Re: rationale for the default of DupAddrDetectTransmits (Re: IPv6 Routing & ND vs. Addressing)

[Erik Kline <[e-mail]>]

On 18 July 2017 at 02:06, Ole Troan <[e-mail]> wrote:
>
>
> On 17 Jul 2017, at 18:42, 神明達哉 <[e-mail]> wrote:
>
> At Mon, 17 Jul 2017 08:58:18 +1200,
> Brian E Carpenter <[e-mail]> wrote:
>
> I looked up RFC4862 to find out how many DAD attempts there were,
>
> because I'd assumed 3 to 4, and if so, then if that many attempts
>
> failed, I'd say the link is well beyond its capacity. Something would
>
> need to be done to remedy a link capacity problem in that case.
>
>
> I was surprised to find, as Ole mentioned, the number of attempts was
>
> only 1 rather than 3 or 4.
>
>
> I'm more than surprised. I think that's a bug and IMHO it should be fixed.
>
>
> Although it's probably true that at the time of RFC4862 we didn't
> expect networks with much higher packet loss to be so dominantly
> deployed, I don't necessarily think the current parameter of RFC4862
> is a "bug".  First off, "1" is just the default of
> DupAddrDetectTransmits, not a fixed constant.  Secondly, my
> understanding is that DAD as defined in RFC4862 (or its predecessors)
> has never been considered a very reliable duplicate detection
> mechanism, so it didn't bother to make it arbitrarily less unreliable
> by default (when 1 is may not be enough, there's no guarantee that 3
> or 4 is enough anyway).  I also thought the "official answer" for a
> high packet-loss environment is to use a more sophisticated mechanism
> such as RFC4429.
>
> (That said, I wouldn't necessarily be opposed to updating the default
> value if and when we update RFC4862 so it will match the latest
> deployments for leaf networks better).
>
>
> It's a tradeoff between how long it takes before the address is usable.
>
> I wasn't aware tgat 4429 was more robust.
>
> During the efficient nd work, Erik did:
> https://tools.ietf.org/html/draft-nordmark-6man-dad-approaches-02
>
> And
> https://tools.ietf.org/html/draft-yourtchenko-6man-dad-issues-01
>
> I think we essentially need ACD.
>
> cheers
> Ole

We could recommend to do two things in concert: use optimistic
addresses and raise DAD to 3,4,5,...