Internet Engineering Task Force (IETF) L. Andersson

Internet Engineering Task Force (IETF) L. Andersson
Request for Comments: 9017 Bronze Dragon Consulting
Updates: 3032, 7274 K. Kompella
Category: Standards Track Juniper Networks
ISSN: 2070-1721 A. Farrel
Old Dog Consulting
April 2021

Special-Purpose Label Terminology

Abstract

This document discusses and recommends terminology that may be used
when MPLS Special-Purpose Labels (SPLs) are specified and documented.

This document applies that terminology change to the relevant IANA
registry and also clarifies the use of the Entropy Label Indicator
(7) when immediately preceded by the Extension Label (15).

This document updates RFCs 3032 and 7274.

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 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9017.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Introduction
1.1. Terminology
2. Background
2.1. GMPLS Special-Purpose Labels
3. Terminology and Abbreviations
4. Clarification on Handling of the Entropy Label Indicator
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

RFC 7274 [RFC7274] made some changes to the terminology used for MPLS
Special-Purpose Labels but did not define consistent terminology.

One thing that RFC 7274 did was to deprecate the use of the term
"reserved labels" when describing a range of labels allocated from a
registry maintained by IANA. The term "Reserved" in such a registry
means "set aside, not to be used", but that range of labels was
available for allocation according to the policies set out in that
registry. The name "Special-Purpose Labels" was introduced in RFC
7274 in place of the previous term, and the abbreviation "SPL" was
recommended.

At the time of writing the first draft version of this document, the
IETF was in the process of allocating the very first SPLs from the
Extended SPL (eSPL) range [RFC8595]. This document discusses and
recommends terminology and abbreviations to be used when talking
about and documenting Special-Purpose Labels.

This document updates RFC 3032 [RFC3032] and RFC 7274 [RFC7274] in
that it changes the terminology for both Base SPLs (previously
referred to simply as "Special-Purpose Labels") and Extended SPLs.

This document applies that terminology change to the relevant IANA
registry and also clarifies the use of the Entropy Label Indicator
(7) when immediately preceded by the Extension Label (15).

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. Background

Two sets of SPLs are defined for use in MPLS:

* The range 0-15 (Base Special-Purpose Labels (bSPLs) as described
in this document) is specified in RFC 3032 [RFC3032].

* The range 0-1048575 of eSPLs is specified in RFC 7274 [RFC7274].

- The values 0-15 have been reserved and are never to be
allocated.

- The values 16-239 are available for allocation.

- The values 240-255 are for experimental use.

- The values 256-1048575 are currently not available for
allocation. A Standards Track RFC would be needed to change
this rule, and that RFC would need to define the ranges that
are made available for allocation and the registration policy
for those ranges.

2.1. GMPLS Special-Purpose Labels

Note that IANA maintains a registry that is called "Special-Purpose
Generalized Label Values". Labels in that registry have special
meaning when present in certain signaling objects, are 32 bits long,
and are not to be confused with MPLS forwarding-plane labels. This
document does not make any changes to the GMPLS registry or to how
labels from that registry are described.

3. Terminology and Abbreviations

Prior to the publication of this document, IANA maintained a name
space for "Special-Purpose Multiprotocol Label Switching (MPLS) Label
Values" code points [SPL-NAME-SPACE]. Within this name space, there
are two registries. One was called the "Special-Purpose MPLS Label
Values" registry [bSPL]. The other was called the "Extended Special-
Purpose MPLS Label Values" registry [eSPL].

The difference in the name of the name space and the first registry
is only that the MPLS abbreviation is expanded. This document makes
no change to the name of the name space itself (i.e., "Special-
Purpose Multiprotocol Label Switching (MPLS) Label Values"). This
document changes the name of the first registry to "Base Special-
Purpose MPLS Label Values" but leaves the name of the latter registry
unchanged as "Extended Special-Purpose MPLS Label Values".

The following conventions will be used in specifications and when
talking about SPLs.

* Collectively, the two (unrelated) ranges (0-15 and 16-1048575) are
known as "Special-Purpose Labels" (SPLs).

* SPLs from the range 0-15 are called "Base Special-Purpose Labels"
(bSPLs).

* SPLs from the range 16-1048575 are called "Extended Special-
Purpose Labels" (eSPLs). (Note that the reserved values 0-15 from
the "Extended Special-Purpose MPLS Label Values" registry do not
need a name, as they are not available for allocation and MUST NOT
be used.)

* The combination of the Extension Label (XL) (value 15, which is a
bSPL and is also called the "xSPL") and an eSPL is called a
"Composite Special-Purpose Label" (cSPL).

