284 lines
9.3 KiB
Plaintext
284 lines
9.3 KiB
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Network Working Group L. Masinter
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Request for Comments: 2397 Xerox Corporation
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Category: Standards Track August 1998
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The "data" URL scheme
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Status of this Memo
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice
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Copyright (C) The Internet Society (1998). All Rights Reserved.
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1. Abstract
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A new URL scheme, "data", is defined. It allows inclusion of small
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data items as "immediate" data, as if it had been included
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externally.
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2. Description
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Some applications that use URLs also have a need to embed (small)
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media type data directly inline. This document defines a new URL
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scheme that would work like 'immediate addressing'. The URLs are of
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the form:
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data:[<mediatype>][;base64],<data>
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The <mediatype> is an Internet media type specification (with
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optional parameters.) The appearance of ";base64" means that the data
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is encoded as base64. Without ";base64", the data (as a sequence of
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octets) is represented using ASCII encoding for octets inside the
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range of safe URL characters and using the standard %xx hex encoding
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of URLs for octets outside that range. If <mediatype> is omitted, it
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defaults to text/plain;charset=US-ASCII. As a shorthand,
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"text/plain" can be omitted but the charset parameter supplied.
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The "data:" URL scheme is only useful for short values. Note that
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some applications that use URLs may impose a length limit; for
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example, URLs embedded within <A> anchors in HTML have a length limit
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determined by the SGML declaration for HTML [RFC1866]. The LITLEN
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(1024) limits the number of characters which can appear in a single
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Masinter Standards Track [Page 1]
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RFC 2397 The "data" URL scheme August 1998
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attribute value literal, the ATTSPLEN (2100) limits the sum of all
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lengths of all attribute value specifications which appear in a tag,
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and the TAGLEN (2100) limits the overall length of a tag.
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The "data" URL scheme has no relative URL forms.
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3. Syntax
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dataurl := "data:" [ mediatype ] [ ";base64" ] "," data
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mediatype := [ type "/" subtype ] *( ";" parameter )
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data := *urlchar
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parameter := attribute "=" value
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where "urlchar" is imported from [RFC2396], and "type", "subtype",
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"attribute" and "value" are the corresponding tokens from [RFC2045],
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represented using URL escaped encoding of [RFC2396] as necessary.
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Attribute values in [RFC2045] are allowed to be either represented as
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tokens or as quoted strings. However, within a "data" URL, the
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"quoted-string" representation would be awkward, since the quote mark
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is itself not a valid urlchar. For this reason, parameter values
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should use the URL Escaped encoding instead of quoted string if the
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parameter values contain any "tspecial".
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The ";base64" extension is distinguishable from a content-type
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parameter by the fact that it doesn't have a following "=" sign.
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4. Examples
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A data URL might be used for arbitrary types of data. The URL
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data:,A%20brief%20note
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encodes the text/plain string "A brief note", which might be useful
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in a footnote link.
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The HTML fragment:
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<IMG
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SRC="data:image/gif;base64,R0lGODdhMAAwAPAAAAAAAP///ywAAAAAMAAw
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AAAC8IyPqcvt3wCcDkiLc7C0qwyGHhSWpjQu5yqmCYsapyuvUUlvONmOZtfzgFz
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ByTB10QgxOR0TqBQejhRNzOfkVJ+5YiUqrXF5Y5lKh/DeuNcP5yLWGsEbtLiOSp
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a/TPg7JpJHxyendzWTBfX0cxOnKPjgBzi4diinWGdkF8kjdfnycQZXZeYGejmJl
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ZeGl9i2icVqaNVailT6F5iJ90m6mvuTS4OK05M0vDk0Q4XUtwvKOzrcd3iq9uis
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F81M1OIcR7lEewwcLp7tuNNkM3uNna3F2JQFo97Vriy/Xl4/f1cf5VWzXyym7PH
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hhx4dbgYKAAA7"
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ALT="Larry">
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Masinter Standards Track [Page 2]
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RFC 2397 The "data" URL scheme August 1998
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could be used for a small inline image in a HTML document. (The
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embedded image is probably near the limit of utility. For anything
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else larger, data URLs are likely to be inappropriate.)
