REFER and NOTIFY Methods
Third-party call control enables a SIP entity to be in control of session signaling while the media is exchanged between other entities. In some situations, the controller will not want to continue monitoring (controlling) the signaling of the session. Instead, the controller will want the other entities to continue the session independently. At this point, the controller requires a mechanism to transfer SIP sessions to another entity.
To provide session transfer functionality, the REFER method was defined. One SIP entity instructs another to perform a certain action. For example, the REFER method instructs a server to send a specific request a certain URL.
Refer to PPL Event Indication IDs 0x0021 and 0x0022 in PPL Event Indications.
The NOTIFY method is defined in SIP to provide asynchronous event notification. The CSP supports the NOTIFY method, which is typically used with the REFER method in consultative and non-consultative Call Transfer applications (IP Centrex or IP PBX).
Important! The REFER and NOTIFY methods are supported for bearer-free and bearer calls.
Refer to the PPL Event Request ID 0x0020 in the PPL Event Requests.
This feature was implemented per the following IETF documents:
• · draft-ietf-sip-cc-transfer-05.txt
• · draft-ietf-sip-refer-02.txt
• · draft-ietf-sip-replaces-00.txt