Subject: [Geopriv] Let us get Geolocation Policy finished!

From: Hannes Tschofenig <[email-masked]>
Date: Sat, 29 Mar 2008 19:14:31 +0200

Hi all,

we have updated a new version of the Geolocation Policy draft. You can 
find it here:
http://www.ietf.org/internet-drafts/draft-ietf-geopriv-policy-15.txt

We believe it addresses the DISCUSSes we received. The draft now 
contains the following aspects:

* Updated Security Consideration section to illustrate a potential 
privacy vulnerability in a mobile scenario when a Target moves from one 
transformation region to another one

* Reworked "Condition" section whereby the Polygon based condition 
element is replaced with a Circle based condition element that is 
simpler to use. Civic Location based condition element remained unchanged.

* Reworked the "Transformation" section: Geodetic transformation 
subsection replaced with a mechanism that returns a circle with a radius 
that is indicated by the Rule Maker. Civic location based transformation 
unchanged.

As part of these changes we had to clean-up a couple of other minor 
things as well.

Please let us know what you think. Are you OK with the suggested approach?

Ciao
Hannes

PS: Here is the relevant diff:
http://tools.ietf.org//rfcdiff?url1=http://tools.ietf.org/id/draft-ietf-geopriv-policy-12.txt&url2=http://www.ietf.org/internet-drafts/draft-ietf-geopriv-policy-15.txt

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From: "Thomson, Martin" <[email-masked]>
Date: Sun, 30 Mar 2008 17:52:15 -0500

Hi Hannes,

I promised you a response to this earlier.  I'll keep it brief.

The changes are good, but I think that the transformation could be
improved.  My main problem relates to the way that a user is expected to
understand this transformation.  I'm not sure how to simply and
succinctly explain the effect that this transformation produces.  It's
almost as if you need to start with how you would explain to a user and
see if you can produce that through a rule.

I'll be more specific on the proposed method.  To keep this short, I'll
use Ro to refer to the radius of the original; Ra refers to the value in
the transformation rule, that is, the "allowed" radius.

For the first case, where Ro >= Ra, the only problem that I see is that
the centroid doesn't move.  Despite knowing well that the centroid isn't
special, it's hard to explain that to a user.  Not seeing any effect on
the result could cause the user some frustration.

For the second case, where Ro < Ra, the actual variation in distance is
by min(Ra/2, Ra - Ro).  The halving is a little arbitrary and only comes
into effect when Ra > 2*Ro.

To propose an alternative, I'd like to concentrate on what I'd tell a
user: "this is the distance that the location is moved by".  Or, to
expand: "if you were here, we move your location in a random direction
by up to this distance".

My proposal is to simplify further and not go for a "target uncertainty"
at all.  Instead, take the number as a "fudge distance".

1) Drop altitude, take centroid and radius, make circle.
2) Move centroid a random distance up to T (the value specified by the
user) in a random direction.
3) Increate the radius of the new region of uncertainty by T.  This new
area will be guaranteed to cover the original area, ensuring that the
confidence doesn't have to be reduced.

It might also make sense to tweak this further by proposing a different
method for acquiring the random distance to limit the density of the
distribution near the original centroid.  For instance, T - (rand(1) *
sqrt(T))^2 would even out the distribution over the resulting area.  But
that could be left up to implementations.

Cheers,
Martin

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]] On
> Behalf Of Hannes Tschofenig
> Sent: Sunday, 30 March 2008 4:15 AM
> To: GEOPRIV
> Subject: [Geopriv] Let us get Geolocation Policy finished!
> 
> Hi all,
> 
> http://tools.ietf.org//rfcdiff?url1=http://tools.ietf.org/id/draft-
> ietf-geopriv-policy-12.txt&url2=http://www.ietf.org/internet-
> drafts/draft-ietf-geopriv-policy-15.txt
> 

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