IPv6 path discovery oddities - flushing the routing cache resolves

[Valentijn Sessink <valentyn@blub.net>]

Hello list,

I'm experiencing difficulties with IPv6 path discovery. The setup is 
quite simple, a machine with native IPv6, no special routing - let's 
call it the "server" for now. Unfortunately, I seem to loose 
connectivity multiple times a day - and after digging in, I found this 
to be "too big" messages that weren't honored at the server. The network 
consists of something like:

server --- hosting --- others ---- SIXXS tunnel with 1280 MTU --- me.

A "ip -6 route list cache" would show a cached route to my "client", but 
one without MTU. Then after "ip -6 route flush cache", and after trying 
to send a large packet (for example issuing "ps uaxww" on an ssh 
prompt), "ip -6 route list cache" will show a correct MTU.

But after a while, things start to go wrong again, and another "ip -6 
route flush cache" is needed.

The server is running 3.8.0-something (ubuntu 12.04 with a newer kernel).

tcpdump shows that on reception of icmpv6 "too big", nothing happens 
(i.e. the "too big" packet will be sent time and again), and after the 
"ip -6 route flush cache", suddenly the "too big" message is honored.

I saw a couple of path discovery issues on this list, more specifically 
one with the subject "IPv6 path MTU discovery broken" earlier this month 
- but I'm not sure it's the same issue (because the original submitter 
specifically mentions kernels 3.10 and 3.11 and has a much more 
complicated routing table).

The "server" just has:

auto br0
iface br0 inet6 static
   address 2a01:xxxx:xxx:xxxx::2
   netmask 64
   gateway fe80::1
   bridge_ports eth0
   bridge_maxwait 0

Nothing special, no special routes, no routing daemons.

If I can do anything to shine more light on this issue, please tell me so.

Best regards,

Valentijn