Next Generation Wireless Applications: Creating Mobile Applications in a Web 2.0 and Mobile 2.0 World, 2nd Edition

14.6 SESSION INITIATION PROTOCOL (SIP)

14.6.1 Making the Connection

SIP is a protocol at the heart of IMS. It comes from an Internet Engineering Task Force (IETF) standard, called RFC 3261. However, SIP is also a wider set of ideas. In order for SIP to work, IP-networked entities are required. These entities, whatever they are, engage in a SIP dialogue using SIP user agents, as shown in Figure 14.2.

Two SIP user agents cannot communicate without outside help. They need help in finding each other the same way that two phones need a phone number (address) in order to connect. In the world of IP, entities talk using IP addresses. People don't have IP addresses, so we can't contact Aunt Sally¹ on her SIP-enabled device via her IP address. What we can do is contact her via a telephone number that her mobile company has assigned to her. However, SIP user agents address each other using IP addresses, not phone numbers. With a mobile network that supports IP, like GPRS or UMTS, it's not a problem to assign an IP address to Aunt Sally's SIP-capable device. However, we then need a mechanism to discover that IP address so that when we tell our SIP device to contact Aunt Sally's device via her phone number, it has a way to turn the number into an IP address in order to route SIP messages over IP packets.

There's an added complication to locating Aunt Sally's device via IP. In these silicon-abundant days, IP is easy to implement in software on nearly any device with a silicon chip in it. Therefore, ...