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

■■ Priority—To specify the priority of the request.

■■ Schemes—To specify which URI schemes the UA supports.

■■ Extensions—To specify which SIP extensions the UA supports.

■■ Methods—To specify the capabilities of the UA (such as voice or IM).

■■ Actor—To express the role a UA performs. Example roles that are

defined in RFC 3840 are principal(direct communication with the

person), attendant(indirect communication through a third person),

msg-taker(a message will be taken and delivered to the principal),

and information(information about the principal is available).

■■ Is Focus—To specify that the UA is a conferencing server or focus.

The following are examples of these classes.

Example for Contact

The caller would specify in the REGISTERmessage or an INVITEmessage:

Contact: HenryS <sip:[email protected]>;audio;video

;text;duplex=”full”;priority=”urgent”

The preferences include audio, video, and text chat in full duplex with a

specification for urgency.

Example for Accept-Contact

The caller would like to speak to a UA that supports the SIPMESSAGEmethod

for page mode instant messaging, and is a business device. The degree of pref-

erence is indicated by the weight factor q.

Accept-Contact: *;methods=”MESSAGE”;class=”business”;q=1.0

Example for Reject-Contact

The caller would not like to communicate with a voicemail server or a device

with video.

Reject-Contact: *;actor=”msg-taker”;video

The Reject-Contactfield can also contain a list of URIs for which no call

setup is desired.

For more examples and use cases of caller prefs, see [3].

156 Chapter 8