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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.

  1. The use of the EXP field was first defined in RFC 3270 [RFC3270], 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 [RFC5129], 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 [RFC2119].