Subject: [MBONED] Multicast and IPv4-IPv6 co-existence

From: Stig Venaas <[email-masked]>
Date: Wed, 12 Sep 2007 11:26:30 +0200

With the transition/migration to IPv6 we will probably for decades have
nodes and networks that either can be IPv4-only, both IPv4 and IPv6, or
IPv6-only. And of course, combinations of those. Below are some thoughts
regarding multicast and IPv4-IPv6 co-existence.

Looking at end nodes (multicast sources and receivers) the principal
problem is that the source and all the receivers must use the same IP
protocol. If you source multicast and some receivers are IPv4-only and
some IPv6-only, you will need to either send the stream twice, both IPv4
and IPv6, or you need some kind of translation.

Having the source send the content twice might be fine for many single
source applications. It becomes more problematic if receivers send
multicast RTCP reports that you want all the other receivers to receive,
or if you have multi-party applications like conferencing. One thing is
bandwidth, the other is that this is not possible if some of the
participants are IPv4-only and some IPv6-only.

We can hope that in the short run all end-points can do IPv4 multicast
(IPv4-only or dual-stack), and that by the time we get many IPv6-only
there will be few IPv4-only. I am far from sure this will be the case,
but this would simplify things.

Assuming that all nodes participating in a multicast session (all
sources and receivers) can speak the same IP protocol, one can avoid
translation. However, while all the end-points might speak the same,
there will probably be a need for encapsulation/tunneling techniques to
make things work through networks that might only support multicast for
one of the IP protocols. It might make sense to encapsulate multicast
from one IP protocol into multicast in the other, however one could
consider unicast encapsulation as well. There could also be some other
interesting aspects, e.g. one might say have IPv6 islands doing ASM
being connected by an IPv4 network using SSM.

There are also some application level issues. It is not quite clear to
me how you for instance in SDP can signal that the same content is
available both from an IPv4 and an IPv6 address and have the application
somehow try one of them and if necessary fall back to the other.

I would like to know whether people think it is worth looking into
such issues. Are we likely to run into such problems in the short term,
should we wait and see whether the need arises, or is it just a waste
of time and not a real problem?

I kind of see some of these problems myself. But so far the networks
and nodes I am exposed to are either IPv4-only or dual-stack, so one
can always reach everyone using IPv4 multicast if one wants to make
sure everyone can receive.

Stig

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