Internet DRAFT - draft-dorner-content-header
draft-dorner-content-header
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Internet Draft: draft-dorner-content-header-01.txt
Category: Informational Rens Troost
Steve Dorner
January 1994
Communicating Presentation Information in
Internet Messages:
The Content-Disposition Header
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of
six months. Internet-Drafts may be updated, replaced, or
obsoleted by other documents at any time. It is not
appropriate to use Internet-Drafts as reference material or
to cite them other than as a "working draft" or "work in
progress".
To learn the current status of any Internet-Draft, please
check the 1id-abstracts.txt listing contained in the
Internet-Drafts Shadow Directories on ds.internic.net,
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1. Abstract
This memo provides a mechanism whereby messages conforming to
the [RFC 1521] ("MIME") specification can convey
presentational information. It specifies a new
"Content-Disposition" header, optional and valid for any
[RFC 1521] 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 [RFC 1521]. As
such, the reader is assumed to be familiar with [RFC 1521],
and [RFC 822]. The information presented herein supplements
but does not replace that found in those documents.
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2. Introduction
[RFC 1521] describes 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.
3. 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 [RFC 822], the
Content-Disposition header field is defined as follows:
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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 [RFC 822] and [RFC 1521].
3.1 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 are
encountered, subject to the normal semantics of multipart
messages.
3.2 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.
3.3 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).
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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 [RFC 1521] 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 [RFC 1521] `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.
3.4 Future Extensions and Unrecognized Disposition Types
In the likely event that new parameters or types are needed,
they should be registered with the IANA, in the manner
specified in [RFC 1521], appendix E.
Once new types and parameters are defined, there is of course
the likelihood that implementations will see types and
parameters they do not understand. Furthermore, since
x-tokens are allowed, implementations may also see entirely
unregistered types and parameters.
Unrecognized parameters should be ignored. Unrecognized 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 value is more likely aiming for something more elaborate
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than inline presentation.
Unless noted otherwise in the definition of a parameter,
Content-Disposition parameters are valid for all
dispositions. (In contrast to [RFC 1521] 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.
3.5 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.
3.6 Content-Disposition and the Main Message
It is permissible to use Content-Disposition on the main body
of an [RFC 822] message.
4. 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
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"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--
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5. 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.
6. 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.
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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
Author: Rens Troost rens@imsi.com
Co-Author: Steve Dorner sdorner@qualcomm.com
9. References
[RFC 1521]
Borenstein N., and N. Freed, "MIME (Multipurpose Internet
Mail Extensions) Part One: Mechanisms for Specifying and
Describing the Format of Internet Message Bodies",
RFC 1521, Bellcore, Innosoft, September 1993.
[RFC 822]
Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC 822, UDEL, August 1982.
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