Interdomain Internet Routing

Abstract

Our goal is to explain how routing between different administrative domains works in the Internet. We discuss how Internet Service Providers (ISPs) exchange routing information, packets, and (above all) money between each other, and how the way in which they buy service from and sell service to each other and their customers influences routing. We discuss the salient features of the Border Gateway Protocol, Version 4 (BGP4, which we will refer to simply as BGP), the current interdomain routing protocol in the Internet. Finally, we discuss a few interesting failures and shortcomings of the routing system. These notes focus only on the essential elements of interdomain routing, often sacrificing detail for clarity and sweeping generality.