This results in label stacks such as the examples shown in Figures 1
and 2.

0 31
| MPLS Label Stack entry |
+--------+--------+--------+--------+
| MPLS Label Stack entry |
+--------+--------+--------+--------+
bSPL | Base SPL |
+--------+--------+--------+--------+
| MPLS Label Stack entry (cont.) |

Figure 1: Example of Label Stack

0 31
| MPLS Label Stack entry |
+--------+--------+--------+--------+
| MPLS Label Stack entry |
+--------+--------+--------+--------+
xSPL | Extension Label (XL) | <--+
+--------+--------+--------+--------+ |--- cSPL
eSPL | Extended SPL | <--+
+--------+--------+--------+--------+
| MPLS Label Stack entry (cont.) |

Figure 2: Example of Label Stack

4. Clarification on Handling of the Entropy Label Indicator

Section 3.1 of [RFC7274] contains two paragraphs that describe the
handling of the Entropy Label Indicator (label 7). These paragraphs
have introduced some confusion about whether the Entropy Label
Indicator can be present when immediately preceded by the Extension
Label. This document updates [RFC7274] by replacing those paragraphs
as follows.

OLD

| 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.

NEW

| Values 0-15 of the "Extended Special-Purpose MPLS Label Values"
| registry are set aside as reserved. Furthermore, an
| implementation MUST NOT place a label with a value in the range
| 0-15 in the label stack immediately following an XL; an LSR
| processing a packet with an XL at the top of the label stack
| immediately followed by a label with a value in the range 0-15
| MUST drop the packet.
|
| When inspecting a label stack to find an Entropy Label Indicator
| (ELI -- label 7), a preexisting implementation may fail to inspect
| the previous label and thus not notice that it is an XL. Such
| systems can continue to process the entropy information and
| forward the packet when the previous label is an XL without
| causing harm. However, the packet will be dropped when the XL
| reaches the top of the stack at another LSR.

END

5. Security Considerations

This document describes the terminology to be used when describing
and specifying the use of SPLs. It does not affect forwarding in the
MPLS data plane, nor does it have any effect on how Label Switched
Paths are established by an MPLS control plane or by a centralized
controller.

This document does not aim to describe existing implementations of
SPLs or potential vulnerabilities of SPLs.

6. IANA Considerations

IANA has changed the name of the registry once called "Special-
Purpose MPLS Label Values" to now be called "Base Special-Purpose
MPLS Label Values" [bSPL].

IANA has also updated the "Base Special-Purpose MPLS Label Values"
registry by changing the description for value 15 from "Extension
Label" to "Extension Label (XL)" and also adding this document as a
reference.

+=======+======================+====================+
| Value | Description | Reference |
+=======+======================+====================+
| 15 | Extension Label (XL) | RFC 7274, RFC 9017 |
+-------+----------------------+--------------------+

Table 1: Updated Entry for Value 15 in the "Base
Special-Purpose MPLS Label Values" Registry

7. References

7.1. Normative References

[bSPL] IANA, "Base Special-Purpose MPLS Label Values",
<https://www.iana.org/assignments/mpls-label-values/>.

[eSPL] IANA, "Extended Special-Purpose MPLS Label Values",
<https://www.iana.org/assignments/mpls-label-values/>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.

[RFC7274] Kompella, K., Andersson, L., and A. Farrel, "Allocating
and Retiring Special-Purpose MPLS Labels", RFC 7274,
DOI 10.17487/RFC7274, June 2014,
<https://www.rfc-editor.org/info/rfc7274>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[SPL-NAME-SPACE]
IANA, "Special-Purpose Multiprotocol Label Switching
(MPLS) Label Values",
<https://www.iana.org/assignments/mpls-label-values/>.

7.2. Informative References

[RFC8595] Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based
Forwarding Plane for Service Function Chaining", RFC 8595,
DOI 10.17487/RFC8595, June 2019,
<https://www.rfc-editor.org/info/rfc8595>.

Acknowledgements

We would like to thank the Routing Directorate reviewer, Eric Gray,
for a detailed, careful, and insightful review, and Tom Petch for
pointing out several issues of clarity.

Contributors

The following individual contributed text to this document:

Stewart Bryant
Futurewei Technologies Inc.

Email: [email protected]

Authors' Addresses

Loa Andersson
Bronze Dragon Consulting

Email: [email protected]

Kireeti Kompella
Juniper Networks

Email: [email protected]

Adrian Farrel
Old Dog Consulting

Email: [email protected]