es SIP the perfect protocol to combine into different services for the user. Specifi- cally, its similarities with HTTP [RFC 2068] and Simple Mail Transfer Pro- tocol (SMTP) [RFC 821] make it easy to combine the most successful Internet services SO far (Web and e-mail) with multimedia. SIP not only integrates services, but it also delivers them to the user's real location. For those in search of the holy grail of unified communications, SIP represents a revolution in combined services because SIP applications integrate Web browsing, e-mail, voice calls, videoconferencing, presence information, and instant messages in a straightforward way. Some people see SIP as the telecommunications industry's next killer app. SIP Is Based on HTTP A SIP implementation is rather similar to an HTTP implementation for the obvious reason that the former is based on the latter. Both use a request/response model, both are text based, and both The Session Initiation Protocol: SIP have a similar format for encoding protocol messages. These similarities enable an implementation to reuse code between the two protocols. The utility in sharing code is highest for devices that have to provide Web browsing and SIP-based services. A SIP mobile phone with wireless Inter- net features will almost certainly implement both (Figure 4-16). Such devices lack the huge hard disks and tons of memory you're used to on the desktop; they're usually thin devices with serious footprint constraints for which the capacity gained by reuse is substantive. SIP Uses URLs to Address SIP Resources Fortunately, the format used by SIP to address SIP entities is identical to the one used on the Web and by e- mail systems. This gives tremendous flexibility to SIP redirections and lets us integrate several forms of communication. A redirect server typically returns an alternative address where the user can possibly be located. The redirect server is actually returning a URL. In most of our examples, it's a SIP URL, but the server has no problem return- ing a Web URL or 