Differentiated Services - Classification and Marking

Classification and Marking

Network traffic entering a DiffServ domain is subjected to classification and conditioning. Traffic may be classified by many different parameters, such as source address, destination address or traffic type and assigned to a specific traffic class. Traffic classifiers may honor any DiffServ markings in received packets or may elect to ignore or override those markings. Because network operators want tight control over volumes and type of traffic in a given class, it is very rare that the network honors markings at the ingress to the DiffServ domain. Traffic in each class may be further conditioned by subjecting the traffic to rate limiters, traffic policers or shapers.

The Per-Hop Behavior is determined by the DS field of the IP header. The DS field contains a 6-bit Differentiated Services Code Point (DSCP) value. Explicit Congestion Notification (ECN) occupies the least-significant 2 bits of the IPv4 Type of Service field (TOS) and IPv6 Traffic Class field (TC).

In theory, a network could have up to 64 (i.e. 26) different traffic classes using different DSCPs. The DiffServ RFCs recommend, but do not require, certain encodings. This gives a network operator great flexibility in defining traffic classes. In practice, however, most networks use the following commonly-defined Per-Hop Behaviors:

• Default PHB (Per hop behavior)—which is typically best-effort traffic
• Expedited Forwarding (EF) PHB—dedicated to low-loss, low-latency traffic
• Assured Forwarding (AF) PHB—gives assurance of delivery under prescribed conditions
• Class Selector PHBs—which maintain backward compatibility with the IP Precedence field.