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     7 Network Working Group                                      L. Masinter

     8 Request for Comments: 2397                           Xerox Corporation

     9 Category: Standards Track                                  August 1998

    10 

    11 

    12                          The "data" URL scheme

    13 

    14 Status of this Memo

    15 

    16    This document specifies an Internet standards track protocol for the

    17    Internet community, and requests discussion and suggestions for

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

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

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

    21 

    22 Copyright Notice

    23 

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

    25 

    26 1. Abstract

    27 

    28    A new URL scheme, "data", is defined. It allows inclusion of small

    29    data items as "immediate" data, as if it had been included

    30    externally.

    31 

    32 2. Description

    33 

    34    Some applications that use URLs also have a need to embed (small)

    35    media type data directly inline. This document defines a new URL

    36    scheme that would work like 'immediate addressing'. The URLs are of

    37    the form:

    38 

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

    40 

    41    The <mediatype> is an Internet media type specification (with

    42    optional parameters.) The appearance of ";base64" means that the data

    43    is encoded as base64. Without ";base64", the data (as a sequence of

    44    octets) is represented using ASCII encoding for octets inside the

    45    range of safe URL characters and using the standard %xx hex encoding

    46    of URLs for octets outside that range.  If <mediatype> is omitted, it

    47    defaults to text/plain;charset=US-ASCII.  As a shorthand,

    48    "text/plain" can be omitted but the charset parameter supplied.

    49 

    50    The "data:" URL scheme is only useful for short values. Note that

    51    some applications that use URLs may impose a length limit; for

    52    example, URLs embedded within <A> anchors in HTML have a length limit

    53    determined by the SGML declaration for HTML [RFC1866]. The LITLEN

    54    (1024) limits the number of characters which can appear in a single

    55 

    56 

    57 

    58 Masinter                    Standards Track                     [Page 1]

    59 

    60 RFC 2397                 The "data" URL scheme               August 1998

    61 

    62 

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

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

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

    66 

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

    68 

    69 3. Syntax

    70 

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

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

    73        data       := *urlchar

    74        parameter  := attribute "=" value

    75 

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

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

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

    79 

    80    Attribute values in [RFC2045] are allowed to be either represented as

    81    tokens or as quoted strings. However, within a "data" URL, the

    82    "quoted-string" representation would be awkward, since the quote mark

    83    is itself not a valid urlchar. For this reason, parameter values

    84    should use the URL Escaped encoding instead of quoted string if the

    85    parameter values contain any "tspecial".

    86 

    87    The ";base64" extension is distinguishable from a content-type

    88    parameter by the fact that it doesn't have a following "=" sign.

    89 

    90 4. Examples

    91 

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

    93 

    94                           data:,A%20brief%20note

    95 

    96    encodes the text/plain string "A brief note", which might be useful

    97    in a footnote link.

    98 

    99    The HTML fragment:

   100 

   101    <IMG

   102    SRC="data:image/gif;base64,R0lGODdhMAAwAPAAAAAAAP///ywAAAAAMAAw

   103    AAAC8IyPqcvt3wCcDkiLc7C0qwyGHhSWpjQu5yqmCYsapyuvUUlvONmOZtfzgFz

   104    ByTB10QgxOR0TqBQejhRNzOfkVJ+5YiUqrXF5Y5lKh/DeuNcP5yLWGsEbtLiOSp

   105    a/TPg7JpJHxyendzWTBfX0cxOnKPjgBzi4diinWGdkF8kjdfnycQZXZeYGejmJl

   106    ZeGl9i2icVqaNVailT6F5iJ90m6mvuTS4OK05M0vDk0Q4XUtwvKOzrcd3iq9uis

   107    F81M1OIcR7lEewwcLp7tuNNkM3uNna3F2JQFo97Vriy/Xl4/f1cf5VWzXyym7PH

   108    hhx4dbgYKAAA7"

   109    ALT="Larry">

   110 

   111 

   112 

   113 

   114 Masinter                    Standards Track                     [Page 2]

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   116 RFC 2397                 The "data" URL scheme               August 1998

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   118 

   119    could be used for a small inline image in a HTML document.  (The

   120    embedded image is probably near the limit of utility. For anything

   121    else larger, data URLs are likely to be inappropriate.)

