Specifications

Appendix
415
Step
Action
Description
F7
ACK—User A to Proxy Server
User A sends a SIP ACK to the proxy
server, The ACK confirms that User A has
received the 200 OK response. The call
session is now active.
F8
ACK—Proxy Server to User B
The proxy server sends the SIP ACK to
User B. The ACK confirms that the proxy
server has received the 200 OK
response. The call session is now active.
F9
INVITE—User C to Proxy Server
User C sends a SIP INVITE message to
the proxy server. The INVITE request is an
invitation to User A to participate in a
call session.
In the INVITE request:
 The IP address of User A is inserted
in the Request-URI field.
 User C is identified as the call
session initiator in the From field.
 A unique numeric identifier is
assigned to the call and is inserted
in the Call-ID field.
 The transaction number within a
single call leg is identified in the
CSeq field.
 The media capability User C is
ready to receive is specified.
 The port on which User A is
prepared to receive the RTP data
is specified.
F10
INVITE—Proxy Server to User A
The proxy server maps the SIP URI in the
To field to User A. The proxy server sends
the INVITE message to User A.
F11
180 Ringing—User A to Proxy
Server
User A sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the user is being
alerted.
F12
180 Ringing—Proxy Server to
User C
The proxy server forwards the 180
Ringing response to User C. User C hears
the ring-back tone indicating that User
A is being alerted.