Routing in the Internet has traditionally implied shunting packets based on layer 2 (network layer of the IP reference model) addresses. While that may have been adequate in the past, the new protocols of the Internet are moving routing to the highest layer — the application layer. These protocols include established ones such as simple mail transport protocol (SMTP) and new ones such as session initiation protocol (SIP). One could argue that e-mail is, in a generalized form, a routing engine: Users can configure it to route e-mail messages to the location where they happen to be. Most recent protocols such as SIP add multiple novel dimensions to application-layer routing, such as mobility and end-user configuration. At its core, a SIP call controller (better known as a SIP proxy) is a pure routing engine. It analyzes an incoming invitation and makes a decision, after accounting for various factors, on how to best route the message. In this paper, we take a deeper look at how these new protocols route messages and, in the process, make the Internet that much more indispensable.3