Reliable Multicast

Many IP multicast tools use a reliable multicast mechanism to maintain consistency between each instance of the tool. Reliable multicast is a mechanism by which data which is multicast from a sender is guaranteed to reach the receivers at some time. The reliability mechanism requires some form of acknowledgement scheme to say which packets did arrive and did not arrive at a receiver, a kind of retransmission scheme to send out lost or late packets, and support within the tools to participate in these schemes.

For media such as video and audio, data arrives so fast it is better to present it quickly and tolerate some reduction in quality if packets do not arrive on time, or are lost in transmission. This can be done because humans are quite tolerant of loss, particularly for video, and the information content is not drastically reduced if loss does occur. Furthermore, with a reliable transmission scheme late or lost data could be resent to the receiver at a significant amount of time after the original send time, which would cause the immediacy of any interaction between users to be eliminated. Clearly, this is not suitable for live, interactive conferences. However, this level of reliability is very important for shared workspace tools where whole objects need appear with all their data eventually, but where there is no immediacy lost if a packet is retransmitted and arrives at some later time.

Both of the well known shared workspace tools, wb and nt, use a reliable multicast mechanism. The scheme used by Wb is described in [#!srm!#] and that for Nte is described in [#!nte!#] Both schemes resend parts of objects that get lost on the network. The RTP protocol is not used because the data is not real-time data, but partial-object data. In nt, and possibly wb, the RTP timestamp does not have enough resolution to hold all the timing information required. Therefore both nt and wb have their own packet format to support their reliability and retransmission mechanisms. It is unfortunate that the schemes are not described in enough detail to build a new tool that could participate in these schemes.