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Versions: 00 01 RFC 2183

Network Working Group                                          R. Troost
Internet-Draft                                       New Century Systems
Expires: 14 August 1997                                        S. Dorner
                                                   QUALCOMM Incorporated
                                                        K. Moore, Editor
                                                 University of Tennessee
                                                           February 1997


               Communicating Presentation Information in
                           Internet Messages:
                  The Content-Disposition Header Field

                     draft-moore-mime-cdisp-00.txt

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  and 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 ``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   ftp.is.co.za   (Africa),   nic.nordu.net   (Europe),
munnari.oz.au  (Pacific  Rim),  ds.internic.net  (US  East  Coast),   or
ftp.isi.edu (US West Coast).

Abstract

     This  memo  provides a mechanism whereby messages conforming to the
MIME specifications [RFC 2045, RFC 2046, RFC 2047,  RFC 2048,  RFC 2049]
can convey presentational information.  It specifies the "Content-Dispo-
sition" header field, which is optional and valid for  any  MIME  entity
("message"  or  "body  part").  Two  values  for  this  header field 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  pro-
cedures are defined for extending this set of values.

     This  document  is  intended as an extension to MIME.  As such, the
reader is assumed to be  familiar  with  the  MIME  specifications,  and
[RFC 822].  The  information  presented  herein supplements but does not



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replace that found in those documents.

     This document is a revision to the Experimental protocol defined in
RFC  1806.  As compared to RFC 1806, this document contains minor edito-
rial updates, adds new parameters needed to support  the  File  Transfer
Body  Part,  and references a separate specification for the handling of
non-ASCII and/or very long parameter values.

[[NOTE IN DRAFT:: Comments on this document  during  the  review  period
should be sent to <ietf-822@imc.org>.]]

1.  Introduction

     MIME  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 inter-
change 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 addi-
tional 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 "file-
name" parameter provides for this.  Further, the creation-date,  modifi-
cation-date,  and  read-date parameters allow preservation of those file
attributes when the file is transmitted over MIME email.

     NB: The keywords  MUST,  MUST  NOT,  REQUIRED,  SHALL,  SHALL  NOT,
SHOULD,  SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in
this document, are to be interpreted as described in [DRAFT-KEYWORDS].




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2.  The Content-Disposition Header Field

     Content-Disposition is an optional header field.  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-type
                       *(";" disposition-parm)

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

        disposition-parm := filename-parm
                          / creation-date-parm
                          / modification-date-parm
                          / read-date-parm
                          / size-parm
                          / parameter

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

        creation-date-parm := "creation-date" "=" quoted-date-time

        modification-date-parm := "modification-date" "=" quoted-date-time

        read-date-parm := "read-date" "=" quoted-date-time

        size-parm := "size" "=" 1*DIGIT

        quoted-date-time := quoted-string
                         ; contents MUST be an RFC 822 `date-time'
                         ; numeric timezones (+HHMM or -HHMM) MUST be used



[[NOTE  IN  DRAFT::  Should  consider  defining all parameters as simply
token=value, without explicit productions for each parameter, and moving
the  restrictions  on  the  use  of  parameter  values  to  the  section



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describing that parameter.]]

     NOTE ON PARAMETER VALUE LENGHTS: A short (length <= 78  characters)
parameter  value  containing  only  non-`tspecials' characters SHOULD be
represented as a single `token'.  A  short  parameter  value  containing
only  ASCII  characters, but including `tspecials' characters, SHOULD be
represented as `quoted-string'.  Parameter values longer than 78 charac-
ters,  or  which contain non-ASCII characters, MUST be encoded as speci-
fied in [DRAFT-PVCSC].

     `Extension-token', `parameter', `tspecials' and `value' are defined
according to [RFC 2045] (which references [RFC 822] in the definition of
some of these  tokens).  `quoted-string'  and  `DIGIT'  are  defined  in
[RFC 822].

2.1  The Inline Disposition Type

     A  bodypart  should be marked `inline' if it is intended to be dis-
played 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 sug-
gested 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



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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 2045] 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 param-
eter.

     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  implemen-
tation  to  write the entity to a separate file. It is perfectly accept-
able for implementations to leave the entity as part of the normal  mail
stream unless the user requests otherwise. As a consequence, the parame-
ter 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 The Creation-Date parameter

     The  creation-date  parameter  MAY  be used to indicate the date at
which the file was created.  If this parameter is included, the paramter
value  MUST  be  a  quoted-string which contains a representation of the
creation date of the file in [RFC 822] `date-time' format.

     UNIX and POSIX implementors are cautioned that the `st_ctime'  file
attribute  of the `stat' structure is not the creation time of the file;
it is thus not appropriate as a source for the  creation-date  parameter
value.

2.5 The Modification-Date parameter

     The modification-date parameter MAY be used to indicate the date at
which the file was last modified.  If the modification-date parameter is
included,  the  paramter  value MUST be a quoted-string which contains a
representation of the last modification date of the  file  in  [RFC 822]
`date-time' format.





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2.6 The Read-Date parameter

     The  read-date  parameter MAY be used to indicate the date at which
the file was last read.  If the read-date  parameter  is  included,  the
parameter  value MUST be a quoted-string which contains a representation
of the last-read date of the file in [RFC 822] `date-time' format.


