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Versions: (draft-masinter-multipart-form-data) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7578

APPSAWG                                                      L. Masinter
Internet-Draft                                                     Adobe
Obsoletes: 2388 (if approved)                             April 10, 2015
Intended status: Standards Track
Expires: October 12, 2015


            Returning Values from Forms: multipart/form-data
               draft-ietf-appsawg-multipart-form-data-11

Abstract

   This specification defines the multipart/form-data Internet Media
   Type, which can be used by a wide variety of applications and
   transported by a wide variety of protocols as a way of returning a
   set of values as the result of a user filling out a form.  It
   obsoletes RFC 2388.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 12, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  percent-encoding option . . . . . . . . . . . . . . . . . . .   3
   3.  Advice for Forms and Form Processing  . . . . . . . . . . . .   3
   4.  Definition of multipart/form-data . . . . . . . . . . . . . .   4
     4.1.  Boundary parameter of multipart/form-data . . . . . . . .   4
     4.2.  Content-Disposition header for each part  . . . . . . . .   4
     4.3.  filename attribute of content-distribution part header  .   4
     4.4.  Multiple files for one form field . . . . . . . . . . . .   5
     4.5.  Content-Type header for each part . . . . . . . . . . . .   5
     4.6.  The charset parameter for text/plain form data  . . . . .   5
     4.7.  The _charset_ field for default charset . . . . . . . . .   6
     4.8.  Content-Transfer-Encoding deprecated  . . . . . . . . . .   6
     4.9.  Other Content- headers  . . . . . . . . . . . . . . . . .   7
   5.  Operability considerations  . . . . . . . . . . . . . . . . .   7
     5.1.  Non-ASCII field names and values  . . . . . . . . . . . .   7
       5.1.1.  Avoid non-ASCII field names . . . . . . . . . . . . .   7
       5.1.2.  Interpreting forms and creating form-data . . . . . .   7
       5.1.3.  Parsing and interpreting form data  . . . . . . . . .   8
     5.2.  Ordered fields and duplicated field names . . . . . . . .   8
     5.3.  Interoperability with web applications  . . . . . . . . .   8
     5.4.  Correlating form data with the original form  . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Media type registration for multipart/form-data . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Changes from RFC 2388  . . . . . . . . . . . . . . .  12
   Appendix B.  Alternatives . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   In many applications, it is possible for a user to be presented with
   a form.  The user will fill out the form, including information that
   is typed, generated by user input, or included from files that the
   user has selected.  When the form is filled out, the data from the
   form is sent from the user to the receiving application.

   The definition of "multipart/form-data" is derived from one of those
   applications, originally set out in [RFC1867] and subsequently
   incorporated into HTML 3.2 [W3C.REC-html32-19970114], where forms are
   expressed in HTML, and in which the form data is sent via HTTP or



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   electronic mail.  This representation is widely implemented in
   numerous web browsers and web servers.

   However, "multipart/form-data" is also used for forms that are
   presented using representations other than HTML (spreadsheets, PDF,
   etc.), and for transport using means other than electronic mail or
   HTTP; it is used in distributed applications which do not involve
   forms at all, or do not have users filling out the form.  For this
   reason, this document defines a general syntax and semantics
   independent of the application for which it is used, with specific
   rules for web applications noted in context.

   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, RFC 2119
   [RFC2119].

2.  percent-encoding option

   Within this specification, "percent-encoding" (as defined in
   [RFC3986]) is offered as a possible way of encoding characters in
   file names that are otherwise disallowed, including non-ASCII
   characters, spaces, control characters and so forth.  The encoding is
   created replacing each non-ASCII or disallowed character with a
   sequence, where each byte of the UTF-8 encoding of the character is
   represented by a percent-sign (%) followed by the (case-insensitive)
   hexadecimal of that byte.

3.  Advice for Forms and Form Processing

   The representation and interpretation of forms and the nature of form
   processing is not specified by this document.  However, for forms and
   form-processing that result in generation of multipart/form-data,
   some suggestions are included.

