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Versions: 00 01 02 RFC 2397

Internet-Draft                                          Larry Masinter
draft-masinter-url-data-02.txt                       Xerox Corporation
Expires in 6 months                                  November 13, 1996


                       The "data" URL scheme

Status of This Memo

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Abstract

A new URL scheme, "data", is defined. It allows inclusion of small
data items as "immediate" data, as if it had been included externally.

Description

Some applications that use URLs also have a need to embed (small)
media type data directly inline. This document defines a new URL
scheme that would work like 'immediate addressing'. The URLs are of
the form:

        data:[<mediatype>][;base64],<data>

The <mediatype> is an Internet media type specification (with optional
parameters, etc.)  The appearance of ";base64" means that the data is
encoded as base64. Without ";base64", the data (as a sequence of
octets) is represented using ASCII encoding for octets inside the
range of safe URL characters and using the standard %xx hex encoding
of URLs for octets outside that range.  If <mediatype> is omitted, it
defaults to text/plain;charset=US-ASCII.  As a shorthand, "text/plain"
can be omitted but the charset parameter supplied.

The "data" URL scheme is intended for short values. Many applications
that use URLs may impose a length limit; for example, URLs embedded
within <A> anchors in HTML have a length limit determined by the SGML
declaration for HTML[RFC1866].

The "data" URL scheme has no relative URL forms.

Syntax

    dataurl    := "data:" [ mediatype [ ";base64" ] "," data
    mediatype  := type "/" subtype *( ";" parameter )
    data       := *urlchar
    parameter  := attribute "=" value

where "urlchar" is imported from [RFC-URL-SYNTAX], and "type",
"subtype", "attribute" and "value" are the corresponding tokens from
[RFC 1522], represented using URL escaped encoding of [RFC-URL-SYNTAX]
as necessary.

Attribute values in [RFC 1522] are allowed to be either represented as
tokens or as quoted strings. However, within a "data" URL, the
"quoted-string" representation would be awkward, since the quote mark
is itself not a valid urlchar. For this reason, parameter values
should use the URL Escaped encoding instead of quoted string if the
parameter values contain any "tspecial".

The ";base64" extension is distinguishable from a content-type
parameter by the fact that it doesn't have a following "=" sign.

Examples

A data URL might be used for arbitrary types of data. The URL

      data:,A%20brief%20note

encodes the text/plain string "A brief note", which might be useful in
a footnote link.

The HTML fragment:

<IMG
SRC="data:image/gif;base64,R0lGODdhMAAwAPAAAAAAAP///ywAAAAAMAAw
AAAC8IyPqcvt3wCcDkiLc7C0qwyGHhSWpjQu5yqmCYsapyuvUUlvONmOZtfzgFz
ByTB10QgxOR0TqBQejhRNzOfkVJ+5YiUqrXF5Y5lKh/DeuNcP5yLWGsEbtLiOSp
a/TPg7JpJHxyendzWTBfX0cxOnKPjgBzi4diinWGdkF8kjdfnycQZXZeYGejmJl
ZeGl9i2icVqaNVailT6F5iJ90m6mvuTS4OK05M0vDk0Q4XUtwvKOzrcd3iq9uis
F81M1OIcR7lEewwcLp7tuNNkM3uNna3F2JQFo97Vriy/Xl4/f1cf5VWzXyym7PH
hhx4dbgYKAAA7"
ALT="Larry">

could be used for a small inline image in a HTML document.  (The
embedded image is probably near the limit of utility. For anything
else larger, data URLs are likely to be inappropriate.)

A data URL scheme's media type specification can include other
parameters; for example, one might specify a charset parameter.

   data:text/plain;charset=iso-8859-7;%be%fg%be

can be used for a short sequence of greek characters.

Some applications may use the "data" URL scheme in order to provide
setup parameters for other kinds of networking applications. For
example, one might create a media type
        application/vnd.xxx-query

whose content consists of a query string and a database identifier for
the "xxx" vendor's databases. A URL of the form:

data:application/vnd.xxx-query,select_vcount,fcol_from_fieldtable/local

could then be used in a local application to launch the "helper" for
application/vnd.xxx-query and give it the immediate data included.

History

This idea was originally proposed August 1995. Some versions of the
data URL scheme have been used in the definition of VRML, and a
version has appeared as part of a proposal for embedded data in HTML.
Various changes have been made, based on requests, to elide the media
type, pack the indication of the base64 encoding more tightly, and
eliminate "quoted printable" as an encoding since it would not easily
yield valid URLs without additional %xx encoding, which itself is
sufficient. The "data" URL scheme is in use in VRML, new applications of
HTML, and various commercial products.

Security

Interpretation of the data within a "data" URL has the same
security considerations as any implementation of the given media type.
An application should not interpret the contents of a data URL which
is marked with a media type that has been disallowed for processing
by the application's configuration.

Sites which use firewall proxies to disallow the retrieval of certain
media types (such as application script languages or types with known
security problems) will find it difficult to screen against the inclusion
of such types using the "data" URL scheme.  However, they should be
aware of the threat and take whatever precautions are considered
necessary within their domain.


References

[RFCSYNTAX]     RFC xxxx. Uniform Resource Locators (URL). R. Fielding,
                L. Masinter, T. Berners-Lee, December 1996.

[RFC1866]       RFC 1866: Hypertext Markup Language - 2.0.
                T. Berners-Lee & D. Connolly.  November 1995.

[RFC1522]       RFC 1522: MIME (Multipurpose Internet Mail Extensions)
                Part One: Mechanisms for Specifying and Describing
                the Format of Internet Message Bodies

Author contact information:

Larry Masinter
Xerox Palo Alto Research Center
3333 Coyote Hill Road
Palo Alto, CA 94304
masinter@parc.xerox.com


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