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SIP Basic Call Flow
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Diagram illustrate a successful SIP IP phone-to-SIP IP phone call.


INVITE: UA-A to UA-B


UA-A sends an INVITE request to the UA-B. The request is an invitation to UA-B to participate in a call session.
 Mandatory Header Fields in an INVITE
• Via
• To
• From
• Call-ID
• CSeq
• Contact
• Max-Forwards

100 Trying: UA-B to UA-A

UA-B sends a 100 Trying response to UA-A so that it stops retransmission of INVITE message.

180 Ringing: UA-B to UA-A

UA-B sends a 180 Ringing response to the UA-A. At this point UA-B starts ringing.


200 OK: UA-B to UA-A

UA-B sends a 200 OK response to UA-A. The response notifies UA-A that UA-B has answered the phone.


ACK: UA-A to UA-B

UA-A sends an ACK to UA-B. The ACK confirms that UA-A has received the 200 OK response from UA-B.


Media Session: UA-A and UA-B

A media session is considered established when the INVITE, 200 OK, and ACK messages have been exchanged between the UA-A and the UA-B.


BYE: UA-B to UA-A

The request indicates that UA-B wants to release the call. Because it is UA-B that wants to terminate the call, the Request-URI field is now replaced with UA-A SIP URL and the From field contains UA-B SIP URL.

200 OK: UA-A to UA-B

This response notifies UA-B that UA-A has received the BYE request.



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