December 2007

"Language is a process of free creation; its laws and principles are fixed, but the manner in which the principles of generation are used is free and infinitely varied."

Noam Chomsky

The objective of this article is to: (i) extend the discussion of licensing to non-software assets and (ii) provide an introduction to rights expression languages (RELs). Licensing is not limited to software. We can associate a license with any kind of asset that holds intellectual value, and can thus be turned into a source of revenue. Here, our interest is on information assets, which include software and software components, but also services, processes, and content. For instance, a song that a user downloads from iTunes is an information asset. So is a web service such as the Google Maps API (application programming interface).

Licensing and DRM

We begin the discussion by defining two key terms, licensing and digital rights management (DRM). Licensing is a fundamental way of controlling the distribution of information assets, and underlies the design of business relationships and strategies. Licensing principles reflect the overall business value of assets to their producers and consumers. Also, licensing is often used to protect the intellectual property rights (IPR) of the producers of an asset. Licensing is both a source of revenue and a strategic tool.

In their book Digital Rights Management: Business and Technology, Rosenblatt, Trippe, and Mooney define DRM as an umbrella term referring to the collection of technologies (hardware, software, and services) that govern the access to information assets through associated rights, and controls their distribution. The foundation of DRM technology relies on our ability to represent the rights over digital assets. RELs represent the rights over assets in a machine-understandable way. RELs describe different aspects of usage control, payment, and access, for a digital access environment.

According to Parrott, a REL consists of four components:

  • Subjects, the actors who perform some operation or action
  • Objects, the content against which a subject wants to perform an operation
  • Operations or what the subjects wants to do to the object
  • A set of constraints or conditions under which an operation can be performed

These components and their relations support a range of models, each describing a way of applying digital rights. In general, a REL expresses the rights of an information asset either in some form of logic or in an XML-based language.

To illustrate these concepts, let us assume that a user wants to download a song from the iTunes store and play it on his iPod. The subject is the user, the object the song, the operation to play the song, and the constraints are that the user has to pay 99 cents for the download and cannot share the song with his friends.

Rights Expression Languages

What follows is a brief history of RELs. A pioneering formal language called DigitalRights describes a mathematical model of simple licenses that consists of payment and rendering events and a formal representation of licenses. LicenseScript is a logic-based REL.

Logic-based RELs express general prepositions of a permissive or obligatory (restrictive) statement. However, these languages cannot express a finer level of granularity of the assets, actors, or actions involved. Logic-based RELs cannot interoperate with other types of RELs.

XML-based RELs support interoperable ways of expressing the rights of an information asset. An XML-based REL allows asset producers to specify flexible expressions. The Extended Rights Markup Language (XrML) and the Open Digital Rights Language (ODRL) are two XML-based RELs which have gained international recognition and are widely used in industry.

XrML is the basis for the REL of the MPEG-21 multimedia framework. It focuses on the license through which a rights holder confers usage rights to a consumer. A license can be digitally signed by the rights holder, now also referred to as the issuer, to confirm that the holder grants the rights contained in the license. An XrML license contains one or multiple grants and the license issuer. A grant is the element within the license that authorizes a subject to exercise a right on some object under some constraints. Note that the actual terminology used by XrML is slightly different from this. ODRL is an open standard language for the expression of terms and conditions over assets in open and trusted environments. ODRL consists of an expression language and a data dictionary. The expression language defines basic terms of rights expressions and their organization using a set of abstract concepts. The data dictionary defines the semantics of the concrete terms used to express an instance of a rights specification.

ODRL is based upon an extensible model for rights expression, and defines the following three core entities and their relationships:

  • Assets, the objects being licensed
  • Rights, the rules concerning permitted activities, the constraints or limits to these permissions, the requirements or obligations needed to exercise the permission, and the conditions or specifications of exceptions that, if true, terminate the permissions and may require re-negotiation of the rights
  • Parties, the information regarding the service provider, consumer, or broker

With these entities, ODRL can express offers (proposals from rights holders for specific rights over their assets) and agreements (contracts or deals between the parties with specific offers). ODRL supports the declaration of a wide range of expressions. It can also be extended to different types of domains. For example, we can use ODRL to specify that a consumer of a geocoding web service can only use this service in a non-commercial context, as well as the number of times the service can be accessed each day. ODRL has been published by the World Wide Web Consortium (W3C), and has received wide acceptance. ODRL is supported by several industry consortia such as the Dublin Core Metadata Initiative (DCMI) and the Open Mobile Alliance (OMA).

Two applications of ODRL are an ODRL profile of the semantics of Creative Commons (CC) licenses and the ODRL profile for services (ODRL-S). The core semantics of CC licenses have been expressed in ODRL. This profile supports extensions to these semantics, and defines an XML Schema. ODRL-S is an extended version of ODRL to express clauses for service licensing, creating a machine-understandable service license.


Information assets are usually accompanied by a license that describes the terms and conditions on the use of this asset imposed by its producer. A license reflects the overall business value of the asset to its producers and consumers. The kind of rights vary based on the nature and context of the assets involved. For example, one of the rights for a multimedia asset is that consumers can play it. The concept of playing can not be directly applied to a web service asset.

Similarly, the rights governing the use of the interface and implementation of a web service are distinct. However, for multimedia or software assets we cannot make such a distinction. In this article we introduced the concept of licensing and RELs, and briefly described XrML and ODRL as the two most prominent RELs.

We thank Dr. Renato Iannella, NICTA, Australia, and Prof. Vincenzo D'Andrea, University of Trento, Italy, for their suggestions and comments.

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