Allocating and Retiring MPLS Reserved Labels
draft-kompella-mpls-special-purpose-labels-00

Network Working Group                                        K. Kompella
Internet-Draft                                          Contrail Systems
Updates: 3032 (if approved)                                 L. Andersson
Intended status: Standards Track                                Ericsson
Expires: April 18, 2013                                        A. Farrel
                                                        Juniper Networks
                                                        October 15, 2012

              Allocating and Retiring MPLS Reserved Labels
             draft-kompella-mpls-special-purpose-labels-00

Abstract

   Some MPLS labels have been allocated for specific purposes.  A block
   of labels (0-15) has been set aside to this end, and are commonly
   called "reserved labels".  They will be called "special purpose
   labels" in this document.  As there are only 16 of these labels,
   caution is needed in the allocation of new special purpose labels,
   yet at the same time allow forward progress when one is called for.
   This memo defines some procedures to follow in the allocation and
   retirement of special purpose labels, as well as a method to extend
   the special purpose label space.  Finally, this memo renames the IANA
   registry for these labels to "Special Purpose MPLS Label Values", and
   creates a new one called the "Extended Special Purpose MPLS Label
   Values" registry.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 18, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions used . . . . . . . . . . . . . . . . . . . . .  3
   2.  Questions  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Answers  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Extended Special Purpose MPLS Label Values . . . . . . . .  5
     3.2.  Process for Retiring Special Purpose Labels  . . . . . . .  6
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informational References . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   The specification of the Label Stack Encoding for Multi-Protocol
   Label Switching (MPLS) [RFC3032] defined four special purpose label
   values (0 to 3), and set aside values 4 through 15 for future use.
   These labels have special significance in both the control and the
   data plane.  Since then, three further values have been allocated
   (values 7, 13, and 14 in [I-D.ietf-mpls-entropy-label], [RFC5586] and
   [RFC3429], respectively), leaving nine unassigned values from the
   original space of sixteen.

   While the allocation of three out of the remaining twelve special
   purpose label values in the space of about 12 years is not in itself
   a cause for concern, the scarcity of special purpose labels is.
   Furthermore, many of the special purpose labels require special
   processing by forwarding hardware, changes to which are often
   expensive, and sometimes impossible.  Thus, documenting a newly
   allocated special purpose label value is important.

   This memo outlines some of the issues in allocating and retiring
   special purpose label values, and defines mechanisms to address
   these.  This memo also extends the space of special purpose labels.

1.1.  Conventions used

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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2.  Questions

   In re-appraising MPLS special purpose labels, the following questions
   come to mind:

   1.  What allocation policies should be applied by IANA for the
       allocation of special purpose labels?  Should Early Allocation
       [RFC4020] be allowed?  Should there be labels for Experimental
       Use or Private Use [RFC5226]?

   2.  What documentation is required for special purpose labels
       allocated henceforth?

   3.  Should a special purpose label ever be retired?  What criteria
       are relevant here?  Can a retired special purpose label ever be
       re-allocated for a different purpose?  What procedures and time
       frames are appropriate?

   4.  The special purpose label value of 3 (the "Implicit Null Label",
       [RFC3032]) is only used in signaling, never in the data plane.
       Could it (and should it) be used in the data plane?  If so, how
       and for what purpose?

   5.  What is a feasible mechanism to extend the space of special
       purpose labels, should this become necessary?

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3.  Answers

   This section provides answers to the questions posed in the previous
   section.

   1.

       A.  Allocation of special purpose MPLS labels is via "Standards
           Action".

       B.  The IANA registry will be renamed "Special Purpose MPLS
           Labels".

       C.  Early allocation may be allowed on a case-by-case basis.

       D.  The current space of 16 special purpose labels is too small
           for setting aside value for experimental or private use.
           However, the extended special purpose labels registry created
           by this document has enough space, and this document defines
           a range for experimental use.

   2.  A Standards Track RFC must accompany a request for allocation of
       special purpose labels, as per [RFC5226].

   3.  The retirement of a special purpose MPLS label value must follow
       a strict and well-documented process.  This is necessary since we
       must avoid orphaning the use of this label value in existing
       deployments.  This process is detailed in Section 3.2.

   4.  The use of the "implicit null label" (label 3) in the data plane
       may be allowed, subject to approval by the MPLS WG, and an
       accompanying Standards Track RFC that details the use of the
       label, and a discussion of possible sources of confusion between
       signaling and data plane, and mitigation thereof.

   5.  The special purpose label (the "extension" label) is to be set
       aside for the purpose of extending the space of special purpose
       labels.  Further details are described in Section 3.1.

   A further question to be settled in this regard is whether a
   "regular" special purpose label retains its meaning if it follows the
   extension label; see Section 3.1.

