N1: Multicasting

Making the MBone Real

Thyagarajan, Ajit ( ajit@ee.udel.edu)

Casner, Stephen ( casner@isi.edu)

Deering, Steve ( deering@parc.xerox.com)

Abstract

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.

• True multicast routing prunes traffic from links that do not have downstream receivers.
• More flexible administrative scoping based on addresses allows restricting multicast traffic within organizations or communities.
• Enhancements to IGMP, the protocol used by hosts to join and leave multicast groups, will enable a host to identify the specific senders that the host wishes to hear, or not to hear, thereby reducing the incidence of traffic from unwanted sources.

• Hierarchically partitioning the topology into multiple, independent routing domains to limit the amount of topological information that any one router must maintain.
• Scaling the routing algorithms to support a large number of sources active simultaneously.

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:

• A rate-limiting mechanism that provides an option of setting a ceiling for multicast traffic to prevent low-bandwidth links from being saturated by multicast traffic.
• A fair queuing mechanism to prevent any one source from consuming all of the capacity of a bottleneck link.
• Other techniques that involve explicit reservation mechanisms to guarantee or deny a certain bandwidth for multicast traffic.
• If high bandwidth data streams are variably encoded into different priorities, a prioritized dropping mechanism will enable maximization of the quality of service depending on available bandwidth.

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:

• Several utilities have been developed for monitoring the health of the routers and the collective state of the MBone, including a mapping utility that builds a complete connectivity diagram of the MBone.
• A multicast traceroute utility has been developed to aid in the quick diagnosis of faults among routers and to provide coarse-grain traffic statistics.
• Reception quality reporting mechanisms in the Real-time Transport Protocol (RTP) will facilitate debugging by employing all of the actual receivers of a multicast distribution to collect and report statistics to network monitors.

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.