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Abstract -- Making the MBone Real Network and Application Engineering Track
N1: Multicasting

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Making the MBone Real

Thyagarajan, Ajit ( ajit@ee.udel.edu)
Casner, Stephen ( casner@isi.edu)
Deering, Steve ( deering@parc.xerox.com)


IP multicasting is a powerful extension of the Internet Protocol to efficiently deliver datagram packets to multiple hosts instead of a single host. Its benefits for applications such as live audio and video conferencing among Internet sites around the world have been clearly demonstrated over the past three years in an experimental deployment called the "Multicast Backbone", or MBone. The time has come for the next step, bringing IP multicast service to production quality and extending deployment to the entire Internet so that the MBone ceases to exist as a separate entity. This paper discusses the enhancements for scalable routing, congestion management, and diagnostic tools that are underway to make the MBone and IP multicast real.

The MBone

The MBone is a virtual network layered on top of the Internet consisting of routers and hosts that support IP multicasting. Ever since its inception in 1992, the MBone has proved to be remarkably successful, having multicast over 100 conferences and international events, with the recent Rolling Stones concert in Dallas being one of the more publicized events. Today, the MBone connects over 1400 subnets in over 25 countries spanning all the continents of the world.

The phenomenal growth of the MBone has been propelled by the development of numerous applications, including multicast netnews and shared whiteboard utilities in addition to real-time audio and video conferencing, subjecting the MBone machinery to a strenuous test. These applications are capable of generating large quantities of real-time data, requiring efficient management of the increased traffic. The popularity of these applications leads to many simultaneous uses, and the popularity of some events transmitted over the MBone draws many participants. To become real, the MBone must accommodate growth in all these dimensions.

Scalable Routing and Traffic Distribution

Significant improvements in controlling multicast traffic distribution have already been implemented over the past year and other mechanisms to enable better sharing of bandwidth are being designed.

Perhaps the most pressing problem is the growth of the number of subnets in the MBone such that it can no longer be managed as a single, flat routing domain. There are two aspects to making the routing scalable: Congestion Management

The high-bandwidth, long-lived, non-flow-controlled traffic typical of the MBone, such as real-time audio and video streams, can cause severe congestion in the Internet, which was designed primarily to support "well-behaved" TCP and short-transaction UDP traffic. A number of new congestion-control mechanisms appear to be necessary or desirable:

The availability of greater bandwidth will alleviate these problems significantly.

Diagnostic Tools

As with any large network, proper diagnostic tools are required to monitor the state of the network and to provide feedback on the operation of the routing and congestion management algorithms, such as the following:


The MBone is another of the Internet's "success disasters", an experiment that has rapidly outgrown the confines of the lab or the testbed, using prototype software that was never meant to operate at the scale that is being demanded by its users. This paper describes a number of steps that must be taken to evolve the MBone into a reliable and manageable multicast service that is available everywhere in the Internet.