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Appendix 4

Multicasting with ShowMe

The following is based mostly on information in the ShowMe manual. It should serve to make clearer the issues surrounding the use of multicasting.

ShowMe provides an option for specifying that a conference should be opened in multicast mode. When this mode is chosen a window appears inviting the user to choose the range over which the conference is to be broadcast. The user has to specify a TTL (Time To Live) number which defines the distance a given network packet travels. The larger the TTL number, the further the data packet travels. The TTL value is decremented each time the data passes through a gateway [the precise definition of what constitutes a 'gateway' is not given].

The manual points out that there are no standards defined for TTL numbers but there are conventions which may be followed by sites. Thus, by convention a TTL number of 1 will mean that data is restricted to the local sub-net, TTL numbers of 32, 64, 128, and 256 will resrtict the data to the site, region, continent, and world (unrestricted) respectively.

Once a TTL number has been selected, ShowMe chooses an IP multicast address for the conference (based on the host machine's IP address) and passes that address to other conference members. This allows the other conference members to send conference messages back to the originator. Current multicasting technology is such that all leaf nodes which are part of a multicast tree receive multicast data regardless of whether they have requested it. If a high TTL number is chosen then, even if only two people are in the conference, all multicast routers are required to carry the traffic if they are within the TTL range. This can obviously have a serious impact on a network.

Multicast technology is under constant development and ways of alleviating the potential problems are being addressed by new versions of the software which avoid sending data to areas of the multicast tree which have not requested data and which allow system administrators to restrict the amount of bandwidth being used on the network.

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