path: root/doc/rfc/rfc8493.txt

Independent Submission                                          J. Kunze
Request for Comments: 8493                    California Digital Library
Category: Informational                                       J. Littman
ISSN: 2070-1721                                       Stanford Libraries
                                                               E. Madden
                                                     Library of Congress
                                                            J. Scancella

                                                                C. Adams
                                                     Library of Congress
                                                            October 2018


                 The BagIt File Packaging Format (V1.0)

Abstract

   This document describes BagIt, a set of hierarchical file layout
   conventions for storage and transfer of arbitrary digital content.  A
   "bag" has just enough structure to enclose descriptive metadata
   "tags" and a file "payload" but does not require knowledge of the
   payload's internal semantics.  This BagIt format is suitable for
   reliable storage and transfer.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8493.












Kunze, et al.                 Informational                     [Page 1]
^L
RFC 8493                          BagIt                     October 2018


Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.








































Kunze, et al.                 Informational                     [Page 2]
^L
RFC 8493                          BagIt                     October 2018


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Structure . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Required Elements . . . . . . . . . . . . . . . . . . . .   6
       2.1.1.  Bag Declaration: bagit.txt  . . . . . . . . . . . . .   6
       2.1.2.  Payload Directory: data/  . . . . . . . . . . . . . .   7
       2.1.3.  Payload Manifest: manifest-algorithm.txt  . . . . . .   7
     2.2.  Optional Elements . . . . . . . . . . . . . . . . . . . .   8
       2.2.1.  Tag Manifest: tagmanifest-algorithm.txt . . . . . . .   8
       2.2.2.  Bag Metadata: bag-info.txt  . . . . . . . . . . . . .   9
       2.2.3.  Fetch File: fetch.txt . . . . . . . . . . . . . . . .  12
       2.2.4.  Other Tag Files . . . . . . . . . . . . . . . . . . .  12
     2.3.  Text Tag File Format  . . . . . . . . . . . . . . . . . .  13
     2.4.  Bag Checksum Algorithms . . . . . . . . . . . . . . . . .  13
   3.  Complete and Valid Bags . . . . . . . . . . . . . . . . . . .  14
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Example of a Basic Bag  . . . . . . . . . . . . . . . . .  15
     4.2.  Example Bag Using fetch.txt . . . . . . . . . . . . . . .  16
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
     5.1.  Special Directory Characters  . . . . . . . . . . . . . .  16
     5.2.  Control of URLs in fetch.txt  . . . . . . . . . . . . . .  17
     5.3.  File Sizes in fetch.txt . . . . . . . . . . . . . . . . .  17
     5.4.  Attacks on Payload File Content . . . . . . . . . . . . .  17
   6.  Practical Considerations (Non-normative)  . . . . . . . . . .  17
     6.1.  Interoperability  . . . . . . . . . . . . . . . . . . . .  17
       6.1.1.  Filename Normalization  . . . . . . . . . . . . . . .  18
       6.1.2.  Windows and Unix File Naming  . . . . . . . . . . . .  18
       6.1.3.  Legacy Checksum Tools . . . . . . . . . . . . . . . .  18
   7.  Augmented Backus-Naur Form (Non-normative)  . . . . . . . . .  21
     7.1.  Bag Declaration: bagit.txt  . . . . . . . . . . . . . . .  21
     7.2.  Payload Manifest: manifest-algorithm.txt  . . . . . . . .  21
     7.3.  Bag Metadata: bag-info.txt  . . . . . . . . . . . . . . .  22
     7.4.  Fetch File: fetch.txt . . . . . . . . . . . . . . . . . .  22
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25







Kunze, et al.                 Informational                     [Page 3]
^L
RFC 8493                          BagIt                     October 2018


1.  Introduction

1.1.  Purpose

   BagIt is a set of hierarchical file layout conventions designed to
   support storage and transfer of arbitrary digital content.  A "bag"
   consists of a directory containing the payload files and other
   accompanying metadata files known as "tag" files.  The "tags" are
   metadata files intended to facilitate and document the storage and
   transfer of the bag.  Processing a bag does not require any
   understanding of the payload file contents, and the payload files can
   be accessed without processing the BagIt metadata.

   The name, BagIt, is inspired by the "enclose and deposit" method
   [ENCDEP], sometimes referred to as "bag it and tag it".  BagIt
   differs from serialized archival formats such as MIME, TAR, or ZIP in
   two general areas:

   1.  Strong integrity assurances.  The format supports cryptographic-
       quality hash algorithms (see Section 2.4) and allows for in-place
       upgrades to add additional manifests using stronger algorithms
       without breaking backwards compatibility.  This provides high
       levels of confidence against data corruption, but it is not
       designed to be secure against active attacks.

   2.  Direct file access.  Because BagIt specifies an actual filesystem
       hierarchy rather than a serialized representation of one, files
       can be accessed using standard operating system utilities,
       implementations do not need to process a potentially large
       archival file to extract a subset of data, and the format imposes
       no size limits for either individual files or a bag.

