Many sophisticated emerging applications, such as distributed interactive simulation, multimedia conferencing, or distributed virtual reality, are difficult to deploy in todays internetworking infrastructure. This is mainly due to one requirement that all these applications share: the need for guaranteed real-time service. These applications not only require high bandwidth, but predictable Quality of Service (QoS) as well.
The QoS requirements at network level are typically specified in terms of bounds on worst-case end-to-end delay and on the worst-case delay jitter for packets of the connection. Other parameters may be specified as well, such as packet loss rate, deadline miss rate, but we will not further address them in this discussion. The desired delivery time for each message across the network is bounded by a deadline, a specific maximum delivery delay. This delay bound is an application-layer, end-to-end timing constraint. If a message arrives after the deadline is expired, the message is useless and is typically discarded. For most real-time applications, it is not important how fast a message is delivered. Indeed, packets arriving early may need to be buffered at the receiver to achieve constant end-to-end delay. Therefore, delay jitter, which is the maximum variation in delay experienced by packets in a single connection, is another performance metric. For example, in video transmission, jitter may cause some frames to arrive early, and others to arrive late. Although the transmission of all frames satisfies the deadline requirement, the displayed movie may appear jittery. Buffers at the receiver can be used to control delay jitter. The amount of buffer space required can be determined from the peak rate and the delay jitter of the delivery process and can be quite large for a network with no control of delay.
Real-time communication services require clients to specify traffic parameters of connections along with their QoS requirements (also called performance requirements) to obtain guarantees. QoS requirements are typically defined by constraints imposed by the application environment, e.g., frame rates for video transmission, interactivity thresholds for audio conferencing, or stability requirements for control applications. During connection establishment, the underlying network service is informed about the QoS requirements, typically as part of a connection-request message. If the connection satisfies the requirements without endangering guarantees given to existing connections, the connection is established for the client. At this point, the client and the network service enter a "real-time service contract": as long as the client adheres to the traffic specification (and the network service can enforce this by policing), the network service must guarantee the agreed-upon QoS requirements.