The basic messages sent in a SIP environment are:
Most of the time, you will use REGISTER, INVITE, BYE, and CANCEL. Some messages are used for other features. As an example, INFO is used for DTMF relay and mid-call signaling information. PUBLISH, NOTIFY, and SUBSCRIBE give support to presence systems. REFER is used for call transfer and MESSAGE for chat applications. Newer messages can appear depending on the protocol standardization process.
Responses to these messages are in text format as in the HTTP protocol. Some of the most important are shown below:
This section introduces some basic SIP operations using a simple ...