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Network Working Group                                          R. TroostRequest for Comments: 2183                           New Century SystemsUpdates:1806                                                  S. DornerCategory: Standards Track                          QUALCOMM Incorporated                                                        K. Moore, Editor                                                 University of Tennessee                                                             August 1997Communicating Presentation Information inInternet Messages:The Content-Disposition Header FieldStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   This memo provides a mechanism whereby messages conforming to the   MIME specifications [RFC 2045,RFC 2046,RFC 2047,RFC 2048,RFC2049] can convey presentational information.  It specifies the   "Content-Disposition" 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 procedures 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   replace that found in those documents.   This document is a revision to the Experimental protocol defined inRFC 1806.  As compared toRFC 1806, this document contains minor   editorial 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.Troost, et. al.             Standards Track                     [Page 1]

RFC 2183                  Content-Disposition                August 19971.  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   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.  Further, the creation-date,   modification-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 [RFC 2119].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.Troost, et. al.             Standards Track                     [Page 2]

RFC 2183                  Content-Disposition                August 1997   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 anRFC 822 `date-time'                      ; numeric timezones (+HHMM or -HHMM) MUST be used   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   characters, or which contain non-ASCII characters, MUST be encoded as   specified in [RFC 2184].   `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].Troost, et. al.             Standards Track                     [Page 3]

RFC 2183                  Content-Disposition                August 19972.1  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 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 parameter.Troost, et. al.             Standards Track                     [Page 4]

RFC 2183                  Content-Disposition                August 1997   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 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.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.Troost, et. al.             Standards Track                     [Page 5]

RFC 2183                  Content-Disposition                August 19972.8  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 inSection 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 presentation.   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, 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.Troost, et. al.             Standards Track                     [Page 6]

RFC 2183                  Content-Disposition                August 19972.10  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 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.Troost, et. al.             Standards Track                     [Page 7]

RFC 2183                  Content-Disposition                August 1997        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.Troost, et. al.             Standards Track                     [Page 8]

RFC 2183                  Content-Disposition                August 19975.  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 not 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 2119]        Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",RFC 2119, March 1997.   [RFC 2184]        Freed, N. and K. Moore, "MIME Parameter value and Encoded Words:        Character Sets, Lanaguage, and Continuations",RFC 2184, August        1997.   [RFC 2045]        Freed, N. and N. Borenstein, "MIME (Multipurpose Internet Mail        Extensions) Part One: Format of Internet Message Bodies",RFC2045, December 1996.Troost, et. al.             Standards Track                     [Page 9]

RFC 2183                  Content-Disposition                August 1997   [RFC 2046]        Freed, N. and N. Borenstein, "MIME (Multipurpose Internet Mail        Extensions) Part Two: Media Types",RFC 2046, December 1996.   [RFC 2047]        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 J. Postel, "MIME (Multipurpose        Internet Mail Extensions) Part Four: Registration Procedures",RFC 2048, December 1996.   [RFC 2049]        Freed, N. and N. Borenstein, "MIME (Multipurpose Internet Mail        Extensions) Part Five: Conformance Criteria and Examples",RFC2049, December 1996.   [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 PritchettTroost, et. al.             Standards Track                    [Page 10]

RFC 2183                  Content-Disposition                August 19978.  Authors' Addresses   You should blame the editor of this version of the document for any   changes sinceRFC 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 ofRFC 1806 are:        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.com9. 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   Experimental documents approved by the Internet Engineering Steering   Group.Troost, et. al.             Standards Track                    [Page 11]

RFC 2183                  Content-Disposition                August 1997   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:     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?)10. Changes sinceRFC 1806   The following changes have been made since the earlier version of   this document, published inRFC 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 reference toRFC 1521; in other cases, a reference toRFC 1521 was simply replaced with the word "MIME".   +    Added  a section on registration procedures, since none of the        procedures inRFC 2048 seemed to be appropriate.   +    Added new parameter types: creation-date, modification-date,        read-date, and size.   +    Incorporated a reference todraft-freed-pvcsc-* for encoding        long or non-ASCII parameter values.   +    Added reference toRFC 2119 to define MUST, SHOULD, etc.        keywords.Troost, et. al.             Standards Track                    [Page 12]