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

for moving existing participants to the centralized conference. The example

provided here also is valid for moving from a two-party call to a centralized

conference.

The following steps involve the move from an ad hoc to a centralized con-

ference:

1. Discover a server that supports ad hoc centralized conferences, such as
   conf.factory.example.com.


2. Create a conference at the server by sending an INVITEto the Confer-
   ence Factory URI [5].


3. The conference server creates the unique conference URI by assigning it
   a random number or string and returning it with the isfocusfeature
   tag to indicate that the server is acting as a focus.


4. The application of the user in charge of migrating the ad hoc conference
   to the centralized server can now use the REFERmethod [8] in SIP call
   control to set up calls between the existing and new participants, and
   the conference server. See Chapter 19, “SIP Component Services,” on
   third-party call control used in component services.

For example, John Doe initiates the call setup for Mary Higgins:

REFER sip:[email protected] SIP/2.0

Via: SIP/2.0/UDP 192.0.0.4:5060;branch=z9hG4bKnashds7

Max-Forwards: 70

From: <sip:[email protected]>;tag=3412349dfa3s

To: <sip:[email protected]>;tag=874726

Call-ID: 4gfjweroiu2aiqjnszd

CSeq: 3432 REFER

Contact: <sip:192.0.0.4>

Refer-To: <sip:[email protected]>

Referred-by: <sip:[email protected]>

Content-Length: 0

As a result, Mary Higgins sets up her call to the conference server:

INVITE sip:[email protected] SIP/2.0

Via: SIP/2.0/UDP phone282.example.net:5060;branch=z9hG4bKnashds7e3

Max-Forwards: 70

From: <sip:[email protected]>;tag=1234d9dfa3s

To: <sip:[email protected]>

Call-ID: 4gfjweroiu2aiqjnszd

CSeq: 54 INVITE

Contact: <sip:phone282.example.net>

Content-Length: ...

Other users will be migrated in the same way to the centralized conference.

250 Chapter 14