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Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" Field
RFC 5462

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
   [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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