netdev - Re: [RFC] xfrm: fix pmtu discovery (kill xfrm6_update

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Date:	Mon, 4 Feb 2013 08:39:14 +0100
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Jiri Bohac <jbohac@...e.cz>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] xfrm: fix pmtu discovery (kill xfrm6_update_pmtu)

On Fri, Feb 01, 2013 at 06:25:29PM +0100, Jiri Bohac wrote:
> On Fri, Feb 01, 2013 at 09:45:15AM +0100, Steffen Klassert wrote:
> > 
> > Subject: [PATCH] ipv6: Don't send packet to big messages to self
> > 
> > Calling icmpv6_send() on a local message size error leads to an
> > incorrect update of the path mtu in the case when IPsec is used.
> > So use ipv6_local_error() instead to notify the socket about the
> > error.
> 
> It fixes the problem I was trying to fix.
> I tested the patch with the PMTU and IPsec parts of the ipv6ready
> testsuite and it passed.
> 
> I wonder if this is going to do everything that should be done
> e.g. in the TCPv6 case.  Now, tcp_v6_err() calls
> tcp_v6_mtu_reduced() which syncs the MSS and forces a retransmit.
> 
> Looking at the code, I don't think this is going to happen with
> this patch, so the MSS update and retransmit will only be done
> when the respective TCP timer expires.
> 
> So perhaps the individual protocols will need to handle the new
> error reporting?
> 

Well, we might loose the fast path retransmit. We could think
about implementing a fast path for such situations, but that's
another issue. Let's fix the bug first, I'll submit the patch
to fix it.

> 
> And out of curiosity, do you know the reason why
> xfrm[46]_update_pmtu changes the non-xfrm route's MTU?
> Is this really intended?
> 

The pmtu is a learned measure of the route, it depends on the
external environment, not on the question whether we add local
overhead. So I think it was intended to change always the
pmtu of the original route, even if we use IPsec.


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