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.
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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
[RFC3032] 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]
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