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Network Working Group                                           J. Kunze
Internet-Draft                                California Digital Library
Intended status: Informational                                J. Littman
Expires: December 6, 2018         George Washington University Libraries
                                                               E. Madden
                                                            J. Scancella
                                                                C. Adams
                                                     Library of Congress
                                                            June 4, 2018

                 The BagIt File Packaging Format (V1.0)


   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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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 inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 6, 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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Structure . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Required Elements . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Bag Declaration: bagit.txt  . . . . . . . . . . . . .   5
       2.1.2.  Payload Directory: data/  . . . . . . . . . . . . . .   6
       2.1.3.  Payload Manifest: manifest-algorithm.txt  . . . . . .   6
     2.2.  Optional Elements . . . . . . . . . . . . . . . . . . . .   7
       2.2.1.  Tag Manifest: tagmanifest-algorithm.txt . . . . . . .   7
       2.2.2.  Bag Metadata: bag-info.txt  . . . . . . . . . . . . .   8
       2.2.3.  Fetch File: fetch.txt . . . . . . . . . . . . . . . .  10
       2.2.4.  Other Tag Files . . . . . . . . . . . . . . . . . . .  11
     2.3.  Text Tag File Format  . . . . . . . . . . . . . . . . . .  11
     2.4.  Bag Checksum Algorithms . . . . . . . . . . . . . . . . .  12
   3.  Complete and Valid bags . . . . . . . . . . . . . . . . . . .  12
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Example of a basic bag  . . . . . . . . . . . . . . . . .  13
     4.2.  Example bag using fetch.txt . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     5.1.  Special directory characters  . . . . . . . . . . . . . .  14
     5.2.  Control of URLs in fetch.txt  . . . . . . . . . . . . . .  15
     5.3.  File sizes in fetch.txt . . . . . . . . . . . . . . . . .  15
   6.  Practical Considerations (non-normative)  . . . . . . . . . .  15
     6.1.  Interoperability  . . . . . . . . . . . . . . . . . . . .  15
       6.1.1.  Filename normalization  . . . . . . . . . . . . . . .  15
       6.1.2.  Windows and Unix file naming  . . . . . . . . . . . .  17
       6.1.3.  Legacy checksum tools . . . . . . . . . . . . . . . .  18
   7.  Augmented Backus-Naur Form (non-normative)  . . . . . . . . .  19
     7.1.  Bag Declaration: bagit.txt  . . . . . . . . . . . . . . .  19
     7.2.  Payload Manifest: manifest-algorithm.txt  . . . . . . . .  19
     7.3.  Bag Metadata: bag-info.txt  . . . . . . . . . . . . . . .  19
     7.4.  Fetch File: fetch.txt . . . . . . . . . . . . . . . . . .  20
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  20

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     11.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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 archive 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.

   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
       archive 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",
   "OPTIONAL" in this document are to be interpreted as described in BCP

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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals as shown here.

   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

   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 which 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 thats maps filepaths to checksums.  A manifest
      can be a payload manifest Section 2.1.3 or a tag manifest
      Section 2.2.1.

   payload  The data encapsulated by the bag as a set of named files,
      which may be organized in sub-directories.  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 which contains metadata about the bag or its
      payload.  This document defines the standard BagIt tag files: the
      bag declaration in "bagit.txt" Section 2.1.1, payload manifests
      Section 2.1.3, tag manifests Section 2.2.1, bag metadata in "bag-
      info.txt" Section 2.2.2, and remote payload in "fetch.txt"
      Section 2.2.3.  This document also allows other arbitrary tag
      files as described in Section 2.2.4.

   complete  A bag which contains every element required by this
      document, every payload file listed in a manifest, and any
      optional files which 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.

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2.  Structure

   A bag MUST consist of a base directory containing:

   1.  a set of required and optional tag files Section 2.2

   2.  a sub-directory named "data", called the payload directory.
       Section 2.1.2

   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 Section 2.1.3 and Section 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.

         <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

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

   _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 [RFC2978].
   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 sub-directory 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 sub-directory structures within the payload
   directory, however for the purpose of this document such sub-
   directory 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

   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


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   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
   according to _algorithm_ over every octet in the file.

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

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

   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

   o  If a _filepath_ includes a line feed (LF), a carriage return (CR),
      carriage return plus line feed (CRLF) or percent sign (%), those
      characters (and only those) MUST be percent-encoded following

   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

   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.

