Following are examples of the Referred-Byheader:
■■ Referred-By: sip:[email protected];ref=http://
headhunters.comprovides the reference source for “headhunters,”
which is a web page.
■■ Referred-By: sip:[email protected]; ref=<htpp://
headhunters.com>; scheme=pgp;pgp-version=”5.0”;
signature=”34a6e328d7cc710f8382”also provides a Pretty Good
Privacy (PGP) signature computed across the URI of referee and the ref-
erence URI. It is recommended that Referred-Byheaders be signed to
prevent unauthorized parties from hijacking calls.Informational reply status codes have been proposed [13] to provide infor-
mation about the progress of the call. Other approaches are also being consid-
ered, including the use of SUBSCRIBEand NOTIFYrequests to request and
receive notification about the final outcome of a REFERrequest.SIP Third-Party Call Control
As mentioned, by contrast to the ITU and CTI standards, the SIP third-party
control presumes intelligent IP end devices instead of dumb terminals, keep-
ing pace with the advent of highly distributed computing. We will show here
how intelligent IP endpoints can be controlled for communication services
using SIP only. The central idea is loose coupling between intelligent IP end-
points and using SIP call setup only to invoke the necessary functions in the
devices (such as phones and desktop and Palm computers). The details of the
device operation are then left to the device itself.
Third-party call control is a basic telephony function and is used for many
services (such as call setup by a controller and call transfers). SIP third-party
call control also can be used to control many other services and integrate com-
munications with various other applications and transactions, as shown in the
application services architecture in Chapter 19, “SIP Component Services.” No
other standard or proprietary protocols are necessary besides SIP.NOTEReaders familiar with computer telephony integration (CTI) can
appreciate the complexity of the technology involved in controlling phones
from computers and desktop PCs. CTI relies on complex schemes that are based
on special application programming interfaces (APIs) that, in turn, depend on
proprietary system implementations and proprietary operating systems.
Any “open” API has, therefore, two claimants on intellectual property rights and
change by the owners at their convenience. We will show here that computers
can control phones in a very simple and completely open, standard manner using
SIP. It is the opinion of the authors that CTI has been made obsolete by SIP.202 Chapter 11