Scripting Media Types
draft-hoehrmann-script-types-03

Network Working Group                                       B. Hoehrmann
Internet-Draft                                              June 6, 2005
Expires: December 8, 2005

                         Scripting Media Types
                    draft-hoehrmann-script-types-03

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes the registration of media types for the
   ECMAScript and JavaScript programming languages and conformance
   requirements for implementations of these types.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conformance and Document Conventions . . . . . . . . . . . . .  3
   3.  Deployed Scripting Media Types and Compatibility . . . . . . .  3
   4.  Character Encoding Scheme Handling . . . . . . . . . . . . . .  5
       4.1   Charset Parameter  . . . . . . . . . . . . . . . . . . .  5
       4.2   Character Encoding Scheme Detection  . . . . . . . . . .  5
       4.3   Character Encoding Scheme Error Handling . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  JavaScript Media Types . . . . . . . . . . . . . . . . . . . . 10
       7.1   text/javascript (obsolete) . . . . . . . . . . . . . . . 10
       7.2   application/javascript . . . . . . . . . . . . . . . . . 11
   8.  ECMAScript Media Types . . . . . . . . . . . . . . . . . . . . 12
       8.1   text/ecmascript (obsolete) . . . . . . . . . . . . . . . 12
       8.2   application/ecmascript . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       9.1   Normative References . . . . . . . . . . . . . . . . . . 14
       9.2   Informative References . . . . . . . . . . . . . . . . . 14
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
       Intellectual Property and Copyright Statements . . . . . . . . 16

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1.  Introduction

   This memo describes media types for the JavaScript and ECMAScript
   programming languages.  Refer to "Brief History" and "Overview" in
   [ECMA] for background information on these languages.

   Programs written in these programming languages have historically
   been interchanged using inapplicable, experimental, and unregistered
   media types.  This document defines four of the most commonly used
   media types for such programs to reflect this usage in the IANA media
   type registry, foster interoperability by defining underspecified
   aspects, and to provide general security considerations.

2.  Conformance and Document Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119] and
   indicate requirement levels for compliant implementations.
   Requirements apply to all implementations unless otherwise stated.

   An implementation is a software module that supports one of the media
   types defined in this document.  Software modules may support
   multiple media types but conformance is considered individually for
   each type.

   Implementations that fail to satisfy one or more "MUST" requirements
   are considered non-compliant.  Implementations that satisfy all
   "MUST" requirements but fail to satisfy one or more "SHOULD"
   requirements are said to be "conditionally compliant".  All other
   implementations are "unconditionally compliant".

3.  Deployed Scripting Media Types and Compatibility

   Various unregistered media type have been used in an ad-hoc fashion
   to label and exchange programs written in ECMAScript and JavaScript.
   These include:

      +-----------------------------------------------------+
      | text/javascript          | text/ecmascript          |
      | text/javascript1.0       | text/javascript1.1       |
      | text/javascript1.2       | text/javascript1.3       |
      | text/javascript1.4       | text/javascript1.5       |
      | text/jscript             | text/livescript          |
      | text/x-javascript        | text/x-ecmascript        |
      | application/x-javascript | application/x-ecmascript |
      | application/javascript   | application/ecmascript   |
      +-----------------------------------------------------+

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   Use of the "text" top-level type for this kind of content is known to
   be problematic.  This document thus defines text/javascript and text/
   ecmascript but marks them as "obsolete".  Use of these types should
   be limited to legacy systems.  The media types

      * application/javascript
      * application/ecmascript

   which are also defined in this document are intended for common use
   and should be used instead.  Use of the experimental counterpart of
   any of the types is discouraged per [RFC2048], section 2.1.4.  Use of
   other types when one of the types defined in this document could be
   used as well is discouraged.

   This document defines equivalent processing requirements for the
   types text/javascript, text/ecmascript and application/javascript.
   Use of and support for the media type application/ecmascript is
   considerably less widespread than for other media types defined in
   this document.  Taking that to its advantage, this document defines
   stricter processing rules for this type to foster more interoperable
   processing.

   The types defined in this document are applicable to scripts written
   in [JS15] and [ECMA] respectively as well as to scripts written in a
   compatible language and corresponding profiles such as [EcmaCompact].

   This document does not address scripts written in other languages, in
   particular, future versions of JavaScript, future editions of [ECMA],
   and extensions to [ECMA] such as [E4X] are not directly addressed.
   This document may be updated take other content into account.

   Updates of this document may introduce new optional parameters,
   implementations MUST consider the impact of such an update.  For the
   application/ecmascript media type, implementations MUST NOT process
   content labeled with a "version" parameter as if no such parameter
   had been specified; this is typically achieved by considering the
   content unsupported.  This error handling behavior allows extending
   the definition of the media type for content that cannot be processed
   by implementations of [ECMA].

