IP tunnel MTU

Mon Oct 22 22:24:24 UTC 2012

Hello,

Several months ago, there was discussion on the list regarding IP
tunnel maximum transmission unit (MTU). Since that time, it has been
brought to my attention by members of my company's network operations
staff that tunnel MTU is a very real problem they need to cope with
on a daily basis - especially with the growing need to depend on
both tunnels and tunnels-within-tunnels to track mobile devices.

Since tunnels always reduce the effective MTU seen by data packets
due to the encapsulation overhead, the only two ways to accommodate
the tunnel MTU is either through the use of path MTU discovery or
through fragmentation and reassembly. Unfortunately, both are known
to be problematic in a non-trivial number of cases.

The discussions on NANOG from back in the June timeframe resulted in
"Operational Issues with Tunnel Maximum transmission Unit":

  https://datatracker.ietf.org/doc/draft-generic-v6ops-tunmtu/

I would like to ask this group to now give this document a look and
post your comments/thoughts/experiences. For example, has the tunnel
MTU problem crept into daily operational considerations to the point
that we should now at least be documenting it and preferably trying
to do something about it? From talking to our staff, I believe the
answer is yes but it would be good to have confirmation from others.

Thanks in advance for your thoughts,

Fred
fred.l.templin at boeing.com