RFC 2836 (rfc2836) - Page 2 of 7

Per Hop Behavior Identification Codes

RFC 2836         Per Hop Behavior Identification Codes          May 2000


   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.

   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.

   For efficient support of diffserv over these link layers, one model
   is for different Behavior Aggregates (BAs) (or sets of Behavior
   Aggregates) to be transported over different connections so that they
   are granted different (and appropriate) forwarding behaviors inside
   the link layer cloud. When those connections are dynamically
   established for the transport of diffserv traffic, it is very useful
   to communicate at connection establishment time what forwarding
   behavior(s) is(are) to be granted to each connection by the link
   layer device so that the BAs transported experience consistent
   forwarding behavior inside the link layer cloud. This can be achieved
   by including in the connection establishment signaling messages the
   encoding of the corresponding PHB, or set of PHBs, as defined in this
   document.  Details on proposed usage of PHB encodings by some MPLS
   label distribution protocols (RSVP and LDP) for support of Diff-Serv
   over MPLS, can be found in [MPLS-DS].



Brim, et al.                Standards Track