RFC 3140 (rfc3140) - Page 2 of 8
Per Hop Behavior Identification Codes
Alternative Format: Original Text Document
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