RFC2388 - Returning Values from Forms: multipart/form-data

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　　Network Working Group L. Masinter
Request for Comments: 2388 Xerox Corporation
Category: Standards Track August 1998

Returning Values from Forms: multipart/form-data

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

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

1. Abstract

This specification defines an Internet Media Type, multipart/form-
data, 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.

2. 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 [HTML40], 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 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.

3. Definition of multipart/form-data

The media-type multipart/form-data follows the rules of all multipart
MIME data streams as outlined in [RFC2046]. 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 are unique.

"multipart/form-data" contains a series of parts. Each part is
expected to contain a content-disposition header [RFC2183] where the
disposition type is "form-data", and where the disposition contains
an (additional) parameter of "name", where the value of that
parameter is the original field name in the form. 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.

Field names originally in non-ASCII character sets may be encoded
within the value of the "name" parameter using the standard method
described in RFC2047.

As with all multipart MIME types, each part has an optional
"Content-Type", which defaults to text/plain. If the contents of a
file are returned via filling out a form, then the file input is
identified as the appropriate media type, if known, or
"application/octet-stream". If multiple files are to be returned as
the result of a single form entry, they should be represented as a
"multipart/mixed" part embedded within the "multipart/form-data".

Each part may be encoded and the "content-transfer-encoding" header
supplied if the value of that part does not conform to the default
encoding.

4. Use of multipart/form-data

4.1 Boundary

As with other multipart types, a boundary is selected that does not
occur in any of the data. Each field of the form is sent, in the
order defined by the sending appliction and form, as a part of the
multipart stream. Each part identifies the INPUT name within the
original form. Each part should be labelled with an appropriate
content-type if the media type is known (e.g., inferred from the file
extension or operating system typing information) or as
"application/octet-stream".

4.2 Sets of files

If the value of a form field is a set of files rather than a single
file, that value can be transferred together using the
"multipart/mixed" format.

4.3 Encoding

While the HTTP protocol can transport arbitrary binary data, the
default for mail transport is the 7BIT encoding. The value supplied
for a part may need to be encoded and the "content-transfer-encoding"
header supplied if the value does not conform to the default
encoding. [See section 5 of RFC2046 for more details.]

4.4 Other attributes

Forms may request file inputs from the user; the form software may
include the file name and other file attributes, as specified in [RFC
2184].

The original local file name may be supplied as well, either as a
"filename" parameter either of the "content-disposition: form-data"
header or, in the case of multiple files, in a "content-disposition:
file" header of the subpart. The sending application MAY supply a
file name; if the file name of the sender's operating system is not
in US-ASCII, the file name might be approximated, or encoded using
the method of RFC2231.

This is a convenience for those cases where the files supplied by the
form might contain references to each other, e.g., a TeX file and its
.sty auxiliary style description.

4.5 Charset of text in form data

Each part of a multipart/form-data is supposed to have a content-
type. In the case where a field element is text, the charset
parameter for the text indicates the character encoding used.

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

5. Operability considerations

5.1 Compression, encryption

Some of the data in forms may be compressed or encrypted, using other
MIME mechanisms. This is a function of the application that is
generating the form-data.

5.2 Other data encodings rather than multipart

Various people have suggested using new mime top-level type
"aggregate", e.g., aggregate/mixed or a content-transfer-encoding of
"packet" to express indeterminate-length binary data, rather than
relying on the multipart-style boundaries. While this would be
useful, the "multipart" mechanisms are well established, simple to
implement on both the sending client and receiving server, and as
efficient as other methods of dealing with multiple combinations of
binary data.

The multipart/form-data encoding has a high overhead and performance
impact if there are many fields with short values. However, in
practice, for the forms in use, for example, in HTML, the average
overhead is not significant.

5.3 Remote files with third-party transfer

In some scenarios, the user operating the form software might want to
specify a URL for remote data rather than a local file. In this case,
is there a way to allow the browser to send to the client a pointer
to the external data rather than the entire contents? This capability
could be implemented, for example, by having the client send to the
server data of type "message/external-body" with "access-type" set
to, say, "uri", and the URL of the remote data in the body of the
message.

5.4 Non-ASCII field names

Note that MIME headers are generally required to consist only of 7-
bit data in the US-ASCII character set. Hence field names should be
encoded according to the method in RFC2047 if they contain
characters outside of that set.

5.5 Ordered fields and duplicated field names

The relationship of the ordering of fields within a form and the
ordering of returned values within "multipart/form-data" is not
defined by this specification, nor is the handling of the case where
a form has multiple fields with the same name. While HTML-based forms
may send back results in the order received, and intermediaries
should not reorder the results, there are some systems which might
not define a natural order for form fields.

5.6 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 (primarly 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.

5.7 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. Security Considerations

The data format described in this document introduces no new security
considerations outside of those introduced by the protocols that use
it and of the component elements. It is important when interpreting
content-disposition to not overwrite files in the recipients address
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 transmission, 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 user's file space, the possibility that a user
might be sent an automated script that fills out a form and then
sends the user's local file to another address arises. Thus,
additional caution is required when executing automated scripting
where form-data might include user's files.

7. Author's Address

Larry Masinter
Xerox Palo Alto Research Center
3333 Coyote Hill Road
Palo Alto, CA 94304

Fax: +1 650 812 4333
EMail: masinter@parc.xerox.com

Appendix A. Media type registration for multipart/form-data

Media Type name:
multipart

Media subtype name:
form-data

Required parameters:
none

Optional parameters:
none

Encoding considerations:
No additional considerations other than as for other multipart
types.

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.

The multipart/form-data type introduces no new security
considerations for recipients beyond what might occur with any of
the enclosed parts.

References

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

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

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

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

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

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

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

[HTML40] D. Raggett, A. Le Hors, I. Jacobs. "HTML 4.0
Specification", World Wide Web Consortium Technical Report
"REC-html40", December, 1997. <http://www.w3.org/TR/REC-
html40/>

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