netdev - Re: [PATCH] ipv6: Limit mtu to 65572 bytes

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Date:	Fri, 11 Apr 2014 04:44:42 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
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, Apr 10, 2014 at 06:42:10PM -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.
> 
> Tested:
> 
> ifconfig lo mtu 70000
> netperf -H ::1
> 
> Before patch :
> 
> -> Throughput :   0.05 Mbits
> 
> After patch :
> 
> -> Throughput : 36624.24 Mbits
> 
> Reported-by: Francois WELLENREITER <f.wellenreiter@...il.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>

Sorry about my wrong statement earlier in this thread, you're right.

Btw. this patch fixes a flaw in the UDP output path, too, where we didn't
generate EMSGSIZE error correctly before in all cases. udp_sendmsg only checks
for INT_MAX - sizeof(udphdr) and defers further checking to ip6_append_data
which seems to be prepared for jumbograms. ;)

Very cool catch with the possible ipv4 performance improvment, too. ;)

Thanks,

  Hannes

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