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IETF RFC 5462
Multiprotocol Label Switching (MPLS) Label Stack Entry: 'EXP' Field Renamed to 'Traffic Class' Field
Last modified on Wednesday, February 25th, 2009
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Network Working Group L. Andersson
Request for Comments: 5462 Acreo AB
Updates: 3032, 3270, 3272, 3443, 3469, R. Asati
3564, 3985, 4182, 4364, 4379, Cisco Systems
4448, 4761, 5129 February 2009
Category: Standards Track
Multiprotocol Label Switching (MPLS) Label Stack Entry:
"EXP" Field Renamed to "Traffic Class" Field
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
The early Multiprotocol Label Switching (MPLS) documents defined the
form of the MPLS label stack entry. This includes a three-bit field
called the "EXP field". The exact use of this field was not defined
by these documents, except to state that it was to be "reserved for
experimental use".
Although the intended use of the EXP field was as a "Class of
Service" (CoS) field, it was not named a CoS field by these early
documents because the use of such a CoS field was not considered to
be sufficiently defined. Today a number of standards documents
define its usage as a CoS field.
Andersson & Asati Standards Track PAGE 1 
RFC 5462 MPLS TC Field Definition February 2009
To avoid misunderstanding about how this field may be used, it has
become increasingly necessary to rename this field. This document
changes the name of the field to the "Traffic Class field" ("TC
field"). In doing so, it also updates documents that define the
current use of the EXP field.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Details of Change . . . . . . . . . . . . . . . . . . . . . . 3
2.1. RFC 3032 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. RFC 3270 . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. RFC 5129 . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. The Scope of This Change . . . . . . . . . . . . . . . . . 7
3. Use of the TC field . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . . 9
1. Introduction
The format of an MPLS label stack entry is defined by RFC 3032
[RFC 3032] to include a three-bit field called the "EXP field". The
exact use of this field is not defined by RFC 3032, except to state
that it is to be "reserved for experimental use".
The EXP field, from the start, was intended to carry "Class of
Service" (CoS) information. The field was actually called the "Class
of Service field" in early versions of the working group document
that was published as RFC 3032. However, at the time that RFC 3032
was published, the exact usage of this "Class of Service" field was
not agreed upon and the field was designated as "experimental use";
hence, the name has since been the "EXP field".
The designation "for experimental use" has led other Standards
Development Organizations (SDOs) and implementors to assume that it
is possible to use the field for other purposes. This document
changes the name of the field to clearly indicate its use as a
traffic classification field.
At first, we discussed using the original "CoS field" as the name for
the field, but it has been pointed out that this name does not cover
the following changes that have occurred with respect to its usage
since RFC 3032 was published.
Andersson & Asati Standards Track PAGE 2
RFC 5462 MPLS TC Field Definition February 2009
1. The use of the EXP field was first defined in RFC 3270 [RFC 3270],
where a method to define a variant of Diffserv Label Switched
Paths (LSP), called EXP-Inferred-PSC LSP (E-LSPs), was specified.
PSC is a two-stage acronym that is expanded as PHB (Per Hop
Behavior) Scheduling Class (PSC).
2. The use of the EXP field as defined in RFC 3270 has been further
extended in RFC 5129 [RFC 5129], where methods for explicit
congestion marking in MPLS are defined.
This document, hence, uses the name "Traffic Class field (TC field)",
which better covers the potential use. The MPLS TC field relates to
an MPLS encapsulated packet the same way as the IPv6 TC field relates
to an IPv6 encapsulated packet or the IPv4 Precedence field relates
to an IPv4 encapsulated packet.
The definitions of how the EXP field is used are perfectly clear in
RFC 3270 and RFC 5129. However, these RFCs do not explicitly state
they update RFC 3032, and this fact was not captured in the RFC
repository until after work on this document was started.
This document updates RFC 3032, RFC 3270, and RFC 5129 to clarify the
intended usage of the TC field. The changes to these RFCs requires
some changes to the actual text in those documents; Section 2
explains the changes.
This document also updates several other RFCs; see Section 2.4. For
these documents, the change is limited to changing the name of the
Label Stack entry field.
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 RFC 2119 [RFC 2119].
2. Details of Change
The three RFCs 3032, 3270, and 5129 are now updated according to the
following.
2.1. RFC 3032
RFC 3032 states on page 4:
3. Experimental Use
This three-bit field is reserved for experimental use.
Andersson & Asati Standards Track PAGE 3
RFC 5462 MPLS TC Field Definition February 2009
This paragraph is now changed to:
3. Traffic Class (TC) field
This three-bit field is used to carry traffic class information,
and the change of the name is applicable to all places it occurs
in IETF RFCs and other IETF documents.