   BagIt is widely used for preserving digital assets originating from
   different domains.  Organizations involved in digital preservation
   with BagIt include the Library of Congress, Dryad Data Repository,
   NSF DataONE, and the Rockefeller Archive Center.  Software
   implementations are available for many languages, including Python,
   Ruby, Java, Perl, and PHP.  It is also used in the libraries of many
   universities, such as Cornell, Purdue, Stanford, Ghent University,
   New York University, and the University of California.

1.2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.



Kunze, et al.                 Informational                     [Page 4]
^L
RFC 8493                          BagIt                     October 2018


   Implementers are strongly encouraged to review the interoperability
   considerations described in Section 6.1.

1.3.  Terminology

   The following terms have precise definitions as used in this
   document:

   bag:  A set of opaque files contained within the structure defined by
      this document.

   bag declaration:  The file required to be in all bags conforming to
      this document.  Contains values necessary to process the rest of a
      bag.  See Section 2.1.1.

   bag checksum algorithm:  The name of a cryptographic checksum
      algorithm that has been normalized for use in a manifest or tag
      manifest file name (e.g., "sha512") as described in Section 2.4.

   manifest:  A tag file that maps filepaths to checksums.  A manifest
      can be a payload manifest (see Section 2.1.3) or a tag manifest
      (see Section 2.2.1).

   payload:  The data encapsulated by the bag as a set of named files,
      which may be organized in subdirectories.  The contents of the
      payload files are opaque to this document, and, with respect to
      BagIt processing, are always considered as sequences of
      uninterpreted octets.  See Section 2.1.2.

   tag directory:  A directory that contains one or more tag files.

   tag file:  A file that contains metadata about the bag or its
      payload.  This document defines the standard BagIt tag files: the
      bag declaration in "bagit.txt" (see Section 2.1.1), payload
      manifests (see Section 2.1.3), tag manifests (see Section 2.2.1),
      bag metadata in "bag-info.txt" (see Section 2.2.2), and remote
      payload in "fetch.txt" (see Section 2.2.3).  This document also
      allows other arbitrary tag files as described in Section 2.2.4.

   complete:  A bag that contains every element required by this
      document, every payload file listed in a manifest, and any
      optional files that are listed in a tag manifest.  See Section 3.

   valid:  A complete bag where every checksum in every manifest has
      been successfully verified against the corresponding file.






Kunze, et al.                 Informational                     [Page 5]
^L
RFC 8493                          BagIt                     October 2018


2.  Structure

   A bag MUST consist of a base directory containing the following:

   1.  a set of required and optional tag files (see Section 2.2);

   2.  a subdirectory named "data", called the payload directory (see
       Section 2.1.2); and

   3.  a set of optional tag directories.

   The tag files in the base directory consist of one or more files
   named "manifest-_algorithm_.txt" (see Sections 2.1.3 and 2.4), a file
   named "bagit.txt" (see Section 2.1.1), and zero or more additional
   tag files (see Section 2.2).  The tag files and directories are in
   arbitrary file hierarchies and MAY have any name that is not reserved
   for a file or directory in this document.

   The base directory can have any name, as illustrated by the figure
   below.

         <base directory>/
         |
         +-- bagit.txt
         |
         +-- manifest-<algorithm>.txt
         |
         +-- [additional tag files]
         |
         +-- data/
         |     |
         |     +-- [payload files]
         |
         +-- [tag directories]/
               |
               +-- [tag files]

2.1.  Required Elements

2.1.1.  Bag Declaration: bagit.txt

   The "bagit.txt" tag file MUST consist of exactly two lines in this
   order:

   BagIt-Version: M.N
   Tag-File-Character-Encoding: ENCODING





Kunze, et al.                 Informational                     [Page 6]
^L
RFC 8493                          BagIt                     October 2018


   _M.N_ identifies the BagIt major (M) and minor (N) version numbers.
   _ENCODING_ identifies the character set encoding used by the
   remaining tag files.  _ENCODING_ SHOULD be "UTF-8", but for backwards
   compatibility it MAY be any other encoding registered in
   [cs-registry].  The bag declaration itself MUST be encoded in UTF-8
   and MUST NOT contain a Byte Order Mark (BOM) [RFC3629].

   The number for this version of BagIt is "1.0".

2.1.2.  Payload Directory: data/

   The base directory MUST contain a subdirectory named "data".

   The payload directory contains the arbitrary digital content within
   the bag.  The files under the payload directory are called payload
   files, or the payload.  Each payload file is treated as an opaque
   octet stream when verifying file correctness.  Payload files MAY be
   organized in arbitrary subdirectory structures within the payload
   directory; however, for the purpose of this document, such
   subdirectory structures and filenames have no given meaning.

2.1.3.  Payload Manifest: manifest-algorithm.txt

   A payload manifest file provides a complete listing of each payload
   file name along with a corresponding checksum to permit data
   integrity checking.  A bag can have more than one payload manifest,
   with each using a different checksum algorithm.  Manifest entries
   MUST satisfy the following constraints:

   o  Every bag MUST contain at least one payload manifest file and MAY
      contain more than one.

   o  Every payload manifest MUST list every payload file name exactly
      once.

   o  A payload manifest file MUST have a name of the form "manifest-
      _algorithm_.txt", where _algorithm_ is a string specifying the
      checksum algorithm used by that manifest as described in
      Section 2.4.

