Subject: [MBONED] IP multicast address selection questions

From: Marcus Ertl <[email-masked]>
Date: Mon, 17 Oct 2011 00:34:46 +0200

Hi list,

I have been pondering over IP (v4 and v6) multicast addresses for quite
a while by now without coming up with a definitive answer, and because
it is an operative issue, the IANA referred me to you, maybe you can
help me?

What would be the preferred method for multicast-addressing several
groups of receivers on a dedicated IP network, where groups are defined
by the application layer as a 32 bit number, but the expected number is
much smaller? Messages to the individual groups are expected to be
frequent, even the majority, during normal operation, but a group ID can
be sent relatively early in the protocol header.

The options I see are:

- use a single multicast group for every receiver and send the ID of
the target group in the header (which means that all members of any
group would need to sort out every group message) or

- use one multicast group per protocol group (in case of IPv4 the
maximum would be limited of course) (which means that a
theoretically huge number of MC groups would be reserved for the protocol)

?

Also, which range should I take those MC addresses from? The datagrams
must not be blocked by switches (I have read that some switches don't
forward the ranges 224.0.0.0 - 224.255.255.255, 239.0.0.0 -
239.255.255.255 and 233.0.0.0 - 233.255.255.255 - maybe there are more?)
and may in rare cases need to cross internal routers (VPN etc.).
Should I

- use a "free for all" range (similar to the 192,168.x.x and similar IP
ranges), (if there is one?), or

- request allocation as part of the IANA considerations or

- use unicast-prefix-based MC addresses as per RFC3306 (IPv4 groups
would be limited to a single octet in that case?)

?

Background info: this is about the I-D "Protocol for Stage
Illumination". Due to feedback it has been modified heavily, but before
uploading a revised version, I would like to sort this out. The protocol
is expected to run on a dedicated network, but in reality some sort of
internet connectivity, use accross VPNs or sharing of the infrastructure
with any sort of unrelated equipment is bound to occur. Therefore, I
would like to be as conservative as reasonable to enable peaceful
coexistance.

Thank you in advance,
Marcus