2. Terminology Used in This Document
Behavior Aggregate: a collection of packets with the same codepoint crossing a link in a particular direction. The terms "aggregate" and "behavior aggregate" are used interchangeably in this document.
Classifier: an entity which selects packets based on the content of packet headers according to defined rules.
Class Selector Codepoint: any of the eight codepoints in the range xxx000 (where x may equal 0 or 1). Class Selector Codepoints are discussed in Sec. 4.2.2.
Class Selector Compliant PHB: a per-hop behavior satisfying the Class Selector PHB Requirements specified in Sec. 4.2.2.2.
Codepoint: a specific value of the DSCP portion of the DS field. Recommended codepoints SHOULD map to specific, standardized PHBs. Multiple codepoints MAY map to the same PHB.
Differentiated Services Boundary: the edge of a DS domain, where classifiers and traffic conditioners are likely to be deployed. A differentiated services boundary can be further sub-divided into ingress and egress nodes, where the ingress/egress nodes are the downstream/upstream nodes of a boundary link in a given traffic direction. A differentiated services boundary typically is found at the ingress to the first-hop differentiated services-compliant router (or network node) that a host's packets traverse, or at the egress of the last-hop differentiated services-compliant router or network node that packets traverse before arriving at a host. This is sometimes referred to as the boundary at a leaf router. A differentiated services boundary may be co-located with a host, subject to local policy. Also DS boundary.
Differentiated Services-Compliant: in compliance with the requirements specified in this document. Also DS-compliant.
Differentiated Services Domain: a contiguous portion of the Internet over which a consistent set of differentiated services policies are administered in a coordinated fashion. A differentiated services domain can represent different administrative domains or autonomous systems, different trust regions, different network technologies (e.g., cell/frame), hosts and routers, etc. Also DS domain.
Differentiated Services Field: the IPv4 header TOS octet or the IPv6 Traffic Class octet when interpreted in conformance with the definition given in this document. Also DS field.
Mechanism: The implementation of one or more per-hop behaviors according to a particular algorithm.
Microflow: a single instance of an application-to-application flow of packets which is identified by source address, destination address, protocol id, and source port, destination port (where applicable).
Per-hop Behavior (PHB): a description of the externally observable forwarding treatment applied at a differentiated services-compliant node to a behavior aggregate. The description of a PHB SHOULD be sufficiently detailed to allow the construction of predictable services, as documented in [ARCH].
Per-hop Behavior Group: 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. Also PHB Group.
Traffic Conditioning: control functions that can be applied to a behavior aggregate, application flow, or other operationally useful subset of traffic, e.g., routing updates. These MAY include metering, policing, shaping, and packet marking. Traffic conditioning is used to enforce agreements between domains and to condition traffic to receive a differentiated service within a domain by marking packets with the appropriate codepoint in the DS field and by monitoring and altering the temporal characteristics of the aggregate where necessary. See [ARCH].
Traffic Conditioner: an entity that performs traffic conditioning functions and which MAY contain meters, policers, shapers, and markers. Traffic conditioners are typically deployed in DS boundary nodes (i.e., not in interior nodes of a DS domain).
Service: a description of the overall treatment of (a subset of) a customer's traffic across a particular domain, across a set of interconnected DS domains, or end-to-end. Service descriptions are covered by administrative policy and services are constructed by applying traffic conditioning to create behavior aggregates which experience a known PHB at each node within the DS domain. Multiple services can be supported by a single per-hop behavior used in concert with a range of traffic conditioners.
To summarize, classifiers and traffic conditioners are used to select which packets are to be added to behavior aggregates. Aggregates receive differentiated treatment in a DS domain and traffic conditioners MAY alter the temporal characteristics of the aggregate to conform to some requirements. A packet's DS field is used to designate the packet's behavior aggregate and is subsequently used to determine which forwarding treatment the packet receives. A behavior aggregate classifier which can select a PHB, for example a differential output queue servicing discipline, based on the codepoint in the DS field SHOULD be included in all network nodes in a DS domain. The classifiers and traffic conditioners at DS boundaries are configured in accordance with some service specification, a matter of administrative policy outside the scope of this document.
Additional differentiated services definitions are given in [ARCH].