Internet DRAFT - draft-ietf-mpls-spl-terminology
draft-ietf-mpls-spl-terminology
MPLS Working Group L. Andersson
Internet-Draft Bronze Dragon Consulting
Updates: 3032, 7274 (if approved) K. Kompella
Intended status: Standards Track Juniper Networks
Expires: July 25, 2021 A. Farrel
Old Dog Consulting
January 21, 2021
Special Purpose Label terminology
draft-ietf-mpls-spl-terminology-06
Abstract
This document discusses and recommends a terminology that may be used
when MPLS Special Purpose Labels (SPL) 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 RFC 7274 and RFC 3032.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 25, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. GMPLS Special Purpose Labels . . . . . . . . . . . . . . 3
3. Terminology and Abbreviations . . . . . . . . . . . . . . . . 3
4. Clarification on Handling of the Entropy Label Indicator . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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 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 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 and Extended SPLs.
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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 of 0-15, Base Special Purpose Labels (bSPLs), 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 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 standard track RFC will be needed to allocate
any labels from this range.
2.1. GMPLS Special Purpose Labels
Note that IANA maintains a registry called "Special Purpose
Generalized Label Values". Labels in that registry have special
meaning when present in certain signalling 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
IANA maintains 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 is called the
'Special-Purpose MPLS Label Values' registry [bSPL]. The other is
called 'Extended Special-Purpose MPLS Label Values' registry [eSPL].
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The difference in the name of the name space and the first registry
is only that the MPLS abbreviation is expanded. 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.
o Collectively, the two (unrelated) ranges (0-15 and 16-1048575) are
known as Special Purpose Labels (SPL).
o Special purpose labels from the range 0-15 are called Base Special
Purpose Labels (bSPL).
o Special purpose labels from the range 16-1048575 are called
Extended Special Purpose Labels (eSPL). (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.)
o The combination of the Extension Label (XL) (value 15 which is a
bSPL, and which is also called the xSPL) and an eSPL is called a
Composite Special Purpose Label (cSPL).
This results in a label stacks such as the illustrative examples
shown in Figure 1 and Figure 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
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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 value 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 value 0-15 MUST drop the packet.
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When inspecting a label stack to find an Entropy Label Indicator
(ELI - label 7) a pre-existing implementation may fail to inspect
the previous label, and so 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
The document describes the terminology to be used when describing and
specifying the use of SPLs. It does not effect the forwarding in the
MPLS data plane, nor does it have any effect on how LSPs 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 is requested to change the name of the registry that today is
called "Special-Purpose MPLS Label Values" is changed to "Base
Special- Purpose MPLS Label Values".
7. Acknowledgements
We like to thank the Routing Directorate reviwer Eric Gray for a
detailed, careful and insightful review, and Tom Petch for pointing
out several issues of clarity.
8. Contributors
The following people contributed text to this document:
Stewart Bryant
Futurewei Technologies Inc.
Email: stewart.bryant@gmail.com
9. References
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9.1. Normative References
[bSPL] "Special-Purpose MPLS Label Values",
<https://www.iana.org/assignments/mpls-label-values/mpls-
label-values.xhtml#special-purpose/>.
[eSPL] "Extended Special-Purpose MPLS Label Values",
<https://www.iana.org/assignments/mpls-label-values/mpls-
label-values.xhtml#extended/>.
[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]
"Special-Purpose Multiprotocol Label Switching (MPLS)
Label Values", <https://www.iana.org/assignments/mpls-
label-values/mpls-label-values.xhtml/>.
9.2. 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>.
Authors' Addresses
Loa Andersson
Bronze Dragon Consulting
Email: loa@pi.nu
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Kireeti Kompella
Juniper Networks
Email: kireeti@juniper.net
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
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