RFC 2836 (rfc2836) - Page 2 of 7
Per Hop Behavior Identification Codes
Alternative Format: Original Text Document
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
