Platform for Internet Content Selection

PICS facilitates the development of technologies to let parents and teachers control what children access on the Internet. This document serves many audiences: hopefully, you'll be able to find the information you're looking for.

The best overview of PICS is the paper, "PICS: Internet Access Controls Without Censorship," to be published in Communications of the ACM, October 1996. A shorter article appeared in Wired Magazine, August 1996, p.109.

PICS maintains two electronic mailing lists for public use:

PICS also maintains special purpose mailing lists for developers.

Information for Web Site Authors

Many authors and web site operators offer materials that they realize will not be appropriate for all audiences. We encourage them to label their materials to make it easier for filtering software to block access. As an added inducement to labeling, we note that some future applications may use labels for searching as well as filtering. Thus, labeling your site will make it easier both for some audiences to avoid your site or documents and for others to find you.

PICS is able to remain value-neutral by refusing to endorse any particular labeling vocabulary. As a web site operator, you will not have that luxury. You'll want to adopt one or more of the rating vocabularies that other sites are using. You may want to use one of the self-rating vocabularies.

The following is a an example of the right way to embed a PICS label in an HTML document:


<META http-equiv="PICS-Label" content='
(PICS-1.1 "http://www.gcf.org/v2.5"
   labels on "1994.11.05T08:15-0500"
          until "1995.12.31T23:59-0000"
          for "http://w3.org/PICS/Overview.html"
   ratings (suds 0.5 density 0 color/hue 1))
<H1>The Top of the Page </H1>


The following is incorrect, because the label is in the body of the document rather than in the HTML header (delimited by <head> and </head>).


<META http-equiv="PICS-Label" content='
(PICS-1.1 "http://www.gcf.org/v2.5"
   labels on "1994.11.05T08:15-0500"
          until "1995.12.31T23:59-0000"
          for "http://w3.org/PICS/Overview.html"
   ratings (suds 0.5 density 0 color/hue 1))

<H1>The Top of the Page 

It is OK to include more than one META tag in a single HTML document, so you can provide labels according to several services. There also is a way to combine several labels into a single label lists. See the technical specifications for details.

Information for Parents

A number of vendors have created filtering software that is compatible with the PICS specifications. Some of the software is built into browsers. Others work independent of the browser. Some of them bundle the software with a set of rating labels; others allow you to use rating labels that come from a separate source. They also differ in ease of use, in level of security, and in cost. We maintain a list of PICS-compatible filtering software (what we call client software) that you may want to investigate, but we do not endorse any particular product or provide product reviews. If you already have PICS-compatible client software, such as a web browser, that is able to read any PICS labels, you may need to tell the client which rating services' labels to pay attention to. Your client software may already have information about some rating services. You may also be interested in others. For each such service, you will need to download a ".rat" file that your client software can work with. We keep a list of rating services and links to their ".rat" files, but do not endorse any particular service. The vendor of your client software should provide documentation on what to do with the .rat file once you have downloaded it.

Information for Schools and Employers

Schools and employers that wish to set access restrictions have two options. They can either install filtering software on each individual computer, as an individual parent would. Alternatively, some vendors offer centralized administration of filtering for a Local Area Network, in some cases with local control in each classroom. We do not endorse particular products, but the list of network-based filtering products may be useful to you.

Information for the Media

Press inquiries about PICS may be directed to any of the following people: You can find a list of product announcements and other PICS-related press releases. These may be a good source of further press contacts.

An overview of the PICS system is available from the paper "PICS: Internet Access Controls Without Censorship," which will appear in Communications of the ACM, in October 1996. A shorter piece appeared in the August issue of Wired Magazine, p. 109.

In addition, the on-line presentations Overview of the PICS effort (presented by Albert Vezza, Associate Director, MIT's Lab for Computer Science at Internet World, October 30, 1995) and Description of PICS Technical effort (presented by Jim Miller (Research Scientist, World Wide Web Consortium at MIT's Lab for Computer Science) at Internet World, October 30, 1995 are good historical sources.

For three short pieces of background there are the original Mission and Scenarios for Content Selection, Technical Committee Charter, and PICS Statement of Principles adopted by PICS at its first meeting in August of 1995.

