Re: [homenet] Site-local multicast [was: Updating DNS]

Juliusz Chroboczek <jch@pps.univ-paris-diderot.fr> Sun, 24 April 2016 20:14 UTC

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Date: Sun, 24 Apr 2016 22:14:11 +0200
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From: Juliusz Chroboczek <jch@pps.univ-paris-diderot.fr>
To: Gabriel Kerneis <kerneis@google.com>
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Cc: HOMENET <homenet@ietf.org>
Subject: Re: [homenet] Site-local multicast [was: Updating DNS]
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> Now you get me curious. How do you do efficient site-local multicast
> when you have multiple wifi and ethernet links?

Assuming you've got only transitive links, then any multicast routing
protocol should work fine at the scale we envision for Homenet as long as
it is able to avoid wireless links whenever possible.  Pierre was
suggesting PIM-BIDIR, and actually provided a suitable implementation [1].
Some people have hinted to me in private conversations that they'd prefer
something simpler, perhaps more akin to DVMRP [2] but without the
automatic tunnelling.

If your Homenet depends on non-transitive links (i.e. has meshy bits in
it), then you cannot use RPF [3], so changes to the data plane are
required (see, for example, Section 5 of [4]).  Thankfully, non-transitive
links are outside the scope of Homenet, so I think it would be fine to use
a solution where multicast doesn't cross the meshy bits.

[1] http://mid.gmane.org/8DAF2006-2661-48E6-A0C2-7A311B5F4DD9@darou.fr
[2] https://en.wikipedia.org/wiki/Distance_Vector_Multicast_Routing_Protocol
[3] https://en.wikipedia.org/wiki/Reverse_path_forwarding
[4] https://tools.ietf.org/html/rfc6621#section-6

-- Juliusz