22
Multicasting
The BC840-MC can be used in a multicast setting. This chapter outlines IP multicast and one
of its methods in particular: source-specific multicast. It also describes the concept of multi-
unicast.
In This Chapter
22.1
Multicast
IP multicast
The BC840-MC 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 BC840-MC 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 BC840-MC 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 BC840-MC, 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 BC840-MC 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.
SSM is implemented on the encoder side, by having the unit transmit a multicast stream to
the 232.x.x.x group (the range assigned to SSM) via RTSP. The Session Description Protocol
(SDP) file generated by the RTSP server includes additional information containing the source
IP (S) and the multicast group (G). The RTSP client in the decoder can then issue an IGMP
join message containing S and G. The intermediary routers can use this information to
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