Subject: MIB Question

From: "David Allan" <[email-masked]>
Date: Fri, 2 Aug 2002 14:37:04 -0400

I've been rummaging through MTU related stuff and not that there is no-where
that record MTU discards (either MPLS MIBs or Interfaces group MIB). Agree
that for IP, it's a big yawn and probably means someone is doing MTU
discovery and is a non-problem.

Issues is PWs where there is no payload fragmentation and VPNs where the
packet payload has no provider routable addresses in it (no ICMP reply
possible). IMHO the "use a big MTU" stipulation won't hold up forever, and
some token effort to prepare for the future may be in order.    

Seems to me that it would be useful to have a counter in the LSR MIB for
non-IP MTU discards (maybe with a threshold trap although I'll let my
betters comment further) so that at least some indication of a problem was
possible as currently packets simply "black hole". I'd suggest that the
non-IP category would also include stack depth > 1 (e.g. 2547) as ideally
path MTU handling for a VPN should be a PE problem.

comments?
Dave

---

From: "Thomas D. Nadeau" <[email-masked]>
Date: Mon, 05 Aug 2002 15:50:50 -0400

[Cross-posting as this discussion has applicability to all three WGs.]

At 02:37 PM 8/2/2002 -0400, David Allan wrote:

>I've been rummaging through MTU related stuff and not that there is 
>no-where that record MTU discards (either MPLS MIBs or Interfaces group 
>MIB). Agree that for IP, it's a big yawn and probably means someone is 
>doing MTU discovery and is a non-problem.
>

         Dave,

         The MPLS-LSR MIB does contain counters that an implementation
may count MTU errors:  mplsInSegmentErrors,  mplsInSegmentDiscards,
  mplsOutSegmentErrors, and mplsOutSegmentDiscards.  However, I do
agree that MTU discards are not explicitly counted.   In the non-PW and
non-VPN cases, I have not seen any requirements to-date to add such
counters from providers, thus we have not added any.

         One question, you mention this within the context of PW/VPN. In 
the case
of PWE3, wouldn't it be  more appropriate to include MTU discards in the 
generic
PW VC counters into the PW-MIB?  For VPN, we could certainly add them into the
PPVPN-MPLS-VPN MIB explicitly as well. The only hesitation I have for adding
these to the PPVPN MIB is that to-date have not had such a requirement from 
any
providers, so I would like to get some more input from that group first.

         --Tom

------------------------------------------------------------------------
Mathematics is the supreme nostalgia of our time.