Re: [DNA] Re: Flash renumbering

[Sathya Narayanan <sathya@research.panasonic.com>]

Erik Nordmark wrote:

> Sathya Narayanan wrote:
>
>>> Based on the comment above, we would also have to forbid the routers
>>> from using learned prefixes as part of the DNA solution.
>>
>> ... why? The problems becomes worse with wrong state for upto 7 days
>> only with reassignment. The above comment is only about flash
>> renumbering which will be corrected within 90 minutes and is not that
>> bad (A)
>
> I guess we need to first agree what goals we are trying to satisfy. I
> see three possible levels:
> 1. do nothing special for flash renumbering and immediate reassignment
> (other than telling network admin to not immediately reassign prefixes)
> 2. do something so that a host can recover, but it might take a while
> (e.g., 90 minutes)
> 3. handle it without any delay
>
> Which goal are you attempting for? (And we should ask the same of the
> whole WG.)

I believe I was going for #2 - not that I clearly thought about it like
this ;-)

I think leaving it as #1 is bad, #3 is too much and will put unnecessary
restrictions on normal operation to address a 'low-probability' problem,
hence we are left with #2.

>> By suggesting that this is suspicious, we are suggesting that the
>> prefix-lifetime being greater than 90 minutes is not necessarily right -
>> in effect putting an upper limit on the prefix-lifetime and overruling
>> 2461. Right? Is it OK to do that?
>
>
> Good question. I wish I had a firm answer. We have an inherent
> conflict between RFC 2461, which was designed we long default
> lifetimes just so that a host wouldn't throw away its addresses and
> prefixes just because a router died for a short time.
>
> But our need to quickly and reliably detect movement, if we combine it
> with the renumbering and reassignment issue, is in conflict. We can't
> solve both.
>
> Hence the best I think we can do is to use the absence of a 'link up'
> as a way to tell the host to stay in the RFC 2461/62 "stability is
> important" mode.
>
> Note that your suggestion to use a RS to reverify each prefix has the
> same issue as triggering a sanity check after 90 minutes, since it
> would also remove a prefix from aggressively than 2461 if the
> advertising router is temporarily down.

Agreed. Also, the periodic RA, if it is complete will not include P1 and
that can be used as indication also. If N CompleteRA messages are
received without the prefix, start getting suspicious.

In conclusion, I suggest we create some text based on your suggestion
from the previous email as a starting point.

>> In support of my argument above marked (A), this solution doesn't
>> address the flash renumbering problem - only flash renumbering with
>> early reassignment. But, I do agree that this simpler defensive
>> scheme can handle the low-probability occurance of flash-renumbering
>> with early
>> reassignment.
>
> Flash renumbering without any reassignment wouldn't cause any
> additional problems for DNA, would it? I think it is only an
> issue/feature of RFC 2461/62.

I am not sure what is a issue and what is a feature anymore ;-). But,
the 90 minute limitation on the learned prefixes will ensure that the
prefix (P2) is removed from the link (link 1). A host that is configured
with P2, but not using it as landmark prefix will continue to use the
address, until a completeRA gives it a 'hint' - while a host configured
with P2 and using P2 for its landmark prefix, will choose a different
prefix for its address after a landmark question/answer exchange. Does
that make sense?

Sathya