   The programming languages defined in [JS15] and [ECMA] share a common
   subset.  Choice of a type for scripts compatible to both languages is
   out of scope of this document.

   This document does not define how fragment identifiers in resource
   identifiers ([RFC3986], [RFC3987]) for documents labeled with one of
   the media types defined in this document are resolved.  An update of
   this document may define processing of fragment identifiers.

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4.  Character Encoding Scheme Handling

   Refer to [RFC3536] for a discussion of terminology in this section.
   Source text (as defined in [ECMA], section 6) can be binary source
   text.  Binary source text is a textual data object that represents
   source text encoded using a character encoding scheme.  A textual
   data object is a whole text protocol message or a whole text
   document, or a part of it, that is treated separately for purposes of
   external storage and retrieval.  An implementation's internal
   representation of source text and source text are not considered
   binary source text.

   Implementations need to determine a character encoding scheme in
   order to decode binary source text to source text.  The media types
   defined in this document allow an optional charset parameter to
   explicitly specify the character encoding scheme used to encode the
   source text.

   How implementations determine the character encoding scheme can be
   subject to processing rules out of scope of this document, transport
   protocols for example can require that a specific character encoding
   scheme is to be assumed if the optional charset parameter is not
   specified, or they can require that the charset parameter is used in
   certain cases.  Such requirements are not considered part of this
   document.

   Implementations that support binary source text MUST support binary
   source text encoded using the UTF-8 [RFC3629] character encoding
   scheme.  Other character encoding schemes MAY be supported.  Use of
   UTF-8 to encode binary source text is encouraged but not required.

4.1  Charset Parameter

   The charset parameter provides a means to specify the character
   encoding scheme of binary source text.  Its value MUST match the
   mime-charset production defined in [RFC2978], section 2.3 and SHOULD
   be a registered charset [CHARSETS].  An illegal value is a value that
   does not match that production.

4.2  Character Encoding Scheme Detection

   It is possible that implementations cannot interoperably determine a
   single character encoding scheme simply by complying with all
   requirements of the applicable specifications.  To foster
   interoperability in such cases, the following algorithm is defined.
   Implementations apply this algorithm until a single character
   encoding scheme is determined.

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   1.  If a charset parameter with a legal value is specified, the value
       determines the character encoding scheme.

   2.  If the binary source text starts with a Unicode encoding form
       signature, the signature determines the encoding.  The following
       octet sequences at the very beginning of the binary source text
       are considered with their corresponding character encoding
       schemes:

          +------------------+----------+
          | Leading sequence | Encoding |
          +------------------+----------+
          | 00 00 FF FE      | UTF-32LE |
          | FE FF 00 00      | UTF-32BE |
          | FF FE            | UTF-16LE |
          | FE FF            | UTF-16BE |
          | EF BB BF         | UTF-8    |
          +------------------+----------+

       The longest matching octet sequence determines the encoding.
       Implementations of this step MUST use these octet sequences to
       determine the character encoding scheme even if the determined
       scheme is not supported.  If this step determines the character
       encoding scheme, the octet sequence representing the Unicode
       encoding form signature MUST be ignored when decoding the binary
       source text to source text.

   3.  The character encoding scheme is determined as UTF-8.

   If the character encoding scheme is determined as UTF-8 through any
   means but step 2 as defined above and the binary source text starts
   with the octet sequence EF BB BF, the octet sequence is ignored when
   decoding the binary source text to source text.  (The sequence will
   also be ignored if step 2 determines the character encoding scheme
   per the requirements in step 2).

   Implementations of the types text/javascript, text/ecmascript, and
   application/javascript SHOULD and implementations of the type
   application/ecmascript MUST implement the requirements defined in
   this section in the cited case.

4.3  Character Encoding Scheme Error Handling

   The following error processing behavior is RECOMMENDED for the media
   types text/javascript, text/ecmascript, and application/javascript,
   and REQUIRED for the media type application/ecmascript.

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   o  If the value of a charset parameter is illegal, implementations
      MAY recover from the error by ignoring the parameter or MAY
      consider the character encoding scheme unsupported.

   o  If binary source text is determined to be encoded using a certain
      character encoding scheme the implementation is unable to process,
      implementations MUST consider the resource unsupported (i.e., they
      MUST NOT decode the binary source text using a different character
      encoding scheme.)

   o  Binary source text can be determined to be encoded using a certain
      character encoding scheme but contain octet sequences that are not
      legal according to that scheme.  This is typically caused by lack
      of proper character encoding scheme information; such errors can
      pose a security risk as discussed in section 5.

      Implementations SHOULD detect such errors as early as possible, in
      particular, they SHOULD detect them before interpreting any of the
      source text; implementations MUST detect such errors and MUST NOT
      interpret any source text after detecting such an error.  Such
      errors MAY be reported, e.g., as syntax errors as defined in
      [ECMA], section 16.

