NCast N-Way Server - Manual (Page 28)

NCast N-Way Reference Manual
6.N-Way System Operation – Detailed Examples
6.1.T
HE
N-W
AY
C
LIENT
If the simple multicast bridging arrangement described in Section 1.5 is not used, then a more in-depth
understanding of how the N-Way system works is required, and this chapter provides greater detail on how the
multicast bridge operates and how packet tunnels are created and used.
Within each Telepresenter multiple system processes are running, and when N-Way activity is enabled a new
process called the “N-Way Client” is launched and activated. The figure below shows the activity and packet flow
through the N-Way Client:
When a multicast session is started, the media stream sources (graphics and audio encoding processes)
generate two types of multicast packets. The first type, RTP (UDP) packets contain encoded graphics or audio
material and are shown as brown arrows in the diagram. The second type, IGMP control packets, are requests
sent to routers to “join” a multicast group, and are depicted as black arrows. Since these are multicast packets,
they are routed to all clients on the LAN (shown as a blue horizontal line).
When the N-Way Client launches it sets up a socket (connection to the internet), which allows it to eavesdrop on
all the packet activity into and out of the Telepresenter. The diagram above shows two separate connections to
the Ethernet LAN, even though there is only one physical connection. This is because two separate and
independent sockets are in use, one for the Media Source and one for the N-Way Client.
When the Client sees an IGMP packet with a source address of its own machine, it encapsulates this request and
forwards it to the server with the intention of telling the server “I have Media Source processes here that are
interested in any traffic on the multicast address group contained in this IGMP packet”. The N-Way Server notes
this request and passes the IGMP request to the LAN on which the server resides. An encapsulated IGMP
request is shown in the diagram as a black arrow encased in an orange box.
If the Client sees an RTP (UDP) packet it checks first if the multicast address in the packet is current (an IGMP
join request has been issued for this multicast group) and then it checks if the source address of the RTP packet
is the same as the source address of its own machine. If so, the RTP packet is encapsulated and sent to the
server. An encapsulated RTP packet is shown in the diagram as a brown arrow encased in an orange box.
When the N-Way Client receives an encapsulated packet from the server, it unpacks it and broadcasts it on the
local LAN connection. Only RTP type packets will be received. The packet which is broadcast will be a multicast
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