Chapter 5: Realizing a Metadata Framework 161 Our ontology describes all basic content types found in a mobile device: contact information, calendar entries, messages (SMS, MMS, and e-mail), images, music, video, and so on. In addition to these physical entities, we have different kinds of collections, such as music playlists and photo albums. The collections can be either user-defi ned or automatic. Typical examples of automatic playlist include recently played songs, and most played songs. 5.6.3 Our Default Ontology This section describes our initial simple ontology for the fi rst version of our metadata framework. The default ontology creates an initial set of metadata attributes that cover the most fundamental usages for image and music applications. The ontology can be easily extended later by adding new metadata attributes. However, care must be taken when adding new schema classes or changing the schema structure, since they may break the backwards compatibility. 5.6.4 Namespace All metadata attributes defi ned here belong to a default namespace. We have reserved the default namespace to be non-extensible to provide the minimum set of metadata defi nitions that every application developer can trust. All extensions must belong to another namespace, freely selectable by the extension developer. Although our framework only requires that namespaces are globally unique strings and the developer is free to choose whatever means to achieve that require- ment, we strongly recommend using URIs as namespaces, as in W3C 22 XML namespace recommendation. 23 As defi ned in RFC 3986, URIs are not globally unique strings, so care must be taken when choosing a proper URI to describe a namespace. For example, the URI http://localhost is a valid URI according to the RFC, but is not usable as a namespace URI. Any namespace URI must be globally identifying, not just in an end-user context. The namespace URI does not need to refer to an existing document, but it would be useful for the URI to point to a human-readable docu- ment explaining the semantics of the metadata defi nitions. This would make it easier for other developers to use or even extend those defi ni- tions. As an example, a good imaginary namespace URI could be 22 http://www.w3.org/TR/2006/REC-xml-names-20060816/ 23 ftp://ftp.rfc-editor.org/in-notes/rfc3986.txt