Speak Freely for Windows

Multicasting to a group

Some implementations of Windows Sockets support "IP Multicasting", a facility which allows the creation of conference groups which individual hosts can join and leave at will. A multicast conference is far more efficient than sending duplicate messages to all recipients, as actual replication of packets is done as close as possible to the actual recipient.

If your Windows network software implements IP Multicasting, you can use the Connection/Multicast Groups... dialogue to join and drop multicast conferences. To join a conference, enter its name or numeric IP address in the "Add group" edit box and press the eponymous button. If the address is a valid multicast address, it will be added to the "Group memberships" list at the left. To leave a conference, select its item in the Group memberships box and click the "Drop Group" button. If you've joined a multicast group and you send sound to it, the sound is normally sent back to your own machine. If you don't like this, or if it doesn't make sense because your sound hardware is half-duplex, uncheck the "Loop back multicasts" button to disable this action. Some Windows Sockets implementations don't allow you control over this behaviour; if that's the case, the Loop back button will be disabled.

You transmit to a multicast group as you would to any other host; create a new connection or open a connection file to the name or numeric IP address of the group. You can specify the extent of distribution of your multicast by entering a number in the "Multicast scope" field of the Options/Connection... dialogue. The following are guidelines for multicast scope values:

       Distribution              Multicast scope
Restricted to the same host              0
Restricted to the same subnet            1
Restricted to the same site             32
Restricted to the same region           64
Restricted to the same continent       128
Unrestricted                           255

The distribution scopes given above should be taken cum grano salis. Their meaning depends entirely upon the implementation of the various intermediate links in the multicast network.

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