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Allocating and Retiring Special Purpose MPLS Labels

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7274.
Authors Kireeti Kompella , Loa Andersson , Adrian Farrel
Last updated 2014-03-18 (Latest revision 2014-02-14)
Replaces draft-kompella-mpls-special-purpose-labels
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Ross Callon
Shepherd write-up Show Last changed 2014-01-30
IESG IESG state Waiting for Writeup::Revised I-D Needed
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Alia Atlas
Send notices to,
IANA IANA review state IANA - Not OK
Network Working Group                                        K. Kompella
Internet-Draft                                          Juniper Networks
Updates: 3032, 3038, 3209, 3811, 4182, 4928, 5331,          L. Andersson
         5586, 5921, 5960, 6391, 6478, 6790 (if approved)         Huawei
                                                               A. Farrel
Intended status: Standards Track                        Juniper Networks
Expires: August 14, 2014                               February 14, 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, and 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 allow forward progress when one is called for.

   This memo defines new 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.

   This document updates a number of previous RFCs that used the term
   "reserved label".

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 149, 2014.

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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
   ( 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 . . . . . . . . . . . . . . . . . . . . .  3
   2.  Questions  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Answers  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Extended Special Purpose MPLS Label Values . . . . . . . .  6
     3.2.  Process for Retiring Special Purpose Labels  . . . . . . .  7
   4.  Updated RFCs . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informational References . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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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 [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

   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   The Extension Label that indicates that an extended special
        purpose label follows.

   ESPL An 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.

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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
       [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

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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

       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 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
       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" (label 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, label 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

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       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.  This answer to this question is provided in Section

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 0-6 and 8-15 of the Extended Special Purpose Label registry
   are set aside as reserved; these MUST NOT appear in the data plane.
   If an LSR encounters such ESPL values it MUST treat the packet as
   malformed and discard it per [RFC3031].

   Label 7 (when received) retains its meaning as ELI whether a regular
   special purpose label or an ESPL; this simplifies a transit LSR's
   task of looking for entropy labels since it may just look for label 7
   and need not verify that the previous label in the stack is not the
   XL 15.  However, an LSR wishing to insert an entropy label SHOULD
   insert label 7 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 SHOULD drop the packet as specified
   for the handling of any unknown label according to [RFC3031].  If an
   LSR encounters an ESPL at the top of stack (after the XL) and does
   not understand the ESPL, it SHOULD drop the packet, again following
   the procedures for unknown labels as set out in [RFC3031].  In either
   case, the LSR MAY log the event, but such logging MUST be rate-

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

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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
   label stack size 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" 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

       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.  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.  Updated RFCs

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

   o [RFC3032] ("MPLS Label Stack Encoding"),
   o [RFC3038] ("VCID Notification for LDP")
   o [RFC3209] ("Extensions to RSVP for LSP Tunnels")
   o [RFC3811] ("MPLS TC MIB")
   o [RFC4182] ("Removing a Restriction on the use of MPLS")
   o [RFC4928] ("Avoiding ECMP Treatment in MPLS Networks")
   o [RFC5331], [RFC5586] ("G-ACh and GAL")
   o [RFC5921] ("MPLS Transport Profile Framework")
   o [RFC5960] ("MPLS-TP Data Plane Architecture")
   o [RFC6391] ("FAT-PW")
   o [RFC6478] ("Pseudowire Status for Static Pseudowires")
   o [RFC6790] ("MPLS Entropy Labels").

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

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5.  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 in Table 1 (using terminology from
       [RFC5226]).  Early allocation following the policy defined in
       [RFC7120] is allowed only for those values assigned by Standards

   | Range               | Allocation Policy                           |
   | 0 - 6, 8 - 15       | Reserved.  Not to be allocated.  MUST NOT   |
   |                     | appear in the data plane.                   |
   |                     |                                             |
   | 7                   | Allocated; meaning is ELI [RFC6790]         |
   |                     |                                             |
   | 16 - 239            | Standards Action                            |
   |                     |                                             |
   | 240 - 255           | Experimental                                |
   |                     |                                             |
   | 256 - 1048575       | Reserved. Not to be allocated without a new |
   |                     | Standards Track RFC to define an allocation |
   |                     | policy.

                                  Table 1

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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, and
   Maria Napierala for useful comments.

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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. and J. Cucchiara, "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., Bryant, S., and M. Bocci, "MPLS Transport
              Profile Data Plane Architecture", RFC 5960, August 2010.

   [RFC6391]  Bryant, S., 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.

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   [RFC6478]  Martini, L., Swallow, G., Heron, G., and M. Bocci,
              "Pseudowire Status for Static Pseudowires", RFC 6478,
              May 2012.

   [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.  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.

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

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

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Authors' Addresses

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


   Loa Andersson


   Adrian Farrel
   Juniper Networks


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