[Help] Last update at http://inet.nttam.com : Mon Aug 7 21:40:35 1995
Abstract -- Making the MBone Real
Making the MBone Real
- Thyagarajan, Ajit
- Casner, Stephen
- Deering, Steve
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
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:
- True multicast routing prunes traffic from links that do not have
- 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:
The availability of greater bandwidth will alleviate these problems
- 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
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.