urrently addressing is how packet-switched networks can support latency- and loss-sensitive traffic such as real-time streams. As discussed in Chapter 8, "The Internet and IP Infrastructures," and Chapter 10, "Next-Generation Networks," quality of service (QoS) mechanisms are a major requirement, and that is where much of our atten- tion is currently focused. With packet switching, packets are routed through a series of intermediate nodes, often involving multiple networks; they are routed in a store-and-forward manner through a series of packet switches (i.e., routers) that ultimately lead to the destination. Information is divided into packets that include a destination address and a sequence number. The secret to understanding the various packet formats is realizing where their strengths and weaknesses lie. They vary as to the number of bits they contain, how much control they give you over delays or losses, and the rules they use to address the highways and the destination points. (See the simple analogy between trans- portation and communications networks in the sidebar "A Transportation Analogy to Packet Switching" in Chapter 3, "Establishing Communications Channels.") Remember from Chapter 3 that packet switching deals with containerized, labeled entities we generically call packets, which vary in size. Packets come from dif- ferent sources-from different users at one customer site or from different users at different customer sites. All these different packets are statistically multiplexed and sent to their destinations over virtual circuits. Also remember from Chapter 1 that a virtual circuit is a set of logical connections that create a pathway between two points; they are not a physical connection that you can trace end to end that belongs to just one conversation. A virtual circuit is therefore a shared communications link that is set up on demand based on negotiated communications parameters. Because packet switching is a store-and-forward process of relaying through a series of intermediate nodes, l