The MBONE is an outgrowth of the first two IETF "audiocast" experiments in which live audio and video were multicast from the IETF meeting site to destinations around the world. The idea is to construct a semipermanent IP multicast testbed to carry the IETF transmissions and support continued experimentation between meetings. This is a cooperative, volunteer effort.
The MBONE is a virtual network. It is layered on top of portions of the physical Internet to support routing of IP multicast packets since that function has not yet been integrated into many production routers. The network is composed of islands that can directly support IP multicast, such as multicast LANs like Ethernet, linked by virtual point-to-point links called "tunnels". The tunnel endpoints are typically workstation class machines having operating system support for IP multicast and running the "mrouted" multicast routing daemon.
IP multicast packets are encapsulated for transmission through tunnels, so that they look like normal unicast datagrams to intervening routers and subnets. A multicast router that wants to send a multicast packet across a tunnel will prepend another IP header, set the destination address in the new header to be the unicast address of the multicast router at the other end of the tunnel, and set the IP protocol field in the new header to be 4 (which means the next protocol is IP). The multicast router at the other end of the tunnel receives the packet, strips off the encapsulating IP header, and forwards the packet as appropriate.
Previous versions of the IP multicast software (before March 1993) used a different method of encapsulation based on an IP Loose Source and Record Route option. This method remains an option in the new software for backward compatibility with nodes that have not been upgraded. In this mode, the multicast router modifies the packet by appending an IP LSRR option to the packet's IP header. The multicast destination address is moved into the source route, and the unicast address of the router at the far end of the tunnel is placed in the IP Destination Address field. The presence of IP options, including LSRR, may cause modern router hardware to divert the tunnel packets through a slower software processing path, causing poor performance. Therefore, use of the new software and the IP encapsulation method is strongly encouraged.
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URL: http://library.fortlewis.edu/~instruct/glosas/mbone41.htm
April 1994
GLOSAS NEWS was orinally posted to the WWW at URL: http://library.fortlewis.edu/~instruct/glosas/cont.htm by Tina Evans Greenwood, Library Instruction Coordinator, Fort Lewis College, Durango, Colorado 81301, e-mail: greenwood_t@fortlewis.edu, and last updated May 7, 1999. By her permission the whole Website has been archived here at the University of Tennessee server directory of GLOSAS Chair Dr. Takeshi Utsumi from July 10, 2000 by Steve McCarty in Japan.