Differentiated services

Differentiated Services (DS) removes the per-flow and per-hop scalability issues, replacing them with a simplified mechanism of classifying packets.

Rather than a dynamic signaling approach, DS uses bits in the IP type of service (TOS) byte to separate packets into classes. The particular bit pattern in the IP TOS byte is called the DS codepoint and is used by routers to define the quality of service delivered at that particular hop, in much the same way routers do IP forwarding using routing table lookups. The treatment given to a packet with a particular DS codepoint is called a per-hop behavior (PHB) and is administered independently at each network node. When the effects of these individual, independent PHBs are concatenated, this results in an end-to-end service.

Differentiated services is being standardized by an IETF working group, which has defined three PHBs: the Expedited Forwarding (EF) PHB, the Assured Forwarding (AF) PHB group, and the Default (DE) PHB. The EF PHB can be used to implement a low latency, low jitter, low loss, end-to-end service such as a virtual leased line (VLL). AF is a family of PHBs, called a PHB group, that is used to classify packets into various drop precedence levels. The drop precedence assigned to a packet determines the relative importance of a packet within the AF class. It can be used to implement the so-called Olympic service, which consists of three classes: bronze, silver, and gold. The DE PHB is the traditional best-effort service model as standardized in RFC 1812.