Multicast is a facility that has been used to some benefit on local and bridge local area networks to reduce the amount of traffic for protocols that are used for information dissemination. Multicast services are starting to appear in Wide Area Networks. The Internet Protocol multicast routing system distributes traffic across a minimum spanning tree of all the networks and routers where there are members of a multicast group. To join a multicast group, a process issues a group join, which is propagated globally. A similar scheme has been proposed for the ISO CLNS. B-ISDN Networks based on the ATM service will necessarily support efficient multicast since they are aimed at Cable TV as well as normal telephony and data services. This scheme is scalable, and works well for such traffic as audio conferences (cf. vat). It makes an assumption that the group is symmetric in that no member of the group is more important than the others, and thus any member of the group can multicast to the group. For audio this is a reasonable assumption, as silence suppression decides which group member sends. If more than one member sends, mixing or filtering can always be performed at the receiver. However for shared windows we do not want more than one site sending simultaneously to a multicast group, and when there are many potential sources, it is the receiver that decides which one to receive from. What is required is for members of a conference to join a group at the start of the conference, but to be able to select from time to time whether they wish to be forwarded traffic from that group. We do not wish to use group join for this purpose as it is propagated globally, which is not necessary as we already know the group members. What is needed is some traffic filter in the multicast routers and mechanism to send an activation message to them. This can be done in two ways:
  1. multicast an ``activate group;SPM_quot; control message to the group. As this is multicast it will only propagate to the existing group routers.
  2. send a message to the source requesting activation of the group. The source then propagates the ``activate group;SPM_quot; message along with the existing stream of window packets to the routers downstream.
For video windows, this is even more critical, and the service must be extended. As the source must also end a synchronisation point in the video for the new receiver, the latter method ensures that this arrives after the filtering of the multicast stream has been removed. This filtering scheme could also be part of a more general scheme for hierarchically encoded video.