On Wed, Mar 02, 2016 at 03:50:16PM +0100, Florian Westphal wrote:
> Jan Engelhardt <jengelh@xxxxxxx> wrote:
> > On Wednesday 2016-03-02 13:10, Florian Westphal wrote:
> > >> case XT_STATISTIC_MODE_RANDOM:
> > >> if ((prandom_u32() & 0x7FFFFFFF) < info->u.random.probability)
> > >>
> > >> --probability seems to check for "less than" the random value.
> > >
> > >Yes. [...]
> > >Other suggestions?
> >
> > "--probability" is meant to represent saying "with a probability
> > of p=10%, ...". This does not mandate any particular operator.
>
> Right, that was my reasoning for making meta random 0.1 behave
> like 'match with a probabiliy of 10%'.
>
> > Furthermore, it surprises me that iptables even supports
> > ! --probability, because you can just express it as 1-p
> > instead.
>
> Yes.
>
> So my suggestion is this:
>
> for nft v2 of meta random support:
>
> - keep the 'implicit LE op' behaviour so that
> meta random 0.1 means '10% probability of matching'.
> - change display to hide the LE detail from the user, i.e.
> don't show 'meta random le 0.1' but 'meta random 0.1'.
> [ I agree with Jan, its detail, users can still see this
> with debug output on ].
>
> Don't change anything else, i.e.
>
> meta random == 0.1 will match with a probability of 1 in 0xfffffff
> on average. It does what you asked it to do ;)
>
> For the translation patch, if ! is given, translate it to the inverse
> as per Jans instruction, e.g.
>
> --probability ! 0.1 is translated to
>
> meta random 0.9
>
> If there are no further comments, I will send a v2 for nft meta random
> side soon.
In all this thread you talk all the time on probability semantics,
however the selector name is 'random'.
Why don't you rename this to 'meta probability' instead?
No changes in the semantics then, just use:
meta probability 0.1
and when expressing the opposite:
meta probability 0.9
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