HP-UX C SIP Stack Programmer's Guide (Novembery 2007)

Session Initiation Protocol (SIP) 9
Entity Interaction
ENTITY
I
NTERACTION
This section describes the interaction between SIP entities in various common
session initiation scenarios.
SESSION
E
STABLISHMENT AND
T
ERMINATION
Figure 1-1 shows the interaction between two user agents during trivial session
establishment and termination.
Figure 1-1 SIP Session Establishment and Call Termination
SESSION ESTABLISHMENT CALL FLOW
1. UA1 sends an INVITE message to Bob’s SIP address:
sip:[e-mail]. This message also contains a packet
describing the media capabilities of the calling terminal.
2. UA2 receives the request and immediately responds with a
100-Trying response message.
3. UA2 starts “ringing” to inform Bob of the new call.
Simultaneously a 180 (Ringing) message is sent to the UAC.
4. UA2 sends a 182 (Queued) call status message to report that
the call is behind two other calls in the queue.
5. UA2 sends a 182 (Queued) call status message to report that
the call is behind one other call in the queue.
User Agent
1
User Agent
2
1: INVITE bob
2: 100/Trying
7: ACK
3: 180/Ringing
4: 182/Queued, 2 callers ahead
5: 182/Queued, 1 caller ahead
6: 200/OK
2: 200/OK
1: BYE