Web Security, Privacy and Commerce, 2nd Edition

D.1 Rating Services

The PICS rating service specifications are designed to enable many different kinds of ratings services on the World Wide Web. A rating service is any person, organization, or other entity that issues ratings. Ratings can be distributed with the document being rated, by a third-party site, on a CD-ROM, or by any other electronic means.

The PICS standard specifies a syntax for text files that describe the different kinds of ratings that a rating service can issue. This lets computer programs automatically parse the kinds of ratings that a service provides.

In their article describing PICS, Resnick and Miller create a sample PICS rating service based on the MPAA's movie-rating scheme:

((PICS-version 1.0) (rating-system "http://moviescale.org/Ratings/Description/") (rating-service "http://moviescale.org/v1.0") (icon "icons/moviescale.gif") (name "The Movies Rating Service") (description "A rating service based on the MPAA's movie rating scale") (category (transmit-as "r") (name "Rating") (label (name "G") (value 0) (icon "icons/G.gif")) (label (name "PG") (value 1) (icon "icons/PG.gif")) (label (name "PG-13") (value 2) (icon "icons/PG-13.gif")) (label (name "R") (value 3) (icon "icons/R.gif")) (label (name "NC-17") (value 4) (icon "icons/NC-17.gif"))))

This rating description indicates a location where information about the rating system and service can be found, gives it a name, and creates a single rating category called Rating. Rated objects can have one of five different ratings: G, PG, PG-13, R, or NC-17. The standard gives each of these ratings a value and an associated icon to be displayed with the rating.

The PICS rating service description is defined to have a MIME file typeapplication/pics-service . The file is formatted as a list.

The PICS format makes extensive use of name/value pairs. These are formatted as (name value). They are interpreted as "name has the value of value." For example, (min 0.0) means that the particular object being described has a minimum value of 0.0.

The following names are used to describe the ratings services themselves:

PICS-version aVersion

The version number of the PICS standard being supported. Should be 1.1.

rating-system aURL

A URL that indicates the location of a human-readable description of the categories, scales, and intended criteria for assigning ratings.

rating-service aURL

A URL that denotes the location of information used by the rating service itself. This URL is used as the basic URL for all icons and database queries.

icon aString

An icon associated with the particular object that is being described.

name aName

A human-readable name of the object being described.

description aDescription

A human-readable description of the object being described.

category

Introduces a list of elements used to denote a particular category that is supported by this rating service.

If a list begins with the atom category, then the list contains a list of name/value pairs that are used to describe a particular ratings category. The following are supported:

transmit-as aString

The name of the category when it is transmitted in a PICS label.

name aName

The name of the category itself.

min aNumber

The minimum value that a label in this category can have.

max aNumber

The maximum value that a label in this category can have.

multivalue aBoolean

Indicates that an object can have more than one label in the category. Has a value of true or false.

unordered aBoolean

Indicates that the order in which labels are reported has no significance. Can be true or false.

label

Introduces a list of elements that describe a particular label.

Integer

Indicates that the label is transmitted as an integer. By default, PICS ratings are not integers.

Each PICS label is further described by a collection of name/value pairs:

name aValue

The name of the label and its value.

Ratings services can operate label bureaus. A label bureau is "a computer system which supplies, via a computer network, ratings of documents. It may or may not provide the documents themselves."