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A data URL scheme's media type specification can include other
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parameters; for example, one might specify a charset parameter.
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data:text/plain;charset=iso-8859-7,%be%fg%be
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can be used for a short sequence of greek characters.
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Some applications may use the "data" URL scheme in order to provide
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setup parameters for other kinds of networking applications. For
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example, one might create a media type
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application/vnd-xxx-query
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whose content consists of a query string and a database identifier
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for the "xxx" vendor's databases. A URL of the form:
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data:application/vnd-xxx-
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query,select_vcount,fcol_from_fieldtable/local
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could then be used in a local application to launch the "helper" for
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application/vnd-xxx-query and give it the immediate data included.
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5. History
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This idea was originally proposed August 1995. Some versions of the
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data URL scheme have been used in the definition of VRML, and a
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version has appeared as part of a proposal for embedded data in HTML.
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Various changes have been made, based on requests, to elide the media
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type, pack the indication of the base64 encoding more tightly, and
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eliminate "quoted printable" as an encoding since it would not easily
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yield valid URLs without additional %xx encoding, which itself is
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sufficient. The "data" URL scheme is in use in VRML, new applications
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of HTML, and various commercial products. It is being used for object
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parameters in Java and ActiveX applications.
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6. Security
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Interpretation of the data within a "data" URL has the same security
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considerations as any implementation of the given media type. An
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application should not interpret the contents of a data URL which is
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marked with a media type that has been disallowed for processing by
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the application's configuration.
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Masinter Standards Track [Page 3]
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RFC 2397 The "data" URL scheme August 1998
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Sites which use firewall proxies to disallow the retrieval of certain
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media types (such as application script languages or types with known
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security problems) will find it difficult to screen against the
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inclusion of such types using the "data" URL scheme. However, they
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should be aware of the threat and take whatever precautions are
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considered necessary within their domain.
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The effect of using long "data" URLs in applications is currently
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unknown; some software packages may exhibit unreasonable behavior
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when confronted with data that exceeds its allocated buffer size.
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7. References
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[RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter,
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"Uniform Resource Identifiers (URI): Generic Syntax", RFC
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2396, August 1998.
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[RFC1866] Berners-Lee, T., and D. Connolly, "Hypertext Markup
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Language - 2.0.", RFC 1866, November 1995.
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[RFC2045] Freed N., and N. Borenstein., "Multipurpose Internet Mail
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Extensions (MIME) Part One: Format of Internet Message
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Bodies", RFC 2045, November 1996.
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Author contact information:
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Larry Masinter
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Xerox Palo Alto Research Center
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3333 Coyote Hill Road
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Palo Alto, CA 94304
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EMail: masinter@parc.xerox.com
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Masinter Standards Track [Page 4]
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RFC 2397 The "data" URL scheme August 1998
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Full Copyright Statement
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Copyright (C) The Internet Society (1998). All Rights Reserved.
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This document and translations of it may be copied and furnished to
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others, and derivative works that comment on or otherwise explain it
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or assist in its implementation may be prepared, copied, published
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and distributed, in whole or in part, without restriction of any
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kind, provided that the above copyright notice and this paragraph are
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included on all such copies and derivative works. However, this
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document itself may not be modified in any way, such as by removing
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the copyright notice or references to the Internet Society or other
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Internet organizations, except as needed for the purpose of
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developing Internet standards in which case the procedures for
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copyrights defined in the Internet Standards process must be
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followed, or as required to translate it into languages other than
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English.
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The limited permissions granted above are perpetual and will not be
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revoked by the Internet Society or its successors or assigns.
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This document and the information contained herein is provided on an
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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Masinter Standards Track [Page 5]
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