SIP communication features six kinds of message requests. These requests, also referred to as methods, enable user agents and network servers to locate, invite, and manage calls. These are the six SIP requests:
This method indicates that the user or service is invited to participate in a session. It includes a session description. For two-way calls, the calling party indicates the media type. A successful response to a two-party INVITE (200 OK response) includes the called party's receive media type. With this simple method, users can recognize the capabilities of the other end and open a conversation session with a limited number of messages and round trips.
These requests correspond to an INVITE request. They represent the final confirmation from the end system and conclude the transaction initiated by the INVITE command. If the calling party includes a session description in the ACK request, no additional parameters are used in the session. If a session description is absent, the session parameters in the INVITE request are used as the default.
This method enables you to query and collect user agents and network server capabilities. However, this request is not used in order to establish sessions.
This method is used by calling and called parties to release a call. Before actually releasing the call, the user agent sends this request to the server indicating the desire to release the session.
This request enables user agents and network servers to cancel any in-progress request. This does not affect completed requests in which final responses were already received.
This method is used by clients to register location information with SIP servers.
SIP message responses are based on the receipt and interpretation of a corresponding request. They are sent in response to requests and indicate call success or failure, including the status of the server. The six classes of responses, their status codes, and explanations of what they do are provided in the table below. The two categories of responses are provisional, which indicates progress, and final, which terminates a request.
Note: In Table 11-3, informational responses are provisional and the remaining five are final responses.