   Example payload manifest filenames:

   manifest-sha256.txt
   manifest-sha512.txt







Kunze, et al.                 Informational                     [Page 7]
^L
RFC 8493                          BagIt                     October 2018


   Each line of a payload manifest file MUST be of the form

   checksum filepath

   where _filepath_ is the pathname of a file relative to the base
   directory, and _checksum_ is a hex-encoded checksum calculated by
   applying _algorithm_ over the file.

   o  The hex-encoded checksum MAY use uppercase and/or lowercase
      letters.

   o  The slash character ('/') MUST be used as a path separator in
      _filepath_.

   o  One or more linear whitespace characters (spaces or tabs) MUST
      separate _checksum_ from _filepath_.

   o  There is no limitation on the length of a pathname.

   o  The payload manifest MUST NOT reference files outside the payload
      directory.

   o  If a _filepath_ includes a Line Feed (LF), a Carriage Return (CR),
      a Carriage-Return Line Feed (CRLF), or a percent sign (%), those
      characters (and only those) MUST be percent-encoded following
      [RFC3986].

   A manifest MUST NOT reference directories.  Bag creators who wish to
   create an otherwise empty directory have typically done so by
   creating an empty placeholder file with a name such as ".keep".

2.2.  Optional Elements

2.2.1.  Tag Manifest: tagmanifest-algorithm.txt

   A tag manifest is a tag file that lists other tag files and checksums
   for those tag files generated using a particular bag checksum
   algorithm.

   A bag MAY contain one or more tag manifests, in which case each tag
   manifest SHOULD list the same set of tag files.

   Each tag manifest MUST list every payload manifest.  Each tag
   manifest MUST NOT list any tag manifests but SHOULD list the
   remaining tag files present in the bag.






Kunze, et al.                 Informational                     [Page 8]
^L
RFC 8493                          BagIt                     October 2018


   A tag manifest file MUST have a name of the form "tagmanifest-
   _algorithm_.txt", where _algorithm_ is a string following the format
   described in Section 2.4 that specifies the bag checksum algorithm
   used in that manifest.

   Tag manifests SHOULD use the same algorithms as the payload manifests
   that are present in the bag.

   Example tag manifest filenames:

   tagmanifest-sha256.txt
   tagmanifest-sha512.txt

   A tag manifest file has the same form as the payload manifest file
   described in Section 2.1.3 but MUST NOT list any payload files.  As a
   result, no _filepath_ listed in a tag manifest begins "data/".

2.2.2.  Bag Metadata: bag-info.txt

   The "bag-info.txt" file is a tag file that contains metadata elements
   describing the bag and the payload.  The metadata elements contained
   in the "bag-info.txt" file are intended primarily for human use.  All
   metadata elements are OPTIONAL and MAY be repeated.  Because "bag-
   info.txt" is intended for human reading and editing, ordering MAY be
   significant and the ordering of metadata elements MUST be preserved.

   A metadata element MUST consist of a label, a colon ":", a single
   linear whitespace character (space or tab), and a value that is
   terminated with an LF, a CR, or a CRLF.

   The label MUST NOT contain a colon (:), LF, or CR.  The label MAY
   contain linear whitespace characters but MUST NOT start or end with
   whitespace.

   It is RECOMMENDED that lines not exceed 79 characters in length.
   Long values MAY be continued onto the next line by inserting a LF,
   CR, or CRLF, and then indenting the next line with one or more linear
   white space characters (spaces or tabs).  Except for linebreaks, such
   padding does not form part of the value.

   Implementations wishing to support previous BagIt versions MUST
   accept multiple linear whitespace characters before and after the
   colon when the bag version is earlier than 1.0; such whitespace does
   not form part of the label or value.







Kunze, et al.                 Informational                     [Page 9]
^L
RFC 8493                          BagIt                     October 2018


   The following are reserved metadata elements.  The use of these
   reserved metadata elements is OPTIONAL but encouraged.  Reserved
   metadata element names are case insensitive.  Except where indicated
   otherwise, these metadata element names MAY be repeated to capture
   multiple values.

   Source-Organization:  Organization transferring the content.

   Organization-Address:  Mailing address of the source organization.

   Contact-Name:  Person at the source organization who is responsible
      for the content transfer.

   Contact-Phone:  International format telephone number of person or
      position responsible.

   Contact-Email:  Fully qualified email address of person or position
      responsible.

   External-Description:  A brief explanation of the contents and
      provenance.

   Bagging-Date:  Date (YYYY-MM-DD) that the content was prepared for
      transfer.  This metadata element SHOULD NOT be repeated.

   External-Identifier:  A sender-supplied identifier for the bag.

