netdev - Re: PMTU discovery is broken on kernel 3.7.1 for UDP sockets

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Message-ID: <1355945864.2676.21.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Wed, 19 Dec 2012 19:37:44 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	"Yurij M. Plotnikov" <Yurij.Plotnikov@...etlabs.ru>
CC:	<netdev@...r.kernel.org>,
	"Alexandra N. Kossovsky" <Alexandra.Kossovsky@...etlabs.ru>
Subject: Re: PMTU discovery is broken on kernel 3.7.1 for UDP sockets

On Wed, 2012-12-19 at 18:27 +0400, Yurij M. Plotnikov wrote:
> On 12/19/12 17:35, Ben Hutchings wrote:
> > On Wed, 2012-12-19 at 17:10 +0400, Yurij M. Plotnikov wrote:
> >    
> >> On kernel 3.7.1 I get strange behaviour of IP_MTU_DISCOVER socket
> >> option. The behaviour in case of IP_PMTUDISC_DO and IP_PMTUDISC_WANT
> >> values of IP_MTU_DISCOVER socket option on SOCK_DGRAM socket are the
> >> same and packet is always sent with "Don't Fragment" bit in case of
> >> IP_PMTUDISC_WANT. Also, the value of IP_MTU socket option is not updated.
> >>      
> > You could try reverting:
> >
> > commit ee9a8f7ab2edf801b8b514c310455c94acc232f6
> > Author: Steffen Klassert<steffen.klassert@...unet.com>
> > Date:   Mon Oct 8 00:56:54 2012 +0000
> >
> >      ipv4: Don't report stale pmtu values to userspace
> >
> >      We report cached pmtu values even if they are already expired.
> >      Change this to not report these values after they are expired
> >      and fix a race in the expire time calculation, as suggested by
> >      Eric Dumazet.
> >
> > Still, PMTU information is not supposed to expire for 10 minutes...
> >
> >    
> With reverted commit there is no such problem on 3.7.1: IP_MTU is 
> updated and DF is set only for the first packet in case of 
> IP_PMTUDISC_WANT.
[...]

So it looks like something is going wrong with the expiry calculation
here.

This change shouldn't affect the PMTU actually used by the kernel, but
could affect Onload since that relies on netlink route updates to keep
in synch.  You didn't say you were using Onload, but if you are then we
should not bother netdev with this until we can demonstrate a problem
that involves only the kernel stack.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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