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
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:
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.
multicast an ``activate group;SPM_quot; control message to the group.
As this is multicast it will only propagate to the existing
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