The idea of a session is closely aligned with the idea of a human activity. A session is a collection of communication exchanges, which together make up a single overall identifiable task. For example, a conversation, or the viewing of a single ``program'', or a collaborative meeting.
Multimedia Sessions come in many shapes and sizes, but there are only really two models for session control: light-weight sessions and tightly coupled conferencing.
Light-weight sessions are multicast based multimedia conferences that lack explicit session membership and explicit conference control mechanisms. Typically a lightweight session consists of a number of many-to-many media streams supported using RTP and RTCP using IP multicast.
The concept of lightweight sessions is explored in more detail in the first interlude after chapter four.
The rendezvous mechanism for light-weight sessions is a multicast based session directory. This distributes session descriptions[#!sdp!#] to all the potential session participants. These session descriptions provide an advertisement that the session will exist, and also provide sufficient information including multicast addresses, ports, media formats and session times so that a receiver of the session description can join the session. As dynamic multicast address allocation can be optimised by knowing which addresses are in use at which times, the session directory is an appropriate agent to perform multicast address allocation.
Tightly coupled conferences may also be multicast based and use RTP and RTCP, but in addition they have an explicit conference membership mechanism and may have an explicit conference control mechanism that provides facilities such as floor control.
Such conferences may be initiated either by invitation (the ``conference'' calls a user), or by user initiation (the user calls the ``conference''). In the latter case the rendezvous mechanism can be handled by the same session directory that handles light-weight sessions, with the addition of a description of the contact mechanism to be used to join the conference to the description of the session. In the former case, a call up mechanism is required which can be combined with the explicit conference membership mechanism.
No standard mechanism currently exists to perform either the conference membership mechanism or the ``dial-up'' mechanism in the Internet, and the many proprietary conferencing systems available all implement this in different ways. At the time of writing, it seems likely that a protocol based on the ITU's T.124 t124 recommendation will be derived for Internet usage.
For both models, a rendezvous mechanism is needed. Note that the conference control model is orthogonal to issues of quality of service and network resource reservation.