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Excerpt
Click here to preview Chapter 3: Internetworking
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In the previous chapter, we saw how to connect one node to another or to an existing network. Many technologies 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 ranges of their radios. To build a global network, we need a way to interconnect these different types of links and 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 fewsubproblems. First of all, we need a way to interconnect links. Devices that interconnect links of the same type areoften called switches, and these devices are the first topic of this chapter. A particularly important class of switches 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. 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 built. 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 bigger and faster routers to handle the ever-increasing traffic load in the Internet’s core.
3.1: SWITCHING AND BRIDGING
In the simplest terms, a switch is amechanismthat allows us to interconnect links to forma larger network. A switch is amulti-input,multi-output device, which transfers packets from an input to one or more outputs. Thus, a switch adds the star topology (see Figure 3.1) to the point-to-point link, bus (Ethernet), and ring topologies established in the last chapter. A star topology has several attractive properties:
- Even though a switch has a fixed number of inputs and outputs, which limits the number of hosts that can be connected to a single switch, large networks can be built by interconnecting a number of switches.
- We can connect switches to each other and to hosts using point-to-point links, which typically means that we can build networks of large geographic scope.
- Adding a new host to the network by connecting it to a switch does not necessarily reduce the performance of the network for other hosts already connected
This last claim cannot be made for the shared-media networks discussed in the last chapter. For example, it is impossible for two hosts on the same 10-Mbps Ethernet segment to transmit continuously at 10Mbps because they share the same transmission medium. Every host on a switched network has its own link to the switch, so it may be entirely possible for many hosts to transmit at the full link speed (bandwidth), provided that the switch is designed with enough aggregate capacity. Providing high aggregate throughput is one of the design goals for a switch; we return to this topic later. In general, switched networks are considered more scalable (i.e., more capable of growing to large numbers of nodes) than shared-media networks because of this ability to support many hosts at full speed.
A switch is connected to a set of links and, for each of these links, runs the appropriate data link protocol to communicate with the node at the other end of the link. A switch’s primary job is to receive incoming packets on one of its links and to transmit them on some other link. This function is sometimes referred to as either switching or forwarding, and in terms of the Open Systems Interconnection (OSI) architecture, it is the main function of the network layer. The question, then, is how does the switch decide which output link to place each packet on? The general answer is that it looks at the header of the packet for an identifier that it uses to make the decision. The details of how it uses this identifier vary, but there are two common approaches. The first is the datagram or connectionless approach. The second is the virtual circuit or connection-oriented approach. A third approach, source routing, is less common than these other two, but it does have some useful applications.
One thing that is common to all networks is that we need to have a way to identify the end nodes. Such identifiers are usually called addresses. We have already seen examples of addresses in the previous chapter, such as the 48-bit address used for Ethernet. The only requirement for Ethernet addresses is that no two nodes on a network have the same address. This is accomplished by making sure that all Ethernet cards are assigned a globally unique identifier. For the following discussions, we assume that each host has a globally unique address. Later on, we consider other useful properties that an address might have, but global uniqueness is adequate to get us started.
Another assumption that we need to make is that there is some way to identify the input and output ports of each switch. There are at least two sensible ways to identify ports: One is to number each port, and the other is to identify the port by the name of the node (switch or host) to which it leads. For now, we use numbering of the ports.
