Network Working Group                                      L. Masinter

Request for Comments: 2397                           Xerox Corporation

Category: Standards Track                                  August 1998

                         The "data" URL scheme

Status of this Memo

   This document specifies an Internet standards track protocol for the

   Internet community, and requests discussion and suggestions for

   improvements.  Please refer to the current edition of the "Internet

   Official Protocol Standards" (STD 1) for the standardization state

   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

1. 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.

2. 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.) 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 only useful for short values. Note that

   some 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 LITLEN

   (1024) limits the number of characters which can appear in a single

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RFC 2397                 The "data" URL scheme               August 1998

   attribute value literal, the ATTSPLEN (2100) limits the sum of all

   lengths of all attribute value specifications which appear in a tag,

   and the TAGLEN (2100) limits the overall length of a tag.

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

3. Syntax

       dataurl    := "data:" [ mediatype ] [ ";base64" ] "," data

       mediatype  := [ type "/" subtype ] *( ";" parameter )

       data       := *urlchar

       parameter  := attribute "=" value

   where "urlchar" is imported from [RFC2396], and "type", "subtype",

   "attribute" and "value" are the corresponding tokens from [RFC2045],

   represented using URL escaped encoding of [RFC2396] as necessary.

   Attribute values in [RFC2045] 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.

4. 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">

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RFC 2397                 The "data" URL scheme               August 1998

   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.

5. 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. It is being used for object

   parameters in Java and ActiveX applications.

6. 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.

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RFC 2397                 The "data" URL scheme               August 1998

   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.

   The effect of using long "data" URLs in applications is currently

   unknown; some software packages may exhibit unreasonable behavior

   when confronted with data that exceeds its allocated buffer size.

7. References

   [RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter,

               "Uniform Resource Identifiers (URI): Generic Syntax", RFC

               2396, August 1998.

   [RFC1866]   Berners-Lee, T., and D. Connolly, "Hypertext Markup

               Language - 2.0.", RFC 1866, November 1995.

   [RFC2045]   Freed N., and N. Borenstein., "Multipurpose Internet Mail

               Extensions (MIME) Part One: Format of Internet Message

               Bodies", RFC 2045, November 1996.

Author contact information:

   Larry Masinter

   Xerox Palo Alto Research Center

   3333 Coyote Hill Road

   Palo Alto, CA 94304

   EMail: masinter@parc.xerox.com

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RFC 2397                 The "data" URL scheme               August 1998

Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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