Summary: Copyright Hari Balakrishnan, 2001-2005, and Nick Feamster, 2005. All rights reserved.
Please do not redistribute without permission.
Interdomain Internet Routing
The goal of this lecture is to explain how routing between different administrative do-
mains works in the Internet. We discuss how Internet Service Providers (ISPs) ex-
change routing information (and packets) between each other, and how the way in which
they buy service from and sell service to each other and their customers influences the
technical research agenda of Internet routing in the real-world. We discuss the salient fea-
tures of the Border Gateway Protocol, Version 4 (BGP4), the current interdomain routing
protocol in the Internet.
4.1 Autonomous Systems
An abstract, highly idealized view of the Internet is shown in Figure 4-1, where end-hosts
hook up to routers, which hook up with other routers to form a nice connected graph
of essentially "peer" routers that cooperate nicely using routing protocols that exchange
"shortest-path" or similar information and provide global connectivity. The same view
posits that the graph induced by the routers and their links has a large amount of re-
dundancy and the Internet's routing algorithms are designed to rapidly detect faults and
problems in the routing substrate and route around them. Some would even posit that
the same routing protocols today perform load-sensitive routing to dynamically shed load