Cisco 7960G Administrator's Manual page 111

Appendix B SIP Call Flows
Step Action
INVITE—Phone A to Phone B
1.
180 Ringing—Phone B to Phone A
2.
200 OK—Phone B to Phone A
3.
ACK—Phone A to Phone B
4.
A two-way RTP channel is established between Phone A and Phone B.
5. INVITE—Phone C to Phone B
6. 180 Ringing—Phone B to Phone C
INVITE—Phone B to Phone A
7.
200 OK—Phone A to Phone B
8.
9. ACK—Phone B to Phone A
Description
Phone A sends a SIP INVITE request to Phone B. The request is an
invitation to User B to participate in a call session. In the INVITE request:
The phone number of User B is inserted in the Request-URI field in the
•
form of a SIP URL. The SIP URL identifies the address of User B and
takes a form similar to an e-mail address (user@host, where user is the
telephone number and host is either a domain name or a numeric
network address). For example, the Request-URI field in the INVITE
request to User B appears as "INVITE sip:555-0199@companyb.com;
user=phone." The "user=phone" parameter distinquishes that the
Request-URI address is a telephone number rather than a username.
Phone A 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 A is ready to receive is specified.
Phone B sends a SIP 180 Ringing response to Phone A.
Phone B sends a SIP 200 OK response to Phone A. The response notifies
Phone A that the connection has been made.
If Phone B supports the media capability advertised in the INVITE message
sent by Phone A, it advertises the intersection of its own and Phone A media
capability in the 200 OK response. If Phone B does not support the media
capability advertised by Phone A, it sends back a 400 Bad Request response
with a 304 Warning header field.
Phone A sends a SIP ACK to Phone B. The ACK confirms that Phone A has
received the 200 OK response from Phone B.
The ACK might contain a message body with the final session description to
be used by Phone B. If the message body of the ACK is empty, Phone B uses
the session description in the INVITE request.
Phone C sends a SIP INVITE request to Phone B. The request is an invitation
to User B to participate in a call session.
Phone B sends a SIP 180 Ringing response to Phone C.
Phone B sends a midcall INVITE to 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 0.0.0.0. This places the call in
hold.
Phone A sends a SIP 200 OK response to Phone B.
Phone B sends a SIP ACK to Phone A. The ACK confirms that Phone B has
received the 200 OK response from Phone A.
Cisco SIP IP Phone Administrator Guide, Release 6.x and 7.x
Call Flow Scenarios for Successful Calls
B-15