3.1.  Extended Special Purpose MPLS Label Values

   An extension label MUST be followed by another label L (and thus MUST
   have the bottom-of-stack bit clear).  L MUST be interpreted as an
   "extended special purpose label" from a new registry created by this

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   document (see Section 4).  Whether or not L has the bottom-of-stack
   bit set depends on whether other labels follow L.

   IANA is asked to set aside label value 15 as the extension label.

   The first 16 values of the extended special purpose label registry
   are duplicated from the pre-existing special purpose label registry.
   This includes the previously allocated values (0-3, 7, 13, and 14),
   the extension label value (15) allocated by this document, and the
   remaining unallocated values (4-6 and 8-12).  Any of these values
   present as an extended special purpose label MUST be interpreted
   exactly as it would if it was presented as a special purpose label.
   In particular, an arbitrary string of consecutive extension labels is
   legal, and semantically equivalent to a single extension label (note
   that this string of extension labels MUST be followed by an extended
   special purpose label that is not the extension label).

3.2.  Process for Retiring Special Purpose Labels

   a.  A label value that has been assigned from the "Special Purpose
       MPLS Label Values" may be deprecated by IETF consensus with
       review by the MPLS working group (or designated experts if the
       working group or a successor does not exist).  An RFC with at
       least Informational status is required.

       The RFC will direct the IANA to mark the label value as
       "deprecated" in the registry, but will not release it at this
       stage.

       Deprecating means that no further specifications using the
       deprecated value will be documented.

       At the same time this is an indication to vendors not to include
       deprecated value in new implementations and to operators to avoid
       including it in new deployments.

   b.  12 months after the RFC deprecating the label value is published,
       an IETF-wide survey may be conducted to determine if the
       deprecated label value is still in use.  If the survey indicates
       that the deprecated label value is in use, the survey may be
       repeated after a further 6 months.

   c.  24 months after the RFC that deprecated the label value was
       published and if the survey indicates that deprecated label value
       is not in use, publication may be requested of an IETF Standards
       Track Internet-Draft that retires the deprecated the label value.
       This document will request IANA to release the label value for
       for future use and assignment.

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4.  IANA Considerations

   This document requests IANA to make the following changes and
   additions to its registration of MPLS Labels.

   1.  Change the name of the "Multiprotocol Label Switching
       Architecture (MPLS) Label Values" registry to the "Special
       Purpose MPLS Label Values".

   2.  Change the allocations policy for the "Special Purpose MPLS Label
       Values" registry to Standards Action.

   3.  Assign label 15 from the "Special Purpose MPLS Label Values"
       registry, naming it the "extension label", and citing this
       document as the reference.

   4.  Create a new registry called the "Extended Special Purpose MPLS
       Label Values" registry.  The ranges and allocation policies for
       this registry are as follows (using terminology from [RFC5226]).
       Early allocation following the policy defined in [RFC4020] is
       allowed only for those values assigned by Standards Action.

   +---------------------+---------------------------------------------+
   | Range               | Allocation Policy                           |
   +---------------------+---------------------------------------------+
   | 0 - 15              | Reserved.  Not to be allocated.  Meaning is |
   |                     | defined by values in the "Special Purpose   |
   |                     | MPLS Label Values" registry.                |
   |                     |                                             |
   | 16 - 1048559        | Standards Action                            |
   |                     |                                             |
   | 1048560 - 1048575   | Experimental                                |
   +---------------------+---------------------------------------------+

                                  Table 1

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5.  Security Considerations

   This document does not make a large change to the operation of the
   MPLS data plane and security considerations are largely unchanged
   from those specified in the MPLS architecture [RFC3031] and in the
   MPLS and GMPLS Security Framework [RFC5920].

   However, it should be noted that increasing the label stack can cause
   packet fragmentation and may also make packets unprocessable by some
   implementations.  This document provides a protocol-legal way to
   arbitrarily increase the label stack and so might provide a way to
   attack some nodes in a network without violating the protocol rules.

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6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC4020]  Kompella, K. and A. Zinin, "Early IANA Allocation of
              Standards Track Code Points", BCP 100, RFC 4020,
              February 2005.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5920]  Fang, L., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, July 2010.

6.2.  Informational References

   [RFC3429]  Ohta, H., "Assignment of the 'OAM Alert Label' for
              Multiprotocol Label Switching Architecture (MPLS)
              Operation and Maintenance (OAM) Functions", RFC 3429,
              November 2002.

   [RFC5586]  Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
              Associated Channel", RFC 5586, June 2009.

   [I-D.ietf-mpls-entropy-label]
              Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              draft-ietf-mpls-entropy-label-06 (work in progress),
              September 2012.

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Authors' Addresses

   Kireeti Kompella
   Contrail Systems
   2350 Mission College Blvd.
   Santa Clara, CA  95054
   US

   Email: kireeti.kompella@gmail.com

   Loa Andersson
   Ericsson

   Email: loa@pi.nu

   Adrian Farrel
   Juniper Networks

   Email: afarrel@olddog.co.uk

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