[Ecrit] LoST civic caching issue

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

During our implementation of caching in LoST, one of my students came  
up with the following interesting issue:

Suppose that a system has cached a mapping for city=New York,  
state=NY and somebody moves the client to a place within that region  
that has its own mapping, e.g., a college campus. (Similar issues  
arise for places where a county has one mapping, with some part,  
e.g., the incorporated area, having a more specific mapping.) In  
those cases, a client would notice that the address has changed, but  
it still matches the old mapping, so it will not re-query.

A related problem occurs if, during a failure, a default response has  
been returned.

For geo coordinates, 'donuts' and other ways to express 'all but this  
area' conditions, which would trigger re-querying. This doesn't work  
for civic coordinates, obviously.

There seem to be several possibilities:

(1) Don't do that. In other words, if an area knows that there are  
more specific mappings, it has to be split up in some way, e.g., by  
enumerating all such areas explicitly. For example, for the  
unincorporated areas, the community name is part of the mapping, even  
though they all share the same county-wide mapping.

This probably works for most cases, but fails for the campus example,  
since the surrounding city doesn't really know about these.

(2) Mark such cases as non-cacheable. Since civic coordinates are  
only used for stationary and nomadic devices, the additional query  
load is rather modest. This is the safe approach.

(3) As a compromise between (1) and (2), mark mappings that are known  
to be leaf nodes. For example, a campus could mark its mapping as  
such since it can be pretty sure that there's nothing below that  
level. That way, laptops moving within a building don't have to  
requery. These could simply be marked as cacheable, so that no  
additional XML tags are needed.

Henning



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