SIP was to some extent derived from HTTP and so the two protocols have much in common. Both are request-response protocols and messages have very similar structure and formats. However, in terms of providing services, there are important differences between the two protocols:
- HTTP services (including HTTP servlet applications) are almost exclusively hosted on HTTP origin servers, that is, on the Web server generating the final response to requests (as opposed to a proxy server). In contrast, an important function of SIP applications is intelligent request routing and the ability to act as a proxy is crucial in this context.
- HTTP is not a peer-to-peer protocol as is SIP and web applications never originate requests. SIP applications, on the other hand, need the ability to initiate requests of their own. An application that accepts an incoming call may have to terminate it by subsequently sending a BYE request towards the caller, a wakeup-call application may have to send the initial INVITE establishing a dialog, and a presence server application needs to be able to initiate NOTIFY requests. A back-to-back user agent (B2BUA) is a type of network based application that achieves a level of control over calls not attainable through proxying, and that requires client functionality, also. These examples demonstrate why client-side functionality is necessarily part of a SIP service infrastructure and explains the presence of such functionality in the SIP Servlet API.
It follows that, in addition to the ability inherited from the Servlet API of responding to incoming requests, the SIP Servlet API MUST support the following capabilities:
- generate multiple response (for example, one or more 1xx followed by a final response)
- proxying requests, possibly to multiple destinations
- initiate requests
- receive responses as well as requests
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