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.
Internet-Drafts are working documents of the Internet Engineering
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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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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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