Having its roots in the military ARPANET, conceived as a data transport network with a focus on resilience, the Internet supports only a best-effort service model, where all packets are treated the same way, therefore providing a single level of service. Now that the Internet is becoming a ubiquitous global communication infrastructure, new applications are emerging with more demanding and diversified requirements than data transport. Internet telephony, for example, has much stricter delay requirements than remote terminal, the most demanding of the original applications. The deployment of other service models providing better quality of service (QoS) is of great importance for the transport of these new applications.
Key Terms in this Chapter
Traffic Shaping: Forcing an input flow to have an output that conforms to a given traffic envelope s by delaying the non-conformant bits (or packets) of the flow in a buffer. At the output of the traffic shaper, the flow has s as an arrival curve.
Label Switching: Technique used in protocols whereby a short identifier (label) is carried in signaling messages and/or data packets in order to allow for easy and efficient classification or access to a given data structure or state information. Along the path, labels may be kept intact, exchanged, or stacked, according to the protocol.
Soft State: Technique whereby the state information is automatically deleted if not refreshed for a given period. It is used to improve the resilience of protocols by providing automatic recovery from faults and changing conditions.
Arrival Curve: The arrival curve of a flow is a wide-sense increasing function a defined for t = 0 , such that the amount of data flowing in any time interval of length t is less than or equal to a(t). It is used to place a constraint on the flow’s arrival process.
Traffic Policing: Forcing an input flow to have an output that conforms to a given traffic envelope s by discarding non-conformant bits (or packets) of the flow or reclassifying them to a different flow or aggregate.
Quality of Service (QoS): Subjectively defined in Recommendation E.800 of the ITU-T as “The collective effect of service performance which determines the degree of satisfaction of a user of the service,” QoS refers to the probability of the network meeting a given traffic contract, which may be quantitatively expressed by parameters such as transfer delay and jitter and probability of packet loss, error, or out-oforder delivery.
Flow Aggregation: Merging of multiple flows, possibly sharing common characteristics, in order to treat them as a single flow in the use of a given resource.
Scalability: The ease with which a system or component can handle increased dimensions of the problem it is designed to solve.