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Versions: 00 01 02 03 draft-ietf-appsawg-multipart-form-data

Network Working Group                                        L. Masinter
Internet-Draft                                                     Adobe
Obsoletes: 2388 (if approved)                         September 21, 2013
Intended status: Standards Track
Expires: March 23, 2014

            Returning Values from Forms: multipart/form-data
                 draft-masinter-multipart-form-data-03

Abstract

   This specification (re)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
   replaces RFC 2388.

NOTE

   There is a GitHub repository for this draft at https://github.com/
   masinter/multipart-form-data along with an issue tracker.  This
   specification has been proposed as a work item of the APPSAWG
   Applications Area working group, apps-discuss@ietf.org.  Please raise
   issues in the tracker, or send to the apps-discuss list.

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
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   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 March 23, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.







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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Definition of multipart/form-data  . . . . . . . . . . . . . .  3
     2.1.  Boundary . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  filename attribute . . . . . . . . . . . . . . . . . . . .  3
     2.3.  Multiple files for one form field  . . . . . . . . . . . .  4
     2.4.  Content-Type . . . . . . . . . . . . . . . . . . . . . . .  4
     2.5.  The charset parameter  . . . . . . . . . . . . . . . . . .  4
     2.6.  The _charset_ field  . . . . . . . . . . . . . . . . . . .  4
     2.7.  Content-Transfer-Encoding  . . . . . . . . . . . . . . . .  4
     2.8.  Other Content- headers . . . . . . . . . . . . . . . . . .  4
   3.  Operability considerations . . . . . . . . . . . . . . . . . .  5
     3.1.  Non-ASCII field names and values . . . . . . . . . . . . .  5
       3.1.1.  Avoid creating forms with non-ASCII field names  . . .  5
       3.1.2.  Ampersand hash encoding  . . . . . . . . . . . . . . .  5
       3.1.3.  Interpreting forms and creating form-data  . . . . . .  5
       3.1.4.  Parsing and interpreting form data . . . . . . . . . .  6
     3.2.  Ordered fields and duplicated field names  . . . . . . . .  6
     3.3.  Interoperability with web applications . . . . . . . . . .  6
     3.4.  Correlating form data with the original form . . . . . . .  6
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   5.  Media type registration for multipart/form-data  . . . . . . .  7
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Appendix A. Changes from RFC 2388  . . . . . . . . . . . . . . . .  8
   Appendix B. Alternatives . . . . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9

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 [HTML3.2] and [HTML4], where forms are expressed in
   HTML, and in which the form values are sent via HTTP or electronic
   mail.  This representation is widely implemented in numerous web


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   browsers and web servers.

   However, multipart/form-data can be used for forms that are presented
   using representations other than HTML (spreadsheets, Portable
   Document Format, etc.), and for transport using other means than
   electronic mail or HTTP. This document defines the representation of
   form values independently of the application for which it is used.

2.  Definition of multipart/form-data

   The media-type multipart/form-data generally follows the model of
   multipart MIME data streams as described in [RFC2046] Section 5.1.

   In forms, there are a series of fields to be supplied by the user who
   fills out the form.  Each field has a name.  Within a given form, the
   names SHOULD be unique.  After a form has been "filled out" and
   "submitted" (processes defined by the form), the result is a set of
   values for each field-- the form-data.

   A "multipart/form-data" body contains a series of parts.  Each part
   MUST contain a "Content-Disposition" header [RFC2183] where the
   disposition type is "form-data", and where the disposition contains
   an (additional) parameter of "name"; the value of the parameter is
   the original field name from the form (encoded, see Section 3.1).
   For example, a part might contain a header:

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

   with the value corresponding to the entry of the "user" field.

2.1.  Boundary

   As with other multipart types, the parts are delimited with a
   boundary, selected such that it does not occur in any of the data.
   Each field of the form is sent, in the order defined by the sending
   application and form, as a part of the multipart stream.  The
   boundary is supplied as a "boundary" parameter to the multipart/form-
   data type, e.g.,

        multipart/form-data;boundary="-AaB03x"

2.2.  filename attribute

   For form data that represents the content of a local file, a name for
   the file SHOULD be supplied as well, by using a "filename" parameter
   of the Content-Distribution header.  (The SHOULD is to allow file
   uploads that result from drag-and-drop in systems where the file name
   is meaningless or private, where the uploaded content is streamed
   directly from a device, or where the file name is not user visible
   and would be unrecognized.)





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   For compatibility with other multipart types, the value of the
   filename parameter MUST be restricted to US-ASCII.  File names
   normally visible to users which contain non-ASCII characters SHOULD
   be encoded using the &#nn; method described in Section 3.1.2.

2.3.  Multiple files for one form field

   If the value of a form field is a set of files rather than a single
   file, that value MUST be transmitted by supplying each in a separate
   part, but all with the same "name", parameter.

