Subject: Re: [PWE3] MPLS PID

From: Curtis Villamizar <[email-masked]>
Date: Wed, 02 Apr 2003 09:26:07 -0500

In message <[email-masked]>, "
David Allan" writes:
> 
> If the goal is that ECMP trumps the MPLS architecture, we have a few things
> to fix, not just the PID. If the goal is that MPLS must be backwards
> compliant with deployed or may be deployed ECMP gear making PW 1st nybbles
> == 0 is only a start. 

Dave,

I never knew that MPLS architecture was a religion.  I always thought
MPLS was deployed and because it solved real world practical problems.

Requiring the martinni control word would seem like a no brainer.
However maybe we don't buy into that for some **technical** reason,
perhaps overhead on the control word in the edge of the network makes
a difference.  If so, then the control word must be added along the
way if the traffic type is known.

The only truly deterministic way to know the traffic type is if L3PID
tells you.  The two common values are IPv4 and MPLS.  MPLS essentially
means "don't know".  The issue comes up when hierarchical LSPs contain
more than one LSP of different type and currently the only choice is
MPLS.  If we add L3PID types of "IP or PWE3 contains control word" or
"PWE3 without control word" a control word of "unknown type" can be
added at the PSC tunnel ingress and stripped at the PSC tunnel egress.
This adding and removing a control word would be a painful solution to
swallow but it would be the cleanest solution and would provide *both*
low overhead on the edge *and* load split (ECMP) in the core without
reordering non-IP flows.

Some people want to depricate L3PID but this issue is proving that it
may be worth keeping it.

This whole argument is predicated on there being a sound reason to
make the control word optional.

By far the simplest solution is to make implementing the control word
a requirement but make enabling the control word optional.  This would
give the provider the option of lower overhead near the edges *or*
ECMP in the core but not both (without risking reordering).

Curtis