media features don’t matter. The feature collection
{foo=A,bar=B,baz=C,bop=F} is in the feature set described by the
predicate, even though the media feature tag "bop" isn’t mentioned.
Feature set predicates are therefore inclusive by default. A feature
collection is present unless the boolean predicate rules it out.
This was a conscious design choice in RFC 2533.
RFC 2533 also talks about matching a preference with a capability
set. This is accomplished by representing both with a feature set.
A preference is a feature set - its a specification of a number of
feature collections, any one of which would satisfy the requirements
of the sender. A capability is also a feature set - its a
specification of the feature collections that the recipient supports.
There is a match when the spaces defined by both feature sets
overlap. When there is overlap, there exists at least one feature
collection that exists in both feature sets, and therefore a modality
or rendering desired by the sender which is supported by the
recipient.
This leads directly to the definition of a match. Two feature sets
match if there exists at least one feature collection present in both
feature sets.
Computing a match for two general feature set predicates is not easy.
Section 5 of RFC 2533 presents an algorithm for doing it by expanding
an arbitrary expression into disjunctive normal form. However, the
feature set predicates used by this specification are constrained.
They are always in conjunctive normal form, with each term in the
conjunction describing values for different media features. This
makes computation of a match easy. It is computed independently for
each media feature, and then the feature sets overlap if media
features specified in both sets overlap. Computing the overlap of a
single media feature is very straightforward, and is a simple matter
of computing whether two finite sets overlap.
Authors’ Addresses
Jonathan Rosenberg
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
EMail: jdrosen@dynamicsoft.com
URI: http://www.jdrosen.net
Henning Schulzrinne
Columbia University
M/S 0401
1214 Amsterdam Ave.
New York, NY 10027
US
EMail: schulzrinne@cs.columbia.edu
URI: http://www.cs.columbia.edu/~hgs
Paul Kyzivat
Cisco Systems
1414 Massachusetts Avenue
BXB500 C2-2
Boxboro, MA 01719
US
EMail: pkyzivat@cisco.com
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