Specifications

Appendix
439
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 A to Proxy Server
User A sends a mid-call INVITE request to
the proxy server with new SDP session
parameters, which are used to place
the call on hold.
F10
INVITE—Proxy Server to User B
The proxy server forwards the mid-call
INVITE message to User B.
F11
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 INVITE is
successfully processed.
F12
200 OK—Proxy Server to User
A
The proxy server forwards the 200 OK
response to User A. The 200 OK response
notifies User A that User B is successfully
placed on hold.
F13
ACK—User A to Proxy Server
User A sends the ACK message to the
proxy server. The ACK confirms that User
A has received the 200 OK response.
The call session is now temporarily
inactive. No RTP packets are being
sent.
F14
ACK—Proxy Server to User B
The proxy server sends the ACK
message to User B. The ACK confirms
that the proxy server has received the
200 OK response.
F15
INVITE—User A to Proxy Server
User A sends a SIP INVITE request to the
proxy server. In the INVITE request, a
unique Call-ID is generated and the
Contact-URI field indicates that User A
requests the call.
F16
INVITE—Proxy Server to User C
The proxy server maps the SIP URI in the
To field to User C. The proxy server sends