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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.


 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.

1. Introduction

 [RFC 1521] 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

Troost & Dorner Experimental [Page 1] RFC 1806 Content-Disposition June 1995

 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 [RFC 822], the Content-Disposition
 header field is defined as follows:
      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].

Troost & Dorner Experimental [Page 2] RFC 1806 Content-Disposition June 1995

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 [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.

Troost & Dorner Experimental [Page 3] RFC 1806 Content-Disposition June 1995

 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 [RFC 1521], 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 [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.

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.

Troost & Dorner Experimental [Page 4] RFC 1806 Content-Disposition June 1995

 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

2.6 Content-Disposition and the Main Message

 It is permissible to use Content-Disposition on the main body of an
 [RFC 822] 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
       Content-Type: multipart/mixed; boundary=outer
       Content-Description: multipart-1
  1. -outer

Troost & Dorner Experimental [Page 5] RFC 1806 Content-Disposition June 1995

         Content-Type: text/plain
         Content-Disposition: inline
         Content-Description: text-part-1
         Some text goes here
  1. -outer

Content-Type: multipart/mixed; boundary=inner

         Content-Disposition: attachment
         Content-Description: multipart-2
  1. -inner

Content-Type: text/plain

           Content-Disposition: inline
           Content-Description: text-part-2
           Some more text here.
  1. -inner

Content-Type: image/jpeg

           Content-Disposition: attachment
           Content-Description: jpeg-1
           <jpeg data>

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

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:

Troost & Dorner Experimental [Page 6] RFC 1806 Content-Disposition June 1995

        + 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.

6. 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.

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

Troost & Dorner Experimental [Page 7] RFC 1806 Content-Disposition June 1995

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

Troost & Dorner Experimental [Page 8]

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