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One trivial algorithm to make worldwide multicast traffic available
everywhere could be to send it... everywhere, despite someone wants it
or not. As this does not seem quite optimized, several routing algorithms
and forwarding techniques have been implemented.
DVMRP (Distance Vector Multicast Routing Protocol) is, perhaps, the one
most multicast routers use now. It is a dense mode routing protocol,
that is, it performs well in environments with high bandwidth and densely
distributed members. However, in sparse mode scenarios, it suffers
from scalability problems.
Together with DVMRP we can find other dense mode routing protocols, such
as MOSPF (Multicast Extensions to OSPF -Open Shortest Path First-)
and PIM-DM (Protocol-Independent Multicast Dense Mode).
To perform routing in sparse mode environments, we have PIM-SM
(Protocol Independent Multicast Sparse Mode) and CBT (Core Based
Trees).
OSPF version 2 is explained in RFC 1583, and MOSPF in RFC 1584.
PIM-SM and CBT specifications can be found in RFC 2117 and 2201,
respectively.
All this routing protocols use some type of multicast forwarding, such
as flooding, Reverse Path Broadcasting (RPB), Truncated
Reverse Path Broadcasting (TRPB), Reverse Path Multicasting (RPM)
or Shared Trees.
It would be too long to explain them here and, as short descriptions
for them are publicly available, I'll just recommend reading the
draft-ietf-mboned-in.txt text. You can find it in the
same places RFCs are available, and it explains in some detail all
the above techniques and policies.
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