Behavior (PHB) for each class. The 6-bit DCSP allows for 26 = 64 possible classes. However, in practice there are three commonly defined classes: Default PHB, which is essentially conventional, best-effort IP forwarding Expedited forwarding for low-latency and low-loss applications such as VoIP Assured forwarding, which uses 12 different DSCPs (four subclasses, each with three different priorities) to give a more flexible PHB (see RFC 2597 for a full explanation) Because DiffServ uses broader classes of service, it is a scalable solution compared to IntServ. In practice, when traffic enters a DiffServ network it is policed (to check that it does not exceed the specified traffic parameters) and is then marked with a DSCP that needs to be supported by routers in the network and by the predefined PHBs. This creates certain problems with the dimensioning of a network to support DiffServ because there is not a reserved amount of resources along a particular path. Instead, it is common to overprovision a DiffServ network such that various possible combinations of edge traffic load will not overburden the core links. In practice it is not always possible to accurately predict which Traffic Engineering Label IP Header with DSCP IP Packets at MPLS Ingress Marked with Different DSCPs LSP 1 Packets with a Particular DSCP LSP 2 Mapped to Individual LSPs at Ingress LSR Fig. 13.12 MPLS mapping different DiffServ classes into unique LSPs. core links will be overburdened because the nature of IP traffic is highly variable, and Diff- Serv does not provide a mechanism to signal traffic demands to the core nodes. MPLS provides support in two ways: First, it provides a manner of mapping DSCPs to either LSPs or labels; second, it provides a traffic trunk that simplifies the dimensioning problem. MPLS provides two ways of mapping DSCPs into its architecture; the first is shown in Figure 13.12 where two different packet flows that need different PHBs (and thus have different DCSPs) are mapped to two unique LSPs. The LSRs assign the PHB