Information for Developers

Anyone interested in developing code based on the PICS specifications is urged to read the call for participation in the PICS developers email list. This email list receives advance notice of proposed changes to the specifications, and serves as a forum for developer issues. Other information of interest to developers:

As described in the summary of the first PICS developers workshop, there is also a working group writing new specifications for a profiles language. The following resource lists are being maintained by members of the PICS developers' community. Contact the maintainer of each individual list with additional links. The maintainers have all agreed to be fast and fair in maintaining these lists (please send any unresolved complaints to pics-ask@w3.org).

PICS Technical Specifications

PICS has produced two specifications. The first specification defines the terms rating service and rating system, and defines a machine-readable description of a rating service that can be used to customize a user interface for parents who wish to use a PICS rating service. The current version of this rating system and rating service specification is dated 5 May 1996 and is version 1.1.

The second specification defines the format of PICS labels, as well as how those labels can be transmitted. It provides for embedding PICS labels into HTML document headers, transmitting PICS labels via RFC822 (electronic mail, HTTP, net news) headers, and requesting PICS labels from a dedicated PICS label bureau. The current version of this label syntax and communication protocols specification is dated 5 May 1996 and is version 1.1.

Frequently Asked Questions

Common Misconceptions

Q: Where do I get the PICS software? How much does PICS cost?

A: Actually, PICS doesn't provide any software. It's just a set of technical specifications that help software and rating services to work together. You would actually use filtering software provided by some organization other than PICS. There is a list of PICS-compatible client software, but W3C does not endorse or evaluate any of the products. Prices vary; some software is free.

Q: What criteria does PICS use in deciding what's safe for my children to view? Isn't that inherently a subjective decision?

A: Again, PICS doesn't actually rate anything. PICS just set technical specifications so that ratings from any source will work with all the filtering software. Rating and labeling services choose their own criteria for rating. Since rating will always involve some amount of subjective judgement, you'll want to choose a rating service whose judgements are close to the ones you would make. There is a list of PICS-compatible rating services, available at xxxx. The filtering software you use may already be configured to use a particular rating service, so you may not have to make this choice separately.

How to label

Q: How difficult is it for a content provider to include a PICS compliant label with the content? Please describe the process.

A: A content provider first needs to choose which rating vocabulary to use. We recommend that you use a vocabulary used by others, to make it easy for parents to understand your labels. A list of self-rating vocabularies is available, but W3C does not endorse any particular vocabulary. Typically, you choose a self-labeling service, connect to its web server and describe your document or web site by filling out an on-line questionnaire. After completing the questionnaire, the service gives you a text label in a special format, which you then paste into the header portion of your HTML document (or the home page for your site).

Q: I have read that an independent rating agency can label sites or documents created by others. Do the documents' creators have to cooperate? Please describe the process.

A: An independent rating agency need not get cooperation from every publisher whose material it labels. As with self-labeling describeed above, the independent labeler first needs to invent or adopt an existing vocabulary. The rater then uses a software tool to create labels that describe particular URLs. Instead of pasting those labels into documents, the independent rater distributes the labels through a separate server, what we call a label bureau. Filtering software will know to check at that label bureau to find the labels, much as consumers know to read particular magazines for reviews of appliances or automobiles.

Q: Can I create my own labels so that I can be sure that my child is only viewing material which I have reviewed?

A: An individual parent or teacher can also act as an independent rating agency. Some filtering software facilitates this process by storing the parent or teacher's labels on the local computer, bypassing the need for a label bureau. Filtering software typically describe this feature as an "override" capability.

How filtering works

Q: I understand that a browser or stand-alone software filter can be set to check labels supplied by an independent rating agency before connecting to a chosen site. Can you explain how this works?

A: When an end-user asks to see a particular URL, the software filter fetches the document but also makes an inquiry to the label bureau to ask for labels that describe that URL. Depending on what the labels say, the filter may block access to that URL.

Q: Does using a PICS compliant software filter slow communication with the Internet?

A: If the filter asks a label bureau for labels, as described above, the extra request will take extra time. There are various techniques, including keeping local copies of labels and making parallel requests, that can reduce this performance penalty. This is likely to be a point of significant competition between vendors.

