netdev - Re: Does IPv6 support Jumbograms ?

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Date:	Thu, 10 Apr 2014 15:28:42 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Francois WELLENREITER <f.wellenreiter@...il.com>,
	netdev@...r.kernel.org
Subject: Re: Does IPv6 support Jumbograms ?

On Wed, Apr 09, 2014 at 05:36:10PM -0700, Eric Dumazet wrote:
> On Thu, 2014-04-10 at 00:35 +0200, Hannes Frederic Sowa wrote:
> > On Wed, Apr 09, 2014 at 02:41:05PM -0700, Eric Dumazet wrote:
> > > What do you mean by blocked ?
> > > 
> > > Please give more details (kernel version, exact mtu...), because it
> > > should not happen !
> > > 
> > > # ifconfig lo mtu 100000
> > > # scp -6 vmlinux edumazet@...-localhost:/tmp
> > > Password: 
> > > vmlinux
> > > 100%   24MB  23.5MB/s   00:00    
> > > # ls -l /tmp/vmlinux
> > > -rwxr-xr-x 1 edumazet eng 24668915 Apr  9 14:37 /tmp/vmlinux
> > 
> > I couldn't test it on my development system with a recent net-next kernel
> > yet, but on my laptop with a distribution 3.13.9 kernel this happened too.
> 
> Oh well, it seems ip6_mtu() needs to cap mtu to max mtu for non
> jumbograms...
> 
> -	return mtu;
> +	return min_t(unsigned int, mtu, IP6_MAX_MTU);
> 
> #define IP6_MAX_MTU (65535 + 40)

I thought ip6_default_advmss will deal with this which correctly limits the
mtu. Capping dst_mtu would make it harder to implement jumbograms some day.

I'll look into it, thank you.

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