   Bag-Size:  The size or approximate size of the bag being transferred,
      followed by an abbreviation such as MB (megabytes), GB
      (gigabytes), or TB (terabytes): for example, 42600 MB, 42.6 GB, or
      .043 TB.  Compared to Payload-Oxum (described next), Bag-Size is
      intended for human consumption.  This metadata element SHOULD NOT
      be repeated.

   Payload-Oxum:  The "octetstream sum" of the payload, which is
      intended for the purpose of quickly detecting incomplete bags
      before performing checksum validation.  This is strictly an
      optimization, and implementations MUST perform the standard
      checksum validation process before proclaiming a bag to be valid.
      This element MUST NOT be present more than once and, if present,
      MUST be in the form "_OctetCount_._StreamCount_", where
      _OctetCount_ is the total number of octets (8-bit bytes) across
      all payload file content and _StreamCount_ is the total number of
      payload files.  This metadata element MUST NOT be repeated.







Kunze, et al.                 Informational                    [Page 10]
^L
RFC 8493                          BagIt                     October 2018


   Bag-Group-Identifier:  A sender-supplied identifier for the set, if
      any, of bags to which it logically belongs.  This identifier
      SHOULD be unique across the sender's content, and if it is
      recognizable as belonging to a globally unique scheme, the
      receiver SHOULD make an effort to honor the reference to it.  This
      metadata element SHOULD NOT be repeated.

   Bag-Count:  Two numbers separated by "of", in particular, "N of T",
      where T is the total number of bags in a group of bags and N is
      the ordinal number within the group.  If T is not known, specify
      it as "?" (question mark): for example, 1 of 2, 4 of 4, 3 of ?, 89
      of 145.  This metadata element SHOULD NOT be repeated.  If this
      metadata element is present, it is RECOMMENDED to also include the
      Bag-Group-Identifier element.

   Internal-Sender-Identifier:  An alternate sender-specific identifier
      for the content and/or bag.

   Internal-Sender-Description:  A sender-local explanation of the
      contents and provenance.

   In addition to these metadata elements, other arbitrary metadata
   elements MAY also be present.

   An example of "bag-info.txt" file is as follows:

   Source-Organization: FOO University
   Organization-Address: 1 Main St., Cupertino, California, 11111
   Contact-Name: Jane Doe
   Contact-Phone: +1 111-111-1111
   Contact-Email: example@example.com
   External-Description: Uncompressed greyscale TIFF images from the
         FOO papers colle...
   Bagging-Date: 2008-01-15
   External-Identifier: university_foo_001
   Payload-Oxum: 279164409832.1198
   Bag-Group-Identifier: university_foo
   Bag-Count: 1 of 15
   Internal-Sender-Identifier: /storage/images/foo
   Internal-Sender-Description: Uncompressed greyscale TIFFs created
         from microfilm and are...










Kunze, et al.                 Informational                    [Page 11]
^L
RFC 8493                          BagIt                     October 2018


2.2.3.  Fetch File: fetch.txt

   For reasons of efficiency, a bag MAY be sent with a list of files to
   be fetched and added to the payload before it can meaningfully be
   checked for completeness.  The fetch file allows a bag to be
   transmitted with "holes" in it, which can be practical for several
   reasons.  For example, it obviates the need for the sender to stage a
   large serialized copy of the content while the bag is transferred to
   the receiver.  Also, this method allows a sender to construct a bag
   from components that are either a subset of logically related
   components (e.g., the localized logical object could be much larger
   than what is intended for export) or assembled from logically
   distributed sources (e.g., the object components for export are not
   stored locally under one filesystem tree).  An OPTIONAL tag file,
   called the fetch file, contains such a list.

   The fetch file MUST be named "fetch.txt".  Every file listed in the
   fetch file MUST be listed in every payload manifest.  A fetch file
   MUST NOT list any tag files.

   Each line of a fetch file MUST be of the form

   url length filepath

   where _url_ identifies the file to be fetched and MUST be an absolute
   URI as defined in [RFC3986], _length_ is the number of octets in the
   file (or "-", to leave it unspecified), and _filepath_ identifies the
   corresponding payload file, relative to the base directory.

   The slash character ('/') MUST be used as a path separator in
   _filepath_. One or more linear whitespace characters (spaces or tabs)
   MUST separate these three values, and any such characters in the
   _url_ MUST be percent-encoded [RFC3986].  If _filename_ includes an
   LF, a CR, a CRLF, or a percent sign (%), those characters (and only
   those) MUST be percent-encoded as described in [RFC3986].  There is
   no limitation on the length of any of the fields in the fetch file.

2.2.4.  Other Tag Files

   A bag MAY contain other tag files that are not defined by this
   document.  Implementations MUST perform standard checksum validation
   on any tag file that is listed in a tag manifest but MUST otherwise
   ignore their contents.








Kunze, et al.                 Informational                    [Page 12]
^L
RFC 8493                          BagIt                     October 2018


2.3.  Text Tag File Format

   All tag files specifically described in this document MUST adhere to
   the text tag file format described below.  Other tag files MAY adhere
   to the text tag file format described below.

