Re: [Ecrit] LoST civic caching issue

[Henning Schulzrinne <hgs at cs.columbia.edu>]

On Apr 14, 2007, at 12:41 PM, Hannes Tschofenig wrote:

> Hi Henning,
>
> thanks for raising this issue and it is in fact a difficult problem.
>
> I see two aspects with the "the donut problem",  i.e., there is a  
> larger service boundary with another service area within (the hole  
> in the donut). There are two cases:
>
> * The entity responsible for the larger service area knows that  
> there is a donut problem. In this case it would be reasonable to  
> assume that this entity could mark the response as "non cachable".  
> For geo one would want to express the service area as a donut --  
> currently we don't have a description how to describe such a shape.  
> I  now believe that we should add something to the PIDF-LO profile  
> draft.

Polygons can be used to describe structures with holes - you just  
have to have the polygon 'wrap around' the hole and overlap in the  
middle. It gets tedious for multiple holes.

Also, it appears that common SQL databases with geo extensions, such  
as the popular PostGIS, do not support such complicated shapes. Thus,  
it may be a better idea to compose this, as a set of coverage  
polygons [we got that] and a set of exceptions.



> * The entity responsible for the larger service area does NOT know  
> about a potential donut problem. This case is actually not related  
> to civic since it can appear in geo as well. I don't have an easy  
> and nice answer for this case.

That is indeed the hard case. However, I believe it is properly the  
responsibility of the exception case to notify the surrounding entity  
that it wants to be considered a 'hole'. My perception is that  
institutions, such as university campuses, with their own police  
force or ambulance corps coordinate with the local civic authorities,  
rather than just freelance. I don't know the politics or legalities  
of whether, say, the NYPD would allow the Columbia police force to be  
declared officially in charge. If all else fails, the local LoST  
resolver would have to implement local policy. This is similar to  
what happens today: If you dial 911 on a cell phone while on the  
Columbia campus, you'll be connected to the NYPD, not campus police.

Henning

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