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IETF RFC 7274
Last modified on Thursday, June 12th, 2014
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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
 Abstract
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
6790.
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
http://www.rfc-editor.org/info/RFC 7274.
Kompella, et al. Standards Track PAGE 1 
RFC 7274 Special-Purpose MPLS Labels June 2014
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
Kompella, et al. Standards Track PAGE 2
RFC 7274 Special-Purpose MPLS Labels June 2014
1. Introduction
The MPLS Label Stack Encoding specification [RFC 3032] 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 [RFC 6790], [RFC 5586],
and [RFC 3429], 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
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
[RFC 7120] be allowed? Should there be labels for experimental
use or private use [RFC 5226]?
Kompella, et al. Standards Track PAGE 3
RFC 7274 Special-Purpose MPLS Labels June 2014
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"
[RFC 3032]) 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
balancing?
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 Label
Values".
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 [RFC 5226].
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.
Kompella, et al. Standards Track PAGE 4
RFC 7274 Special-Purpose MPLS Labels June 2014
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
required.
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. [RFC 6790] 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.
Kompella, et al. Standards Track PAGE 5
RFC 7274 Special-Purpose MPLS Labels June 2014
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 [RFC 3031]. 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.
Kompella, et al. Standards Track PAGE 6
RFC 7274 Special-Purpose MPLS Labels June 2014
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 [RFC 3032] ("MPLS Label Stack Encoding")
o [RFC 3038] ("VCID Notification over ATM link for LDP")
o [RFC 3209] ("RSVP-TE: Extensions to RSVP for LSP Tunnels")
o [RFC 3811] ("Definitions of Textual Conventions (TCs) for
Multiprotocol Label Switching (MPLS) Management")
o [RFC 4182] ("Removing a Restriction on the use of MPLS Explicit
NULL")
o [RFC 4928] ("Avoiding Equal Cost Multipath Treatment in MPLS
Networks")
o [RFC 5331] ("MPLS Upstream Label Assignment and Context-Specific
Label Space")
o [RFC 5586] ("MPLS Generic Associated Channel")
o [RFC 5921] ("A Framework for MPLS in Transport Networks")
o [RFC 5960] ("MPLS Transport Profile Data Plane Architecture")
o [RFC 6391] ("Flow-Aware Transport of Pseudowires over an MPLS
Packet Switched Network")
o [RFC 6478] ("Pseudowire Status for Static Pseudowires")
o [RFC 6790] ("MPLS Entropy Labels")
All such references should be read as "special-purpose labels".
Kompella, et al. Standards Track PAGE 7
RFC 7274 Special-Purpose MPLS Labels June 2014
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 [RFC 5226]). Early allocation following
the policy defined in [RFC 7120] 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 [RFC 3031] and in the
MPLS and GMPLS Security Framework [RFC 5920].
Kompella, et al. Standards Track PAGE 8
RFC 7274 Special-Purpose MPLS Labels June 2014
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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC 3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC 3038] Nagami, K., Katsube, Y., Demizu, N., Esaki, H., and P.
Doolan, "VCID Notification over ATM link for LDP", RFC
3038, January 2001.
[RFC 3209] 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.
[RFC 3811] Nadeau, T., Ed., and J. Cucchiara, Ed., "Definitions of
Textual Conventions (TCs) for Multiprotocol Label
Switching (MPLS) Management", RFC 3811, June 2004.
[RFC 4182] Rosen, E., "Removing a Restriction on the use of MPLS
Explicit NULL", RFC 4182, September 2005.
Kompella, et al. Standards Track PAGE 9
RFC 7274 Special-Purpose MPLS Labels June 2014
[RFC 4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
Cost Multipath Treatment in MPLS Networks", BCP 128, RFC
4928, June 2007.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC 5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space", RFC
5331, August 2008.
[RFC 5960] Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS
Transport Profile Data Plane Architecture", RFC 5960,
August 2010.
[RFC 6391] 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.
[RFC 6478] Martini, L., Swallow, G., Heron, G., and M. Bocci,
"Pseudowire Status for Static Pseudowires", RFC 6478, May
2012.
[RFC 6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, November 2012.
[RFC 7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, January 2014.
8.2. Informative References
[RFC 3429] Ohta, H., "Assignment of the 'OAM Alert Label' for
Multiprotocol Label Switching Architecture (MPLS)
Operation and Maintenance (OAM) Functions", RFC 3429,
November 2002.
[RFC 5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586, June 2009.
[RFC 5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
[RFC 5921] 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.
Kompella, et al. Standards Track PAGE 10
RFC 7274 Special-Purpose MPLS Labels June 2014
Authors' Addresses
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
US
EMail: kireeti.kompella@gmail.com
Loa Andersson
Huawei
EMail: loa@mail01.huawei.com
Adrian Farrel
Juniper Networks
EMail: adrian@olddog.co.uk
Kompella, et al. Standards Track PAGE 11
RFC TOTAL SIZE: 23442 bytes
PUBLICATION DATE: Thursday, June 12th, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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