   In a form, there is generally a sequence of fields, where each field
   is expected to be supplied with a value, e.g. by a user who fills out
   the form.  Each field has a name.  After a form has been filled out,
   and the form's data is "submitted": the form processing results in a
   set of values for each field-- the "form data".

   In forms that work with multipart/form-data, field names could be
   arbitrary Unicode strings; however, restricting field names to ASCII
   will help avoid some interoperability issues (see Section 5.1).

   Within a given form, ensuring field names are unique is also helpful.
   Some fields may have default values or presupplied values in the form
   itself.  Fields with presupplied values might be hidden or invisible;



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   this allows using generic processing for form data from a variety of
   actual forms.

4.  Definition of multipart/form-data

   The media-type "multipart/form-data" follows the model of multipart
   MIME data streams as specified in [RFC2046] Section 5.1; changes are
   noted in this document.

   A "multipart/form-data" body contains a series of parts, separated by
   a boundary.

4.1.  Boundary parameter of multipart/form-data

   As with other multipart types, the parts are delimited with a
   boundary delimiter, constructed using CRLF, "--", the value of the
   boundary parameter.  The boundary is supplied as a "boundary"
   parameter to the "multipart/form-data" type.  As noted in [RFC2046]
   Section 5.1, the boundary delimiter MUST NOT appear inside any of the
   encapsulated parts, and it is often necessary to enclose the boundary
   parameter values in quotes on the Content-type line.

4.2.  Content-Disposition header for each part

   Each part MUST contain a "content-disposition" header [RFC2183] and
   where the disposition type is "form-data".  The "content-disposition"
   header MUST also contain an additional parameter of "name"; the value
   of the "name" parameter is the original field name from the form
   (possibly encoded; see Section 5.1).  For example, a part might
   contain a header:

           Content-Disposition: form-data; name="user"

   with the body of the part containing the form data of the "user"
   field.

4.3.  filename attribute of content-distribution part header

   For form data that represents the content of a file, a name for the
   file SHOULD be supplied as well, by using a "filename" parameter of
   the "content-disposition" header.  The file name isn't mandatory for
   cases where the file name isn't available or is meaningless or
   private; this might result, for example, from selection or drag-and-
   drop or where the form data content is streamed directly from a
   device.

   If a filename parameter is supplied, the requirements of [RFC2183]
   Section 2.3 for "receiving MUA" apply to recievers of "multipart/



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   form-data" as well: Do not use the file name blindly, check and
   possibly change to match local filesystem conventions if applicable,
   do not use directory path information that may be present.

   In most multipart types, the MIME headers in each part are restricted
   to US-ASCII; for compatibility with those systems, file names
   normally visible to users MAY be encoded using the percent-encoding
   method in Section 2, following how a "file:" URI
   [I-D.ietf-appsawg-file-scheme] might be encoded.

   NOTE: The encoding method described in [RFC5987], which would add a
   "filename*" paramter to the "Content-Disposition" header, MUST NOT be
   used.

   Some commonly deployed systems use multipart/form-data with file
   names directly encoded including octets outside the US-ASCII range.
   The encoding used for the file names is typically UTF-8, although
   HTML forms will use the charset associated with the form.

4.4.  Multiple files for one form field

   The form data for a form field might include multiple files.

   [RFC2388] suggested that multiple files for a single form field be
   transmitted using a nested multipart/mixed part.  This usage is
   deprecated.

   To match widely deployed implementations, multiple files MUST be sent
   by supplying each file in a separate part, but all with the same
   "name" parameter.

   Receiving applications intended for wide applicability (e.g.
   multipart/form-data parsing libraries) SHOULD also support the older
   method of supplying multiple files.

4.5.  Content-Type header for each part

   Each part MAY have an (optional) "content-type", which defaults to
   "text/plain".  If the contents of a file are to be sent, the file
   data SHOULD be labeled with an appropriate media type, if known, or
   "application/octet-stream".

