ale is Differentiated Services (DiffServ), which is a bit more costly and complex. With DiffServ, packets carry a class or prior- ity ID, and the routers use per-class forwarding. At the top of the heap is Integrated Services (IntServ). This is the most costly and most complex level, but it allows per- flow state maintenance, uses RSVP signaling, and provides for guaranteed service Cost and Complexity Best Effort DiffServ IntServ Fair access to all Class/priority ID Per-flow state maintenance FIFO queuing Per-class forwarding RSVP signaling No priority treatment in routers Guaranteed service Bounded delay Figure 8.27 The IP QoS continuum Chapter 8 The Internet and IP Infrastructures and bounded delay. Next Steps in Signaling (NSIS) addresses the introduction of QoS on an end-to-end basis. The following sections briefly describe the DiffServ and Serv QoS schemes as well as NSIS; Chapter 10 describes the first two in more detail. DiffServ DiffServ is a prioritization model with preferential allocation of resources based on traffic classification. Figure 8.28 shows the DiffServ architecture, in which different packets are of different sizes and colors, suggesting that they have different priori- ties and queuing. DiffServ supports multiple service levels over an IP-based net- work. It uses a DSCP to select the service-that is, the per-hop behavior-that the packet will receive at each DiffServ-capable node. DSCP is a field in the packets transported over DiffServ networks that classifies the packets according to priority. (The DSCP field was formerly known as the Type of Service field in IPv4 and as the Traffic Class field in IPv6.) DiffServ classifies traffic by marking the IP header at the ingress to the network with flags corresponding to a small number of per-hop behaviors. The per-hop behaviors map to the DSCPs. DiffServ then sorts the pack- ets into queues via the DSCP. The various queues get different treatment in terms of priority, share of bandwidth, and probability of discard. There are several defined Diff