The MPLS WG Archive

[Anoop Ghanwani <anoop@cosinecom.com>]

Title: RE: bug in draft-ietf-mpls-label-encaps-08

I don't think that intended use of the Explicit NULL
labels is to get around the inability of a router
to pop labels, but I could be wrong.

The Explicit NULL labels are used when the penultimate
router doesn't want to do IP forwarding of data packets and
there is no label distribution protocol (RSVP, CR-LDP, LDP)
running between the penultimate and ultimate hops for whatever
reason.  In fact, earlier versions of the encaps draft
used to say that such a label is valid only when it is
the sole entry in the label stack.  I don't know why
that statement was removed.

-Anoop

> -----Original Message-----
> From: Puneet Agarwal [mailto:puneet@pluris.com]
> Sent: Friday, October 13, 2000 6:44 PM
> To: 'erosen@cisco.com'; 'mpls@uu.net'; 'tappan@cisco.com';
> 'yakov@cisco.com'
> Subject: bug in draft-ietf-mpls-label-encaps-08
>
>
> Looks like "draft-ietf-mpls-label-encaps-08.txt" is inconsistent with
> hierarchical tunnels.
>
> On pg 5 it states:
>               i. A value of 0 represents the "IPv4 Explicit
> NULL Label".
>                  This label value is only legal at the bottom of the
>                  label stack.  It indicates that the label
> stack must be
>                  popped, and the forwarding of the packet must then be
>                  based on the IPv4 header.
>
>  My understanding was that when a LSR got a label 3 from the
> egress LSR
> during signaling, if the LSR was able to do a pop then it
> would perform a
> penultimate hop pop otherwise it would just swap the incoming
> label with
> label 0 and then the egress LSR would do the actual pop.
>
> However, the draft states that the label 0 is only valid at
> the bottom of
> the stack. This implies that to support hierarchical tunnels one of 2
> conditions have to be true:
>
> (a) All LSRs supporting hierarchical tunnels, need to support
> PHP (so they
> never swap label 0 on the stack)
>
> OR
>
> (b) The "label value is only legal at the bottom of the label stack"
> constraint should be removed from the draft for label 0.
>
> Since support for PHP is not a requirement for LSRs, the
> draft should be
> updated to remove the constraint for label 0, to support hierarchical
> tunnels.
>
> -Puneet
>