Internet Core Protocols: The Definitive Guide

Prioritization and Service-Based Routing

One of the key differences between IP and other networking protocols is that IP offers direct support for prioritization, allowing network hosts and routers to send important packets before less important packets. This feature is particularly crucial with applications that are sensitive to high levels of delay resulting from network congestion.

For example, assume that an organization has two high-speed networks that are interconnected by a relatively slow wide area network (WAN), and that a lot of data has to cross the WAN frequently. In this example, the routers could forward data across the WAN only at whatever rate was allowed by the WAN itself. If the WAN were fixed at a maximum throughput of 256 Kb/s, then the routers on the WAN could only send 262,144 bits across the WAN in a single second. This may be plenty of bandwidth for a few terminal emulation sessions—or even for a couple of simultaneous database updates—but it would not be enough for several simultaneous streaming video feeds in conjunction with those other applications.

The problem is that the routers just wouldn’t be able to forward enough data across the WAN for all of the applications to work smoothly. The routers would have to start dropping packets once their buffers began filling up or as the queuing delays exceeded the maximum Time-to-Live values on some of the packets. UDP-based applications may not care much about these dropped packets, but TCP-based applications ...