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The core SIP specification provides a way to set up and manage sessions between two user agents. It is possible to create and control a multi-party conference using this specification. However in such a scenario, referred to as the loosely coupled conference model, there does not exist a relationship between every participant in the conference. Such conference situations can be accomplished by using multicast
Alternatively, a UA can maintain multiple dialogs with multiple UAs while also acting as a media mixer. While the UA that is acting as the conference controller/mixer has knowledge of the other UAs involved in the conference, the other UAs do not know about each other. Additionally this scenario puts extra strain on the resources of the controlling UA by forcing it to both
RFC 4353 "A Framework for Conferencing with SIP" introduces a structure by which a central entity, called a focus, provides a variety of conference functions and mixing of media. In this type of conference, referred to as the tightly coupled conference model, each UA involved in the conference connects to the focus and maintains its own SIP dialog with it.This specification also defines a logical function called a conference policy server that stores conference policy, which is simply a set of rules governing a particular conference. The focus must be able to access this conference policy to determine how the conference should operate, such as if a particular UA is allowed to join the conference. The specification also defines a second logical function called a conference notification service. This is a service that a conference participant can subscribe to and receive notifications when changes in conference state occur. In this model, a UA participating in a conference can SUBSCRIBE to the conference URI and be alerted via SIP NOTIFY messages when the state of the conference changes, such as when participants enter and leave the conference. Often the conference focus, policy server, and notification service are located in the same physical entity. RFC 4575 "A SIP Event Package for Conference State" defines an event package for notifying participants of a tightly coupled conferences of the conference state. RFC 4579 "SIP Call Control - Conferencing for User Agents" uses the concepts from RFC 4353 and RFC 4575 to define a set of recommended practices for creating and controlling