   Text tag files are line oriented, and each line MUST be terminated by
   an LF, a CR, or a CRLF.  It is RECOMMENDED that the last line in a
   tag file also end with LF, CR, or CRLF.  Text tag file names MUST end
   in the extension ".txt".

   In all text tag files except for the bag declaration file, text MUST
   use the character encoding specified in the "bagit.txt" bag
   declaration file.  Text tag files except for the bag declaration file
   MAY include a Byte Order Mark (BOM) only if the specified encoding
   requires it for proper decoding.  In accordance with [RFC3629], when
   "bagit.txt" specifies UTF-8, the tag files MUST NOT begin with a BOM.
   See Section 2.1.1.

   The use of UTF-8 for text tag files is strongly RECOMMENDED.  A
   future version of BagIt may disallow encodings other than UTF-8.

2.4.  Bag Checksum Algorithms

   The payload manifest and tag manifest permit validating the integrity
   of the payload and tag files in a bag produced by the checksum
   algorithms.  Checksum values MUST be encoded so as to conform to the
   manifest format specified in Section 2.1.3.  However, the internal
   details of a checksum are outside the scope of this document.

   To avoid future ambiguity, the checksum algorithm SHOULD be
   registered in IANA's "Named Information Hash Algorithm Registry"
   [ni-registry] according to [RFC6920] but MAY, for backwards
   compatibility, also be MD5 [RFC1321] or SHA-1 [RFC3174].

   The name of the checksum algorithm MUST be normalized for use in the
   manifest's filename by lowercasing the common name of the algorithm
   and removing all non-alphanumeric characters.  Following is a partial
   list that maps common algorithm names to normalized names:

   o  MD5: md5

   o  SHA-1: sha1

   o  sha-256: sha256

   o  sha-512: sha512




Kunze, et al.                 Informational                    [Page 13]
^L
RFC 8493                          BagIt                     October 2018


   Starting with BagIt 1.0, bag creation and validation tools MUST
   support the SHA-256 and SHA-512 algorithms [RFC6234] and SHOULD
   enable SHA-512 by default when creating new bags.  For backwards
   compatibility, implementers SHOULD support MD5 [RFC1321] and SHA-1
   [RFC3174].  Implementers are encouraged to simplify the process of
   adding additional manifests using new algorithms to streamline the
   process of in-place upgrades.

3.  Complete and Valid Bags

   A _complete_ bag MUST meet the following requirements:

   1.  Every required element MUST be present (see Section 2.1).

   2.  Every file listed in every tag manifest MUST be present.

   3.  Every file listed in every payload manifest MUST be present.

   4.  For BagIt 1.0, every payload file MUST be listed in every payload
       manifest.  Note that older versions of BagIt allowed payload
       files to be listed in just one of the manifests.

   5.  Every element present MUST conform to BagIt 1.0.

   A _valid_ bag MUST meet the following requirements:

   1.  The bag MUST be _complete_.

   2.  Every checksum in every payload manifest and tag manifest has
       been successfully verified against the contents of the
       corresponding file.




















Kunze, et al.                 Informational                    [Page 14]
^L
RFC 8493                          BagIt                     October 2018


4.  Examples

4.1.  Example of a Basic Bag

   This is the layout of a basic bag containing an image and a companion
   Optical Character Recognition (OCR) file.  Lines of file content are
   shown with added parentheses to indicate each complete line.  For
   brevity, this example uses MD5 rather than the recommended SHA-512.

   myfirstbag/
   |
   |   manifest-md5.txt
   |    (49afbd86a1ca9f34b677a3f09655eae9 data/27613-h/images/q172.png)
   |    (408ad21d50cef31da4df6d9ed81b01a7 data/27613-h/images/q172.txt)
   |
   |   bagit.txt
   |    (BagIt-version: 1.0                                           )
   |    (Tag-File-Character-Encoding: UTF-8                           )
   |
   \--- data/
        |
        |   27613-h/images/q172.png
        |    (... image bytes ...                                     )
        |
        |   27613-h/images/q172.txt
        |    (... OCR text ...                                        )
        ....
























Kunze, et al.                 Informational                    [Page 15]
^L
RFC 8493                          BagIt                     October 2018


4.2.  Example Bag Using fetch.txt

   This is the layout of a bag that expects the receiver to download the
   files listed in the payload manifests prior to validation.  Lines of
   file content are shown with added parentheses to indicate each
   complete line.  For brevity, this example uses MD5 rather than the
   recommended SHA-512.

   highsmith-tahoe/
   |
   |   manifest-md5.txt
   |    (102b0e6effe208ef9b29864946de9e22 data/23364a.tif             )
   |
   |    fetch.txt
   |     (https://cdn.loc.gov/master/pnp/highsm/23300/23364a.tif
   |         216951362 data/23364a.tif                                )
   |
   |   bagit.txt
   |    (BagIt-version: 1.0                                           )
   |    (Tag-File-Character-Encoding: UTF-8                           )
   |
   |   bag-info.txt
   |    (Internal-Sender-Description: Download link found at          )
   |    (  https://www.loc.gov/resource/highsm.23364/                 )

5.  Security Considerations

5.1.  Special Directory Characters

   The paths specified in the payload manifests, tag manifests, and
   fetch files do not prohibit special directory characters that have
   special meaning on some operating systems.  Implementers MUST ensure
   that files outside the bag directory structure are not accessed when
   reading or writing files based on paths specified in a bag.

