SUBSCRIBE method is used when an UA entity wishes to subscribe to a service whereby he would receive event notifications. The type of event is described in the header Event. The entity which accepts the subscription returns the response 200 OK containing the duration of the subscription in the header Expires. The subscription has to be renewed by transmitting a new SUBSCRIBE request. In the absence of renewal, the subscription terminates automatically. 2.6.1.9. NOTIFY method The NOTIFY method enables an entity to notify the occurrence of an event. This entity needs to receive a response 200 OK which gives the assurance that the request has been properly received. Receipt of the response 481 Dialog/Transaction Does Not Exist automatically terminates the subscription. 2.6.1.10. REFER method The REFER method can be used to transfer media established between two UA entities (e.g. Alice and Bob) to someone else. The REFER request sent by Alice (transferor) to Carol to resume communication. The header Refer-to of the request contains the SIP URI of Bob (transferee). It Signaling Protocols should be noted that in this scenario, communication is not established between Alice and Carol. In a second scenario, Alice establishes an additional communication with Carol. Alice then sends the REFER request to Bob to transfer the communication that she has established with Carol. When Bob notifies her that the transfer has been successful, Alice releases the communication established with Bob. 2.6.1.11. MESSAGE method The MESSAGE method is used to transmit short message service (SMS), contained in a message body, between the two UA entities involved. This message is acknowledged by a response 200 OK. The response must not contain the message body. If the recipient wishes to respond, he must in turn generate a new MESSAGE request. 2.6.2. Responses The response begins with a line containing the version of the protocol, followed by a code for the type of response and a textual description of the code. SIP/2. . 200 OK The different t