A dialog cannot be started until the two endpoints have exchanged information
regarding the parameters for the dialog and have acknowledged interest in
participating in the dialog. This is accomplished through a handshake procedure.
For the purposes of this discussion, a handshake takes place when an endpoint sends
a request to a destination, the destination agrees to the session via a successful response
(200 OK), and the requestor returns an acknowledgment. Once this sequence of events
has been completed, the dialog is complete. The handshake procedure is discussed in
more detail in Chapter 5 when we talk about establishing a session in the IMS.
The method used determines the type of request being made. We have already discussed
the different methods defined for SIP. Each of these methods is used for specific needs. For
example, if content such as voice or video is to be exchanged using a connection-orientedtype
service, then INVITE is the method used. However, if the message is carried within
the body of the method itself, then perhaps the MESSAGE method is used.
The first line of a request is the start line, followed by the headers. Here is the format
of the start line for a request:
METHOD (space) REQUEST URI (space) SIP VERSION (crlf)
The request-URI is the address that the request is being sent to, either in the form
of a URI or in the form of a TEL URI.
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