RFC 3270 and RFC 5129 update the definition of the TC field and
describe how to use the field.
In Figure 1 on page 3 in RFC 3032, the format of a label stack entry
is specified as:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
| Label | Exp |S| TTL | Stack
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry
Label: Label Value, 20 bits
Exp: Experimental Use, 3 bits
S: Bottom of Stack, 1 bit
TTL: Time to Live, 8 bits
Figure 1
Figure 1 in RFC 3032 is now changed to match the change of name to TC
field:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
| Label | TC |S| TTL | Stack
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry
Label: Label Value, 20 bits
TC: Traffic Class field, 3 bits
S: Bottom of Stack, 1 bit
TTL: Time to Live, 8 bits
Figure 1 (new)
Note: The designation of the picture above as "Figure 1 (new)" is
introduced as a way to distinguish the figures in this document. It
will still be "Figure 1" in RFC 3032.
Andersson & Asati Standards Track PAGE 4
RFC 5462 MPLS TC Field Definition February 2009
2.2. RFC 3270
RFC 3270 says on page 6:
1.2 EXP-Inferred-PSC LSPs (E-LSP)
A single LSP can be used to support one or more OAs. Such LSPs
can support up to eight BAs of a given FEC, regardless of how many
OAs these BAs span. With such LSPs, the EXP field of the MPLS
Shim Header is used by the LSR to determine the PHB to be applied
to the packet. This includes both the PSC and the drop
preference.
We refer to such LSPs as "EXP-inferred-PSC LSPs" (E-LSP), since
the PSC of a packet transported on this LSP depends on the EXP
field value for that packet.
The mapping from the EXP field to the PHB (i.e., to PSC and drop
precedence) for a given such LSP, is either explicitly signaled at
label set-up or relies on a pre-configured mapping.
Detailed operations of E-LSPs are specified in section 3 below.
RFC 3270 is now updated like this:
a. A new paragraph is added at the end of Section 1 "Introduction":
The EXP field has been renamed the TC field, and thus all
references in RFC 3270 to the EXP field now refer to the TC
field.
b. A new term is added to Section 1.1 "Terminology":
TC Traffic Class (replaces the term EXP)
c. In Section 1.1 "Terminology", the acronym E-LSP is now understood
to mean:
E-LSP Explicitly TC-encoded-PSC LSP
Section 1.2 on page 6 in RFC 3270 is now changed to:
1.2 Explicitly TC-encoded-PSC LSPs (E-LSP)
The EXP field has been renamed to the TC field, and thus all
references in RFC 3270 to EXP field now refer to the TC field.
However, we retain the acronym E-LSP (Explicitly TC-encoded-PSC
LSP) as the acronym is in widespread use.
Andersson & Asati Standards Track PAGE 5
RFC 5462 MPLS TC Field Definition February 2009
A single LSP can be used to support one or more OAs. Such LSPs
can support up to eight BAs of a given FEC, regardless of how many
OAs these BAs span. With such LSPs, the TC field of the MPLS Shim
Header is used by the LSR to determine the PHB to be applied to
the packet. This includes both the PSC and the drop preference.
We refer to such LSPs as "Explicitly TC-encoded-PSC LSPs"
(E-LSPs), since the PSC of a packet transported on this LSP
depends on the TC field (previously called the EXP field) value
for that packet.
The mapping from the TC field to the PHB (i.e., to PSC and drop
precedence) for a given such LSP is either explicitly signaled at
label set-up or relies on a pre-configured mapping.
This is an update to RFC 3032 [RFC 3032], in line with the original
intent of how this field in the MPLS Shim Header should be used
(as a TC field). RFC 3270 has itself been updated by RFC 5129
[RFC 5129].
Detailed operations of E-LSPs are specified in Section 3 of RFC
3270.
2.3. RFC 5129
RFC 5129 is now updated like this:
A new paragraph is added at the end of Section 1.1 "Background":
The EXP field has been renamed to the TC field, and thus all
references in RFC 5129 to the EXP field now refer to the TC field.
Section 2 (bullet 5) on page 7 of RFC 5129 says:
o A third possible approach was suggested by [Shayman]. In this
scheme, interior LSRs assume that the endpoints are ECN-capable,
but this assumption is checked when the final label is popped. If
an interior LSR has marked ECN in the EXP field of the shim
header, but the IP header says the endpoints are not ECN-capable,
the edge router (or penultimate router, if using penultimate hop
popping) drops the packet. We recommend this scheme, which we
call `per-domain ECT checking', and define it more precisely in
the following section. Its chief drawback is that it can cause
packets to be forwarded after encountering congestion only to be
dropped at the egress of the MPLS domain. The rationale for this
decision is given in Section 8.1.
