📄 multicasting.htm
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<H2><A NAME="multicasting"></A>Multicasting to a group</H2>
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<P>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 network does not support
Multicasting, you may be able to use the Speak Freely's <A
HREF="broadcasting.htm">Broadcast</A> facility to transmit
audio to multiple destinations.</P>
<P>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 <A HREF="hardduplex.htm">half-duplex</A>, 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.</P>
<P>You transmit to a multicast group as you would to any other
host; create a <A HREF="newconnection.htm">new connection</A>
or <A HREF="openconnection.htm">open a connection file</A> to
the name or numeric IP address (and port number, if
nonstandard) of the group. You can specify the extent of
distribution of your multicast by entering a number in the
"Multicast scope" field of the <A
HREF="connproperties.htm">Options/Connection</A> dialogue. The
following are guidelines for multicast scope values:</P>
<p>
<pre>
<b> Distribution Multicast scope</b>
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
</pre>
<P>The distribution scopes given above should be taken <I>cum
grano salis</I>. Their meaning depends entirely upon the
implementation of the various intermediate links in the
multicast network.</P>
<P>Note that in order to use multicasting across the Internet
(as opposed to solely on your local area network), your
Internet Service Provider must also support multicasting and
grant you access to that capability. Relatively few commercial
Internet Service Providers support multicasting; if you're at
an educational site with good Internet connectivity, there's a
higher probability you'll have access to multicasting. In any
case, consult your system administrator or service provider's
help desk for further information.</P>
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