SIP Call Flow
The call flows between SIP gateways can be direct or through a proxy or redirect server.
Direct Call Setup
When a UA recognizes the address of a terminating endpoint from cached information,
or has the capacity to resolve it by some internal mechanism, the UAC might initiate
direct (UAC-to-UAS) call setup procedures. If a UAC recognizes the destination UAS, the
client communicates directly with the server, as illustrated in Figure 5-24. In situations in
which the client is unable to establish a direct relationship, the client solicits the assistance
of a network server.
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Invite (SDP)
ACK
200 OK
100 Trying
180 Ringing
SIP
Gateway
Calling Party Called Party
IP
BYE
RTP Stream
SIP Signaling and SDP
(UDP or TCP)
Bearer or Media
(UDP)
200 OK Signaling
V
SIP
Gateway
V
Figure 5-24 Direct Call Setup
Figure 5-24 illustrates the following steps of a direct call setup procedure:
1. The originating UAC sends an invitation (INVITE) to the UAS of the recipient. The
message includes an endpoint description of the UAC and SDP.
2. If the UAS of the recipient determines that the call parameters are acceptable, it
responds positively to the originator UAC.
3. The originating UAC issues an ACK.
At this point, the UAC and UAS have all the information that is required to establish RTP
sessions between them.
Call Setup Using a Proxy Server
The proxy server procedure is transparent to a UA. The proxy server intercepts and forwards
an invitation to the destination UA on behalf of the originator, as depicted in
Figure 5-25.
300 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Invite (SDP)
ACK ACK
200 OK 200 OK
Invite (SDP)
100 Trying 100 Trying
180 Ringing 180 Ringing
SIP
Gateway
Proxy
Server
Calling Party Called Party
IP
BYE
RTP Stream
SIP Signaling and SDP
(UDP or TCP)
Bearer or Media
(UDP)
BYE
200 OK 200 OK
V
SIP
Gateway
IP V
Figure 5-25 Call Setup Using a Proxy Server
A proxy server responds to the issues of the direct method by centralizing control and
management of call setup and providing a more dynamic and up-to-date address resolution
capability. The benefit to the UA is that it does not need to learn the coordinates of
the destination UA, yet it can still communicate with the destination UA. The disadvantages
of this method are that using a proxy server requires more messaging and creates a
dependency on the proxy server. If the proxy server fails, the UA is incapable of establishing
its own sessions.
Note Although the proxy server acts on behalf of a UA for call setup, the UAs establish
RTP sessions directly with each other.
When a proxy server is used, the call setup procedure, as illustrated in Figure 5-25, uses
the following steps:
1. The originating UAC sends an invitation (INVITE) to the proxy server.
2. The proxy server, if required, consults the location server to determine the path to
the recipient and its IP address.
3. The proxy server sends the INVITE to the UAS of the recipient.
4. If the UAS of the recipient determines that the call parameters are acceptable, it
responds positively to the proxy server.
5. The proxy server responds to the originating UAC.
6. The originating UAC issues an ACK.
7. The proxy server forwards the ACK to the recipient UAS.
The UAC and UAS now have all the information that is required to establish RTP sessions
between them.