RFC 3140 (rfc3140) - Page 2 of 8

Per Hop Behavior Identification Codes

RFC 3140         Per Hop Behavior Identification Codes         June 2001


1. Introduction

   Differentiated Services [RFC 2474, RFC 2475] introduces the notion of
   Per Hop Behaviors (PHBs) that define how traffic belonging to a
   particular behavior aggregate is treated at an individual network
   node.  In IP packet headers, PHBs are not indicated as such; instead
   Differentiated Services Codepoint (DSCP) values are used.  There are
   only 64 possible DSCP values, but there is no such limit on the
   number of PHBs.  In a given network domain, there is a locally
   defined mapping between DSCP values and PHBs.  Standardized PHBs
   recommend a DSCP mapping, but network operators may choose
   alternative mappings.

   In some cases it is necessary or desirable to identify a particular
   PHB in a protocol message, such as a message negotiating bandwidth
   management or path selection, especially when such messages pass
   between management domains.  Examples where work is in progress
   include communication between bandwidth brokers, and MPLS support of
   diffserv.

   In certain cases, what needs to be identified is not an individual
   PHB, but a set of PHBs.  One example is a set of PHBs that must
   follow the same physical path to prevent re-ordering.  An instance of
   this is the set of three PHBs belonging to a single Assured
   Forwarding class, such as the PHBs AF11, AF12 and AF13 [RFC 2597].

   This document defines a binary encoding to uniquely identify PHBs
   and/or sets of PHBs in protocol messages.  This encoding MUST be used
   when such identification is required.

   This document replaces RFC 2836, which omitted considerations for the
   Class Selector codepoints.

   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].

1.1. Usage Scenarios

   Diffserv services are expected to be supported over various
   underlying technologies which we broadly refer to as "link layers"
   for the purpose of this discussion.  For the transport of IP packets,
   some of these link layers make use of connections or logical
   connections where the forwarding behavior supported by each link
   layer device is a property of the connection.  In particular, within
   the link layer domain, each link layer node will schedule traffic
   depending on which connection the traffic is transported in.
   Examples of such "link layers" include ATM and MPLS.



Black, et al.               Standards Track