Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 6.6 Session cancellation example using CANCEL

Call Control

The SIP architecture is one of peer-to-peer communication and end-to-end con-

trol. For example, a proxy may not issue a BYErequest terminating a call. It can

only be issued by one of the user agents (end devices) participating in the call.

However, the ability for a third party to direct or control a call between two

other parties can be extremely useful in various service implementations. For

example, an embedded SIP URI in a web page, when clicked, could cause a

desktop SIP phone to place a call to the desired URI. Or, third-party call con-

trol could be used to implement a web call center or ACD feature, which is use-

ful for handling calls to customer service numbers, where the controller

receives the call and routes it based on a number of factors such as available

agents, time of day, and other factors.

SIP User Agent

1 INVITE

Proxy Server SIP User Agent

9 CANCEL

11 CANCEL

2 INVITE

3 100 Trying 4 INVITE

5 100 Trying

6 180 Ringing

7 180 Ringing

8 180 Ringing

10 200 OK

12 200 OK

17 487 Request Cancelled

No Media Session

Established

Phone stops ringing

on receipt of CANCEL

CANCEL is used to

cancel a pending

session.

13 CANCEL

16 ACK

18 ACK

19 487 Request Cancelled

20 ACK

14 200 OK

15 487 Request Cancelled

Proxy Server

118 Chapter 6