Operation Manual

IP
SIP Software Release 6-0-0
J613-M0524-00
25
Multicasting is not the same as broadcasting on the Internet or on a LAN. In
networking jargon, broadcast data are sent to every possible receiver, while
multicast packets are sent only to receivers that want them.
The mutlicast approach uses up a LOT less bandwidth. Not only does it
make better use of available bandwidth it means that there is no limit to the
number of hosts that can ‘tune in’. Consider the case of sending video on a
LAN using the ‘multiple-unicast’ approach. For full-motion, full-screen
viewing, a video stream requires approximately 1.5 Mbps of server-to-client
bandwidth.
In a unicast environment, the server must send a separate video stream to
the network for each client (this consumes 1.5 x n Mbps of link bandwidth
where n = number of client viewers). With a 10-Mbps Ethernet interface on
the server, it takes only six or seven server-to-client streams to completely
saturate the network interface. With the multicast approach, there is no limit
to the number of recipient hosts – as the server never has to send more than
one stream, whether ther is one recipient or 1000 recipients.
Of course, multicasting has to be a connectionless process. The server simply
sends out its multicast UDP packets, with no idea who will be receiving
them, and whether they get received. It would be quite impossible for the
server to have to wait for ACKs from all the recipients, and remember to
retransmissions to those recipients from whom it does not receive ACKs.
Apart from anything else – the server does not know who the recipients are,
or how many there are.
What is IGMP?
IGMP (Internet Group Management Protocol) is the protocol whereby hosts
indicate that they are interested in receiving a particular multicast stream.
When a host wants to receive a stream (in multicast jargon, this is called
‘joining a group’) it sends to its local router an IGMP packet containing the
address of the group it wants to join – this is called an IGMP Membership
report (sometimes called a Join packet).
Now – the local router generally going to be a long way from the server that
is generating the stream. So, having received the IGMP join packet, the
router then knows that it has to forward the multicast stream onto its LAN
(if it is not doing so already). However, if the router is not already receiving
the multicast stream from the server (probably many hops away) what does
the router do next in order to ensure that the multicast stream gets to it?
Does it just forward the IGMP packet on up the chain back up to the server?
No, it does not, because it does not necessarily know the correct path to get
to the server. There is a far more elaborate process involving multicast
routing protocols like PIM, DVMRP, MOSPF, etc. However, the operation of
these protocols is well beyond the scope of this manual. All we need to
understand here is that once the router has received the IGMP join message,