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Protocol Independent Multicast (PIM).
PIM works with all existing unicast routing protocols and supports two types of traffic distribution topologies: dense and sparse. Dense mode is most useful when:
  Senders and receivers are in close proximity to one another.
  There are few senders and many receivers.
  The volume of multicast traffic is high.
  The multicast groups do not change very often.

Dense mode PIM uses reverse path forwarding and is similar to DVMRP. Sparse mode PIM is most useful in network topologies scattered over a large geographic area when:
  There are a only a few receivers in a group.
  Senders and receivers are geographically separated by WAN links.
  The groups change often.

Sparse mode PIM is designed for environments where there are many data streams destined for group members, as shown in Exhibit 7. Each stream goes to a relatively small number of group members. For this topology, reverse path forwarding techniques waste bandwidth.
Sparse mode PIM defines a RP. Receiving members of the group join to a particular RP to create a shared distribution tree. There can be many RPs in a single group. Sources must register with all RPs in a group. Once the data stream begins to flow from sender to RP to receiver, the routers optimize the path automatically to remove any unnecessary hops. Sparse mode PIM assumes that no hosts want the multicast traffic unless they specifically request it.
PIM simultaneously supports dense and sparse mode for different groups. PIM was authored by Cisco Systems, Inc. (San Jose CA) and researchers from the University of Southern California, Lawrence B. Livermore Laboratory, and Xerox Parc. Work is continuing in the Internet community to improve multicast routing protocols.

APPLICATIONS

Dynamic Groups

Groups defined by Class D addresses can be created and torn down in seconds. (Groups that are easily created and torn down are referred to as dynamic.) Changes to the group can also be performed in seconds, which is useful in many applications.

Videoconferencing

Videoconferencing is a temporary group setup that may exist for the duration of a meeting of any length. It is essentially an electronic meeting.

Dataconferencing

Dataconferencing is the same as videoconferencing without the video. Dataconferencing often involves a whiteboard, such as the program used by the academic community in the Mbone.

Real-Time Distribution of Multimedia Information

There are several new services that provide real-time business news to the desktop. Some of the information provided includes audio, video, and data. Subscribers to these services are constantly entering and leaving the group, therefore the ability to change the size and composition of the group dynamically is very important for this application.

Multicast File Transfer.
There are several multicast file transfer applications that involve one-to-many transmission of information in the form of files.
Subscription-based Information Delivery.
Subscription-based information delivery is a service that distributes information electronically using a multicast network and a multicast file transfer program. The information might be, for example, news in text and image form or financial trend analysis in audio form.
Information is in the form of files that need guaranteed delivery. Dynamic groups are important in this application because subscribers are constantly changing.
Electronic subscription delivery of information could become a popular service once the Internet is fully multicast enabled. Today, much time is wasted searching for information on the Internet, even with the advent of sophisticated browsing tools. Consumers have newspapers and magazines delivered as print subscriptions; they should be able to receive the equivalent electronically.
Electronic Software Distribution.
Software is usually distributed at the time of its release. As the number of hosts on networks grows, software updates become more of a burden, which has led to great interest in the electronic distribution of software using a multicast file transfer program that is guaranteed.
Because distribution of software is not a daily or even a weekly event, it is desirable to terminate the group right after the transfer.
Distributed Publishing.
National newspapers and other print publications use regional printing facilities to reduce shipping costs. They send information using a series of point-to-point transfers. Multicast file transfer is a more efficient and less costly solution. In this application, the membership group is relatively static.
Kiosk Applications.
Kiosks are being planned to provide a number of new services, most of which need to be updated with information common for all of the kiosks. For example, the U.S. Department of Transportation is sponsoring a project to implement kiosks at various urban and suburban locations. Information about traffic will be periodically downloaded to the kiosks so that travelers can obtain information about traffic patterns. These kiosks would also be used to disperse information of general public interest, such as facts about major sporting events such as the Olympic games.
Kiosks may also be placed in retail stores as a means of showcasing products using multimedia presentations. The latest fashions and sales promotions can be downloaded into the kiosks using multicast networks and a file transfer program.
Database Updates.
Linked databases in remote locations can be updated easily using a multicast file transfer program on a multicast network infrastructure.

VENDOR OFFERINGS

There are not many network infrastructures on a wide area basis that support multicast. There are, however, a number of companies providing low-cost videoconferencing products, and many of them support multicast IP or plan to support it. Intel Corp. (Santa Clara, CA), Connectix Corp. (San Mateo, CA), PictureTel Corp. (Danvers, MA), Insoft, Inc. (Mechanicsburg, PA), and Xing Technology Corp. (Arroyo Grande, CA) all have PC-based videoconferencing products.

Multicast file transfer products are available from StarBurst Communications Corp. (Concord, MA) and Legent, a division of Computer Associates International, Inc. (Islandia, NY).

CONCLUSION

Multicast network technology and the associated applications are poised to dramatically change the use of networked PCs. Videoconferencing and dataconferencing over IP data networks will become common as network speeds increase and multicast IP router networks become pervasive. Multicast file transfer over multicast IP networks makes electronic information delivery on a subscription basis a reality and electronic delivery of information more like the model for printed information delivery.

Multicast file transfer also facilitates software maintenance by providing the ability to easily distribute software updates. Other distributed applications such as groupware and E-mail directory updates can also benefit, proving that multicast technology will be a major component of information distribution in the not-too-distant future.


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