proxy in HTTP and other Internet protocols. A SIP proxy does not set up or terminate sessions, but sits in the middle of a SIP message exchange, receiving messages and forwarding them. This example shows one proxy, but there can be multiple proxies in a signaling path. SIP has two broad categories of URIs: ones that correspond to a user, and ones that correspond to a single device or end point. The user URI is known as an address of record (AOR) and a request sent to an address of record will require database lookups and service and feature operations and can result the request being sent to one or more end devices. A device URI is known as a contact, and typically does not require database lookups. An address of record URI is usually used in To and From header fields, as this is the general way to reach a person and is suitable for storing in address books and in returning missed calls. A device URI is usually used in a Contact header field and is associated with a particular user for a shorter period of time. The method of relating (or binding) a contact URI with an address of record URI will be discussed in Section 2.3. Because Schroedinger does not know exactly where Heisenberg is cur- rently logged on and which device they are currently using, a SIP proxy server is used to route the INVITE. First, a DNS lookup of Heisenberg's SIP URI domain name (munich. de) is performed, which returns the IP address of the proxy server proxy munich. de, which handles that domain. The INVITE is then sent to that IP address: Introduction to SIP Schroedinger Proxy server Heisenberg INVITE INVITE 180 Ringing 180 Ringing 200 OK 200 OK Media session 200 OK Figure 2.2 SIP call example with proxy server. INVITE sip:werner.heisenberg@munich.de SIP/2.0 Via: SIP/2.0/UDP 100.101.102.103:5060; branch=z9hG4bKmp17a Max-Forwards: 70 To: Heisenberg <sip:werner.heisenberg@munich.de> From: E. Schroedinger <sip:schroed5244@aol.com>;tag=4 Call-ID: 10@100.101.102.103 CSeq: 1 INVITE Subject: Where are you exactly? Contact: <sip:schroed5244@pc33.a