Re: Question about tcp hash function tcp_hashfn()

Evgeniy Polyakov
2006-06-01T08:38:49Z

On Thu, Jun 01, 2006 at 01:11:25AM -0600, Brian F. G. Bidulock (bidulock@openss7.org) wrote:
> Evgeniy,
>
> On Thu, 01 Jun 2006, Evgeniy Polyakov wrote:
>
> > On Thu, Jun 01, 2006 at 12:46:08AM -0600, Brian F. G. Bidulock (bidulock@openss7.org) wrote:
> > > > Since pseudo-randomness affects both folded and not folded hash
> > > > distribution, it can not end up in different results.
> > >
> > > Yes it would, so to rule out pseudo-random effects the pseudo-
> > > random number generator must be removed.
> > >
> > > >
> > > > You are right that having test with 2^48 values is really interesting,
> > > > but it will take ages on my test machine :)
> > >
> > > Try a usable subset; no pseudo-random number generator.
> >
> > I've run it for 2^30 - the same result: folded and not folded Jenkins
> > hash behave the same and still both results produce exactly the same
> > artifacts compared to XOR hash.
>
> But not without the pseudo-random number generation... ?

How can I obtain (2^30)*6 bytes of truly random bytes?

> > Btw, XOR hash, as completely stateless, can be used to show how
> > Linux pseudo-random generator works for given subset - it's average of
> > distribution is very good.
>
> But its distribution might auto-correlate with the Jenkins function.
> The only way to be sure is to remove the pseudo-random number generator.
>
> Just try incrementing from, say, 10.0.0.0:10000 up, resetting port number
> to 10000 at 16000, and just incrementing the IP address when the port
> number wraps, instead of pseudo-random, through 2^30 loops for both.
> If the same artifacts emerge, I give in.

I've run it with following source ip/port selection algo:
if (++sport == 0) {
saddr++;
sport++;
}

Starting IP was 1.1.1.1 and sport was 1.
Destination IP and port are the same 192.168.0.1:80

Jenkins hash started to show different behaviour:
it does not have previous artefacts, but instead it's dispersion is
_much_ wider than in XOR case.

With following ip/port selection algo:
if (++sport == 0) {
//saddr++;
sport += 123;
}

I see yet another jenkins artefacts, but again different from previous
two.

But each time both folded and not folded hashes behave exactly the same.

> Can you show the same artifacts for jenkins_3word?

What should be used as starting point there?
If I use 0 it is the same as jhash_2words().
If I use 123123 - artefacts are the same, just slighly shifted (I tested
only the latest test above though).

Looking into the code we can see that jhash_2words() is jhash_3words()
with zero "C" value, so it will show the same nature.

--
Evgeniy Polyakov