Re: [v6ops] draft-van-beijnum-multi-mtu-05

[Nick Hilliard <[e-mail]>]

Philip Homburg wrote:
> Assuming two nodes (hosts or routers) have announced such a larger MTU. And
> they have verified (using ICMP, or whatever) that is is actually possible
> to exchange packets at a bigger MTU.
> 
> Then what problems do you expect in todays networks?

for starters, multicast and broken implementations.  If there are
routers doing this negotiation, then this may change the path mtu
mid-flight, which could cause problems if the path mtu drops
unexpectedly, which it might easily if lots of things are dynamically
negotiated.  Or if the MTU negotiation takes place at the first hop and
the endpoints are left thinking that they can handle an mtu of 9200
bytes while the intermediate path might only support 1500 and the
fragmenting router is already rate limiting icmp6 toobig responses.  Or
several of the situations mentioned in rfc2923.

The issue is that the use case is not sufficiently compelling to warrant
introducing low level changes in a protocol which has been around for 20
years and has a wide deployment, where operational experience tells us
that problems relating to MTU path mismatches are a huge headache which
are difficult to debug.

Nick