Information Technology Reference
In-Depth Information
Figure 1.9
Application layer multicast.
act as relays, forwarding the content to other users in the group. Application
layer multicasting is increasingly promising because of its scalability, flex-
ibility, and ease of deployment. There are basically two kinds of ALM: end
system multicast and overlay multicast.
With end system multicast, the membership management and multicast trees
are maintained by participating end hosts. It does not require extra infrastruc-
ture to support ALM. All peers share the load and cost of the whole service.
With overlay multicast, proxies and intermediate nodes are deployed into
the network as the “backbone overlay” for content delivery [19,20,24]. Overlay
multicast tries to take advantage of both IP and ALM. Since the topologi-
cal information of the proxies is known, a high performance overlay can be
achieved by careful design. In addition, the proxies are well controlled and
are less likely to fail or improperly leave the overlay. Therefore, it is more
reliable than ALM. Another advantage of overlay multicast is that unlike
ALM, overlay multicast can host several multicast groups. However, all these
advantages are built upon the cost of deploying and maintaining the proxies
over the Internet. Typical overlay multicast protocols include Scattercast [20]
and OverCast [19]. Figure 1.10 shows a conceptual diagram for Scattercast,
where SCX stands for Scattercast proXies.
The difference between overlay multicast and a CDN is that in a CDN, the
client node requests data from CDN edge nodes; in contrast, a client node
could request data either from the overlay proxy or from other nodes.
