A number of SIP messaging scenarios have already been described in this chapter, and most of them have described fairly straightforward call scenarios. We have also seen a number of extensions to SIP. In fact, one of the great advantages of SIP is the fact that it enables the easy addition of backwards-compatible extensions. Another great advantage of SIP is the fact that it can be used to provide a number of advanced features and services. For example, we have already seen how the REFER method can be used for a call-transfer application. We have also seen that SIP inherently supports personal mobility through the use of registration. We have seen how a forking proxy can enable a one-number service and we have also seen how the Retry-After: header can be used to support call-completion services. In many ways, these are just the tip of the iceberg when it comes to the features that SIP can offer.

It is possible for SIP messages to carry MIME content as well as an SDP description. Therefore, a response to an INVITE could include a piece of text, an HTML document, an image, and so on. Equally, because of the fact that a SIP address is a URL, it can easily be included in web content for click-to-call applications. Basically, SIP is well suited to integration with existing IP-based applications and can leverage those applications in the creation of new services. SIP not only...