3Com 4007, Руководство по внедрению, Страница: 429 / 684

Key Concepts 429
Internet Support for
IP Multicast
The MBONE is the Internet’s experimental multicast backbone network. It
is an interconnected set of Internet routers, subnetworks, and tunnels
that support the delivery of IP multicast traffic.
The MBONE was first configured in 1992 as a test zone to enable IP
multicast applications to be deployed without waiting for multicast
routers to replace unicast routers across the entire Internet. The MBONE is
actually a virtual network located within portions of the physical Internet.
Its construction reflects several multicast zones connected together via IP
multicast tunnels. When it was created in 1992, the MBONE spanned
four countries and 40 subnetworks; today it spans over 25 countries and
thousands of subnetworks.
You can connect to the MBONE through most Internet service providers
(ISPs). You can use it to test multicast applications and technology or to
connect private multicast LANs. Some organizations broadcast public
information over the MBONE; examples include IETF (Internet Engineering
Task Force) meetings and NASA (National Aeronautics and Space
Administration, United States) space shuttle launches.
Key Concepts
This section describes several terms and concepts related to IP multicast
routing.
Traffic Movement Application sources generate the majority of IP multicast packets, but
group members and routers that are communicating (DVMRP and IGMP
messages) to establish the delivery path also generate IP multicast
packets.
Traffic from application sources always travels in one direction —
downstream from the source to group members. Using various protocols,
network devices are responsible for determining where group members
exist and coordinating a loop-free delivery path from the source to them.
Traffic that relates to the delivery path can travel both
upstream
and
downstream — between routers and between routers and group
members.