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

On Wed, 2003-12-17 at 12:07, Robert Elz wrote:
>   | 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?

> 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%).

You're missing the point. The spec is telling us to probe the SAME
ADDRESS three times. The retransmissions might make sense if the link
has a high loss probability, but address collision probability has
nothing to do with it.

Please read this bit again:

Here's my proposal (three fold):
     1. Let's specify DAD probe in the same way as in IPv6, defaulting
        to a single probe packet
     2. In the event of a conflict (slightly more probable with v4LL
        compared to IPv6), allow the node to select a new v4LL and retry
        up to, e.g., AddrSelectRetries times
     3. Let's get rid of the text describing shorter timeouts. If we
        default to a single probe, DAD only takes 1..2 seconds (with a
        high probability). That is good enough from the usability point
        of view.

Regards,

	MikaL