Re: WG ACTION: 1 week to discuss [LL37] Aggressive Time-outs

> From: "Robert Elz" <[email protected]>
> 
>   | Instead of trying to figure out how long it takes to send three probes,
>   | I would like to step back a little and ask the question WHY exactly do
>   | we have to send THREE PROBES? What exactly in v4LL necessitates sending
>   | three DAD probes when IPv6 can get away with only one?
> 
> v6 has 64 bits of address space, it is incredibly unlikely that something
> doing random address assignment (which is what is relevant here) is ever
> going to clash with something else - so one check, no reply, and all is OK.
> 
> v4LL has only 16 bits to play in (slightly less because of the exclusions of
> the first and last /24's from contention) - the birthday paradox tells us
> that with perfect random assignments we're going to get a clash as soon as
> the number of hosts on the link gets to around 256 (2^8 being sqrt(2^16) which
> is a very rough approximation of the point where probability goes > 50%).
> 
> 256 hosts is not at all implausible on a link (2^32 is), so v4LL has to
> actually expect to get duplicates, where in v6 they are only likely to occur
> due to human stupidity.   v4LL simply has to try harder to detect those
> duplicates.

In addition to this, it is probably the case that IPv4 systems do not cope so well as IPv6 with clashes and changing addresses, so having a higher confidence level that the address you have picked is unique on the link is probably going to make life that bit easier for the user.

The final and probably strongest argument against Mika's suggestion is that it is way too late in the process to be re-opening this algorithm at this point. Unless you can show that three probes will break where one won't (which I doubt) then we should tidy up the arithmetic so at least we get the answer to 2+2 right, and get it out of the door.

Philip