Internet DRAFT - draft-arcmedia-type



Network Working Group                                         S. Leonard
Internet-Draft                                             Penango, Inc.
Intended Status: Standards Track                               M. Kerwin
Expires: April 30, 2015                                 October 27, 2014

            The Archive Primary Media Type for File Archives


   This document defines a new primary content-type to be known as
   "archive", which defines a fundamental type of content with unique
   presentational, hardware, and processing aspects.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.2.  Notational Conventions . . . . . . . . . . . . . . . . . .  2
   2.  Definition of an archive . . . . . . . . . . . . . . . . . . .  2
   3.  Consultation Mechanisms  . . . . . . . . . . . . . . . . . . .  5
   4.  Encoding and Transport . . . . . . . . . . . . . . . . . . . .  5
   5.  Common Required and Optional Parameters  . . . . . . . . . . .  7
   6.  Split Archives . . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Fragment Identifier Syntax . . . . . . . . . . . . . . . . . .  8
   8.  Piped-Composite Type Suffix Syntax . . . . . . . . . . . . . .  8
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   10. Normative References . . . . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Expected Subtypes . . . . . . . . . . . . . . . . . .  9

1.  Introduction

   The purpose of this memo is to propose an update to [RFC2045] to
   include a new primary content-type to be known as "archive". 
   [RFC2045] describes mechanisms for specifying and describing the
   format of Internet Message Bodies via content-type/subtype pairs. 
   "archive" defines a fundamental type of content with unique
   presentational, hardware, and processing aspects.  Various subtypes
   of this primary type are immediately anticipated, and will be covered
   under separate documents.

1.1.  Overview

   This document will outline what an archive is, show examples of
   archives, and discuss the benefits of grouping archives together.

   This document is a discussion document for an agreed definition,
   intended eventually to form a standard accepted extension to

1.2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Definition of an archive

   An archive primary media type identifies data that represents one or
   more files [FILE] along with metadata.  Archives are used to collect

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   multiple data files together into a single file for easier
   portability and storage. Archive formats can provide many optional
   services, including:

   1. compression
   2. encryption
   3. authentication
   4. backup
   5. filesystem imaging
   6. software packaging and distribution
   7. volume-splitting (archive split into multiple contents)
   8. block storage

   Formats and techniques that perform one or more of these services
   already exist under separate registrations. For example, the Content-
   Encoding header can be used to compress Internet message content. The
   distinguishing feature of the archive primary type is that these
   services are integrated into the format itself, along with the
   inclusion of file-specific metadata. Virtually all formats
   contemplated under this primary type are designed to concatenate
   multiple files into a single data stream, along with filenames and
   other metadata. When an Internet-facing application handles content
   labeled with this type, it SHOULD provide handling consistent with
   the archive as a discrete data item. For example, an Internet mail
   user agent would display an archive-labeled type with an archive
   icon, possibly with a preview of the files contained therein (as
   opposed to automatically traversing its contents, as it would for
   multipart-labeled content).

   Common operations include creating an archive, identifying files in
   an archive, adding to an archive, backing up to an archive,
   extracting an archive, restoring from an archive, deleting from an
   archive, mounting and unmounting an archive, [[TODO: executing an
   archive?]], and installing and uninstalling an archive.

   * Creating: taking files from a filesystem and representing those
        files in an archive.

   * Identifying files: parsing an archive's format, extracting
        information about files represented in the archive.

   * Adding: parsing an archive's format, adding files or non-file data
        to the archive. In virtually all cases, at least some part of
        the archive's content will be modified (though perhaps only at
        the end). Unlike, for instance, text media types, concatenating
        two separate archive contents *never* yields a valid composite


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   * Backing up: taking some or all of a filesystem and representing the
        filesystem in an archive, with the express intention of
        recording the files as they exist in a source filesystem at the
        time of backing up. For example, the compression, encryption,
        and access control list (permissions) properties of the files
        would be preserved.