   All implementations SHOULD have a test suite to guard against special
   directory characters.

   For example, a maliciously crafted "tagmanifest-sha512.txt" file
   might contain entries that begin with a path character such as "/",
   "..", or a "~username" home directory reference in an attempt to
   cause a naive implementation to leak or overwrite targeted files on a
   POSIX operating system.

   Windows implementations SHOULD test their implementations to ensure
   that safety checks prevent use of drive letters and the less commonly
   used namespace sequences (e.g., "\\?\C:\...") described in [MSFNAM].




Kunze, et al.                 Informational                    [Page 16]
^L
RFC 8493                          BagIt                     October 2018


   To assist implementers, the Library of Congress conformance suite
   [LC-CONFORMANCE-SUITE] has some tests for invalid bags that are
   expected to fail on POSIX or Windows clients.

5.2.  Control of URLs in fetch.txt

   Implementers of tools that complete bags by retrieving URLs listed in
   a fetch file need to be aware that some of those URLs might point to
   hosts, intentionally or unintentionally, that are not under control
   of the bag's sender.  Moreover, older checksum algorithms, even if
   reasonable for detecting corruption during transit, may not offer
   strong cryptographic protection against intentional spoofing.

5.3.  File Sizes in fetch.txt

   The size of files, as optionally reported in the fetch file, cannot
   be guaranteed to match the actual file size to be downloaded.
   Implementers SHOULD take steps to monitor and abort transfer when the
   received file size exceeds the file size reported in the fetch file.
   Implementers SHOULD NOT use the file size in the fetch file for
   critical resource allocation, such as buffer sizing or storage
   requisitioning.

5.4.  Attacks on Payload File Content

   The integrity assurance provided by manifests is designed to provide
   high levels of confidence against data corruption but is not designed
   to be secure against active attacks.  Organizations that need to
   secure bags against such threats SHOULD agree on additional measures,
   such as digital signatures, that are out of scope for this
   specification.

6.  Practical Considerations (Non-normative)

6.1.  Interoperability

   This section lists practical considerations for implementers and
   users.  None of the points below are required, but they are
   recommended for general-purpose usage.

   Upon discovering errors in bags, an implementation is free to take
   action (for example, logging or reporting) in an application-specific
   manner.  This document does not mandate any particular action.

   The Library of Congress conformance suite [LC-CONFORMANCE-SUITE] is
   provided as a public resource to test new implementations for
   compatibility and error handling.




Kunze, et al.                 Informational                    [Page 17]
^L
RFC 8493                          BagIt                     October 2018


6.1.1.  Filename Normalization

   This section provides background information on various challenges
   caused by differences in how operating systems, filesystems, and
   common tools handle filenames.  This section is followed by a list of
   recommendations for implementers in Section 6.1.1.3.

6.1.1.1.  Case Sensitivity

   There are three challenges for interoperability related to filename
   case:

   o  Filesystems such as File Allocation Table (FAT) or Extended File
      Allocation Table (EXFAT) always convert filenames to uppercase:
      "example.txt" will be stored as "EXAMPLE.TXT".

   o  Many Unix filesystems save filenames exactly as provided, which
      allows multiple files that differ only in case: "example.txt" and
      "Example.txt" are separate files.

   o  New Technology File System (NTFS) and Apple's Hierarchical File
      System (HFS) Plus usually preserve case when storing files but are
      case insensitive when retrieving them.  A file saved as
      "Example.txt" will be retrieved by that name but will also be
      retrieved as "EXAMPLE.TXT", "example.txt", etc.

6.1.1.2.  Unicode Normalization

   The Unicode specification has common cases where different character
   sequences produce the same human-meaningful text.  These are referred
   to as "canonically equivalent" and the Unicode specification defines
   different normalization forms - see [UNICODE-TR15] for the full
   details.

   The example below shows the common surname "Nunez" normalized in
   different forms.

   Normalization Form D (Decomposition)

   Char      UTF8 Hex  Name
   ----------------------------------------------
   N               4e  LATIN CAPITAL LETTER N
   u               75  LATIN SMALL LETTER U
   \u0301        cc81  COMBINING ACUTE ACCENT
   n               6e  LATIN SMALL LETTER N
   \u0303        cc83  COMBINING TILDE
   e               65  LATIN SMALL LETTER E
   z               7a  LATIN SMALL LETTER Z



Kunze, et al.                 Informational                    [Page 18]
^L
RFC 8493                          BagIt                     October 2018


   Normalization Form C (Canonical Composition)

