Subject: RE: Label Spaces and Uniqueness

From: "Don Fedyk" <[email-masked]>
Date: Tue, 10 Jul 2001 15:41:23 -0400

Hi

So what I think I heard is either:

Standard says: All labels from the same space. No problems.

But in practice I thought VPNs used:

Assume per/interface outer labels, the next inner stacked label 
from per node space. PHP to be avoided adjacent to boxes doing this.

In the end it is a local decision of the box. No other box 
cares what algorithm the allocating box is using. 

So should the standard be updated?

Don

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From: David Charlap <[email-masked]>
Date: Tue, 10 Jul 2001 16:03:22 -0400

> cares what algorithm the allocating box is using.

What it ultimately boils down to is: If multiple label spaces are used
on a box, that box must be able to unambiguously determine which label
space to use for looking up labels.

Any mechanism that allows this is legal.

Under certain circumstances (eg. a label stack where the egress of the
inner LSP is using PHP) it may not be possible to unambiguously
determine the label space.  In which case, the LSPs involved in those
situations must all draw labels from the same label space.

-- David

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From: Eric Gray <[email-masked]>
Date: Tue, 10 Jul 2001 16:06:46 -0400

Don,

    While some are hunting on the details of when it is a
local matter and when it is not, I think that most are not
of the opinion that this is of such overwhelming importance
that the specification must be changed at once (if ever).
I once made the point on this list that there is unlikely
to be a reason to update the MPLS Architecture RFC.

    In defense of the RFC, however, the original question
neglected to include the context associated with the text
quoted (speaking of labels and context).  A few paragraphs
above the quoted text (from the next to last paragraph of
section 3.14), the section sets context for prohibiting
assignment of different meanings to the same labels when
certain conditions are not met.  The paragraph from which
the text is quoted is continuing in this context.  Therefore,
the quoted text is not as sweeping as it at first appears -
even though (taken out of context as it was) it appears to
be more restrictive than it needs to be in the general case.

--
Eric Gray