   * Extracting: parsing an archive's format, copying file data (or file
        metadata) out of the archive into one or more files on a
        destination filesystem. This operation implies that at least
        some file metadata will be preserved, while other file metadata
        may be adjusted or added to adapt to the local environment.

   * Restoring: parsing an archive's format, copying file data out of
        the archive into the destination filesystem, with the express
        intention of recreating the files as they existed in a source
        filesystem at the time of backing up. For example, the
        compression, encryption, and access control list (permissions)
        properties of the files would be preserved.

   * Deleting: parsing an archive's format, removing file data (or
        metadata) from the archive, requiring changes to the archive's
        contents. Some archive formats permit orphan data in the archive
        content; other formats require re-serializing some or all of the

   * Mounting and unmounting: Mapping an archive's semantics directly to
        a filesystem, so that the files represented in the archive can
        be accessed using the filesystem's namespace with typical
        filesystem APIs. Rather than being backed by a physical block
        storage device, that part of the filesystem is backed by the

   * Executing [[NB: this may be controversial; it is worth
        discussing]]: Identifying executable semantics of an archive,
        and causing code to execute.

   * Installing and uninstalling [[NB: this may be controversial; it is
        worth discussing]]: Treating the archive as a software package,
        extracting certain contents in the archive and executing other
        contents in the archive, according to some software packaging


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3.  Consultation Mechanisms

   Before proposing a subtype for the archive/* primary type, it is
   suggested that the subtype author examine the definition (above) of
   what an archive/* is and the listing (below) of what an archive/* is
   not.  Additional consultations with the authors of the existing
   archive/* subtypes is also suggested.

4.  Encoding and Transport

   Unrecognized subtypes of archive SHOULD at a minimum be treated as
   "archive/file".  Like "application/octet-stream", the purpose of the
   "archive/file" is to provide default handling; it does not represent
   a particular archive format. Implementations SHOULD pass subtypes of
   archive that they do not specifically recognize to a robust
   general-purpose archive viewing application, if such an application
   is available.

   If default archive (archive/file) handling is not supported, it is
   appropriate to treat the archive like "application/octet-stream".

   Unless noted in the subtype registration, subtypes of archive SHALL
   be assumed to contain binary data, implying a content encoding of
   base64 for email and binary transfer for ftp and http.

   The formal syntax for the subtypes of the model primary type SHOULD
   look like this:

      Type name:


      Subtype name:


      Required parameters:


      Optional parameters:


      Encoding considerations:

         base64 encoding is recommended when transmitting archive/*
         documents through MIME electronic mail.

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      Security considerations:

         see Section 5 below

      Interoperability considerations:


      Published specification:


      Applications that use this media type:


      Fragment identifier considerations:

         The considerations of this document, plus any extra syntaxes
         not inconsistent with this document.

      Additional information:

         Deprecated alias names for this type:
            (Include non-archive alias names,
             such as those in application.)
         Magic number(s): TBD
         File extension(s): TBD
         Macintosh file type code(s): TBD

      See Appendix A for references to some of the expected subtypes.

      Person and email address to contact for further information:


      Intended usage: TBD (COMMON will be the most common)

      Restrictions on usage: TBD

      Author: TBD

      Change controller: TBD

      Provisional registration? (standards tree only): (Yes/No)

      (Any other information that the author deems interesting may be
      added below this line.)

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   The optional parameters consist of starting conditions and variable
   values used as part of the subtypes.

5.  Common Required and Optional Parameters

   Unlike the text primary media type (for instance), virtually all
   archive formats have been designed with almost all of the information
   required for interpretation contained within the format. Therefore,
   parameters are NOT RECOMMENDED; registrants are not expected to
   register additional parameters.

