Special-Purpose Label TerminologyBronze Dragon Consultingloa@pi.nuJuniper Networkskireeti@juniper.netOld Dog Consultingadrian@olddog.co.ukMPLSExtended Special-Purpose LabelBase Special-Purpose LabelReserved LabelEntropy Label IndicatorThis 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.IntroductionRFC 7274 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 . This document discusses and recommends
terminology and abbreviations to be used when talking about and
documenting Special-Purpose Labels.This document updates RFC 3032 and RFC 7274
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).TerminologyThe 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
when, and only
when, they appear in all capitals, as shown here.BackgroundTwo 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
.
The range 0-1048575 of eSPLs is specified in RFC 7274 .
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.
GMPLS Special-Purpose LabelsNote 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.
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 . Within this name space, there are two
registries. One was called the "Special-Purpose MPLS Label Values"
registry . The other was called the "Extended Special-Purpose
MPLS Label Values" registry .
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 and .Clarification on Handling of the Entropy Label Indicator
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
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.
ENDSecurity ConsiderationsThis 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.IANA ConsiderationsIANA has changed the name of the registry once called
"Special-Purpose MPLS Label Values" to now be called "Base Special-Purpose MPLS Label Values" . 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.
Updated Entry for Value 15 in the "Base Special-Purpose MPLS Label Values" Registry
Value
Description
Reference
15
Extension Label (XL)
RFC 7274, RFC 9017
ReferencesNormative ReferencesSpecial-Purpose Multiprotocol Label Switching (MPLS) Label ValuesIANABase Special-Purpose MPLS Label ValuesIANAExtended Special-Purpose MPLS Label ValuesIANAInformative ReferencesAcknowledgementsWe would like to thank the Routing Directorate reviewer, ,
for a detailed, careful, and insightful review, and for
pointing out several issues of clarity.ContributorsThe following individual contributed text to this document:Futurewei Technologies Inc.stewart.bryant@gmail.com