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Appendix
279
Step Action Description
A response notifies User A that the
connection has been made.
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
The proxy server forwards the 180