netdev - PMTU problem with >2.6.38 and IPIP tunnels

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Date:	Sat, 23 Jul 2011 21:40:49 -0400
From:	Dan Siemon <dan@...erfire.com>
To:	netdev@...r.kernel.org
Subject: PMTU problem with >2.6.38 and IPIP tunnels

I'm running into some MTU related problems with kernels newer than
2.6.38. I've tried bisecting and it appears the problem was introduced
between 2.6.39rc2 and rc3 but I'm not 100% certain because the problem
does not always present itself immediately. I am certain this does not
occur with 2.6.38.8.

I tunnel all of my home network traffic over an IPIP tunnel. My Internet
access is DSL based so there's a PPPoE connection as well. Both ends of
the IPIP tunnel are Linux.

When the gateway is first booted with the problem kernel everything is
fine. Using 'show ip route cache' I see MTU entries with the correct
values. After a short period the MTU on all the routes starts dropping
in what often appears to be 20 byte increments.

show ip route cache | egrep -i mtu
cache <src-direct>  expires 435sec ipid 0x28de mtu 712 iif eth1
cache <src-direct>  expires 542sec ipid 0x66e8 mtu 672 iif eth1

Until eventually all routes show a MTU of 552.

I'd be happy to run tests if anyone has an idea of what is causing this.

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