proposal for a compromise on DNS discovery

[Alain Durand <Alain.Durand@Sun.COM>]

I have circulated this among several people yesterday and it may be time
to bring this to the list.

Our community is basically divided in two camps,
the "conservatives", coming from v4 where the operational practice
is to use DHCP for DNS discovery and would like to operate the same way
for v6, and the "liberals", coming from v6 where the autoconfiguration
story is build on Neighbor Discovery and RA/RS, and who would like to
expand on this.

As I explain in an earlier mail, the differences in the mode of 
operation are:
- DHCPv6 can also pass the address of the NTP server
- RA/RS have the unsolicited multicast provision that helps save 
bandwidth
   when a large number of nodes are autoconfiguring at the same time.

The compromise solution I'd like to bring forward is to optionally 
piggy back the
unsolicited multicast mode to DHCPv6-lite.

More specifically, this would work the following way:

At boot time, the client wait a bit (random 0..MAX1 seconds) and listen
to unsolicited DHCPv6 DNS recursive server advertisements sent to the
link-local-all-node multicast address.
If noting comes, it send a DHCPv6 request.
The DHCPv6 server answer to the unicast address of the requester
and, if configured to do so (this would be optional), send the same 
thing to the
link-local-all-node multicast address.
Those multicast responses will be rate limited by the server,
that is, if a second request come within MAX2 seconds,
only the unicast answer will be sent.

I'm sure there would be many details to work out.

	- Alain.

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