The different levels of multicast support as discussed in Section 
can be applied to an internet situation. The internet consists of
interconnected LANs. A LAN may or may not have native multicasting support and
the same holds for the IP layer on top of the LAN.
In this thesis, the problems of an IP
layer with support for multicasting and a LAN with limited
support for multicasting (ATM) are discussed.
To get a complete picture of multicasting in the Internet Protocol,
the implications of multicasting in a network view of an internet. In
Figure two versions of an internet are drawn, both
versions consist of LANs connected by IP-routers. In both versions, the
transmission will be shown of a message from one host (A) to a group of
hosts (B, C, D, E, F, G) connected to the internet.
Figure: Multiple unicast versus multicasting on an internet
In the unicast IP network, host A transmits a copy of the message to each destination host. Obviously, the network host A is connected to will have to carry all these transmissions. If A transmits all the copies at the same time, six times the bandwidth of one message is required on A's network and also on the routers connected to A's network. The chain of protocol entities that take care of the transmission process also use processing capacity on the host for each transmission.
Host A may also decide to transmit the message sequentially to save the use of bandwidth on A's network and directly reachable routers. Although bandwidth usage is reduced to that for a single copy, the sending of six copies of the message takes six times as long as the transmission of one copy. The sequential transmission will use network resources for a much longer time.
The multicast capable version of the network shown in
Figure consists of LANs with native multicast
support and those LANs are connected by IP multicast capable routers.
Host A can now address a host group by addressing the host group's
Class D address. The message is sent out onto host A's
network where it is picked up by the multicast routers. The
multicast router knows
where to forward the multicast transmission of the message
to. Since all LANs support native multicasting, the minimum required
bandwidth for the transmission of the message to all group members
connected to each network is used. The copying of the message is done
by the multicast method of the LANs.
The multicast transmission takes the same bandwidth on host A's network as a single copy, regardless of how many clients are members of the host group on the other side of the internet.
The obvious difference between the multiple unicast and multicasting is that multicasting scales very well and multiple unicasting is not scalable at all. Even if not all LANs have native multicast support, the added cost of transmitting copies will be limited to a single LAN. The rest of the internet will have very little negative effect of this local multiple-unicast.
Further details and backgrounds of IP multicasting and a more detailed explanation of how it works can be found on the web-site of the IP multicasting Initiative: http://www.ipmulticast.com/