MBONE (Multicast Backbone)

Jean Bunn Geneva University

jean@ibm.unige.ch

Introduction

Imagine being able to participate in a conference in Amsterdam whilst physically sitting in your office in Geneva. How about watching live video, on your workstation of the astronauts installing a new co-processor on the Hubble Space Telescope 320 nautical miles above the earth. Imagination became reality in the Services Informatiques at Geneva University during the second half of last year (1993), as a result of a connection to the MBONE network.

The MBONE originated from experiments during IETF (Internet Engineering Task Force) meetings in which live audio and video were transmitted around the world. The MBONE is a network of hosts connected to the Internet communicating using a technique called IP multicast and is used to develop protocols and applications.

In order to participate in the MBONE network it is necessary to have a workstation supporting IP multicast and have a network connection with a reasonable bandwidth (typically around 1 Mbps). The necessary IP multicast and application software is freely available.

IP Multicasting

When a packet is broadcast, the packet is delivered to all attached hosts. When a packet is unicast, the packet is delivered from the source host to the destination host. The term multicast [1] refers to a technique which allows a single packet to be passed to selected destinations and can be considered as a generalisation of the broadcast (selected destinations are all the possible destinations) and unicast (only one destination is selected) techniques, see Figure 1.

Each host on an Ethernet network is assigned a 48 bit integer known as the Ethernet address which allows the host to recognise which packets are meant for it. The Ethernet address however can also be a network broadcast address or a multicast address. The low-order bit of the high-order octet is used to differentiate between a multicast address (1) and a unicast (0) address.

IP multicasting [2] is an extension of Local Area Network (LAN) multicasting to a TCP/IP network. In order to participate fully in IP multicast , a host must be able to send and receive multicast datagrams. An IP multicast datagram specifies an IP host group address in the destination field rather than an individual IP address. A host communicates its group membership by using IGMP (Internet Group Management Protocol).

The IP host group address uses class D IP addresses (the first 4 high order bits are 1110- in dotted decimal notation the range is 224.0.0.0 to 239.255.255.255) followed by the particular multicast group in the remaining 28 bits. The mapping of the IP host group address to the Ethernet multicast address is done by placing the low order 23 IP host group address bits into the low order 23 bits of the Ethernet multicast address.

IP Multicast Software

In order to receive IP multicast packets it is necessary to have an operating system which understands IP multicast. Some system kernels support IP multicast as part of the standard distribution, others such as SUNOS 4.1.3 need to be patched to add the multicast support. As most conventional routers only support routing of unicast packets, the mrouted (multicast routing daemon) encapsulates the IP multicast packets so that they look like standard unicast packets to the intervening routers. Hosts running mrouted are linked by virtual point to point links called tunnels. These tunnels establish a link between a local IP address and a remote IP address.

Application Software

Application tools for video, audio and white board applications are available in the public domain and are pointed to in the FAQ (Frequently Asked Questions)[3] .

The session directory package sd[4] produces a window showing session announcements. Clicking on a session name gives information about the session such as time and date of transmission. Double clicking on the session name starts up the appropriate tools ( e.g. vat, nv,) for the desired session.

The network video package (nv)[5] is a videoconferencing tool. For a default bandwidth of 128kbps, typical frame rates of 3-5 frames per second can be transmitted or received.

The visual audio tool vat[4] allows host to host or multihost audio conferencing and can use a variety data compression formats. The vat utility produces a conference hosts window which shows all hosts currently participating in the conference, see Figure 2. The name of the host currently transmitting is displayed in a box that is video-reversed. It is not necessary to speak in order to participate in a conference! In fact, the normal etiquette is to wait until the speaker has finished and has opened questions to the floor or to the net, before speaking. Using vat requires a certain amount of prudence to avoid accidental transmissions during conference sessions!

The white board utility wb [4] can be used as a shared white board drawing surface and it can be used to export and view postscript files. Speakers can make their slides available as postscript files during a conference session. The camera can be directed at the speaker while the slides can be viewed via wb.

