SIP methods

• INVITE — Used to invite and account to participate in a call session.
• ACK — Acknowledge an INVITE request.
• CANCEL — Cancel a pending request.
• REGISTER — Register user with a SIP server.
• OPTIONS — Lists information about the capabilities of a caller.
• BYE — Terminates a session between two users in a call.
• REFER — Indicates that the recipient(identified by the Request URI) should contact a third party using the contact information provided in the request.
• SUBSCRIBE — The SUBSCRIBE method is used to request current state and state updates from a remote node.
• NOTIFY — The NOTIFY method is used to notify a SIP node that an event which has been requested by an earlier SUBSCRIBE method has occurred.

Example SIP INVITE request

INVITE sip:201@192.168.1.104 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.102;rport;branch=z9hG4bKvbxaoqar
Max-Forwards: 70 
 
To: 
From: "NightRanger" ;tag=eihgg
Call-ID: hfxsabthoymshub@backtrack
CSeq: 649 INVITE
Contact: 
Content-Type: application/sdp 
 
Allow: INVITE,ACK,BYE,CANCEL,OPTIONS,PRACK,REFER,NOTIFY,SUBSCRIBE,INFO,MESSAGE
Supported: replaces,norefersub,100rel
User-Agent: Twinkle/1.2 
 
Content-Length: 310