RFC1806 - Communicating Presentation Information in Internet Messages: The Conte

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　　Network Working Group R. Troost
Request for Comments: 1806 New Century Systems
Category: Experimental S. Dorner
QUALCOMM Incorporated
June 1995

Communicating Presentation Information in
Internet Messages:

The Content-Disposition Header

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Abstract

This memo provides a mechanism whereby messages conforming to the
[RFC1521] ("MIME") specification can convey presentational
information. It specifies a new "Content-Disposition" header,
optional and valid for any [RFC1521] entity ("message" or "body
part"). Two values for this header are described in this memo; one
for the ordinary linear presentation of the body part, and another to
facilitate the use of mail to transfer files. It is expected that
more values will be defined in the future, and procedures are defined
for extending this set of values.

This document is intended as an extension to [RFC1521]. As such, the
reader is assumed to be familiar with [RFC1521], and [RFC822]. The
information presented herein supplements but does not replace that
found in those documents.

1. Introduction

[RFC1521] specifies a standard format for encapsulating multiple
pieces of data into a single Internet message. That document does not
address the issue of presentation styles; it provides a framework for
the interchange of message content, but leaves presentation issues
solely in the hands of mail user agent (MUA) implementors.

Two common ways of presenting multipart electronic messages are as a
main document with a list of separate attachments, and as a single
document with the various parts expanded (displayed) inline. The
display of an attachment is generally construed to require positive
action on the part of the recipient, while inline message components

are displayed automatically when the message is viewed. A mechanism
is needed to allow the sender to transmit this sort of presentational
information to the recipient; the Content-Disposition header provides
this mechanism, allowing each component of a message to be tagged
with an indication of its desired presentation semantics.

Tagging messages in this manner will often be sufficient for basic
message formatting. However, in many cases a more powerful and
flexible approach will be necessary. The definition of such
approaches is beyond the scope of this memo; however, such approaches
can benefit from additional Content-Disposition values and
parameters, to be defined at a later date.

In addition to allowing the sender to specify the presentational
disposition of a message component, it is desirable to allow her to
indicate a default archival disposition; a filename. The optional
"filename" parameter provides for this.

2. The Content-Disposition Header Field

Content-Disposition is an optional header; in its absence, the MUA
may use whatever presentation method it deems suitable.

It is desirable to keep the set of possible disposition types small
and well defined, to avoid needless complexity. Even so, evolving
usage will likely require the definition of additional disposition
types or parameters, so the set of disposition values is extensible;
see below.

In the extended BNF notation of [RFC822], the Content-Disposition
header field is defined as follows:

disposition := "Content-Disposition" ":"
disposition-type
*(";" disposition-parm)

disposition-type := "inline"
/ "attachment"
/ extension-token
; values are not case-sensitive

disposition-parm := filename-parm / parameter

filename-parm := "filename" "=" value;

`Extension-token', `parameter' and `value' are defined according to
[RFC822] and [RFC1521].

2.1 The Inline Disposition Type

A bodypart should be marked `inline' if it is intended to be
displayed automatically upon display of the message. Inline bodyparts
should be presented in the order in which they occur, subject to the
normal semantics of multipart messages.

2.2 The Attachment Disposition Type

Bodyparts can be designated `attachment' to indicate that they are
separate from the main body of the mail message, and that their
display should not be automatic, but contingent upon some further
action of the user. The MUA might instead present the user of a
bitmap terminal with an iconic representation of the attachments, or,
on character terminals, with a list of attachments from which the
user could select for viewing or storage.

2.3 The Filename Parameter

The sender may want to suggest a filename to be used if the entity is
detached and stored in a separate file. If the receiving MUA writes
the entity to a file, the suggested filename should be used as a
basis for the actual filename, where possible.

It is important that the receiving MUA not blindly use the suggested
filename. The suggested filename should be checked (and possibly
changed) to see that it conforms to local filesystem conventions,
does not overwrite an existing file, and does not present a security
problem (see Security Considerations below).

The receiving MUA should not respect any directory path information
that may seem to be present in the filename parameter. The filename
should be treated as a terminal component only. Portable
specification of directory paths might possibly be done in the future
via a separate Content-Disposition parameter, but no provision is
made for it in this draft.

Current [RFC1521] grammar restricts parameter values (and hence
Content-Disposition filenames) to US-ASCII. We recognize the great
desirability of allowing arbitrary character sets in filenames, but
it is beyond the scope of this document to define the necessary
mechanisms. We expect that the basic [RFC1521] `value'
specification will someday be amended to allow use of non-US-ASCII
characters, at which time the same mechanism should be used in the
Content-Disposition filename parameter.

Beyond the limitation to US-ASCII, the sending MUA may wish to bear
in mind the limitations of common filesystems. Many have severe
length and character set restrictions. Short alphanumeric filenames
are least likely to require modification by the receiving system.

The presence of the filename parameter does not force an
implementation to write the entity to a separate file. It is
perfectly acceptable for implementations to leave the entity as part
of the normal mail stream unless the user requests otherwise. As a
consequence, the parameter may be used on any MIME entity, even
`inline' ones. These will not normally be written to files, but the
parameter could be used to provide a filename if the receiving user
should choose to write the part to a file.

