the media connection. This decomposition is transparent to SIP, and the protocols used to decompose a gateway are not described in this book. Another difference between a user agent and a gateway is the number of users supported. While a user agent typically supports a single user (although perhaps with multiple lines), a gateway can support hundreds or thousands of users. A PSTN gateway could support a large corporate customer, or an entire geographic area. As a result, a gateway does not REGISTER every user it sup- ports in the same way that a user agent might. Instead, a non-SIP protocol can be used to inform proxies about gateways and assist in routing. One proto- col that has been proposed for this is the Telephony Routing over IP (TRIP) protocol [6], which allows an interdomain routing table of gateways to be devel- oped. Another protocol called Telephony Gateway Registration Protocol (TGREP) [7] has also been developed to allow a gateway to register with a proxy server within a domain. 3.5 SIP Servers SIP servers are applications that accept SIP requests and respond to them. A SIP server should not be confused with a user agent server or the client-server nature of the protocol, which describe operation in terms of clients (originators of requests) and servers (originators of responses to requests). A SIP server is a dif- ferent type of entity. The types of SIP servers discussed in this section are logical entities. Actual SIP server implementations may contain a number of server types, or may operate as a different type of server under different conditions. Because servers provide services and features to user agents, they must support both TCP, TLS, and UDP for transport. Figure 3.2 shows the interaction of user agents, servers, and a location service. Note that the protocol used between a server and the location service or database is not in general SIP and is not dis- cussed in this book. 3.5.1 Proxy Servers A SIP proxy server receives a SIP request from a user agent or another proxy and acts on behalf