g with SIP are RTP and SDP. RTP is used to carry the real-time multimedia data, including audio, video, and text. RTP makes it possible to encode and split the data into packets and transport the packets over the Internet. SDP is used to describe and encode the capabilities of session participants. This description is then used to negotiate the characteristics of the session SO that all the devices can participate. For example, the description is used in negotiation of the codecs used to encode media so all the participants will be able to decode it and in negotiation of the transport protocol to be used. Chapter 9 IP Services LAN A LAN B Figure 9.6 SIP SIP Network Elements Basic SIP elements include user agents, proxies, regis- trars, and redirect servers (see Figure 9.7). User agents usually but not necessarily reside on a user's computer in the form of an application, but user agents can also be cellular phones, PSTN gateways, PDAs, automated integrated voice response systems, and so on. User agents are often referred to as the user agent server (UAS) and user agent client (UAC). The UAS and UAC are logical entities only. Each user agent actually contains both. The UAC is the part of the user agent that sends requests and receives responses; the UAS is the part of the user agent that receives requests and sends responses. A proxy server is an optional SIP component. Proxy servers handle routing of SIP signaling, but they don't initiate SIP messages. User agents can send messages to a proxy server. The most important task of a proxy server is to route session invitations closer to the callee. The session invitation usually traverses a set of User Agent User Agent User Agent IP-Based Network User Agent Proxy Server Figure 9.7 SIP network elements proxies until it finds one that knows the actual location of the callee. Such a proxy then forwards the session invitation directly to the callee, and the callee then accepts or declines the session invitation. A registrar server is another optional SIP component. It is