Subject: RE: Updated MTU Signalling Extensions for LDP

From: "David Allan" <[email-masked]>
Date: Fri, 9 Nov 2001 09:37:51 -0500

Ben:

(long winded comment)

I agree that there is potentially a problem, and you have a solution, but
(and first to conceed there is no real data to back up my suspicion) that in
any heterogeneous network, routing changes will result in a signicant load
of advertisments of MTU changes. For example, in a liberal label retention
network, previously on a routing change there would be no or minimal
additional label advertisments on a routing change. I merely query my IGP
and select the best label.

Now I have the potential of requiring a refresh of MTU size on those
advertisments I already have, which is a lot of transactions I would not
normally have received. And further, I may not care as the new MTU may still
exceed my service requirement.

I discussed the problem with a few people and in general observed/concluded:

- stuff in general punts today (note Martini/kompella L2VPN caveats)
- I only really care if the MTU drops below my service requirement (or
whatever Ive picked as my network MTU). I don;t want a lot of messages
telling me the MTU changed from "humungous value x" to "humungous value y"
- adding MTU to CSPF is overkill

and stuff considered was:
- an LDP notification of packet discard occurring, e.g. only radiate out an
MTU advertisment when it's a problem.

Somthing not entirely satisfactory as stuff is breaking before you know to
fix it.

So to me a real solution is your advertisment, coupled with some means of
establishing a network wide cut off value (above which no-one cares and
changes will not be radiated). Section 2.2 partially addresses this, but I
am still going to generate a load of messages as FECs are incrementally
discovered (and potentially new lowest values are found).  Again with no
real data to back me up, I suspect the network will naturally converge on
the lowest MTU available, which I could have saved a lot of time and
messages by merely set all ingresses to fragement to that value anyway. IMHO
I may force a lot of unnecessary framentation with this approach.

So I think this is a corner case problem, and network scalability could be
enhannced if the MTU advertisment was only generated if the path MTU dropped
below some provisioned value.

cheers
Dave

> -----Original Message-----
> From: Ben Black [mailto:[email-masked]]
> Sent: Thursday, November 08, 2001 9:02 PM
> To: [email-masked]
> Subject: Updated MTU Signalling Extensions for LDP
> 
> 
> This has been submitted, but not yet posted.
> 
> http://www.layer8.net/~ben/drafts/draft-black-ldp-mtu-extensio
> ns-01.txt
> 
> I've integrated a number of suggested changes from the original (now
> expired) version.  Please comment as appropriate so this draft can be
> moved ahead.
> 
> 
> Ben
> 
>

---

From: Ben Black <[email-masked]>
Date: Fri, 9 Nov 2001 15:05:42 -0800

Certainly, if it was desired by an operator, the MTU for the network could
be the global minimum, but there is a lot of room for policy in the draft.
As long as the resultant MTU is valid for the path in question, it really
doesn't matter how nodes pick their MTU.  

Ben