hey appear as a single application to the user: a videoconference application. The separation between the SIP UA handling the delivery of a session description and the media tools actually handling the contents of the ses- sion description is powerful. This separation enables SIP to establish any type of session. What Does a SIP User Agent Look Like? SIP UAs are implemented on top of many different systems. They can run, for instance, in a computer as one among many applications, or they can be implemented in a dedicated device, such as a SIP phone. The device type will not affect SIP. Media tools might vary from device to device depending on the type of sessions invoked, but the SIP behavior is always the same. The Session Initiation Protocol: SIP ferent from each other. This is because the user interface varies with the Nonetheless, from the user point of view, SIP devices can look very dif- kind of device. The user interface of a videoconference program running click, on a but SIP phone will probably resemble a traditional telephone with computer will most likely be a window with a selection of buttons to the buttons a 0 through 9, *, and #. SIP devices range from powerful computers devices accessing the Internet , with a high-bandwidth connection to small with low-bit rate wireless connections. Figure 4-9 shows some examples. I should mention that work to adapt SIP for household appliances could is ongoing. Therefore, future examples of devices with SIP User Agents include refrigerators, toasters, and lamps. We are focusing on telephony examples because they are easier to under- stand and more immediately relevant for most readers; however, remember kind that SIP is powerful precisely because it can be used to establish any of session. Voice sessions are just one example. Figure 4-9 Examples of devices that have SIP user agents. Chapter 4 Redirect Servers Redirect servers help locate SIP UAs by providing alternative locations where the user can be reachable. For example, Laura wants to call Bob. On her monito