RE: [pmtud] I-D ACTION:draft-savola-mtufrag-network-tunneling-00.txt(fwd)

[Pekka Savola <pekkas at netcore.fi>]

On Fri, 18 Jun 2004 Mika.Joutsenvirta at nokia.com wrote:
> this is the statement I've been looking for. In most of tunneling
> descriptions this statement is left out (should be common understanding 
> I guess). With this restriction there shouldn't be problems.

I added as an appendix:

        <section title="MTU of the Tunnel">
                <t>Different tunneling mechanisms may treat the tunnel
links as having different kind of MTU values.  Some might use the same
default MTU as for other interfaces; some others might use the default
MTU minus the expected IP overhead (e.g., 20, 28, or 40 bytes); some
others might even treat the tunnel as having "indefinite MTU", e.g.,
64 kilobytes.</t>
                <t>As <xref target="I-D.ietf-v6ops-mech-v2"/>
describes, having an indefinite MTU, i.e., fragmenting the outer
packet (and never the inner packet) and never performing PMTUD is a
very bad idea, especially in host-to-router scenarios. (It could be
argued that if the nodes are sure that this is a host-to-host tunnel,
a larger MTU might make sense if fragmentation and reassembly is more
efficient than just sending properly sized packets -- but this seems
like a stretch.)
        </section>

And a referring note in section 3.2:

                <t>Note that this only works if the MTU of the tunnel
is of reasonable size, e.g., 1480 bytes: see <xref
target="tunnel-mtu"/> for more.</t>

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings


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