HP-UX Java SIP Stack Programmer's Guide (February 2008)

Figure 1-2 illustrates the interaction between user agents while a session is being
established and terminated.
Figure 1-2 Establishing and Terminating a Call
The following steps describe the sequence of messages exchanged between the user
agents:
• The following messages are exchanged while establishing a session:
1. UA1 sends an INVITE request message to invite UA2 to participate in the
session.
2. UA2 responds with a 100 - Trying response message to indicate that the
request has been received.
3. UA2 sends the 180-Ringing response to indicate that UA2 is ringing.
4. UA2 sends the 200-OK response to inform that the user at UA2 has picked
up the call.
5. UA1 sends the ACK request to confirm receipt of the 200-OK response.
• The following messages are exchanged while terminating a session:
6. When the user at UA1 ends the call, a BYE request message is sent to UA2.
7. UA2 responds with the 200-OK message and informs the user at UA2 that
the session has ended.
Using the Proxy Server
A proxy server exists between the users. This server handles requests and responses
between the users. Figure 1-3 illustrates the interaction between users that use the proxy
server.
30 Introduction