Preferences of the Called Party
The preferences of the called party are generally invoked by an incoming
proxy server that handles incoming calls for the called party. For example,
calls to sip:[email protected] be routed through the SIP proxy server
specified in the DNS SRV records for the pulver.comdomain.
Since this server will be making decisions on the called party’s behalf, a
mechanism has been developed in SIP for a user to upload preferences and
services into a SIP proxy server. This mechanism is the REGISTERmessage. It
is the means for specifying preferences and services using CPL [3], as intro-
duced in Chapter 7, “SIP Service Creation.”
Many of the switches in CPL use the Caller Preferences parameters in the
Contactheaders of the caller’s INVITEand the called party’s REGISTER.
Server Support for User Preferences and for Policies
Servers can use the Contact, Accept-Contact, and Reject-Contact
headers to make the following decisions:
■■ Should it proxy or redirect the request?
■■ Which URIs to proxy or redirect to.
■■ Should it fork the request?
■■ How to search (recursively or not), or to search in parallel or
sequentially.
Administrative policies can also be exercised at the server to exclude, for
example, certain URIs or to exclude video for certain callers to conserve band-
width.
Summary
This chapter has shown how the combination of SIP caller preferences and
CPL scripting provide a powerful capability for processing calls and designing
services.
User Preferences 157