   This document does not define facilities that allow to specify the
   character encoding scheme used to encode binary source text in a
   conflicting manner; there are only two sources for character encoding
   scheme information, the charset parameter and the Unicode encoding
   form signature; if a charset parameter is specified, binary source
   text is processed as defined for that character encoding scheme.

5.  Security Considerations

   Refer to [RFC3552] for a discussion of terminology in this section.
   Examples in this section, discussions of interactions of host
   environments with scripts and extensions to [ECMA] are to be
   understood as non-exhaustive and of purely illustrative nature.

   The programming language defined in [ECMA] is not intended to be
   computationally self-sufficient, it is rather expected that the
   computational environment provides facilities to programs to enable
   specific functionality.  Such facilities constitute unknown factors
   and are thus considered out of scope of this document.

   Derived programming languages are permitted to include additional
   functionality that is not described in [ECMA], such functionality
   constitutes an unknown factor and is thus considered out of scope of
   this document.  In particular, extensions to [ECMA] defined for the
   JavaScript programming language are not discussed in this document.

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   Uncontrolled execution of scripts can be exceedingly dangerous.
   Implementations that execute scripts MUST give consideration to their
   application's threat models and those of the individual features they
   implement; in particular, they MUST ensure that untrusted content is
   not executed in an unprotected environment.

   Specifications for host environment facilities and for derived
   programming languages should include security considerations.  If an
   implementation supports such facilities, the respective security
   considerations apply.  In particular, if scripts can be referenced
   from or included in specific document formats, the considerations for
   the embedding or referencing document format apply.

   Scripts embedded in application/xhtml+xml [RFC3236] documents for
   example could be enabled through the host environment to manipulate
   the document instance, which could cause the retrieval of remote
   resources; security considerations regarding retrieval of remote
   resources of the embedding document would apply in this case.

   This circumstance can further be used to make information, that is
   normally only available to the script, available to a web server by
   encoding the information in the resource identifier of the resource,
   which can further enable eavesdropping attacks.  Implementation of
   such facilities is subject to the security considerations of the host
   environment as discussed above.

   The facilities defined in [ECMA] do not include provisions for input
   of external data, output of computed results, or modification of
   aspects of the host environment.  An implementation of only the
   facilities defined in [ECMA] is not considered to support dangerous
   operations.

   The programming language defined in [ECMA] does include facilities to
   loop, cause computationally complex operations, or consume large
   amounts of memory; this includes, but is not limited to, facilities
   that allow dynamically generated source text to be executed (e.g.,
   the eval() function); uncontrolled execution of such features can
   cause denial of service which implementations MUST protect against.

   A host environment can provide facilities to access external input,
   scripts that pass such input to the eval() function can be vulnerable
   to code injection attacks; scripts must protect against such attacks.

   A host environment can provide facilities to output computed results
   in a user-visible manner, for example, host environments supporting a
   graphical user interface can provide facilities that enable scripts
   to present certain messages to the user; implementations MUST take
   steps to avoid confusion of the origin of such messages; in general,

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   the security considerations for the host environment apply in such a
   case as discussed above.

   Implementations are required to support the UTF-8 character encoding
   scheme; the security considerations of [RFC3629] apply.  Additional
   character encoding schemes may be supported; support for such schemes
   is subject to the security considerations of those schemes.

   Source text is expected to be in Unicode Normalization Form C;
   scripts and implementations MUST consider security implications of
   unnormalized source text and data; for a detailed discussion of such
   implications refer to the security considerations in [RFC3629].

   Scripts can be executed in an environment that is vulnerable to code
   injection attacks; for example, a CGI script [RFC3875] echoing user
   input could allow the inclusion of untrusted scripts which could be
   executed in an otherwise trusted environment; this threat scenario is
   subject to security considerations out of scope of this document.

   The "data" resource identifier scheme [RFC2397] in combination with
   the types defined in this document could be used to cause execution
   of untrusted scripts through the inclusion of untrusted resource
   identifiers in otherwise trusted content.  Security considerations of
   [RFC2397] apply.

   Implementations can fail to implement a specific security model or
   other means to prevent possibly dangerous operations; such failure
   could possibly be exploited to gain unauthorized access to a system
   or sensitive information; such failure constitutes an unknown factor
   and is thus considered out of scope of this document.

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6.  IANA Considerations

   This document registers four new media types as defined in the
   following sections.

7.  JavaScript Media Types

7.1  text/javascript (obsolete)

   Type name:               text
   Subtype name:            javascript
   Required parameters:     none
   Optional parameters:     charset, see section 4.1.
   Encoding considerations:
      The same as the considerations in section 3.1 of [RFC3023].

   Security considerations: See section 5.
   Interoperability considerations:
      None except as noted in other sections of this document.

   Published specification: [JS15]
   Applications which use this media type:
      Script interpreters as discussed in this document.