4.6.  The charset parameter for text/plain form data

   In the case where the form data is text, the charset parameter for
   the "text/plain" Content-Type MAY be used to indicate the character
   encoding used in that part.  For example, a form with a text field in




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   which a user typed "Joe owes <eu>100" where <eu> is the Euro symbol
   might have form data returned as:

       --AaB03x
       content-disposition: form-data; name="field1"
       content-type: text/plain;charset=UTF-8
       content-transfer-encoding: quoted-printable

       Joe owes =E2=82=AC100.
       --AaB03x

   In practice, many widely deployed implementations do not supply a
   charset parameter in each part, but, rather, they rely on the notion
   of a "default charset" for a multipart/form-data instance.
   Subsequent sections will explain how the default charset is
   established.

4.7.  The _charset_ field for default charset

   Some form processing applications (including HTML) have the
   convention that the value of a form entry with entry name "_charset_"
   and type "hidden" is automatically set when the form is opened; the
   value is used as the default charset of text field values (see form-
   charset in Section 5.1.2).  In such cases, the value of the default
   charset for each text/plain part without a charset parameter is the
   supplied value.  For example:

       --AaB03x
       content-disposition: form-data; name="_charset_"

       iso-8859-1
       --AaB03x--
       content-disposition: form-data; name="field1"

       ...text encoded in iso-8859-1 ...
       AaB03x--

4.8.  Content-Transfer-Encoding deprecated

   Previously, it was recommended that senders use a "Content-Transfer-
   Encoding" encoding (such as "quoted-printable") for each non-ASCII
   part of a multipart/form-data body, because that would allow use in
   transports that only support a "7BIT" encoding.  This use is
   deprecated for use in contexts that support binary data such as HTTP.
   Senders SHOULD NOT generate any parts with a "Content-Transfer-
   Encoding" header.





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   Currently, no deployed implementations that send such bodies have
   been discovered.

4.9.  Other Content- headers

   The "multipart/form-data" media type does not support any MIME
   headers in the parts other than Content-Type, Content-Disposition,
   and (in limited circumstances) Content-Transfer-Encoding.  Other
   headers MUST NOT be included and MUST be ignored.

5.  Operability considerations

5.1.  Non-ASCII field names and values

   Normally, MIME headers in multipart bodies are required to consist
   only of 7-bit data in the US-ASCII character set.  While [RFC2388]
   suggested that non-ASCII field names be encoded according to the
   method in [RFC2047], this practice doesn't seem to have been followed
   widely.

   This specification makes three sets of recommendations for three
   different states of workflow.

5.1.1.  Avoid non-ASCII field names

   For broadest interoperability with existing deployed software, those
   creating forms SHOULD avoid non-ASCII field names.  This should not
   be a burden, because in general the field names are not visible to
   users.  The field names in the underlying need not match what the
   user sees on the screen.

   If non-ASCII field names are unavoidable, form or application
   creators SHOULD use UTF-8 uniformly.  This will minimize
   interoperability problems.

5.1.2.  Interpreting forms and creating form-data

   Some applications of this specification will supply a character
   encoding to be used for interpretation of the multipart/form-data
   body.  In particular, HTML 5 [W3C.REC-html5-20141028] uses:

   o  The content of a '_charset_' field, if there is one.

   o  the value of an accept-charset attribute of the <form> element, if
      there is one,

   o  the character encoding of the document containing the form, if it
      is US-ASCII compatible,



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   o  otherwise UTF-8.

   Call this value the form-charset.  Any text, whether field name,
   field value, or (text/plain) form data which is uses characters
   outside the ASCII range MAY be represented directly encoded in the
   form-charset.

