I do v6 over PPP, but both sides are routers.  I have addresses
assigned on an Ethernet on the dialing side, and run ripngd
(zebra/quagga) on both ends.

So, I only have link-local addresses assigned, and this all works fine
- I had no difficulties.  I realize this isn't what you are trying to
do.

  First, to get rtadvd to send RA's out the ppp interface, the ppp interface
  needs to be listed on rtadvd's command line.  But it can only be there
  when the ppp interface in use actually exists, or rtadvd aborts.
  That means, the ip-up script at the server end has to restart rtadvd
  adding in the new ppp interface name when the link comes up, then it
  needs to be restarted again, without that ppp interface name when the
  link goes down again.   This is all possible, but really very ugly.

Or, rtadvd should be fixed to cope with interfaces coming and going.
This is possible not only with ppp but pcmcia cards.  It may be that
generic library support to watch the routing socket and provide
up/down callbacks for interfaces would be a good idea.  But, one would
have to glue this into the select loop.

  Second, the "client" in my case (and I suspect for may of us here) is
  not a host, but a router.   To get this address configuration to work,
  I need to tell my router to accept RAs - which is something that routers
  aren't normally supposed to do.    That's OK, I can do that, in my
  case I know there won't be anything sending RA's on any of the other
  interfaces on the client.

Ah - if you have a router, you should be running a routing protocol.
But, you want a dynamic prefix - presumably copied somehow to the
other interfaces, so perhaps you are really feeling pain from the lack
of router renumbering.

  Of course, this one assumes that the RS/RA model is the appropriate way
  for PPP servers to inform clients (temporary attach) clients of
  addressing available to them - which I actually doubt, I'd think it
  would make much more sense for this to be done using LCP.

I'd say that just using the link-local addresses and having a real
routing/router renumbering protocol run over it would be the right
thing.

  Has no-one else run into this kind of issue before (does no-one else do
  IPv6 over dial up from a network, rather than an isolated host) ?

My PPP dialup is static - I only dial into one place.  So I just
allocated prefixes and had no trouble.

I am not aware of any implementations of router renumbering (RFC2894).

The v6 wg lists "Proxy Router Advertisement" as a work item, to be
completed by July 2003, but I can't find any drafts or RFCs about it.

-- 
        Greg Troxel <gdt@ir.bbn.com>