2.4.  Content-Type

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

2.5.  The charset parameter

   In the case where a field value 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
   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=windows-1250
         content-transfer-encoding: quoted-printable

         Joe owes =80100.
         --AaB03x

2.6.  The _charset_ field

   Forms have the convention that the value of a form entry with entry
   name "_charset_" and type "hidden" is automatically set to the name
   of the form-charset.  In this case, the value of the default charset
   of each text/plain part without a charset parameter is the supplied
   value.

2.7.  Content-Transfer-Encoding

   When used in transports which do not allow arbitrary binary data,
   each part that cannot be represented within the transport SHOULD be
   encoded and the "Content-Transfer-Encoding" header supplied in that
   part.  For example, some email transports use a 7BIT encoding.  (See
   section 5 of [RFC2046] for more details.) When transferred via HTTP,
   Content-Transfer-Encoding the form-data values SHOULD NOT be used.

2.8.  Other Content- headers


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   The "multipart/form-data" media type does not support any MIME
   headers in the parts other than Content-Type, Content-Disposition,
   and (when appropriate), Content-Transfer-Encoding.

3.  Operability considerations

3.1.  Non-ASCII field names and values

   MIME headers in multipart/form-data are required to consist only of
   7-bit data in the US-ASCII character set.  While [RFC2388] suggested
   that non-ASCII field names should be encoded according to the method
   in [RFC2047] if they contain characters outside of US-ASCII, practice
   varies.

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

3.1.1.  Avoid creating forms with 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.

3.1.2.  Ampersand hash encoding

   Within this specification, the "ampersand hash encoding" is used for
   representing characters that are not allowed in a context: replace
   each disallowed character character by a string consisting of an
   ampersand (&), a hash mark (#), one or more ASCII digits representing
   the Unicode code point of the character in base ten, and finally a
   semicolon (;).

3.1.3.  Interpreting forms and creating form-data

   Some applications of this specification will supply a character
   encoding to be used for creation of the multipart/form-data result.
   In particular, [HTML5] uses:

   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,

   o  otherwise UTF-8.

   Call this the form-charset.  Any field name or file name which is not
   in US-ASCII must be encoded using the &#nn; encoding in Section 3.1.2

   multipart/form-data parts which do not have a Content-Type header and
   which are not the result of supplying a local file MUST be
   transformed by the same algorithm.

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3.1.4.  Parsing and interpreting form data

   While this specification provides guidance for creation of multipart/
   form-data, interpreters of multipart/form-data should be aware of the
   variety of implementations.  Currently, deployed browsers differ as
   to how they encode multipart/form-data.  For this reason the matching
   of form elements to form-data parts may rely on a fuzzier match.  In
   particular, some form-data generators might have followed the 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] or to send unencoded UTF-8
   or even unencoded strings in the form-charset.

   Generally, 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.

3.2.  Ordered fields and duplicated field names

   Form processors given forms with a well-defined ordering SHOULD send
   back results in the order received and preserve duplicate field
   names, in order.  Intermediaries MUST NOT reorder the results.(Note
   that there are some forms which do not define a natural order of
   appearance.)

3.3.  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
   application might need to optionally support both the application/x
   -url-encoded format as well.

3.4.  Correlating form data with the original form









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   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.)

4.  Security Considerations

   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,
   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 result of 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.

5.  Media type registration for multipart/form-data

   Media Type name: multipart

   Media subtype name: form-data

   Required parameters: boundary

   Optional parameters: none

   Encoding considerations: For use in transports that restrict the
      encoding to 7BIT or 8BIT, each part is encoded separately.

   Security considerations: 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 by the
      recipient.  This is a consideration for any application that
      generates a multipart/form-data.  See Section 4 of this document.


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6.  References

6.1.  Normative References

   [RFC1806]  Troost, R. and S. Dorner, "Communicating Presentation
              Information in Internet Messages: The Content-Disposition
              Header", RFC 1806, June 1995.

   [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.

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

   [RFC2184]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions: Character Sets, Languages, and
              Continuations", RFC 2184, August 1997.

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

6.2.  Informative References

   [HTML3.2]  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>.

   [HTML4]    Raggett, D., Hors, A. and I. Jacobs, "HTML 4.0
              Recommendation", World Wide Web Consortium REC-
              html40-971218, December 1997, <http://www.w3.org/TR/REC-
              html40-971218>.

   [HTML5]    Berjon, R., Faulkner, S., Leithead, T., Navara, E.,
              O'Connor, E. and S. Pfeiffer, "HTML5", September 2013,
              <http://www.w3.org/html/wg/drafts/html/CR/>.

   [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.

Appendix A.  Changes from RFC 2388




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   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 not recommended.

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

   The handling of non-ASCII field names is changed significantly.  Few
   if any implemented the =?charset:string?= method of [RFC2047].

   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.

   More prescriptive about order and duplicates.

   Remove obsolete discussion of alternatives.

Appendix B.  Alternatives

   There are numerous alternative ways in which form data can be
   encoded; many are listed in [RFC2388] under "Other data encodings
   rather than multipart."  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 is the ambiguity introduced because implementations
   did not follow [RFC2388] because it used "may" instead of "MUST" when
   specifying encoding of field names, and for other unknown reasons, so
   now, parsers need to be more complex for fuzzy 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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