   Char      UTF8 Hex  Name
   ----------------------------------------------
   N               4e  LATIN CAPITAL LETTER N
   u             c3ba  LATIN SMALL LETTER U WITH ACUTE
   n             c3b1  LATIN SMALL LETTER N WITH TILDE
   e               65  LATIN SMALL LETTER E
   z               7a  LATIN SMALL LETTER Z

   Unicode normalization is relevant to BagIt implementors because
   different systems have different standards for normalization:

   o  Apple's HFS Plus filesystem always normalizes filenames to a fully
      decomposed form based on the Unicode 2.0 specification (see
      [TN1150]).

   o  Windows treats filenames as opaque character sequences (see
      [MSFNAM]) and will store and return the encoded bytes exactly as
      provided.

   o  Linux and other common Unix systems are generally similar to
      Windows in storing and returning opaque byte streams, but this
      behavior is technically dependent on the filesystem.

   o  Utilities used for file management, transfer, and archiving may
      ignore this issue, apply an arbitrary normalization form, or allow
      the user to control how normalization is applied.

   In practice, this means that the encoded filename stored in a
   manifest may fail a simple file existence check because the
   filename's normalization was changed at some point after the manifest
   was written.  This situation is very confusing for users because the
   filenames are visually indistinguishable, and the "missing" file is
   obviously present in the payload directory.

6.1.1.3.  Recommendations

   o  Implementations SHOULD discourage the creation of bags containing
      files that differ only in case.

   o  Implementations SHOULD prevent the creation of bags containing
      files that differ only in normalization form.

   o  BagIt implementations SHOULD tolerate differences in normalization
      form by comparing both the list of filesystem and manifest names
      after applying the same normalization form to both.




Kunze, et al.                 Informational                    [Page 19]
^L
RFC 8493                          BagIt                     October 2018


   o  Implementations SHOULD issue a warning when multiple manifests are
      present that differ only in case or normalization form.

6.1.2.  Windows and Unix File Naming

   As specified above, only the Unix-based path separator ('/') may be
   used inside filenames listed in BagIt manifest and fetch.txt files.
   When bags are exchanged between Windows and Unix platforms, the path
   separator SHOULD be translated as needed.  Receivers of bags on
   physical media SHOULD be prepared for filesystems created under
   either Windows or Unix.  Besides the fundamental difference between
   path separators ('\' and '/'), generally, Windows filesystems have
   more limitations than Unix filesystems.

   Windows path names have a maximum of 255 characters, and none of
   these characters may be used in a path component:

       < > : " / | ? *

   Windows also reserves the following names, with or without a file
   extension:

       CON, PRN, AUX, NUL
       COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9
       LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, LPT9

   See [MSFNAM] for more information and possible alternatives.

6.1.3.  Legacy Checksum Tools

   Some bags have been manually assembled using checksum utilities such
   as those contained in the GNU Coreutils package (md5sum, sha1sum,
   etc.), collectively referred to here as "md5sum".  Implementers who
   desire wide support of legacy content should be aware of some known
   quirks of these tools.

   md5sum can be run in "text mode", which causes it to normalize line
   endings on some operating systems.  On Unix-like systems, both modes
   will usually produce the same results; on systems like Windows, they
   can produce different results based on the file contents.  The md5sum
   output format has two characters between the checksum and the
   filepath: the first is always a space, and the second is an asterisk
   ("*") for binary mode and a space for text mode.

   A final note about md5sum-generated manifests is that, for a
   _filepath_ containing a backslash ('\'), the manifest line will have
   a backslash inserted in front of the _checksum_ and, under Windows,
   the backslashes inside _filepath_ can be doubled.



Kunze, et al.                 Informational                    [Page 20]
^L
RFC 8493                          BagIt                     October 2018


   Implementers MAY wish to accept this format by ignoring a leading
   asterisk or handling differences in line termination gracefully but,
   if so, implementations MUST warn the user that the bag in question
   will fail strict validation.  In such cases, it is RECOMMENDED that
   tools provide an easy option to update the bag with valid manifests.

7.  Augmented Backus-Naur Form (Non-normative)

   The Augmented Backus-Naur Form (ABNF) rules provided below are non-
   normative.  If there is a discrepancy between requirements in the
   normative sections and the ABNF, the requirements in the normative
   sections prevail.  Some definitions use the core rules (e.g., DIGIT,
   HEXDIG, etc) as defined in [RFC5234].

7.1.  Bag Declaration: bagit.txt

   bagit.txt ABNF rules:

   bagit-txt = "BagIt-Version: " 1*DIGIT "." 1*DIGIT ending
               "Tag-File-Character-Encoding: " encoding ending
   encoding  = 1*CHAR
   ending    = CR / LF / CRLF

