RFC 3260: New Terminology and Clarifications for Diffserv | RFC Editor

Skip to content Primary navigationThe RFC SeriesFor AuthorsAbout UsHomeRFC 3260 View as: Plain Text, Compact HTML Info RFC 3260: New Terminology and Clarifications for DiffservterminologyDiffserv1 April 2002. D. GrossmanInformationalSubscribe14 subscribed Add to Set Added to 3 Sets Read aloud Network Working Group D. Grossman Request for Comments: 3260 Motorola, Inc. Updates: 2474, 2475, 2597 April 2002 Category: Informational New Terminology and Clarifications for Diffserv Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This memo captures Diffserv working group agreements concerning new and improved terminology, and provides minor technical clarifications. It is intended to update RFC 2474, RFC 2475 and RFC 2597. When RFCs 2474 and 2597 advance on the standards track, and RFC 2475 is updated, it is intended that the revisions in this memo will be incorporated, and that this memo will be obsoleted by the new RFCs. 1. Introduction As the Diffserv work has evolved, there have been several cases where terminology has needed to be created or the definitions in Diffserv standards track RFCs have needed to be refined. Some minor technical clarifications were also found to be needed. This memo was created to capture group agreements, rather than attempting to revise the base RFCs and recycle them at proposed standard. It updates in part RFC 2474, RFC 2475 and RFC 2597. RFC 2598 has been obsoleted by RFC 3246, and clarifications agreed by the group were incorporated in that revision. 2. Terminology Related to Service Level Agreements (SLAs) The Diffserv Architecture [2] uses the term "Service Level Agreement" (SLA) to describe the "service contract... that specifies the forwarding service a customer should receive". The SLA may include traffic conditioning rules which (at least in part) constitute a Traffic Conditioning Agreement (TCA). A TCA is "an agreement Grossman Informational [Page 1] RFC 3260 New Terminology and Clarifications for Diffserv April 2002 specifying classifier rules and any corresponding traffic profiles and metering, marking, discarding and/or shaping rules which are to apply...." As work progressed in Diffserv (as well as in the Policy WG [6]), it came to be believed that the notion of an "agreement" implied considerations that were of a pricing, contractual or other business nature, as well as those that were strictly technical. There also could be other technical considerations in such an agreement (e.g., service availability) which are not addressed by Diffserv. It was therefore agreed that the notions of SLAs and TCAs would be taken to represent the broader context, and that new terminology would be used to describe those elements of service and traffic conditioning that are addressed by Diffserv. - A Service Level Specification (SLS) is a set of parameters and their values which together define the service offered to a traffic stream by a DS domain. - A Traffic Conditioning Specification (TCS) is a set of parameters and their values which together specify a set of classifier rules and a traffic profile. A TCS is an integral element of an SLS. Note that the definition of "Traffic stream" is unchanged from RFC 2475. A traffic stream can be an individual microflow or a group of microflows (i.e., in a source or destination DS domain) or it can be a BA. Thus, an SLS may apply in the source or destination DS domain to a single microflow or group of microflows, as well as to a BA in any DS domain. Also note that the definition of a "Service Provisioning Policy" is unchanged from RFC 2475. RFC 2475 defines a "Service Provisioning Policy as "a policy which defines how traffic conditioners are configured on DS boundary nodes and how traffic streams are mapped to DS behavior aggregates to achieve a range of services." According to one definition given in RFC 3198 [6], a policy is "...a set of rules to administer, manage, and control access to network resources". Therefore, the relationship between an SLS and a service provisioning policy is that the latter is, in part, the set of rules that express the parameters and range of values that may be in the former. Further note that this definition is more restrictive than that in RFC 3198. Grossman Informational [Page 2] RFC 3260 New Terminology and Clarifications for Diffserv April 2002 3. Usage of PHB Group RFC 2475 defines a Per-hop behavior (PHB) group to be: "a set of one or more PHBs that can only be meaningfully specified and implemented simultaneously, due to a common constraint applying to all PHBs in the set such as a queue servicing or queue management policy. A PHB group provides a service building block that allows a set of related forwarding behaviors to be specified together (e.g., four dropping priorities). A single PHB is a special case of a PHB group." One standards track PHB Group is defined in RFC 2597 [3], "Assured Forwarding PHB Group". Assured Forwarding (AF) is a type of forwarding behavior with some assigned level of queuing resources and three drop precedences. An AF PHB Group consists of three PHBs, and uses three Diffserv Codepoints (DSCPs). RFC 2597 defines twelve DSCPs, corresponding to four independent AF classes. The AF classes are referred to as AF1x, AF2x, AF3x, and AF4x (where 'x' is 1, 2, or 3 to represent drop precedence). Each AF class is one instance of an AF PHB Group. There has been confusion expressed that RFC 2597 refers to all four AF classes with their three drop precedences as being part of a single PHB Group. However, since each AF class operates entirely independently of the others, (and thus there is no common constraint among AF classes as there is among drop precedences within an AF class) this usage is inconsistent with RFC 2475. The inconsistency exists for historical reas