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

    Date:        Tue, 16 Dec 2003 23:27:16 +0200
    From:        Mika Liljeberg <[email protected]>
    Message-ID:  <1071610036.12327.17.camel@hades>

  | 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.

kre