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RFC 7274 - Allocating and Retiring Special-Purpose MPLS Labels

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Internet Engineering Task Force (IETF)                       K. Kompella
Request for Comments: 7274                              Juniper Networks
Updates: 3032, 3038, 3209, 3811, 4182, 4928, 5331,          L. Andersson
         5586, 5921, 5960, 6391, 6478, 6790                       Huawei
Category: Standards Track                                      A. Farrel
ISSN: 2070-1721                                         Juniper Networks
                                                               June 2014

          Allocating and Retiring Special-Purpose MPLS Labels


   Some MPLS labels have been allocated for specific purposes.  A block
   of labels (0-15) has been set aside to this end; these labels are
   commonly called "reserved labels".  They will be called "special-
   purpose labels" in this document.

   As there are only 16 of these special-purpose labels, caution is
   needed in the allocation of new special-purpose labels; yet, at the
   same time, forward progress should be allowed when one is called for.

   This memo defines new procedures for the allocation and retirement of
   special-purpose labels, as well as a method to extend the special-
   purpose label space and a description of how to handle extended
   special-purpose labels in the data plane.  Finally, this memo renames
   the IANA registry for special-purpose labels to "Special-Purpose MPLS
   Label Values" and creates a new registry called the "Extended
   Special-Purpose MPLS Label Values" registry.

   This document updates a number of previous RFCs that use the term
   "reserved label".  Specifically, this document updates RFCs 3032,
   3038, 3209, 3811, 4182, 4928, 5331, 5586, 5921, 5960, 6391, 6478, and

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................3
   2. Questions .......................................................3
   3. Answers .........................................................4
      3.1. Extended Special-Purpose MPLS Label Values .................5
      3.2. Process for Retiring Special-Purpose Labels ................6
   4. Updated RFCs ....................................................7
   5. IANA Considerations .............................................8
   6. Security Considerations .........................................8
   7. Acknowledgments .................................................9
   8. References ......................................................9
      8.1. Normative References .......................................9
      8.2. Informative References ....................................10

1.  Introduction

   The MPLS Label Stack Encoding specification [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 [RFC6790], [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 in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Two new acronyms are introduced:

   XL    Extension Label.  A label that indicates that an extended
         special-purpose label follows.

   ESPL  Extended Special-Purpose Label.  A special-purpose label that
         is placed in the label stack after the Extension Label.  The
         combination of XL and ESPL might be regarded as a new form of
         "compound label" comprising more than one consecutive entry in
         the label stack.

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
       [RFC7120] 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?

   6.  Should extended special-purpose labels be used for load

3.  Answers

   This section provides answers to the questions posed in the previous


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

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

       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 values for experimental or private use.
           However, the "Extended Special-Purpose MPLS Label Values"
           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
       Standards Action 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.  For now, the use of the "Implicit NULL Label" (value 3) in the
       data plane will not be allowed.  If this decision is revisited
       later, an accompanying Standards Track RFC that details the use
       of the label, a discussion of possible sources of confusion
       between signaling and data plane, and mitigation thereof shall be

   5.  A special-purpose label (the "Extension Label", XL, value 15) is
       set aside for the purpose of extending the space of special-
       purpose labels.  Further details are described in Section 3.1.

   6.  [RFC6790] says that special-purpose labels MUST NOT be used for
       load balancing.  The same logic applies to extended special-
       purpose labels (ESPLs).  Thus, this document specifies that ESPLs
       MUST NOT be used for load balancing.  It is noted that existing
       implementations would violate this, as they do not recognize XL
       as anything other than a single special-purpose label and will
       not expect an ESPL to follow.  The consequence is that if ESPLs
       are used in some packets of a flow, these packets may be
       delivered on different paths and so could be re-ordered.
       However, it is important to specify the correct behavior for
       future implementations, hence the use of "MUST NOT".

   A further question that needed to be settled in this regard was
   whether a "regular" special-purpose label retains its meaning if it
   follows the XL.  The answer to this question is provided in
   Section 3.1.

3.1.  Extended Special-Purpose MPLS Label Values

   The XL MUST be followed by another label L (and thus MUST have the
   bottom-of-stack bit clear).  L MUST be interpreted as an ESPL and
   interpreted as defined in a new registry created by this document
   (see Section 5).  Whether or not L has the bottom-of-stack bit set
   depends on whether other labels follow L.  The XL only assigns
   special meaning to L.  A label after L (if any) is parsed as usual
   and thus may be a regular label or a special-purpose label; if the
   latter, it may be the XL and thus followed by another ESPL.

   The label value 15 is set aside as the XL as shown in Section 5.

   Values 0-15 of the "Extended Special-Purpose MPLS Label Values"
   registry are set aside as reserved.  Furthermore, values 0-6 and 8-15
   MUST NOT appear in the data plane following an XL; an LSR processing
   a packet with an XL at the top of the label stack followed by a label
   with value 0-6 or 8-15 MUST drop the packet.

   Label 7 (when received) retains its meaning as Entropy Label
   Indicator (ELI) whether a regular special-purpose label or an ESPL;
   this is because of backwards compatibility with existing implemented
   and deployed code and hardware that looks for the ELI without
   verifying if the previous label is XL or not.  However, when an LSR
   inserts an entropy label, it MUST insert the ELI as a regular
   special-purpose label, not as an ESPL.

3.1.1.  Forwarding Packets with Extended Special-Purpose Labels

   If an LSR encounters the XL at the top of stack and it doesn't
   understand extension labels, it MUST drop the packet as specified for
   the handling of an invalid incoming label according to [RFC3031].  If
   an LSR encounters an ESPL at the top of stack (after the XL) that it
   does not understand, it MUST drop the packet, again following the
   same procedure.  In either case, the LSR MAY log the event, but such
   logging MUST be rate-limited.

