(n.) Defined in terms of appropriate key performance indicators, such as the average transactional response time, the total number of concurrent sessions, the outage (planned or unplanned) per month or year, and the completion time for a batch job. The appropriate indicators vary from service to service. Nonetheless, QoS is intended to capture a set of metrics that adequately reflects the delivery of a service and provides the basis for decisions regarding the overall performance of a given service. The IETF defines QoS more specifically within the context of network service delivery, as follows:
At a high level of abstraction, QoS refers to the ability to deliver network services according to the parameters specified in a service level agreement. Quality is characterized by service availability, delay, jitter, throughput and packet loss ratio. At a network resource level, QoS refers to a set of capabilities that enable a service provider to prioritize traffic, control bandwidth, and network latency. There are two different approaches to QoS on IP networks: integrated services and differentiated service. Integrated services require policy control over the creation of signaled reservations that provide specific quantitative end-to-end behavior for an individual flow or set of flows. In contrast, differentiated services require policy to define the correspondence between code points in the packet's DS-field and individual per-hop behaviors (to achieve a specified per-domain behavior). A maximum of 64 per-hop behaviors limit the number of classes of service traffic that can be marked at any point in a domain. These classes of service signal the treatment of the packets with respect to various QoS aspects, such as flow priority and packet drop precedence. In addition, a policy can be used to specify the routing of packets based on various classification criteria. Policies control the set of configuration parameters and routing for each class in differentiated service, and the admission conditions for reservations in integrated services.