   A tag manifest file MUST have a name of the form "tagmanifest-
   _algorithm_.txt", where _algorithm_ is a string following the format

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   described in Section 2.4 specifying 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:


   A tag manifest file has the same form as the payload file 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, terminated
   with a line feed (CR), carriage return (LF) or carriage return plus
   line feed (CRLF).

   The label MUST NOT contain colon (:), line feeds (LF) or carriage
   returns (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 line
   feed (LF), a carriage return (CR), or carriage return plus line feed
   (CRLF) and indenting the next line with one or more linear white
   space (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 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.

   The following are reserved metadata elements.  The use of these
   reserved metadata elements are OPTIONAL but encouraged.  Reserved
   metadata element names are case-insensitive.  Except where indicated

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   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

   External-Description  A brief explanation of the contents and

   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  Size or approximate size of the bag being transferred,
      followed by an abbreviation such as MB (megabytes), GB, or TB; 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, 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.

   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 recognizable
      as belonging to a globally unique scheme, the receiver SHOULD make
      an effort to honor reference to it.  This metadata element SHOULD
      NOT be repeated.

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   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).  Examples: 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 "bag-info.txt" file

   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...

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

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   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 a
   line feed (LF), a carriage return (CR), carriage return plus line
   feed (CRLF) or percent sign (%), those characters (and only those)
   MUST be percent-encoded following [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 which is listed in a tag manifest but MUST otherwise
   ignore their contents.

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
   a line feed (LF), a carriage return (CR), or carriage return plus
   newline (CRLF).  It is RECOMMENDED that the last line in a tag file
   also ends 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

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   requires it for proper decoding.  In accordance with [RFC3629], when
   "bagit.txt" specifies UTF-8 the tag files MUST NOT begin with a byte-
   order mark (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 manifests 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

   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 mapping common algorithm names to normalized names:

   o  MD5: md5

   o  SHA-1: sha1

   o  sha-256: sha256

   o  sha-512: sha512

   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 (Section 2.1).

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   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.

4.  Examples

4.1.  Example of a basic bag

   This is the layout of a basic bag containing an image and a companion
   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.

   |   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 ...                                        )

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4.2.  Example bag using fetch.txt

   This is the layout of a bag which 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.

   |   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 which 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 which 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].

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   To assist implementers, the Library of Congress conformance suite
   [LC-CONFORMANCE-SUITE] has some tests for invalid bags which 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

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.

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 followed by a list of recommendations
   for implementers in Section

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   There are two challenges for interoperability related to filename

   o  Filesystems such as FAT or EXFAT always convert filenames to
      uppercase: "example.txt" will be stored as "EXAMPLE.TXT"

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

   o  NTFS and Apple's 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.  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 and a brief example below:

   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

   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

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   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

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

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

   o  Utilities used for file management, transfer, and archival 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.  Recommendations

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

   o  Implementations SHOULD prevent the creation of bags containing
      files which 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.

   o  Implementations SHOULD issue a warning when multiple manifests are
      present which 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

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   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

       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 but 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.

   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.

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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 [RFC4234]

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

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

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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.  Contributors

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

9.  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.

10.  IANA Considerations

   This draft does not request any action from IANA.

11.  References

11.1.  Normative References

              IANA, "Named Information Hash Algorithm Registry", 9 2016,

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <https://www.rfc-editor.org/info/rfc2978>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,

   [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,

   [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,

   [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,

   [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>.

11.2.  Informative References

   [ENCDEP]   Tabata, K., "A Collaboration Model between Archival
              Systems to Enhance the Reliability of Preservation by an
              Enclose-and-Deposit Method", 2005,

              The Library of Congress, "BagIt Conformance Suite", 2016-,

   [MSFNAM]   Microsoft, Inc., "Naming a File", 2008,

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   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, DOI 10.17487/RFC4234,
              October 2005, <https://www.rfc-editor.org/info/rfc4234>.

   [TN1150]   Apple Inc., "Technical Note TN1150: HFS Plus Volume
              Format", 3 2004,

              Unicode Consortium, "Unicode(R) Standard Annex #15:
              Unicode Normalization Forms", 2 2016,

Authors' Addresses

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

   Email: jak@ucop.edu

   Justin Littman
   George Washington University Libraries
   2130 H St NW
   Washington, DC  20052

   Email: justinlittman@gwu.edu

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

   Email: emad@loc.gov

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   John Scancella
   Library of Congress
   101 Independence Avenue SE
   Washington, DC  20540

   Email: jsca@loc.gov

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

   Email: cadams@loc.gov

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