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Re: OWL S&AS comment - owl:imports

From: Dave Beckett <dave.beckett@bristol.ac.uk>
Date: Tue, 24 Jun 2003 15:42:40 +0100
To: Jeff Heflin <heflin@cse.lehigh.edu>
Cc: public-webont-comments@w3.org
Message-Id: <20030624154240.2a36a189.dave.beckett@bristol.ac.uk>

On Fri, 20 Jun 2003 10:05:35 -0400
Jeff Heflin <heflin@cse.lehigh.edu> wrote:

> Dear Dave,
> 
> Thank you for you comment. As the original issue owner for imports, I
> have been asked to respond to you.
> 
> You say:
> 
> > Firstly it seems that owl:imports potentially will import the entire
> > semantic web of OWL Ontologies.  Have you considered the security and
> > denial-of-service implications of this mechanism?
> 
> It may be true that the imports closure of some documents will be very
> large. However, it is not likely to be much larger than the set of
> documents found by following all the URIs in each document, and in many
> cases will
> be smaller than that. Still the result may be so large that applications
> have to give up after a certain point and like search engines, which
> don't truly cover the whole Web, admit that they only provide partial
> results. To make it clear that not every OWL tool need to load every
> ontology, we will add the following text to "OWL Web Ontology Language
> 1.0 Reference":
> 
> "Note that whether or not an OWL tool must load an imported ontology
> depends on the purpose of the tool. If the tool is a complete reasoner
> (including complete consistency checkers) then it must load all of the
> imported ontologies. Other tools, such as simple editors and incomplete
> reasoners, may choose to load only some or even none of the imported
> ontologies."
> 
> I assume by denial-of-service implications you are imagining a scenario
> where someone creates an ontology or set of ontologies that imports a
> target some large number of times, causing any application that loads
> the ontology to "attack" the target. Note that an application gains no
> benefit from reloading an ontology, so it is expected that efficient
> applications will cache their ontologies and only load the target a
> single time, severely hampering any attempt at denial-of-service via the
> imports mechanism.

It also affects the client.  network delays and failures.  What happens
when an ontology can't be downloaded?  When I read about owl:imports I
was reminded of how the IETF makes all RFCs have to address security
implications, and since OWL includes this network-based processing
model, I saw an analogy.

> You also comment:
> 
> > Secondly, is not clear at what stage that this (Imports Closure
> > http://www.w3.org/TR/2003/WD-owl-semantics-20030331/rdfs.html#5.3 )
> > should be done.  In an example where you have some RDF/XML that will
> > map to RDF triples describing an OWL ontology, what are the steps
> > that you expect to happen?
> 
> There was a minor omission in 5.3 that explains how imports relates to
> entailment. The last definition should say that K and Q are
> imports-closed. I propose to change it to:
> 
> "Definitions: Let K and Q be imports-closed collections of RDF graphs.
> Then K OWL Full entails Q whenever every OWL Full interpretation (of any
> vocabulary V that includes the RDF and RDFS vocabularies and the OWL
> vocabulary that satisfies all the RDF graphs in K also satisfies all the
> RDF graphs in Q. K is OWL Full consistent if there is some OWL Full
> interpretation that satisfies all the RDF graphs in K."
> 
> There would also be a similar change in section 5.4 for OWL DL.
> 
> Note that these changes mean that in order to compute entailment, you
> must first compute the import closure (as defined earlier in Section
> 5.3). As I mentioned before not every tool needs to process imports. For
> those that do, a straight-forward approach is to recursively load the
> documents imported by each document you load and then execute any
> reasoning. However, it is not the purpose of the Semantics
> document to mandate a particular processing strategy.

It might be nice to state that in a declarative form rather than infered
form after interpreting the satisfaction definitions.

> Finally, thank you for pointing out that the issues list did not include
> a link to the formal objection. It has now been updated.
> 
> Thank you again for you comments. Please respond to this mailing list to
> let me know if I have adequately addressed your concerns.

Thank you - this is satisfactory.

Dave
Received on Tuesday, 24 June 2003 10:45:24 GMT

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