2.7 The Size parameter

     The size parameter indicates an approximate size  of  the  file  in
octets.   It  can  be  used,  for  example, to pre-allocate space before
attempting to store the file,  or  to  determine  whether  enough  space
exists.

[[NOTE IN DRAFT: The above text is just a guess. The real `size' parame-
ter needs to be aligned with the FTBP specifications  for  compatibility
with draft-ietf-mixer-bodymap-06.txt]]

2.8  Future Extensions and Unrecognized Disposition Types

     In  the  likely  event that new parameters or disposition types are
needed, they should be registered with  the  Internet  Assigned  Numbers
Authority (IANA), in the manner specified in Section 9 of this memo.

     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 presenta-
tion.

     Unless noted otherwise in the definition of a  parameter,  Content-
Disposition  parameters are valid for all dispositions.  (In contrast to
MIME 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.

2.9  Content-Disposition and Multipart

     If  a  Content-Disposition header is used on a multipart body part,



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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 dis-
played as normal; however, an `attachment' subpart should require action
from the user to display.

     If the `attachment' disposition is used, presentation of the multi-
part 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.

2.10  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".  The recipient's user might
also  choose to set the last-modified date of the stored file to date in
the modification-date parameter:

        Content-Type: image/jpeg
        Content-Disposition: attachment; filename=genome.jpeg;
          modification-date="Wed, 12 Feb 1997 16:29:51 -0500";
        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 immedi-
ately, 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



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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 `attach-
ment'.  `Inline' indicates that the entity should  be  immediately  dis-
played 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.



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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  file-
name,  a  receiving MUA must take care that the sender's suggested file-
name 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 explic-
itly 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

[DRAFT-KEYWORDS]
     Bradner, S. "Key words for use in RFCs to Indicate Requirement Lev-
     els".  Internet-Draft draft-bradner-key-words-03.txt, January 1997.

[DRAFT-PVCSC]
     Freed, N. and Moore, K.  "MIME Parameter value and  Encoded  Words:
     Character  Sets,  Lanaguage,  and  Continuations".   Internet-Draft
     draft-freed-pvcsc-01.txt, November 1996.

[RFC 2045]
     Freed, N. and Borenstein, N.,  "MIME  (Multipurpose  Internet  Mail
     Extensions) Part One: Format of Internet Message Bodies", RFC 2045,
     December 1996.

[RFC 2046]
     Freed, N. and Borenstein  N.,  "MIME  (Multipurpose  Internet  Mail
     Extensions) Part Two: Media Types", RFC 2046, December 1996.

[RFC 2047]



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     Moore, K. "MIME (Multipurpose Internet Mail Extensions) Part Three:
     Message Header Extensions for non-ASCII Text", RFC  2047,  December
     1996.

[RFC 2048]
     Freed, N., Klensin, J. and Postel, J., "MIME (Multipurpose Internet
     Mail Extensions) Part Four:  Registration  Procedures",  RFC  2048,
     December 1996.

[RFC 2049]
     Freed,  N.  and  Borenstein  N.,  "MIME (Multipurpose Internet Mail
     Extensions) Part Five:  Conformance  Criteria  and  Examples",  RFC
     2049, December 1996.

[RFC 822]
     Crocker,  D.,  "Standard  for the Format of ARPA Internet Text Mes-
     sages", 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


8.  Authors' Addresses

     You should blame the editor of this version of the document for any
changes since RFC 1806:

        Keith Moore
        Department of Computer Science
        University of Tennessee, Knoxville
        107 Ayres Hall
        Knoxville TN  37996-1301
        USA

        Phone: +1 (423) 974-5067
        Fax: +1 (423) 974-8296
        Email: moore@cs.utk.edu


     The authors of RFC 1806 are:



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        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

        EMail: sdorner@qualcomm.com


9. Registration of New Content-Disposition Values and Parameters

     New Content-Disposition values (besides "inline" and  "attachment")
may be defined only by Internet standards-track documents, or in Experi-
mental documents approved by the Internet Engineering Steering Group.

     New content-disposition parameters may be registered  by  supplying
the  information in the following template and sending it via electronic
mail to IANA@IANA.ORG [[is this the right address?]]:

        To: IANA@IANA.ORG
        Subject: Registration of new Content-Disposition parameter

        Content-Disposition parameter name:

        Allowable values for this parameter:
                (If the parameter can only assume a small number of values,
                list each of those values.  Otherwise, describe the values
                that the parameter can assume.)
        Description:
                (What is the purpose of this parameter and how is it used?)


[[Question: should there be a faceted name space for parameters like the
one for content-types?]]






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10. Changes since RFC 1806

     The  following  changes have been made since the earlier version of
this document, published in RFC 1806 as an Experimental protocol:

+    Updated references to MIME documents.  In some cases this  involved
     substituting a reference to one of the current MIME RFCs for a ref-
     erence to RFC 1521; in other cases, a reference  to  RFC  1521  was
     simply replaced with the word "MIME".

+    Added  a section on registration procedures, since none of the pro-
     cedures in RFC 2048 seemed to be appropriate.

+    Added new parameter types: creation-date, modification-date,  read-
     date, and size.

+    Incorporated  a  reference to draft-freed-pvcsc-* for encoding long
     or non-ASCII parameter values.

+    Added reference to draft-bradner-key-words-03.txt to  define  MUST,
     SHOULD, etc. keywords.






























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