cteriza- tions include definitions of the delay tolerance and elasticity for that application. They can also associate delay tolerance and elasticity with applications and users and potentially even time-of-day, day-of-week scenarios. It is necessary to be able to ensure various levels of service; the availability of bandwidth, end-to-end delay, delay variances, and packet losses that support the application in question; and the relative priority of traffic. Also, QoS is associated with policy admission control and policing of traffic streams. There are two ways of implementing QoS. Implicit QoS means that the applica- tion chooses the required QoS. Explicit QoS means that the network manager con- trols that decision. Chapter 10 Next-Generation Networks There are three main approaches to QoS. The first is an architected approach, and ATM falls under this category. The second is per-flow services, where the QoS is administered per flow, or per session. This includes the reservation protocol (Resource Reservation Protocol [RSVP]) that is part of the IETF IntServ specifica- tion, as well as MPLS. The third approach is packet labeling, in which each individ- ual packet is labeled with an appropriate QoS or priority mark; the techniques that use this approach include IEEE 802.1 and 802. 1pq, as well as the IETF DiffServ specification. The following sections describe various QoS tactics, including ATM QoS, IP QoS, and Class-Based Queuing (CBQ), as well as policy-based management and related protocols, such as COPS and Lightweight Directory Access Protocol (LDAP). ATM QoS ATM QoS includes four different service levels (one of which has two variations) that define a series of specific QoS parameters that tailor cells to fit video, data, voice, or mixed-media traffic. The following are the four service classes: Constant bit rate (CBR)-CBR provides a constant, guaranteed rate to real-time applications such as streaming video-so it is continuous band- width. It emulates a circuit-switched approach and is associated with min