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 A is ready
to receive is specified.
The port on which User B is prepared
to receive the RTP data is specified.
INVITE—Proxy Server to User B
The proxy server maps the SIP URI in the
To field to User B. The proxy server sends
the INVITE message to User B.
100 Trying—User B to Proxy
Server
User B sends a SIP 100 Trying response to
the proxy server. The 100 Trying response
indicates that the INVITE request has been
received by User B.
100 Trying—Proxy Server to
User A
The proxy server forwards the SIP 100
Trying to User A to indicate that the
INVITE request has been received by User
B.
180 Ringing—User B to Proxy
Server
User B sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the User B is being
alerted.
180 Ringing—Proxy Server to
User A
The proxy server forwards the 180 Ringing
response to User A. User A hears the ring-
back tone indicating that User B is being
alerted.
200 OK— User B to Proxy Server
User B sends a SIP 200 OK response to the
proxy server. The 200 OK response
notifies User A that the connection has
been made.
200OK—Proxy Server to User A
The proxy server forwards the 200 OK
message to User A. The 200 OK response
notifies User A that the connection has
been made.
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