5.1.3.  Parsing and interpreting form data

   While this specification provides guidance for creation of multipart/
   form-data, parsers and interpreters should be aware of the variety of
   implementations.  File systems differ as to whether and how they
   normalize Unicode names, for example.  The matching of form elements
   to form-data parts may rely on a fuzzier match.  In particular, some
   multipart/form-data generators might have followed the previous
   advice of [RFC2388] and used the [RFC2047] "encoded-word" method of
   encoding non-ASCII values:

    encoded-word = "=?" charset "?" encoding "?" encoded-text "?="

   Others have been known to follow [RFC2231], to send unencoded UTF-8,
   or even strings encoded in the form-charset.

   For this reason, interpreting "multipart/form-data" (even from
   conforming generators) may require knowing the charset used in form
   encoding, in cases where the _charset_ field value or a charset
   parameter of a text/plain Content-Type header is not supplied.

5.2.  Ordered fields and duplicated field names

   Form processors given forms with a well-defined ordering SHOULD send
   back results in order (note that there are some forms which do not
   define a natural order.)  Intermediaries MUST NOT reorder the
   results.  Form parts with identical field names MUST NOT be
   coalesced.

5.3.  Interoperability with web applications

   Many web applications use the "application/x-url-encoded" method for
   returning data from forms.  This format is quite compact, e.g.:

      name=Xavier+Xantico&verdict=Yes&colour=Blue&happy=sad&Utf%F6r=Send

   However, there is no opportunity to label the enclosed data with
   content type, apply a charset, or use other encoding mechanisms.

   Many form-interpreting programs (primarily web browsers) now
   implement and generate multipart/form-data, but an existing



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   application might need to optionally support both the application/x-
   url-encoded format as well.

5.4.  Correlating form data with the original form

   This specification provides no specific mechanism by which multipart/
   form-data can be associated with the form that caused it to be
   transmitted.  This separation is intentional; many different forms
   might be used for transmitting the same data.  In practice,
   applications may supply a specific form processing resource (in HTML,
   the ACTION attribute in a FORM tag) for each different form.
   Alternatively, data about the form might be encoded in a "hidden
   field" (a field which is part of the form but which has a fixed value
   to be transmitted back to the form-data processor.)

6.  IANA Considerations

   Please update the Internet Media Type registration of multipart/form-
   data to point to this document, using the template in Section 8.  In
   addition, please update the registrations of the "name" parameter and
   the "form-data" value in the "Content Disposition Values and
   Parameters" registry to both point to this document.

7.  Security Considerations

   All form processing software should treat user supplied form-data
   with sensitivity, as it often contains confidential or personally
   identifying information.  There is widespread use of form "auto-fill"
   features in web browsers; these might be used to trick users to
   unknowingly send confidential information when completing otherwise
   innoccuous tasks.  Multipart/form-data does not supply any features
   for checking integrity, ensuring confidentiality, avoiding user
   confusion, or other security features; those concerns must be
   addressed by the form-filling and form-data-interpreting
   applications.

   Applications which receive forms and process them must be careful not
   to supply data back to the requesting form processing site that was
   not intended to be sent.

   It is important when interpreting the filename of the Content-
   Disposition header to not overwrite files in the recipient's file
   space inadvertently.

   User applications that request form information from users must be
   careful not to cause a user to send information to the requestor or a
   third party unwillingly or unwittingly.  For example, a form might
   request 'spam' information to be sent to an unintended third party,



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   or private information to be sent to someone that the user might not
   actually intend.  While this is primarily an issue for the
   representation and interpretation of forms themselves (rather than
   the data representation of the form data), the transportation of
   private information must be done in a way that does not expose it to
   unwanted prying.

   With the introduction of form-data that can reasonably send back the
   content of files from a user's file space, the possibility arises
   that a user might be sent an automated script that fills out a form
   and then sends one of the user's local files to another address.
   Thus, additional caution is required when executing automated
   scripting where form-data might include a user's files.

   Files sent via multipart/form-data may contain arbitrary executable
   content, and precautions against malicious content are necessary.

   The considerations of [RFC2183] Sections 2.3 and 5 with respect to
   the filename parameter of the Content-Disposition header also apply
   to its usage here.