7.2.  Payload Manifest: manifest-algorithm.txt

   Payload Manifest ABNF rules:

   payload-manifest      = 1*payload-manifest-line
   payload-manifest-line = checksum 1*WSP filepath ending
   checksum              = 1*case-hexdig
   case-hexdig           = DIGIT / "A" / "a" / "B" / "b" / "C" / "c" /
                           "D" / "d" / "E"/ "e"/ "F" / "f"
   filepath              = "data/"
                           1*( unreserved / pct-encoded / sub-delims )
   unreserved            = ALPHA / DIGIT / "-" / "." / "_" / "~"
   sub-delims            = "!" / "$" / "&" / DQUOTE / "'" / "(" / ")" /
                           "*" / "+" / "," / ";" / "=" / "/"
   pct-encoded           = "%0D" / "%0d" / "%0A" / "%0a" / "%25"
   ending                = CR / LF / CRLF












Kunze, et al.                 Informational                    [Page 21]
^L
RFC 8493                          BagIt                     October 2018


7.3.  Bag Metadata: bag-info.txt

   bag-info.txt ABNF rules:

   metadata      = 1*metadata-line
   metadata-line = key ":" WSP value ending *(continuation ending)
   key           = 1*non-reserved
   value         = 1*non-reserved
   continuation  = WSP 1*non-reserved
   non-reserved  = VCHAR / WSP
                   ; any valid character for the specific encoding
                   ; except those that match "ending"
   ending        = CR / LF / CRLF

7.4.  Fetch File: fetch.txt

   fetch.txt ABNF rules:

   fetch      = 1*fetch-line
   fetch-line = url 1*WSP length 1*WSP filepath ending
   url        = <absolute-URI, see [RFC3986], Section 4.3>
   length     = 1*DIGIT / "-"
   filepath   = ("data/"
                 1*( unreserved / pct-encoded / sub-delims ))
   ending     = CR / LF / CRLF

8.  IANA Considerations

   This document has no IANA actions.

9.  References

9.1.  Normative References

   [cs-registry]
              IANA, "Character Set",
              <https://www.iana.org/assignments/character-sets>.

   [ni-registry]
              IANA, "Named Information Hash Algorithm",
              <https://www.iana.org/assignments/named-information>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.






Kunze, et al.                 Informational                    [Page 22]
^L
RFC 8493                          BagIt                     October 2018


   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
              <https://www.rfc-editor.org/info/rfc3174>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC6920]  Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
              Keranen, A., and P. Hallam-Baker, "Naming Things with
              Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
              <https://www.rfc-editor.org/info/rfc6920>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [ENCDEP]   Tabata, K., Okada, T., Nagamori, M., Sakaguchi, T., and S.
              Sugimoto, "A Collaboration Model between Archival Systems
              to Enhance the Reliability of Preservation by an Enclose-
              and-Deposit Method", 2005,
              <https://web.archive.org/web/20060508015635/
              http://www.iwaw.net/05/papers/iwaw05-tabata.pdf>.

   [LC-CONFORMANCE-SUITE]
              The Library of Congress, "Test cases for validating Bagit
              Implementations", commit 43bcbdf, November 2017,
              <https://github.com/LibraryOfCongress/
              bagit-conformance-suite/>.





Kunze, et al.                 Informational                    [Page 23]
^L
RFC 8493                          BagIt                     October 2018


   [MSFNAM]   Microsoft, Inc., "Naming Files, Paths, and Namespaces",
              May 2018,
              <http://msdn2.microsoft.com/en-us/library/aa365247.aspx>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [TN1150]   Apple Inc., "Technical Note TN1150: HFS Plus Volume
              Format", March 2004, <https://developer.apple.com/legacy/
              library/technotes/tn/tn1150.html>.

   [UNICODE-TR15]
              Unicode Consortium, "Unicode Standard Annex #15: Unicode
              Normalization Forms", Technical Report, Unicode 11.0.0,
              May 2018, <http://www.unicode.org/reports/tr15/>.

Acknowledgements

   BagIt benefitted from the thoughtful assistance of Stephen Abrams,
   Mike Ashenfelder, Dan Chudnov, Dave Crocker, Scott Fisher, Brad
   Hards, Erik Hetzner, Keith Johnson, Leslie Johnston, David Loy, Mark
   Phillips, Tracy Seneca, Stian Soiland-Reyes, Brian Tingle, Adam
   Turoff, and Jim Tuttle.

Contributors

   Additional contributors to the authoring of BagIt are Andy Boyko,
   David Brunton, Rosie Storey, Ed Summers, Brian Vargas, and Kate
   Zwaard.




















Kunze, et al.                 Informational                    [Page 24]
^L
RFC 8493                          BagIt                     October 2018


Authors' Addresses

   John A. Kunze
   California Digital Library
   415 20th St, 4th Floor
   Oakland, CA  94612
   United States of America

   Email: jak@ucop.edu


   Justin Littman
   Stanford Libraries
   518 Memorial Way
   Stanford, CA  94305
   United States of America

   Email: justinlittman@stanford.edu


   Liz Madden
   Library of Congress
   101 Independence Avenue SE
   Washington, DC  20540
   United States of America

   Email: emad@loc.gov


   John Scancella

   Email: john.scancella@gmail.com


   Chris Adams
   Library of Congress
   101 Independence Avenue SE
   Washington, DC  20540
   United States of America

   Email: cadams@loc.gov










Kunze, et al.                 Informational                    [Page 25]
^L