1. A machine readable non-transitory storage medium storing instructions which, when executed by a processor, cause the processor to perform the steps of: receiving a time based metadata track associated with a time based media track containing media data, the time based metadata track comprising a set of identifiers that specify one or more types of metadata in the time based metadata track; and indexing the time based metadata track as a non-time based description of a set of types of metadata in the time based metadata track, the non-time based description comprising the set of identifiers that specify the one or more types of metadata in the time based metadata track, each of the identifiers in the set of identifiers uniquely specifying a type of metadata relative to all other types of metadata in the metadata track.