SIP MESSAGE
-SIP is a text based protocol (like HTTP, the Web protocol or SMTP)

SIx message are defined as below:
☆Invite
☆Ack
☆Bye
☆Options
☆Cancel
☆Register

SIP MESSAGE
●REGISTER - Used when a user agent first goes online and registers their
●SIP address and IP address with a Registrar server.
●INVITE - used to invite another User agent to communicate, and then
establish a SIP session between them.
●ACK - used to accept a session and confirm reliable message exchanges.
●OPTIONS - used to obtain information on the capabilities of another user
agent, so that a session can be established between them.
●CANCEL - used to cancel a pending request without terminating the session.
●BYE - used to terminate the session. Either the user agent who initiated the
session, or the one being called can use the BYE command at any time to
terminate the session.