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Choosing multicast addresses and ports

Multicast (as opposed to unicast) is used to send UDP packets from 1 source to multiple destination servers. This is useful for example for streaming from a satellite/DVB-T receiver to multiple receiving PCs for playback.  Multicast can also be used on the output of an encoder (MComms Transcode) to feed multiple streaming servers.  Multicast only works with UDP and is not possible with TCP due to the 2 way nature of TCP, most commonly multicast is used with RTP and MPEG2-TS.

A multicast IP address must be chosen according to IANA information, we recommend using an address in the range 239.0.0.0 to 239.255.255.255 as this is reserved for private use.  Using multicast addresses in the 224.0.0.0 range may clash with existing services and cause your stream to fail.  For more details see http://www.iana.org/assignments/multicast-addresses/multicast-addresses.xml

Choosing a UDP port number for multicast streams is also important.  Even if you use a different multicast IP for each of your streams, we strongly recommend using different UDP port numbers as well. This is because a server and all software running on the server receives ALL multicast traffic on an open port and extra processing is required to filter out the required traffic.  If the each stream arrives on a different port, the server can safely ignore any traffic on ports that are not open.  Port numbers MUST be chosen so that don't clash with any existing services or ephemeral ranges.  The ephemeral range for Windows Vista, 7, 2008 is 49152 to 65535, for older Windows it is 1025 to 5000 and for Linux it is 32768 to 61000.  For more information on Windows see http://support.microsoft.com/kb/929851  Care should also be taken to avoid system ports 0 to 1024.  See http://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xml Generally one of the unassigned User Ports (1024-49151) should be used, you can run the netstat -abn command to see which ports are currently in use.

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