Nature seems ... to reach many of her ends by long circuitous routes. —Rudolph Lotze
Problem: Not All Networks are Directly Connected
As we have seen, there are many technologies that can be used to build last-mile links or to connect a modest number of nodes together, but how do we build networks of global scale? A single Ethernet can interconnect no more than 1024 hosts; a point-to-point link connects only two. Wireless networks are limited by the range of their radios. To build a global network, we need a way to interconnect these different types of links and multi-access networks. The concept of interconnecting different types of networks to build a large, global network is the core idea of the Internet and is often referred to as internetworking.
We can divide the internetworking problem up into a few subproblems. First of all, we need a way to interconnect links. Devices that interconnect links of the same type are often called switches, or sometimes Layer 2 (L2) switches. These devices are the first topic of this chapter. A particularly important class of L2 switches in use today are those used to interconnect Ethernet segments. These switches are also sometimes called bridges.
The core job of a switch is to take packets that arrive on an input and forward (or switch) them to the right output so that they will reach their appropriate destination. There are a variety of ways that the switch can determine the "right" output for a packet, which can be broadly categorized as connectionless and connection-oriented approaches. These two approaches have both found important application areas over the years.
Given the enormous diversity of network types, we also need a way to interconnect disparate networks and links (i.e., deal with heterogeneity). Devices that perform this task, once called gateways, are now mostly known as routers, or alternatively, Layer 3 (L3) switches. The protocol that was invented to deal with interconnection of disparate network types, the Internet Protocol (IP), is the topic of our second section.
Once we interconnect a whole lot of links and networks with switches and routers, there are likely to be many different possible ways to get from one point to another. Finding a suitable path or route through a network is one of the fundamental problems of networking. Such paths should be efficient (e.g., no longer than necessary), loop free, and able to respond to the fact that networks are not static—nodes may fail or reboot, links may break, and new nodes or links may be added. Our third section looks at some of the algorithms and protocols that have been developed to address these issues.
Once we understand the problems of switching and routing, we need some devices to perform those functions. This chapter concludes with some discussion of the ways switches and routers are implemented. While many packet switches and routers are quite similar to a general-purpose computer, there are many situations where more specialized designs are used. This is particularly the case at the high end, where there seems to be a never-ending need for more switching capacity that can handle the ever-increasing traffic load in the Internet's core.