.. index:: BGP: Border Gateway Protocol
Chapter |BGP|: Interdomain Routing
The previous chapter described the mechanisms that are used to assemble a set of independent networks into a logical internet. This makes end-to-end packet delivery possible, but doesn't say anything about how routes among the independent networks are calculated and distributed. Whereas our discussion of routing in Chapter |Routing| focused on finding paths with the lowest cost according to some metric, routing among independent networks is largely about satisfying the policies that govern how networks are used. This chapter looks at this issue by describing how routing policies are specified and enforced. The answer is intimately tied up with the mechanism routers use to learn what paths are available, and decide which paths to use to deliver their traffic.
Calculating and sharing routes among autonomously operated networks has proven to be one of the greatest challenges facing the Internet. The protocol responsible for providing a solution, the Border Gateway Protocol (BGP), now carries about one million pieces of routing information, and bears the responsibility of allowing Internet Service Providers and their customers to set and enforce business rules about whose traffic is carried by whom. Moreover, establishing trust in that information, by providing defenses against attempts to hijack and "black hole" traffic, is a critical aspect of the solution. We explore these challenges in this chapter.
The routing algorithms described in Chapter |Routing| are not up to the challenge, for two main reasons. The first is the issue of scale. There are tens of thousands of autonomous organizations in the Internet and they span the globe, so it's not clear that the routing protocols we have seen to date could scale to that level even if we treated each organization as a simple point in a graph. But more importantly, the problem definition is not "find the shortest path to destination X". "Shortest path" is not even well defined, since each network can use independent metrics for their internal routing. Instead, the problem is "find a path to X that matches the policies of the providers who can deliver traffic to X". Ever since the Internet transitioned from being a research network to a commercially viable communication substrate in the 1990s, BGP has been the protocol used to answer questions of that kind.