Re: Source addresses and multicast (was Re: Fwd: Minutes / Notes)

[Erik Nordmark <Erik.Nordmark@sun.com>]

> I have to admit my ignorance here.  Any pointers that would allow
> me to better understand what you say above?  Especially, I don't
> understand if the "source" address you talk about is the source
> address in a packet, or the addresses (locators) of the members
> of a multicast group.

Steve Deerings PhD thesis explains it all :-)
Google finds some easier things at:
http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=reverse+path+forwarding+multicast
but no good summary as far as I could tell.
(I think there was an mboned intro to multicast I-D somewhere but I can't find 
it).

In summary there is a source-rooted distribution tree which is independent
of the multicast group (which is carried in the destination address field).
The source rooted tree conceptually exists all the time and there is
a separate tree for every source address, but the actual tree is built
on demand - details depending on the multicast routing protocol which is
used.

The tree is then pruned based on the precence of members in the multicast group
to try to avoid to deliver packets down braches where there are no members
i.e. no interested receivers. The details of the tree pruning are different
for different multicast routing protocols.

The resulting issue for multihoming appears if a site wants
to forward packets towards the rest of the internet from more than
one attachment point.
If the same packet ends up with different source addresses/locators 
when exiting through N different border routers (e.g. due to source
locator rewriting by the routers) then the RPF checks will
operate on two different trees and as a result each receiver will receive
N copies of every packet.

  Erik