   An LSR SHOULD NOT make forwarding decisions on labels not at the top
   of stack.  For load-balancing decisions, see Answer 6 in Section 3.

3.1.2.  Choosing a New Special-Purpose Label

   When allocating a new special-purpose label, protocol designers
   should consider whether they could use an extended special-purpose
   label.  Doing so would help to preserve the scarce resources of
   "normal" special-purpose labels for use in cases where minimizing the
   size of the label stack is particularly important.

3.2.  Process for Retiring Special-Purpose Labels

   While the following process is defined for the sake of completeness,
   note that retiring special-purpose labels is difficult.  It is
   recommended that this process be used sparingly.

   a.  A label value that has been assigned from the "Special-Purpose
       MPLS Label Values" registry 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 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
       the deprecated value in new implementations and to operators to
       avoid including it in new deployments.

   b.  Twelve 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 an additional 6 months.

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

4.  Updated RFCs

   The following RFCs contain references to the term "reserved labels":

   o  [RFC3032] ("MPLS Label Stack Encoding")
   o  [RFC3038] ("VCID Notification over ATM link for LDP")
   o  [RFC3209] ("RSVP-TE: Extensions to RSVP for LSP Tunnels")
   o  [RFC3811] ("Definitions of Textual Conventions (TCs) for
      Multiprotocol Label Switching (MPLS) Management")
   o  [RFC4182] ("Removing a Restriction on the use of MPLS Explicit
   o  [RFC4928] ("Avoiding Equal Cost Multipath Treatment in MPLS
   o  [RFC5331] ("MPLS Upstream Label Assignment and Context-Specific
      Label Space")
   o  [RFC5586] ("MPLS Generic Associated Channel")
   o  [RFC5921] ("A Framework for MPLS in Transport Networks")
   o  [RFC5960] ("MPLS Transport Profile Data Plane Architecture")
   o  [RFC6391] ("Flow-Aware Transport of Pseudowires over an MPLS
      Packet Switched Network")
   o  [RFC6478] ("Pseudowire Status for Static Pseudowires")
   o  [RFC6790] ("MPLS Entropy Labels")

   All such references should be read as "special-purpose labels".

5.  IANA Considerations

   IANA has made the following changes and additions to its registration
   of MPLS labels.

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

   2.  Changed the allocation policy for the "Special-Purpose MPLS Label
       Values" registry to Standards Action.

   3.  Assigned value 15 from the "Special-Purpose MPLS Label Values"
       registry, naming it the "Extension Label" and citing this
       document as the reference.

   4.  Created a new registry called the "Extended Special-Purpose MPLS
       Label Values" registry.  The registration procedure is Standards
       Action, and the ranges for this registry are as shown in Table 1
       (using terminology from [RFC5226]).  Early allocation following
       the policy defined in [RFC7120] is allowed only for those values
       assigned by Standards Action.

   | Range               | Allocation Policy                           |
   | 0 - 15              | Reserved.  Never to be made available for   |
   |                     | allocation.                                 |
   |                     |                                             |
   | 16 - 239            | Unassigned                                  |
   |                     |                                             |
   | 240 - 255           | Reserved for Experimental Use               |
   |                     |                                             |
   | 256 - 1048575       | Reserved.  Not to be made available for     |
   |                     | allocation without a new Standards Track    |
   |                     | RFC to define an allocation policy.         |

                                  Table 1

6.  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
   increase the label stack through the insertion of additional
   {XL,ESPL} pairs at a greater rate than insertion of single "rogue"
   labels.  This might provide a way to attack some nodes in a network
   that can only process label stacks of a certain size without
   violating the protocol rules.

   This document also describes events that may cause an LSR to issue
   event logs at a per-packet rate.  It is critically important that
   implementations rate-limit such logs.

7.  Acknowledgments

   Thanks to Pablo Frank and Lizhong Jin for useful discussions.  Thanks
   to Curtis Villamizar, Mach Chen, Alia Atlas, Eric Rosen, Maria
   Napierala, Roni Even, Stewart Bryant, John Drake, Andy Malis, and Tom
   Yu for useful comments.

8.  References

8.1.  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.

   [RFC3038]  Nagami, K., Katsube, Y., Demizu, N., Esaki, H., and P.
              Doolan, "VCID Notification over ATM link for LDP", RFC
              3038, January 2001.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

   [RFC3811]  Nadeau, T., Ed., and J. Cucchiara, Ed., "Definitions of
              Textual Conventions (TCs) for Multiprotocol Label
              Switching (MPLS) Management", RFC 3811, June 2004.

   [RFC4182]  Rosen, E., "Removing a Restriction on the use of MPLS
              Explicit NULL", RFC 4182, September 2005.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128, RFC
              4928, June 2007.

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

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space", RFC
              5331, August 2008.

   [RFC5960]  Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS
              Transport Profile Data Plane Architecture", RFC 5960,
              August 2010.

   [RFC6391]  Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
              Regan, J., and S. Amante, "Flow-Aware Transport of
              Pseudowires over an MPLS Packet Switched Network", RFC
              6391, November 2011.

   [RFC6478]  Martini, L., Swallow, G., Heron, G., and M. Bocci,
              "Pseudowire Status for Static Pseudowires", RFC 6478, May

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, November 2012.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, January 2014.

8.2.  Informative 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., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586, June 2009.

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

   [RFC5921]  Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau,
              L., and L. Berger, "A Framework for MPLS in Transport
              Networks", RFC 5921, July 2010.

Authors' Addresses

   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089

   EMail: kireeti.kompella@gmail.com

   Loa Andersson

   EMail: loa@mail01.huawei.com

   Adrian Farrel
   Juniper Networks

   EMail: adrian@olddog.co.uk


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