions making progress.
> 
> We do not support (yet ?) IPv6 Jumbograms and cook corrupted packets.
> 
> We must limit the IPv6 MTU to (65535 + 40) bytes in theory.
> 
> In practice, its better to align to a multiple of 4 for optimal TCP
> performance.

Same story for IPv4 btw : current lo mtu=65536 is suboptimal


lpq83:~# ifconfig lo mtu 65536
lpq83:~# ./netperf 
MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
localhost () port 0 AF_INET
Recv   Send    Send                          
Socket Socket  Message  Elapsed              
Size   Size    Size     Time     Throughput  
bytes  bytes   bytes    secs.    10^6bits/sec  

 87380  16384  16384    10.00    33020.76   


lpq83:~# ifconfig lo mtu 65533
lpq83:~# ./netperf 
MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
localhost () port 0 AF_INET
Recv   Send    Send                          
Socket Socket  Message  Elapsed              
Size   Size    Size     Time     Throughput  
bytes  bytes   bytes    secs.    10^6bits/sec  

 87380  16384  16384    10.00    39010.09   


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