Internet DRAFT - draft-kompella-mpls-reserved-labels
draft-kompella-mpls-reserved-labels
Network Working Group K. Kompella
Internet-Draft Contrail Systems
Updates: 3032 (if approved) September 28, 2012
Intended status: Standards Track
Expires: April 1, 2013
Allocating and Retiring MPLS Reserved Labels
draft-kompella-mpls-reserved-labels-01
Abstract
There are a limited number of reserved labels defined for Multi-
Protocol Label Switching. Thus one must be cautious in the
allocation of new reserved labels, yet at the same time allow forward
progress when a new reserved label is called for. This memo suggests
some procedures to follow in the allocation and retirement of
reserved labels.
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 1, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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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. Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
[RFC3032] defined four reserved label values for Multi-Protocol Label
Switching (MPLS), namely, 0 to 3, and set aside values 4 through 15
for future use. These labels have special significance in both the
control and data plane. Since then, two further values have been
allocated, leaving ten unassigned values from the original space of
sixteen.
While the allocation of two out of the remaining twelve reserved
label values in the space of about 12 years is not in itself a cause
for concern, the scarcity of reserved labels is. Furthermore, many
of the reserved labels require special processing by forwarding
hardware, changes to which are often expensive, and sometimes
impossible. Thus, documenting a newly allocated reserved label value
is important.
This memo outlines some of the issues in allocating and retiring
reserved label values, and will eventually suggest mechanisms to
address these. Furthermore, this memo proposes a means of extending
the space of reserved 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
1. How should reserved labels be allocated? The current IANA number
space for "MPLS Label Values" has the allocation policy of "IETF
Consensus". Would "Expert Review" be better? Should the name of
this space be changed to reflect its use? Should there be Early
Allocation? Experimental Use? Private Use?
2. What documentation is required for reserved labels allocated
henceforth?
3. Should a reserved label ever be retired? What criteria are
relevant here? What procedures and time frames are appropriate?
4. The reserved label value of 3 is only used in signaling, never in
the data plane. Could it (and should it) be used in the data
plane? If so, how?
5. What is a feasible mechanism to extend the space of reserved
labels, should this become necessary?
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3. Proposal
To answer question 5 from the previous section, the following
proposal is made: set aside label value 15 (the "extension" label)
for the purpose of extending the space of reserved labels. An
extension label 15 MUST be followed by another label L (and thus MUST
have the bottom-of-stack bit clear). L MUST be interpreted as an
"extended reserved label". Furthermore, the interpretation and
special processing of such labels MUST be documented, and they MUST
be registered with IANA (policies TBD).
A further question to be settled in this regard is whether a "plain"
reserved label retains its meaning if it follows the extension label.
That is, does a label value of 0 mean "Explicit IPv4 NULL" whether or
not it is preceded by an extension label? The suggestion here is
"yes" for the currently allocated reserved labels (i.e., 0-3, 7, 13,
and 14). The documentation accompanying a newly allocated reserved
label MUST say whether or not it retains its meaning if preceded by
the extension label.
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4. IANA Considerations
There is already an IANA registry for MPLS label values. This memo
may eventually suggest:
1. a change in the name of the registry to "Reserved MPLS Label
Values
2. some changes to the procedures for allocating values
3. a new registry for "Extended Reserved MPLS Label Values".
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5. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
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Author's Address
Kireeti Kompella
Contrail Systems
2350 Mission College Blvd.
Santa Clara, CA 95054
US
Email: kireeti.kompella@gmail.com
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