Overview of Multicast Streaming - Scalable Continuous Media Streaming Systems - page 289

a

m

Router

b

Server load = 1 unit

m

m

m

Router

c

m

m

Router d

m

1×bandwidth/link 1×bandwidth/link 1×bandwidth/link

Figure 16.2 Sending the same content to four receivers using multicast data delivery

outgoing link a copy of the packet, without incurring any additional bandwidth overhead at
the upstream links/routers or the media server. In other words, the cost to the service provider
will not scale up linearly with the number of receivers in the system. In fact, the media server
cost is fixed, irrespective of the size of the user population. Depending on the pricing model,
the network cost may still increase when there are more receivers because more bandwidth
will be consumed in the downstream networks but nonetheless the total network bandwidth
consumed will still be substantially lower than using the unicast delivery model.
The current Internet, however, does not yet support native multicast due to a number of
practical limitations. First, as Figure 16.2 illustrates routers/switches in the network must all
have multicast capabilities to support end-to-end multicast data delivery. The early routers
deployed in the Internet do not support native multicast, and the replacement of them by newer
multicast-capable routers (which are only widely available in recent years) will likely take a
long time.
Second, in addition to havingmulticast-capable routers, we also need to configure the routers
to enable multicast, such as setting up appropriate multicast routing protocols, addressing
allocation schemes, installing multicast-capable network monitoring systems, etc. This is a
very difficult issue to resolve in the Internet as the Internet is not owned by a single party and
so co-operation from many Internet service providers will be needed. Therefore, this becomes
an administrative and business issue rather than a technical issue. It is worth noting that the
Internet2 [7-8] does have native multicast support although it is not yet available to the general
public. Nevertheless it shows the feasibility of multicast data delivery in the global Internet
once the administrative and business issues are resolved.
Finally, to make use of network multicast, the application software (e.g., media server
software and media client software) will need to be modified accordingly. This is less of an
issue compared to enabling multicast in the network as many programming libraries (e.g.,
sockets) already have support for multicast data delivery.