Re: draft-van-beijnum-multi-mtu-05.txt

[Mikael Abrahamsson <[e-mail]>]

On Thu, 14 Apr 2016, David Lamparter wrote:

> That's actually the crux of it: this isn't Path MTU discovery.  The MTU 
> discovered by this mechanism is the first-hop MTU for a lot of paths in 
> case of a router and it's going to be distinct from the final PMTU 
> towards a particular destination.  Plus, state on this only ever exists 
> for direct neighbors, not for any random internet host.

I don't believe the draft says this.

What it does is discover the MTU to an IPv6 address on the same LAN. To 
then infer that any packets sent VIA this node, is not something the draft 
(at least explicitly) says. The case could be made to say that any packets 
sent via the "discovered-to-support-larger-MTU-router" could use this 
higher MTU, but that's something that isn't obvious, and I think it's up 
for discussion.

So if I have this network:

H1 - R1 - R2 - Internet

If R1 announces in RA to the LAN where H1 is, that MTU is 1500, but H1 but 
R1 also sends HintMTU of 9180, should the default route H1 installs 
based on this RA and points to R1 also have this higher MTU? Maybe. Maybe 
not.

As I said in the session, I would like more fine granular MTU, so that I 
can have separate MTU for separate entities.

For instance, if I know that most of my devices in my home support 9180, 
but none of my Internet connections do (they only support 1500), then I 
would like H1 to have this in its routing table:

Local /64 HintMTU 9180
RIO with home /56 HintMTU 9180
RIO with ::/0 with HintMTU 1500

Because of this, H1 wouldn't need to spend a lot of time trying to send 
packets to the Internet with larger MTU than 1500.

This would however require that we allow MTUs tied to not only PIO, but 
also RIO. I still don't know what to do about the implicit default route 
that one points to anyone one sees an RA from. Personally I consider this 
design of the IPv6 protocol to be unfortunate.

-- 
Mikael Abrahamsson    email: [e-mail]