Issues in Differentiated Services: A Review

Abstract

Differentiated services is the most advanced method for traffic management. Differentiated services utilize 6 bits of the 8 bit type of service field of the IP header. This allows up to 64 possible classes. In Differentiated Services this field is referred to as differentiated services code point. Differentiated Services standards define two types of per hop behaviors (PHB's): Expedited forwarding and Assured forwarding. EF is the highest priority traffic. Packets marked with EF PHB should be forwarded with highest rate or rate equal to its arrival. AF defined four classes with three drop precedence. Edge routers perform two main functions, traffic classification and traffic conditioning, also known as admission control. Both functions are governed by service level agreement (SLA). Generally, packets conforming to SLA are considered as in-profile and packets exceeding SLA are considered as out of profile. The classification process examines incoming packets at the ingress routers against the rules specified by SLA. Packets are assigned the appropriate class (EF or one of the AF classes) by marking them with corresponding DSCP field value. The conditioning process ensures that flows stay within the SLA. Depending on flow characteristics and network conditions, out-of-profile packets are marked with higher drop precedence, delayed in the queue or dropped. A commonly used admission control technique is token bucket algorithm. With complexity pushed to edge routers, core routers have simpler functions in Differentiated Services. PHB's are defined for each class. The router simply checks DSCP field and performs the appropriate action.