135A major driver for many service providers adopting SIP is the advantages and
flexibility of service creation using the protocol. Some of the typical
approaches will be discussed in this chapter, including server implementation,
called user agent implementation, and calling user agent implementation. Call
Processing Language (CPL) and SIP Common Gateway Interface (CGI) will
also be introduced in this chapter. The various options for service creation,
such as CPL, CGI, SIP Java Servlets, Java Integrated Network (JAIN), and
Voice Extended Markup Language (VoiceXML) will also be discussed.
Services in SIP
The basic functions of the SIP protocol involved in establishing sessions
between two endpoints over the Internet was discussed in Chapter 6, “SIP
Overview.” This chapter discusses implementations of additional functional-
ity in relation to session establishment, henceforth referred to generically as
“services.” A classic example of a telephony service is call forwarding, which
results in an endpoint being contacted that is different from the one that was
dialed.
More advanced services can be implemented using SIP than can be imple-
mented in the PSTN because of the increased amount of signaling information
available during a call setup in SIP. Many of these advanced features and