From: Jenny Liu (jen@cs.washington.edu)
Date: Wed Nov 10 2004 - 04:29:21 PST
"The PIM Architecture for Wide Area Multicast Routing" presents an
approach and protocol for sending multicast data over a wide area,
particularly when receivers are sparsely distributed. PIM combines
ideas from the CBT protocol (namely shared trees) and the SPT (shortest
path tree) from the traditional IP multicast model by allowing routers
with receivers as local members along PIM's RP (shared) tree to choose
between a shared tree and a source-specific tree.
By giving the choice of multicast districution tree type to routers
directly connected to receiving members, PIM provides for
end-point-based choice in the type of tree used for multicasting. The
choice of tree type can even be changed on the fly, allowing receivers
(or their first hop routers) to choose what's best for them depending on
the specific multicast application. However, the question of how first
hop routers know whether they want RP or SPT paths within their networks
still remains. It seems that in the end, a human will have to decide on
(at the least) some parameters to govern the state of the tree choice
within each network, and it's not clear how those parameters are to be
chosen.
The paper points out that multicasting on RP and SPT type trees behaves
well under different circumstances. How often do those circumstances
change for the same set of senders and receivers? It's not clear that
the flexibility provided by being able to switch from one type of tree
to the other is actually very useful.
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