2.4 Future Extensions and Unrecognized Disposition Types

In the likely event that new parameters or disposition types are
needed, they should be registered with the IANA, in the manner
specified in [RFC1521], appendix E.

Once new disposition types and parameters are defined, there is of
course the likelihood that implementations will see disposition types
and parameters they do not understand. Furthermore, since x-tokens
are allowed, implementations may also see entirely unregistered
disposition types and parameters.

Unrecognized parameters should be ignored. Unrecognized disposition
types should be treated as `attachment'. The choice of `attachment'
for unrecognized types is made because a sender who goes to the
trouble of producing a Content-Disposition header with a new
disposition type is more likely aiming for something more elaborate
than inline presentation.

Unless noted otherwise in the definition of a parameter, Content-
Disposition parameters are valid for all dispositions. (In contrast
to [RFC1521] content-type parameters, which are defined on a per-
content-type basis.) Thus, for example, the `filename' parameter
still means the name of the file to which the part should be written,
even if the disposition itself is unrecognized.

2.5 Content-Disposition and Multipart

If a Content-Disposition header is used on a multipart body part, it
applies to the multipart as a whole, not the individual subparts.
The disposition types of the subparts do not need to be consulted
until the multipart itself is presented. When the multipart is
displayed, then the dispositions of the subparts should be respected.

If the `inline' disposition is used, the multipart should be
displayed as normal; however, an `attachment' subpart should require
action from the user to display.

If the `attachment' disposition is used, presentation of the
multipart should not proceed without explicit user action. Once the
user has chosen to display the multipart, the individual subpart
dispositions should be consulted to determine how to present the
subparts.

2.6 Content-Disposition and the Main Message

It is permissible to use Content-Disposition on the main body of an
[RFC822] message.

3. Examples

Here is a an example of a body part containing a JPEG image that is
intended to be viewed by the user immediately:

Content-Type: image/jpeg
Content-Disposition: inline
Content-Description: just a small picture of me

<jpeg data>

The following body part contains a JPEG image that should be
displayed to the user only if the user requests it. If the JPEG is
written to a file, the file should be named "genome.jpg":

Content-Type: image/jpeg
Content-Disposition: attachment; filename=genome.jpeg
Content-Description: a complete map of the human genome

<jpeg data>

The following is an example of the use of the `attachment'
disposition with a multipart body part. The user should see text-
part-1 immediately, then take some action to view multipart-2. After
taking action to view multipart-2, the user will see text-part-2
right away, and be required to take action to view jpeg-1. Subparts
are indented for clarity; they would not be so indented in a real
message.

Content-Type: multipart/mixed; boundary=outer
Content-Description: multipart-1

--outer

Content-Type: text/plain
Content-Disposition: inline
Content-Description: text-part-1

Some text goes here

--outer
Content-Type: multipart/mixed; boundary=inner
Content-Disposition: attachment
Content-Description: multipart-2

--inner
Content-Type: text/plain
Content-Disposition: inline
Content-Description: text-part-2

Some more text here.

--inner
Content-Type: image/jpeg
Content-Disposition: attachment
Content-Description: jpeg-1

<jpeg data>
--inner--
--outer--

4. Summary

Content-Disposition takes one of two values, `inline' and
`attachment'. 'Inline' indicates that the entity should be
immediately displayed to the user, whereas `attachment' means that
the user should take additional action to view the entity.

The `filename' parameter can be used to suggest a filename for
storing the bodypart, if the user wishes to store it in an external
file.

5. Security Considerations

There are security issues involved any time users exchange data.
While these are not to be minimized, neither does this memo change
the status quo in that regard, except in one instance.

Since this memo provides a way for the sender to suggest a filename,
a receiving MUA must take care that the sender's suggested filename
does not represent a hazard. Using UNIX as an example, some hazards
would be:

+ Creating startup files (e.g., ".login").

+ Creating or overwriting system files (e.g.,
"/etc/passwd").

+ Overwriting any existing file.

+ Placing executable files into any command search path
(e.g., "~/bin/more").

+ Sending the file to a pipe (e.g., "| sh").

In general, the receiving MUA should never name or place the file
such that it will get interpreted or executed without the user
explicitly initiating the action.

It is very important to note that this is not an exhaustive list; it
is intended as a small set of examples only. Implementors must be
alert to the potential hazards on their target systems.

6. References

[RFC1521]
Borenstein N., and N. Freed, "MIME (Multipurpose Internet
Mail Extensions) Part One: Mechanisms for Specifying and
Describing the Format of Internet Message Bodies",
RFC1521, Bellcore, Innosoft, September 1993.

[RFC822]
Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC822, UDEL, August 1982.

7. Acknowledgements

We gratefully acknowledge the help these people provided
during the preparation of this draft:

Nathaniel Borenstein
Ned Freed
Keith Moore
Dave Crocker
Dan Pritchett

8. Authors' Addresses

Rens Troost
New Century Systems
324 East 41st Street #804
New York, NY, 10017 USA

Phone: +1 (212) 557-2050
Fax: +1 (212) 557-2049
EMail: rens@century.com

Steve Dorner
QUALCOMM Incorporated
6455 Lusk Boulevard
San Diego, CA 92121
USA

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