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23
Multicast, multi-unicast, and port
numbers
The TC620-PID can be used in a multicast setting. This chapter outlines IP multicast and one of
its methods in particular: source-specific multicast. It then describes the concept of
multi-unicast. You also learn about assigning valid port numbers.
In This Chapter
Multicast ........................................................................... 153
Multi-unicasting ................................................................. 154
Port numbers .................................................................... 154
23.1
Multicast
IP multicast
The TC620-PID supports IP multicast. This is a method for 'one-to-many' real-time
communication over an IP network. The technique can be used to send TC620-PID media
streams to a group of interested receivers in a single transmission. The intermediary network
switches and routers replicate the data packets to reach the multiple receivers on the network.
The switches and other network devices used must be carefully configured for, and capable of
handling multicasting and its associated protocols (most notably IGMP). Packets should be sent
over each link in the network only once. If not, broadcasting will occur, which can put a very
heavy load on the network. This is a phenomenon inherent to multicasting and the facilities of
network devices, not of the TC620-PID itself, although it is compounded by the density of the
UDP streams used.
Multicast group
A multicast group is used by the source, that is - the TC620-PID, and the receivers to send and
receive multicast messages. To define a multicast group, the source unit should be assigned a
valid multicasting ('destination') TX stream address and the destination units should get this
same address as source. IPv4 uses the address range 224.0.0.0 through 239.255.255.255 for
multicast applications. The source unit has no knowledge of how many receivers there are. The
group vanishes when the source is disabled, but the source will
not
automatically be disabled
when the last remaining destination is cancelled and will keep transmitting at least towards the
nearest switch. Additionally, it is possible to have the multicast group units send unsolicited
membership reports, keeping it alive even if only one - any - unit of the group is still active.
Source-specific multicast
The TC620-PID also supports source-specific multicast (SSM). This technique allows a receiver
to specify a specific source sending to the multicast group and receive traffic originating from
that source only. Singling out the source in this way can considerably reduce the network load.
Note that SSM must be supported by the last-hop router and the receiver's operating system,
and that the receiver requires IGMPv3 to be able to specify the specific source.
