netdev - RE: Possible IPv6 fragmentation/pmtud bug

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Date:	Thu, 5 Dec 2013 20:13:25 +0000
From:	"Templin, Fred L" <Fred.L.Templin@...ing.com>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: Possible IPv6 fragmentation/pmtud bug

Another clue that I should have mentioned in my initial message; the
"hosts", "routers" and "links" are not physical nodes but rather
network namespace virtual nodes running within a single linux 3.10.17
emulation server. Could there be some sort of weird "cross-talk"
between VMs that is making the router believe it is a host?

Thanks - Fred
fred.l.templin@...ing.com 

> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On Behalf Of Templin, Fred L
> Sent: Thursday, December 05, 2013 11:40 AM
> To: netdev@...r.kernel.org
> Subject: Possible IPv6 fragmentation/pmtud bug
> 
> Hi,
> 
> I have the following test network configuration (all linux nodes
> running the 3.10.17 kernel):
> 
> 
> h1 <-----------> r1 <-----------> r2 <-----------> h2
>    8000 MTU link    4000 MTU link    8000 MTU link
> 
> When host h1 ping6's host h2, routers r1 and r2 forward the ICMPv6
> echo request/reply packets as expected. But, when I set a ping6
> packet size that is larger than the router-to-router link MTU:
> 
>   h1# ping6 -s 5000 h2
> 
> router r1 does NOT send an ICMPv6 Packet Too Big message back
> to host h1 but rather fragments the ICMPv6 echo request packets
> itself before forwarding on to router r2 then ultimately host h1.
> The ping6's still succeed because of IPv6 fragmentation, but it
> is *router r1* that is doing the fragmentation instead of host h1.
> 
> This seems to be related to the setting of the "skb->local_df" flag,
> which should only be set for locally-generated packets; not for
> packets that are being forwarded by a router. Does anyone have any
> idea as to why my router is performing fragmentation when it forwards
> an ICMPv6 packet?
> 
> Thanks - Fred
> fred.l.templin@...ing.com
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