   Additional information:

      Magic number(s):             n/a
      File extension(s):           .js
      Macintosh File Type Code(s): TEXT

   Person & email address to contact for further information:
      See Author's Address section.

   Intended usage:          OBSOLETE
   Restrictions on usage:   n/a
   Author:                  See Author's Address section.
   Change controller:       The IESG.

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7.2  application/javascript

   Type name:               application
   Subtype name:            javascript
   Required parameters:     none
   Optional parameters:     charset, see section 4.1.
   Encoding considerations:
      The same as the considerations in section 3.2 of [RFC3023].

   Security considerations: See section 5.
   Interoperability considerations:
      None except as noted in other sections of this document.

   Published specification: [JS15]
   Applications which use this media type:
      Script interpreters as discussed in this document.

   Additional information:

      Magic number(s):             n/a
      File extension(s):           .js
      Macintosh File Type Code(s): TEXT

   Person & email address to contact for further information:
      See Author's Address section.

   Intended usage:          COMMON
   Restrictions on usage:   n/a
   Author:                  See Author's Address section.
   Change controller:       The IESG.

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8.  ECMAScript Media Types

8.1  text/ecmascript (obsolete)

   Type name:               text
   Subtype name:            ecmascript
   Required parameters:     none
   Optional parameters:     charset, see section 4.1.
   Encoding considerations:
      The same as the considerations in section 3.1 of [RFC3023].

   Security considerations: See section 5.
   Interoperability considerations:
      None except as noted in other sections of this document.

   Published specification: [ECMA]
   Applications which use this media type:
      Script interpreters as discussed in this document.

   Additional information:

      Magic number(s):             n/a
      File extension(s):           .es
      Macintosh File Type Code(s): TEXT

   Person & email address to contact for further information:
      See Author's Address section.

   Intended usage:          OBSOLETE
   Restrictions on usage:   n/a
   Author:                  See Author's Address section.
   Change controller:       The IESG.

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8.2  application/ecmascript

   Type name:               application
   Subtype name:            ecmascript
   Required parameters:     none
   Optional parameters:     charset, see section 4.1.

      Note: Section 3 defines error handling behavior for content
      labeled with a "version" parameter.

   Encoding considerations:
      The same as the considerations in section 3.2 of [RFC3023].

   Security considerations: See section 5.
   Interoperability considerations:
      None except as noted in other sections of this document.

   Published specification: [ECMA]
   Applications which use this media type:
      Script interpreters as discussed in this document.

   Additional information:

      Magic number(s):             n/a
      File extension(s):           .es
      Macintosh File Type Code(s): TEXT

   Person & email address to contact for further information:
      See Author's Address section.

   Intended usage:          COMMON
   Restrictions on usage:   n/a
   Author:                  See Author's Address section.
   Change controller:       The IESG.

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9.  References

9.1  Normative References

   [CHARSETS]
              IANA, "Assigned character sets", <http://www.iana.org/
              assignments/character-sets>.

   [ECMA]     European Computer Manufacturers Association, "ECMAScript
              Language Specification 3rd Edition", December 1999,
              <http://www.ecma.ch/ecma1/stand/ecma-262.htm>.

   [RFC2048]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, October 2000.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3536]  Hoffman, P., "Terminology Used in Internationalization in
              the IETF", RFC 3536, May 2003.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

9.2  Informative References

   [E4X]      European Computer Manufacturers Association, "ECMAScript
              for XML (E4X)", June 2004, <http://www.ecma.ch/ecma1/
              stand/ecma-357.htm>.

   [EcmaCompact]
              European Computer Manufacturers Association, "ECMAScript
              3rd Edition Compact Profile", June 2001,
              <http://www.ecma.ch/ecma1/stand/ecma-327.htm>.

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   [JS15]     Netscape Communications Corp., "Core JavaScript Reference
              1.5", September 2000, <http://web.archive.org/*/http://
              devedge.netscape.com/library/manuals/2000/javascript/1.5/
              reference/>.

   [RFC2397]  Masinter, L., "The "data" URL scheme", RFC 2397,
              August 1998.

   [RFC3236]  Baker, M. and P. Stark, "The 'application/xhtml+xml' Media
              Type", RFC 3236, January 2002.

   [RFC3875]  Robinson, D. and K. Coar, "The Common Gateway Interface
              (CGI) Version 1.1", RFC 3875, October 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

Author's Address

   Bjoern Hoehrmann
   Weinheimer Strasse 22
   Mannheim  D-68309
   Germany

   Email: mailto:bjoern@hoehrmann.de
   URI:   http://bjoern.hoehrmann.de

   Note: Please write "Bjoern Hoehrmann" with o-umlaut (U+00F6) wherever
   possible, e.g. as "Bj&#246;rn H&#246;hrmann" in HTML and XML.

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