8.  Media type registration for multipart/form-data

   This section is the [RFC6838] media type registration.

   Type name:  multipart

   Subtype name:  form-data

   Required parameters:  boundary

   Optional parameters:  none

   Encoding considerations:  Common use is BINARY.
      In limited use (or transports that restrict the encoding to 7BIT
      or 8BIT each part is encoded separately using Content-Transfer-
      Encoding Section 4.8.

   Security considerations:  See Section 7 of this document.

   Interoperability considerations:  This document makes several
      recommendations for interoperability with deployed
      implementations, including Section 4.8.

   Published specification:  This document.

   Applications that use this media type:  Numerous web browsers,
      servers, and web applications.



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   Fragment identifier considerations:  None: Fragment identifiers are
      not defined for this type.

   Additional information:  None: no deprecated alias names, magic
      numbers, file extensions or Macintosh ssssfile type codes.

   Person & email address to contact        for further information
      Author of this document.

   Intended Usage:  COMMON

   Restrictions on usage:  none

   Author:  Author of this document.

   Change controller:  IETF

   Provisional registration:  N/A

9.  References

9.1.  Normative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

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

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions:
              Character Sets, Languages, and Continuations", RFC 2231,
              November 1997.

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





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9.2.  Informative References

   [I-D.ietf-appsawg-file-scheme]
              Kerwin, M., "The file URI Scheme", draft-ietf-appsawg-
              file-scheme-00 (work in progress), January 2015.

   [RFC1867]  Nebel, E. and L. Masinter, "Form-based File Upload in
              HTML", RFC 1867, November 1995.

   [RFC2388]  Masinter, L., "Returning Values from Forms: multipart/
              form-data", RFC 2388, August 1998.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, August 2010.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, January 2013.

   [W3C.REC-html32-19970114]
              Raggett, D., "HTML 3.2 Reference Specification", World
              Wide Web Consortium Recommendation REC-html32-19970114,
              January 1997, <http://www.w3.org/TR/REC-html32-19970114>.

   [W3C.REC-html5-20141028]
              Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
              Navara, E., O&#039;Connor, E., and S. Pfeiffer, "HTML5",
              World Wide Web Consortium Recommendation REC-
              html5-20141028, October 2014,
              <http://www.w3.org/TR/2014/REC-html5-20141028>.

Appendix A.  Changes from RFC 2388

   The handling of non-ASCII field names changed-- no longer
   recommending the RFC 2047 method, instead suggesting senders send
   UTF-8 field names directly, and file names directly in the form-
   charset.

   The handling of multiple files submitted as the result of a single
   form field (e.g.  HTML's <input type=file multiple> element) results
   in each file having its own top level part with the same name
   parameter; the method of using a nested "multipart/mixed" from
   [RFC2388] is no longer recommended for creators, and not required for
   receivers as there are no known implementations of senders.

   The _charset_ convention and use of an explicit form-data charset is
   documented.



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   'boundary' is a required parameter in Content-Type.

   The relationship of the ordering of fields within a form and the
   ordering of returned values within multipart/form-data was not
   defined before, nor was the handling of the case where a form has
   multiple fields with the same name.

   Editorial: Removed obsolete discussion of alternatives in appendix.
   Update references.  Move outline of form processing into
   Introduction.

Appendix B.  Alternatives

   There are numerous alternative ways in which form data can be
   encoded; many are listed in [RFC2388] section 5.2.  The multipart/
   form-data encoding is verbose, especially if there are many fields
   with short values.  In most use cases, this overhead isn't
   significant.

   More problematic are the differences introduced when implementors
   opted to not follow [RFC2388] when encoding non-ASCII field names
   (perhaps because "may" should have been "MUST").  As a result,
   parsers need to be more complex for matching against the possible
   outputs of various encoding methods.

Author's Address

   Larry Masinter
   Adobe

   Email: masinter@adobe.com
   URI:   http://larry.masinter.net



















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