Cisco 7960 Administrator's Manual page 163

Appendix B SIP Call Flows
Step Action
3
200 OK—Cisco SIP IP phone B
to Cisco SIP IP phone A
4
ACK—Cisco SIP IP phone A to
Cisco SIP IP phone B
A two-way RTP channel is established between Cisco SIP IP phone A and Cisco SIP IP phone B.
5 INVITE—Cisco SIP IP phone B
to Cisco SIP IP phone A
6 200 OK—Cisco SIP IP phone A
to Cisco SIP IP phone B
7
ACK—Cisco SIP IP phone B to
Cisco SIP IP phone A
The RTP channel between Cisco SIP IP phone A and Cisco SIP IP phone B is torn down. User A is put
on hold.
78-10497-02
Description
Cisco SIP IP phone B sends a SIP 200 OK response to
Cisco SIP IP phone A. The 200 OK response notifies Cisco
SIP IP phone A that the connection has been made.
If Cisco SIP IP phone B supports the media capability
advertised in the INVITE message sent by Cisco SIP IP
phone A, it advertises the intersection of its own and Cisco
SIP IP phone A's media capability in the 200 OK response.
If Cisco SIP IP phone B does not support the media
capability advertised by Cisco SIP IP phone A, it sends
back a 400 Bad Request response with a 304 Warning
header field.
Cisco SIP IP phone A sends a SIP ACK to Cisco SIP IP
phone B. The ACK confirms that Cisco SIP IP phone A has
received the 200 OK response from Cisco SIP IP phone B.
The ACK might contain a message body with the final
session description to be used by Cisco SIP IP phone B. If
the message body of the ACK is empty, Cisco SIP IP phone
B uses the session description in the INVITE request.
Cisco SIP IP phone B sends a mid-call INVITE to Cisco
SIP IP phone A with new SDP session parameters
(IP address), which are used to place the call on hold.
Call_ID=1
SDP: c=IN IP4 0.0.0.0
The c= SDP field of the SIP INVITE contains an 0.0.0.0.
This places the call in limbo.
Cisco SIP IP phone A sends a SIP 200 OK response to
Cisco SIP IP phone B.
Cisco SIP IP phone B sends a SIP ACK to Cisco SIP IP
phone A. The ACK confirms that Cisco SIP IP phone B has
received the 200 OK response from Cisco SIP IP phone A.
Cisco SIP IP Phone 7960 Administrator Guide
Call Flow Scenarios for Successful Calls
B-55