   Regrettably, not all archive formats are as "universal" or "complete"
   as one might assume at first glance. This is because some archive
   formats are very old or are based on older formats where backwards-
   compatibility was a design goal; thus they were not designed with
   transport across the Internet in mind. The ZIP file is an example:
   although the modern ZIP supports Unicode [CITE], the default encoding
   of ZIP filenames has always been Code Page 437. Since "archive"
   contents are literally archives of computing history, sometimes
   communicating the archive as-is, rather than updating the archive to
   a more universal format, is necessary.

   Implementations that are archive-type aware MUST support the
   following parameters for maximum compatibility. At the same time, new
   archives SHOULD NOT rely on these parameters for disambiguation; new
   archives SHOULD be created in such a way that "universal"
   interoperability is achieved with the archive's self-contained
   information. [[TODO: code page--it's like charset but only applies to
   certain strings in the archive, when the archive format is ambiguous;
   do NOT attempt to apply this parameter as one would apply charset to
   text/*. Endian-ness? Time/Y2K representation issues? Anything else?]]

6.  Split Archives

   Several archive formats (notably RAR and ZIP) support split archives.
   A "split archive" is an archive that is stored in multiple files
   (when stored as multiple files), or more generally, across multiple
   storage media.

   The ZIP format, for example, actually has two types of splits: "split
   archive" and "spanned archive". A "split archive" is a standard ZIP
   archive split over multiple files with the file extensions .z01,
   .z02, etc.; the .zip file is the last file. A "spanned archive" is
   the original format designed for use with swapping floppy disks. All
   archive files have the same filename; the format uses volume labels
   (presumably on floppy disks) to store disk numbers. Neither sub-
   format is merely a naive division of the octet stream: each ZIP file
   is parseable in its own right, and contains its own offset values.

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   The TAR format (or family of formats, including cpio and ustar) was
   originally designed for streaming to and from tape devices, so
   splitting is accomplished differently.

   [[TODO: Consider how to label this content. archive/zip^01?
   archive/zip; split=01? Something else? How shall 01 be associated
   with 02, 03, etc., when the Content-Disposition: ; filename=""
   parameter is "presentation-information" and may be separated from the
   Content-Type header information?]]

7.  Fragment Identifier Syntax

   Because all archives represent files, archives can serve as virtual
   filesystems. Respondents have noted that an archive's files can be
   addressed by a fragment syntax that resembles a filesystem path. At
   the same time, archives may record files in different ways (along
   with different types of metadata), suggesting that a common baseline
   with flexible extension points is more appropriate than a fixed
   universal syntax. [[TODO: This will be explored in future drafts.
   Note the similarities with this and the file: URI...]]

   [[TODO: consider how to provide a fragment for content in the
   archive. NB: most archives do NOT provide Content-Type/media type
   information! So /foo.html being an HTML file is just an *assumption*,
   and possibly a very wrong one at that. There is no IETF registry for
   file extensions.]]

8.  Piped-Composite Type Suffix Syntax

   [[TODO: discuss tar piped through bzip2, gzip, etc. as a distinct
   file format, rather than an application of the Content-Encoding:
   header. Suggest common suffix like archive/tar|bzip2, where | is some
   useful character but not + since + is for structured syntaxes.]]

9.  Security Considerations

   Archives represent files, file metadata, and filesystems; thus,
   security issues loom large because archives can contain just about
   anything. These concerns are magnified by the arbitrary transport of
   such data across the Internet. [[TODO: complete.]]

10. Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate

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              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6868]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, January 2013.

Appendix A.  Expected Subtypes

   The following archive formats will be explored for registration as
   subtypes along with this effort:

   Archiving Only


   Multipurpose (archiving, compression, encryption)

      ZIP, ACE, RAR, 7-Zip, StuffIt, FreeArc

   Software Packaging


   Disk Imaging


Authors' Addresses

   Sean Leonard
   Penango, Inc.
   5900 Wilshire Boulevard
   21st Floor
   Los Angeles, CA  90036


   Matthew Kerwin


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