18 Developing SIP and IP Multimedia Subsystem (IMS) Applications
Step 3 - 4
The response to the REGISTER message is positive, which is indicated by
the 200 (OK) message.
Step 5 - 6
Alice sends a SIP INVITE to Bob. Let us assume that Alice doesn't know
where Bob has registered, and hence the To address field in the SIP INVITE
header is left blank.
Step 7 - 8
The Redirect Server responds with a status code 302 (moved temporarily).
This response has field Contact in the header which is filled with an address
where Alice should try as an alternative. Though not shown in the diagram, it
is assumed that Bob has pre-registered before the call flow.
Step 9 - 10
Alice acknowledges (ACK) the 302 message.
Alice now sends a new SIP INVITE message for Bob.
The role of the proxy server for this message becomes crucial. The proxy
server may map the Request-URI message to a different address, if it knows
that the recipient is at a different address. In our case, Alice knows Bob
address, and hence this won't be necessary. In this case, the Proxy server
just inspects the domain part of the received Request-URI and determines
the next hop in the path from the caller (Alice) to the callee (Bob). After the
initial INVITE request and response, it is possible that subsequent messages
are sent end to end (without traversing a Proxy Server). However, a proxy
server can also ensure that it remains in the signaling path for all subsequent
requests as well.
The Proxy Server sends a Trying message. Any 1XX response is a
provisional response and it indicates that a session has not yet been
established, but a dialog is on. This is an early state of a dialog. There could
also be an optional SIP 180 Ringing Message (Not shown in this call flow).
Step 14 - 15
Bob answers the call with a 200 OK response, indicating that he is ready to
take a call.