the transmission of the data it carries than an Hypertext Transfer Protocol (HTTP) session browsing the Internet. Latency This is another common way of referring to end-to-end delay as experienced by traffic originating from point A in the network and destined to travel to point B. Voice communication is a good example. Once voice latency goes beyond 150 ms from mouth to ear, it is perceptible by the average human operator and becomes distracting. If the delay becomes longer than a certain value, it may even negate a network application based on voice transmission. Videoconferencing is a noto- rious example where sound may not be synchronized with the video images of a participant open- ing and closing his or her mouth. This can become very irritating to customers and thereby prohibit further business opportunities. Jitter This is essentially the variation in delay between two consecutive packets. A typical exam- ple would be the interfacing of an IP realm with Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) or time-division multiplexing (TDM) networks, where the slightest jitter may cause a loss of read or written slots in time and therefore cause application errors. Packet loss This is immaterial in some applications, whereas it is undesirable in others. Sometimes it is simply not an option. Link availability. Because link availability is an issue that transcends the network layer or data link protocol and spans concerns as low as the physical layer itself, we will not spend much time on it. The network- processing computational systems cannot do much to ensure it. It remains undoubtedly in the list of obligations that a carrier may have when serving a corporate customer. However, this is not a prob- lem that can be addressed by the mechanisms and techniques we will be discussing that purport to deliver QoS along the other four dimensions of the puzzle. Now that the fundamental idea is clear, how would we go about implementing a realm that enables its realization? Several tools of the trad