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Re: failure detection

Paul Jakma wrote:

We already have protocols for this, there is at least one possibility, IPv6 RAs, so:

- Use IPv6 RAs to advertise both prefixes, with a preferred lifetime
  set according to how quickly you want to switch, eg set it equal
  to twice the RA interval, or even equal to it.

  When the DSL router detects a link has gone down, it simply stops
  advertising the relevant prefix.

- We may need to invent protocols to carry valid-source prefix
  information across multiple routers in a site. (to update
  prefix-advertisement information). I think that's a general IPv6 RA
  problem though.

- Other stateful configuration protocols may be used, eg DHCPv6
  the server would need to become multi-address aware, an
  implementation detail.


Paul,

I had one minor comment on the above.

It might very well be a useful optimization for routers to be able to express to hosts which prefixes are currently not working for going outside the site. But I think it would be a mistake to overload the preferred/deprecated notion for this purpose; if we want to do this we can just as well add a "temporarily unavailable" bit to the prefix information options, and that way avoid confusing any hosts that are not shim6 aware.

It remains to be seen how hard it would be to provide this for a site with multiple routers. We have the router renumbering RFC, which in theory could be used to carry this from a border router to other routers in the site, but nobody is implementing this protocol AFAIK.
So it's not clear to me whether such a mechanism would be limited to the single link site where the RAs come directly from the border router.

   Erik