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
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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

    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-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

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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         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

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

           Some text goes here

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

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

             Some more text here.

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

             <jpeg data>

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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

    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

    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.,

          + 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

    Co-Author: Steve Dorner

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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