Internet Core Protocols: The Definitive Guide

Datagram Independence

In the preceding section, we used an analogy of a telephone number to illustrate how routers are able to route datagrams to their final destination quickly, based on the destination IP address. However, we should also point out that IP packets are not at all like telephone calls.

Telephone networks use the concept of “circuits” to establish a point-to-point connection between two users. When two people establish a telephone call, a dedicated point-to-point connection is established and is preserved for the duration of the call. In contrast, IP networks treat every individual IP datagram as a totally unique entity, each of which is free to travel across whatever route is most suitable at that moment.

For example, if a user were to retrieve a document from a remote web server, the server would probably need to generate several IP datagrams in order to return the requested material. Each of these datagrams is considered to be a unique and separate entity, totally unrelated to the datagrams sent before or after.

Each of these datagrams may take whatever path is deemed most appropriate by the routers that are forwarding them along. Whereas the first datagram sent from the web server to the requesting client may travel across an underground fiber-optic cable, the second datagram may be sent across a satellite link, while a third may travel over a conventional network. This concept is illustrated in Figure 2.6.

Figure 2.6. Every IP datagram is an individual entity and ...