The Internet has quickly evolved into a vast global network owned and operated by thousands of different administrative entities. During this time, it became apparent that vanilla shortest-path routing would be insufficient to handle the myriad operational, economic, and political factors involved in routing. ISPs began to modify routing configurations to support routing policies, i.e. goals held by the router's owner that controlled which routes were chosen and which routes were propagated to neighbors. BGP, originally a simple path-vector protocol, was incrementally modified over time with a number of mechanisms to support policies, adding substantially to the complexity. Much of the mystery in BGP comes not only from the protocol complexity but also from a lack of understanding of the underlying policies and the problems ISPs face which they address. In this paper we shed light on goals operators have and their resulting routing policies, why BGP evolved the way it did, and how common policies are implemented using BGP. We also discuss recent and current work in the field that aims to address problems that arise in applying and supporting routing policies.





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