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Date:	Thu, 10 Apr 2014 20:14:50 -0700
From:	Eric Dumazet <>
To:	Hannes Frederic Sowa <>
Cc:	YOSHIFUJI Hideaki <>,
	David Miller <>,
	Francois WELLENREITER <>,
Subject: Re: [PATCH] ipv6: Limit mtu to 65572 bytes

On Fri, 2014-04-11 at 04:57 +0200, Hannes Frederic Sowa wrote:
> On Fri, Apr 11, 2014 at 11:30:44AM +0900, YOSHIFUJI Hideaki wrote:
> > Eric Dumazet wrote:
> > 
> > > 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.
> > 
> > It is a TCP issue.  We should not limit the mtu to 65532+40.
> > I am for 65535+40. Otherwise, other protocol such as UDP cannot
> > use full mtu as before.
> I have not seen problems with max ipv6 mtu limit of 65535+40 and tcp.
> I agree this would be a better approach, and maybe & ~3 the mtu/mss
> in tcp code? I assume people expect maximum udp packet sizes working
> over loopback?

Yes, recent Intel cpus have really fast mem copy, even with different
alignments for source/dest.

But old cpus or other arches might have an issue here. Apparently nobody
really cares, so I wont argue and send a V2.

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