Subject: RE: Traffic Engineering for multicast traffic

From: Masataka Ohta <[email-masked]>
Date: Wed, 26 May 99 4:23:39 JST

Joel;

> For example, one mechanism would be to allow the path to be overridden
> once you arrived at the tree.  That is, once the join reaches an on-tree
> router, there is no poin in creating a second path from there to the
> root.  So simply stop.  A variation on this technique was used with PNNI
> support for ATM Leaf Initiated Joins for pt-to-multipoint.
> There are some problems with this, and probably some better refinements.
> But the fundemental observation is that there is no reason to have the
> multicast packets take two paths from the root to another spot on the
> tree.  Some one path will suffice.  Picking which one may be harder than
> just "accepting the first".

Are you aware of the fundamental observation that PNNI is not
used in the real world?

							Masataka Ohta

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From: "R. Venkateswaran" <[email-masked]>
Date: Tue, 25 May 1999 14:15:49 -0500 (Central Daylight Time)

> just "accepting the first".

Yes, the problem here is that each node now has to remember the
explicit path from itself to the root. Otherwise, it cannot decide
whether a new requested explicit path (in a new PIM JOIN message) to
the root is "better" or "worse" than the existing path.

Further, if the node decides that the new explicit pathi is "better",
it has to then convey this information to all the downstream routers
on the multicast tree, because each node now has to update its
explicit path to the root.

Regards,
venki

Disclaimer: The above comments are personal views, and may not
necessarily reflect those of my organization.
-- 
R. Venkateswaran (Venki)           [email -> mailto:[email-masked] ]
[Lucent Technologies Bell Labs (IHP), Room 2E-241,Ph # (630)-979-6253]

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From: "R. Venkateswaran" <[email-masked]>
Date: Tue, 25 May 1999 14:15:49 -0500 (Central Daylight Time)

Regards,
venki

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From: Masataka Ohta <[email-masked]>
Date: Wed, 26 May 99 4:23:39 JST

Joel;

							Masataka Ohta

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From: "R. Venkateswaran" <[email-masked]>
Date: Tue, 25 May 1999 14:15:49 -0500 (Central Daylight Time)

Regards,
venki

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From: Joel Halpern <[email-masked]>
Date: Tue, 25 May 1999 14:48:11 -0400

Actually, depending upon the "meaning" of the explicit route, there is
no need for the RP to keep all the state.

Yours,
Joel M. Halpern

-----Editted Original Message-----
R. Venkateswaran [mailto:[email-masked]] wrote:

I see some serious problems with this approach. The main concern is
that the resulting "multicast tree" may no longer be a "tree" by
graph-theory definitions. That is, an intermediate router will receive
the same multicast data on more than one incoming link and will have
to forward them multiple times to different receivers because each
receiver wants a certain path for their data. Eventually, you may end
up with a bunch of unicast paths to the receivers, as opposed to a
multicast tree. 

To eliminate these "loops" in the multicast tree, some node
has to remember the entire multicast tree structure and eliminate
duplicate paths at the time of PIM JOIN. I would assume it would be
the RP that has to maintain this information.  For large networks,
this will involve too much of state information for the RP to
maintain.

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From: Liwen Wu <[email-masked]>
Date: Wed, 26 May 1999 08:35:01 -0400

> the root is "better" or "worse" than the existing path.

Yes, each on-tree nodes remember ONLY 1 explicit path from
itself to the root.

> explicit path to the root.

Yes, a  new PIM  JOIN-ACK msg(very similar to RSVP PATH msg)
can be defined to flow from the root to all the downstream routers.