The instructions in the Request-Dispositionheader are explained here.
The caller can request the server to:
■■ Proxy—Proxy or redirect the call.
■■ Cancel—Handle CANCELrequests on behalf of the caller or let the caller
do it.
■■ Fork—Fork call requests to different URIs.
■■ Recurse—When receiving addresses to redirect the call to, the server
should try the new addresses, or return them to the caller to make the
decision to try again.
■■ Parallel—Try multiple addresses in parallel, or in sequential order, and
wait for a response before trying the next address.
■■ Queue—Queue the call if the called party is busy and return the provi-
sional response 182 Queued. Waiting in the queue can be terminated
by CANCELor BYErequests.The caller can also express preferences for how certain URIs should be han-
dled by using the headers Contact, Accept-Contact, and Reject-Con-
tact.
User preferences that relate to the same URI can have a range of classes to
specify such preferences as:
■■ Audio—To specify that the UA supports audio sessions.
■■ Application—To specify that the UA supports application sessions.
■■ Data—To specify that the UA supports data sessions.
■■ Control—To specify that the UA supports control sessions.
■■ Video—To specify that the UA supports video sessions.
■■ Text—To specify that the UA supports text sessions.
■■ Automata—To specify that the UA is an automata (such as a voicemail
server).
■■ Class—To specify the class of the UA (business or residential class). So
the caller can avoid calling someone at home.
■■ Duplex—To specify for certain types of lectures or conferences.
■■ Mobility—To set preference for mobile phone or fixed phone.
■■ Description—To specify a description of the UA.
■■ Event Packages—To specify which SIP event packages the UA supports.User Preferences 155