Figure 2.18 REGISTER message flow
2.2.2.6.2 Session invitation
A session invitation consists of one INVITE request which is usually sent to a proxy.The proxy sends immediately a 100 Trying reply to stop re-transmissions and forwards the request further.
All provisional responses generated by the called party are sent back to the calling party. See teh 180 Ringing response in the call flow.The response is generated when the called party's phone starts ringing.
A 200 OK is generated once the called party picks up the phone and it is re-transmitted by the called party's user agent until it receives an ACK from the calling party.The session is established at this point.
2.2.2.6.3 Session termination
Session termination is accomplished by sending a BYE request within the dialogue established by INVITE. BYE messages are sent directly from one user agent to the other, unless a proxy on the path of the INVITE request has indicated that it wishes to stay on the path by using record routing (see Section 2.2.2.6.4).
A party wishing to tear down a session sends a BYE request to the other party involved in the session.The other party sends a 200 OK response to confirm the BYE and the session is terminated. See Figure 2.20, left message flow.
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[IP Telephony Cookbook] / Technological Background
Calling party
SIP Proxy
Called party
INVITE
100 Trying
INVITE
100 Trying
180 Ringing
180 Ringing
200 OK
200 OK
ACK