   122 

   123    A data URL scheme's media type specification can include other

   124    parameters; for example, one might specify a charset parameter.

   125 

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

   127 

   128    can be used for a short sequence of greek characters.

   129 

   130    Some applications may use the "data" URL scheme in order to provide

   131    setup parameters for other kinds of networking applications. For

   132    example, one might create a media type

   133            application/vnd-xxx-query

   134 

   135    whose content consists of a query string and a database identifier

   136    for the "xxx" vendor's databases. A URL of the form:

   137 

   138    data:application/vnd-xxx-

   139    query,select_vcount,fcol_from_fieldtable/local

   140 

   141    could then be used in a local application to launch the "helper" for

   142    application/vnd-xxx-query and give it the immediate data included.

   143 

   144 5. History

   145 

   146    This idea was originally proposed August 1995. Some versions of the

   147    data URL scheme have been used in the definition of VRML, and a

   148    version has appeared as part of a proposal for embedded data in HTML.

   149    Various changes have been made, based on requests, to elide the media

   150    type, pack the indication of the base64 encoding more tightly, and

   151    eliminate "quoted printable" as an encoding since it would not easily

   152    yield valid URLs without additional %xx encoding, which itself is

   153    sufficient. The "data" URL scheme is in use in VRML, new applications

   154    of HTML, and various commercial products. It is being used for object

   155    parameters in Java and ActiveX applications.

   156 

   157 6. Security

   158 

   159    Interpretation of the data within a "data" URL has the same security

   160    considerations as any implementation of the given media type.  An

   161    application should not interpret the contents of a data URL which is

   162    marked with a media type that has been disallowed for processing by

   163    the application's configuration.

   164 

   165 

   166 

   167 

   168 

   169 

   170 Masinter                    Standards Track                     [Page 3]

   171 

   172 RFC 2397                 The "data" URL scheme               August 1998

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   174 

   175    Sites which use firewall proxies to disallow the retrieval of certain

   176    media types (such as application script languages or types with known

   177    security problems) will find it difficult to screen against the

   178    inclusion of such types using the "data" URL scheme.  However, they

   179    should be aware of the threat and take whatever precautions are

   180    considered necessary within their domain.

   181 

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

   183    unknown; some software packages may exhibit unreasonable behavior

   184    when confronted with data that exceeds its allocated buffer size.

   185 

   186 7. References

   187 

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

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

   190                2396, August 1998.

   191 

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

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

   194 

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

   196                Extensions (MIME) Part One: Format of Internet Message

   197                Bodies", RFC 2045, November 1996.

   198 

   199 Author contact information:

   200 

   201    Larry Masinter

   202    Xerox Palo Alto Research Center

   203    3333 Coyote Hill Road

   204    Palo Alto, CA 94304

   205 

   206    EMail: masinter@parc.xerox.com

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   226 Masinter                    Standards Track                     [Page 4]

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   228 RFC 2397                 The "data" URL scheme               August 1998

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   231 Full Copyright Statement

   232 

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

   234 

   235    This document and translations of it may be copied and furnished to

   236    others, and derivative works that comment on or otherwise explain it

   237    or assist in its implementation may be prepared, copied, published

   238    and distributed, in whole or in part, without restriction of any

   239    kind, provided that the above copyright notice and this paragraph are

   240    included on all such copies and derivative works.  However, this

   241    document itself may not be modified in any way, such as by removing

   242    the copyright notice or references to the Internet Society or other

   243    Internet organizations, except as needed for the purpose of

   244    developing Internet standards in which case the procedures for

   245    copyrights defined in the Internet Standards process must be

   246    followed, or as required to translate it into languages other than

   247    English.

   248 

   249    The limited permissions granted above are perpetual and will not be

   250    revoked by the Internet Society or its successors or assigns.

   251 

   252    This document and the information contained herein is provided on an

   253    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

   254    TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

   255    BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

   256    HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

   257    MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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