Other tools available include:
ivs [6]package (audio and video conferencing).This package is used to participate in MICE (Multimedia Integrated Conferencing for Europe) seminars.
A high resolution, low bandwidth image server providing live images of the earth from geostationary satellites can be viewed using the image multicaster client imm [7].
nevot (audio tool) [8].

Platform availability

IP multicast extensions are available [9] for SUNOS 4.1.1,2,3, Ultrix 4.1, 4.2a and HP-UX 9.01 and 386bsd UNIX and also for DEC OSF/1 V1.3 [10] . Silicon Graphics machines support IP Multicast, as do Solaris and SUNOS 5.x operating systems. Alpha machines running OSF 2.0 should support IP multicast without any kernel modifications. An experimental IP multicast enhancement for the RS6000 systems running AIX 3.2.5 is available [11]. mrouted has been reported to run under AIX3.2.5.

vat binaries are available for SUN SPARCstations, DEC5000 and SGI Indigo. nv sources are available and binaries are available for SUN, HP, SGI and DEC machines. nv has also recently been ported to the RS6000 platform running AIX3.2.5.

The next version of MacTCP( 3.x?) is rumoured to support IP multicast and PC/TCP v2.3 from FTP Software Inc. supports IP multicast.

No special hardware (apart from a workstation with a loud speaker!) is required for receiving video and audio. To send video requires a camera and a frame grabber.

Events

One of the most exciting events on the MBONE was the broadcast of the Endeavour mission to repair the Hubble telescope. These broadcasts arrive courtesy of NASA Select (NASA's cable channel).Watching the space walks live on the desktop turned out to be much more interesting than the usual work schedule!

Events on the MBONE have included the IETF meetings (of course), MICE seminars, Xerox PARC lectures and various other conferences mainly on computing and scientific subjects.

How to join the MBONE

In order to join the MBONE in Switzerland, your network manager should contact SWITCH [12].At the moment, a tunnel into Switzerland has been set up from Washington to CERN using the T1 link across the Atlantic. Geneva University has a tunnel with CERN.

A possible ATM connection is for furture consideration.

Acknowledgments

I would like to thank A. Schindler for suggesting this project and for his advice in setting up our participation in the MBONE and A. Hugentobler for analysing the network packets. I would also like to thank Daniel Karrenberg (RIPE) for initially arranging a tunnel from Holland and of course all the people who originally conceived and set up the MBONE network. Last but not least, thanks to all the people on the MBONE mailing list who sent me comments concerning this article.

8/11/94

References

  1. Internetworking with TCP/IP Volume 1 by Douglas E. Comer ISBN :0-13-474321-0.
  2. Deering Stephen, Host Extensions for IP Multicasting, RFC 1112 August 1989 .
  3. The FAQ can be viewed on the World Wide Web at address: http://www.research.att.com/mbone-faq.html.
  4. The visual audio tool (vat), whiteboard (wb) and session directory (sd) have been written by Steve McCanne and Van Jacobson from Lawrence Berkeley Laboratory. They are available at ftp.ee.lbl.gov.
  5. nv was written by Ron Frederick at Xerox Paolo Alto Research Center. Binaries for DEC5K,SGI and SUN4 and sources are available from parcftp.xerox.com.
  6. ivs written by Thierry Turletti at INRIA and is available at zenon.inria.fr.
  7. imm was written by Winston Dang from Hawaii University and can be found on ftp.hawaii.edu.
  8. nevot was written by Henning Schulzrinne and is compatible with vat but it also supports the Real Time Protocol ( a protocol being worked on by the IETF audio/video transport working group). It is available at gaia.cs.umass.edu.
  9. Deering Stephen, IP multicast extensions are available from gregorio.Stanford.edu for HPUX SUNOS et ULTRIX . 386bsd extensions are available from ftp.ee.lbl.gov.
  10. IP multicast extensions are available from ftp.adelaide.edu.au.
  11. RS6000 AIX 3.2.5 IP multicast enhancements are available from gated.cornell.edu.
  12. Marcel Wiget (wiget@chx400.switch.ch) Swiss Academic and Research Network.