PICS Superseded by POWDER

Please note that PICS has now been superseded by the Protocol for Web Description Resources (POWDER). This means that W3C does not recommend any further development of software that uses PICS, the use cases for which are covered by POWDER which offers significant advantages as detailed below.

Like PICS, POWDER has the following capabilities:

  1. Descriptions (labels) can be applied to one or more resources. Software agents can use efficient processing methods to apply the descriptions to any resource in the group without querying the data directly each time.
  2. Formally-defined vocabularies are used to provide any sort of description.
  3. Descriptions (labels) are explicitly attributed to their publisher.
  4. Descriptions may be digitally signed.

The specific capabilities offered by POWDER that are not available in PICS are:

  1. The method used to define the group of resources to be described is substantially more flexible. Whereas PICS relies on the idea of a URL prefix, POWDER allows groups to be defined in terms of any characteristic of an IRI. This is the subject of a standalone Recommendation.
  2. POWDER may be processed in a semantic web environment. The output of a POWDER processor is one or more RDF triples. POWDER documents may also be transformed, via a defined process, into an OWL ontology and processed as such, subject to implementing a semantic extension. The output is, again, RDF triples. The POWDER document suite includes a detailed treatment of formal semantics.
  3. POWDER offers a more robust concept of a default description for a set of resources. An ordered list of descriptions can be defined such that a given URI is described only by the first Description Resource in the list for which a match is found, even if subsequent items in the list would also apply.
  4. Descriptions (labels) can be encoded in XML and any other syntax that is used for encoding RDF/OWL, such as N3. It does not have a specific syntax of its own as PICS does.
  5. In addition to formally-defined descriptions, POWDER allows free-text 'tags' to be associated with resource groups.
  6. POWDER allows descriptions to be published in such a way that they can be used by any publisher to to describe any resource within an outer limit set by the original creator of the description. See Section 2.6 of the Description Resources document for details.

These points comprise the key reasons for POWDER superseding PICS. The one capability that PICS offers that is not supported by POWDER is that descriptions cannot be embedded directly in HTML. This is deliberate as it keeps both maintenance and authentication of the description separate from maintenance of the described resources which is felt is a more robust design.

Transport Mechanisms Compared

PICS labels were designed to be transmitted as part of the client-server interaction over HTTP. As noted above, it is also possible to embed PICS labels in HTML which is done by using the http-equiv meta tag. POWDER takes a different approach with Description Resources encoded within a discrete XML document. Links can be made to such documents using the link element in HTML and ATOM (POWDER defines the 'describedby' relationship type for this purpose) or using RDFa. Alternatively, links can be made using the HTTP Link header. This was defined in RFC 2068 but dropped from RFC 2616. At the time of writing it is expected to be re-instated through a new RFC: Web Linking in the near future. See Section 4 of the Description Resources document for details.

Comments on POWDER should be sent to (with public archive).

Last updated: $Date: 2009/11/25 10:43:28 $

Phil Archer