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Network Working Group                                      L. MasinterRequest for Comments: 2397                           Xerox CorporationCategory: Standards Track                                  August 1998The "data" URL schemeStatus 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 singleMasinter                    Standards Track                     [Page 1]

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">Masinter                    Standards Track                     [Page 2]

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.Masinter                    Standards Track                     [Page 3]

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",RFC2396, 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.comMasinter                    Standards Track                     [Page 4]

RFC 2397                 The "data" URL scheme               August 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Masinter                    Standards Track                     [Page 5]