Q: Can an Inernet access service provider limit access for all subscribers to things labeled a particular way or can PICS compliant labels only be used to limit access at the user level? Can a country limit access to all its Internet access service providers to particular categories of labels?

A: That depends on how the network is configured. If an access provider, or a country, has choke points through which all requests travel, it could install the filtering software at the choke points. It is a more natural and effective use of the technology to put control in the hands of end-users, because different users will want to limit access to different things.

Q: Does PICS recommend any particular software filter?

A: No. The World Wide Web Consortium, which developed PICS, is strictly vendor neutral. A list of client software filters is available.

Q: I am worried that some classification systems may occasionally mislabel adult material as suitable for a child. I would feel confident if two separate classification systems both labelled the material as suitable for a child. Can I use a PICS compliant software filter to only permit access to material that has appropriate labels from two classification systems?

A: In principle, a filter can pay attention to labels from more than one source. The actual implementation may vary from vendor to vendor.

Q: I run a small business and several of my employees require regular access to a few Internet sites. Can I label those sites and run a software filter to ensure that my employees do not access any other sites? Is that difficult to do? How much time will it take me?

A: Many vendors offer a "block unrated sites" option. The difficulty of creating your own zone of acceptable materials for employees will vary from vendor to vendor. We also expect teachers and textbook publishers to create lesson-specific zones of the Internet. At least one vendor has adopted this "inclusive" method as its general filtering technique.

Advanced questions

Q: If the content contains the label, can the label be altered or removed from the content?

A: Yes, a label in an HTML document can be removed. Embedding a label in a document requires the cooperation of the publisher and all those who handle the document.

Q: If the label and content are physically separated, how are they reliably linked to each other? What happens if the content is moved or cached to another site?

A: A label bureau that stores only labels and not documents associates the labels with URLs. Even if the contents retrieved from that URL are cached, the label bureau can still provide a label describing the URL. If the contents are moved to a new URL, the label bureau may try to keep track of synonyms (alternate URLs that point to the same document) but it may not always keep up. A document that has been labeled at one URL may effectively be unlabeled if it moves to another URL.

Q: Does PICS work with communication protocols other than http? In other words, can a PICS compliant software filter be defeated by a user whose access is restricted to documents of a certain label classification, through the user obtaining other documents by FTP or E-mail?

A: PICS labels can describe anything that can be named with a URL. That includes FTP and Gopher, but not e-mail.

Q: Can PICS compliant labels be attached to particular IRC channels and Usenet discussions?

A: Usenet newsgroups, and even individual messages, have URLs, and hence can be labeled. There is not yet an official URL scheme for IRC, but the PICS specifications defined a preliminary scheme, and a more robust URL scheme for IRC is being worked on.

Q: How can I be sure that a PICS label is not false or misleading? Can a digital signature be attached to the label so that I can be sure that the label is genuine?

A: A label can include a cryptographic signature. This mechanism lets you check that the label was authorized by the service you subscribed to. You have to decide for yourself whether the service is trustworthy; if it frequently puts out misleading labels, you probably will want to switch to another rating service.

Q: How can I be sure that the content of a site has not changed since it was labelled?

A: A label can include a cryptographic checksum on the contents of the document. If the checksum matches a checksum of the current contents, then the label is valid. If not, then the document has changed since the label was created.

Q: Can I ask a search service to omit responses that will be blocked by my filter?

A: Not yet, but we're working on it. We agree that it could be frustrating for a child to use a search service, get back ten responses, and then find that clicking on any one of the ten leads to a message saying that the URL is blocked. We're defining a standard format for filtering rules, so that client software will be able to tell the search engine what would be blocked. Some search engines may ignore that information, but others will tailor their searches based on the client's filtering rules.

Q: Setting the filtering rules is too much of a bother for me. Can't I just find someone I trust and install their rules.

A: Some vendors will offer this feature. When the standard format for filtering rules is agreed on (see previous question), many more vendors are likely to offer this feature.

Do you have a question not answered in this FAQ? If so, please send email to pics-ask@w3.org.

Comments to PICS-ask@w3.org.
Created 11 September 1995 by Rohit Khare
Last updated 20 August 1996