d to the alternative priorities for transfer across the Diffserv domain. These priorities are called 'per-hop behaviour' (PHB). PHB is thus a name for a certain way to treat a packet stream. The treatment contains essentially loss, delay and bandwidth characteristics. Packet Backbone The routers offer a number of PHBs to the traffic, and this selection is called a PHB group. A PHB can also be called a forwarding class. Thus, a forwarding class has a certain DSCP value and a certain PHB. The byte used for carrying the DSCP is the "type of service" field in IP version 4 and 'traffic class' field in version 6. See Figure 12.16. Only six of the eight bits are used, which gives a maximum of 64 classes. Every network has a traffic limit for each forwarding class. The need for traffic policing at the entrance to a Diffserv domain is obvious, as well as agreements between the traffic sources and the Diffserv domain operator on setting of priorities. See Figure 12.17. All nodes will sort incoming packets according to their service class and treat them differently depending on their needs. The sorting processes in the traffic plane are called packet classification and traffic conditioning. Type of Total Traffic Flow label service length class Identification Flags Frag offset Payload length Next header Hop limit Time to Protocol Header checksum Source address Source address Destination address IP options Destination address v4 Header = 20 bytes + options v6 Header = 40 bytes Figure 12.16 Code point locations in IPv4 and IPv6 headers Quality-of-service mechanism for IP networks Every network user has a traffic limit for each forwarding class Only the permitted Forwarding class encoded in packet header volumes of traffic pass the policing Forwarding classes mechanism Traffic policing Traffic limits Traffic limitations are often realized as a service level agreement user and network provider Figure 12.17 Diffserv principles Understanding Changing Telecommunications - Building a Successful Telecom Business Packet classification