Andersson & Asati Standards Track PAGE 6
RFC 5462 MPLS TC Field Definition February 2009
Section 2 (bullet 5) of RFC 5129 is now updated to:
o A third possible approach was suggested by [Shayman]. In this
scheme, interior LSRs assume that the endpoints are ECN-capable,
but this assumption is checked when the final label is popped. If
an interior LSR has marked ECN in the TC field of the shim header,
but the IP header says the endpoints are not TC-capable, the edge
router (or penultimate router, if using penultimate hop popping)
drops the packet. We recommend this scheme, which we call `per-
domain ECT checking', and define it more precisely in the
following section. Its chief drawback is that it can cause
packets to be forwarded after encountering congestion only to be
dropped at the egress of the MPLS domain. The rationale for this
decision is given in Section 8.1. This scheme is an update to RFC
3032 [RFC 3032] and RFC 3270 [RFC 3270].
2.4. The Scope of This Change
There are several places in the RFCs that are explicitly updated by
this document that reference the "Exp field", sometimes they refer to
the field as "Exp bits", "EXP bits", or "EXP". In all those
instances, the references now reference the TC field.
There are also other RFCs (e.g., RFC 3272 [RFC 3272], RFC 3443
[RFC 3443], RFC 3469 [RFC 3469], RFC 3564 [RFC 3564], RFC 3985
[RFC 3985], RFC 4182 [RFC 4182], RFC 4364 [RFC 4364], RFC 4379
[RFC 4379], RFC 4448 [RFC 4448], and RFC 4761 [RFC 4761]) that reference
the "Exp field"; sometimes they refer to the field as "Exp bits",
"EXP bits", and "EXP". For all RFCs, including but not limited to
those mentioned in this paragraph, such references now reference the
TC field.
3. Use of the TC field
Due to the limited number of bits in the TC field, their use for QoS
and ECN (Explicit Congestion Notification) functions is intended to
be flexible. These functions may rewrite all or some of the bits in
the TC field.
Current implementations look at the TC field with and without label
context, and the TC field may be copied to the label stack entries
that are pushed onto the label stack. This is done to avoid label
stack entries that are pushed onto an existing label stack having
different TC fields from the rest of the label stack entries.
Andersson & Asati Standards Track PAGE 7
RFC 5462 MPLS TC Field Definition February 2009
4. Security Considerations
This document only changes the name of one field in the MPLS shim
header, and thus does not introduce any new security considerations.
5. Acknowledgments
The authors would like to thank Stewart Bryant, Bruce Davie, George
Swallow, and Francois Le Faucheur for their input to and review of
the current document.
The authors would also like to thank George Swallow, Khatri Paresh,
and Phil Bedard for their help with grammar and spelling; a special
thanks to Adrian Farrel for his careful review and help trawling the
RFC-sea for RFCs that reference the EXP field.
6. References
6.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC 3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC 3272] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and X.
Xiao, "Overview and Principles of Internet Traffic
Engineering", RFC 3272, May 2002.
[RFC 3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks",
RFC 3443, January 2003.
[RFC 3469] Sharma, V. and F. Hellstrand, "Framework for Multi-
Protocol Label Switching (MPLS)-based Recovery", RFC 3469,
February 2003.
[RFC 3564] Le Faucheur, F. and W. Lai, "Requirements for Support of
Differentiated Services-aware MPLS Traffic Engineering",
RFC 3564, July 2003.
Andersson & Asati Standards Track PAGE 8
RFC 5462 MPLS TC Field Definition February 2009
[RFC 3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC 4182] Rosen, E., "Removing a Restriction on the use of MPLS
Explicit NULL", RFC 4182, September 2005.
[RFC 4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC 4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.
[RFC 4448] Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, April 2006.
[RFC 4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling",
RFC 4761, January 2007.
[RFC 5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
6.2. Informative References
[Shayman] Shayman, M. and R. Jaeger, "Using ECN to Signal Congestion
Within an MPLS Domain", Work in Progress, November 2000.
Authors' Addresses
Loa Andersson
Acreo AB
EMail: loa@pi.nu
Rajiv Asati
Cisco Systems
EMail: rajiva@cisco.com
Andersson & Asati Standards Track PAGE 9
Multiprotocol Label Switching (MPLS) Label Stack Entry: 'EXP' Field Renamed to 'Traffic Class' Field
RFC TOTAL SIZE: 19372 bytes
PUBLICATION DATE: Wednesday, February 25th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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