Problem: Scaling to Billions
We have now seen how to build an internetwork that consists of a number of networks of different types. That is, we have dealt with the problem of heterogeneity. The second critical problem in internetworking—arguably the fundamental problem for all networking—is scale. To understand the problem of scaling a network, it is worth considering the growth of the Internet, which has roughly doubled in size each year for 30 years. This sort of growth forces us to face a number of challenges.
Chief among these is how do you build a routing system that can handle hundreds of thousands of networks and billions of end nodes? As we will see in this chapter, most approaches to tackling the scalability of routing depend on the introduction of hierarchy. We can introduce hierarchy in the form of areas within a domain; we also use hierarchy to scale the routing system among domains. The interdomain routing protocol that has enabled the Internet to scale to its current size is BGP. We will take a look at how BGP operates, and consider the challenges faced by BGP as the Internet continues to grow.
Closely related to the scalability of routing is the problem of addressing. Even two decades ago it had become apparent that the 32-bit addressing scheme of IP version 4 would not last forever. That led to the definition of a new version of IP—version 6, since version 5 had been used in an earlier experiment. IPv6 primarily expands the address space but also adds a number of new features, some of which have been retrofitted to IPv4.
While the Internet continues to grow in size, it also needs to evolve its functionality. The final sections of this chapter cover some significant enhancements to the Internet’s capabilities. The first, multicast, is an enhancement of the basic service model. We show how multicast—the ability to deliver the same packets to a group of receivers efficiently—can be incorporated into an internet, and we describe several of the routing protocols that have been developed to support multicast. The second enhancement, Multiprotocol Label Switching (MPLS), modifies the forwarding mechanism of IP networks. This modification has enabled some changes in the way IP routing is performed and in the services offered by IP networks. Finally, we look at the effects of mobility on routing and describe some enhancements to IP to support mobile hosts and routers. For each of these enhancements, issues of scalability continue to be important.