re is no need to keep a per-flow or per-customer state table inside the network. Main concepts of DiffServ are as follows: Classification. As explained before, this means the selection of a packet based on the contents of one or more fields in the header. Chapter 2 System components, requirements, and interfaces 99 Conditioning. Metering, marking, dropping, shaping, or a subset of these. Metering means the measurement of the characteristics of an incoming traffic flow, by using, for example, a token bucket (Single-Rate Tri Color Marking or srTCM). Marking means modification of a packet to indicate conformance with a preagreed traffic contract. If packets do not comply with the traffic contract, they can be discarded (dropped) or shaped to comply with the contract, for example, by delaying them in time. A DiffServ network is built around a domain, which is a contiguous network under the same administrative ownership. Inside this domain we differentiate between edge nodes and core nodes. An edge (or boundary) node provides an interface to a non-DiffServ network or a different DiffServ domain, while the core (or interior) nodes have no interaction outside the DiffServ domain. As the traffic enters the DiffServ domain, it is classified and possibly conditioned (condi- tioning can also take place at the egress though). After this, enough information is available to assign the traffic to a DiffServ Behavior Aggregate using a DSCP. Inside the network, traffic is routed according to the specified PHB for this DSCP. The PHB is defined as the "externally observable behavior of a DiffServ node to a particular aggregate." This means that the PHB is the means by which a node allocates its egress bandwidth to an aggregate. A simple PHB would be, for example, a percentage bandwidth allocation. It is implemented by means of buffer management and packet scheduling mechanisms. The use of the DSCP is defined in RFC2474. As it is impossible to add an extra field to a currently existing IP header, the DSCP is carried in the existing 