Network Working Group                                           A. Boyko
Internet-Draft                                       Library of Congress
Expires: September 25, 2008                                     J. Kunze
                                              California Digital Library
                                                               L. Madden
                                                              J. Littman
                                                     Library of Congress
                                                          March 24, 2008

                 The BagIt File Package Format (V0.93)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2008).

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   This document specifies BagIt, a hierarchical file package format for
   the exchange of generalized digital content.  A "bag" has just enough
   structure to safely enclose a brief "tag" and a payload but does not
   require any knowledge of the payload's internal semantics.  This
   BagIt format should be suitable for disk-based or network-based file
   package transfer.  One important use case is the possibility of
   eventual safe return of a received bag.  Tag information consists of
   a small number of top-level reserved file names, checksums for
   transfer validation, and optional small metadata blocks.

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1.  Introduction

   BagIt is a hierarchical file package format for the exchange of
   generalized digital content.  A "bag" has just enough structure to
   safely enclose a brief "tag" and a payload but does not require any
   knowledge of the payload's internal semantics.  This BagIt format
   should be suitable for disk-based or network-based file package
   transfer.  Use cases include long-term storage and the possibility of
   eventual safe return of a received bag.  Tag information consists of
   a small number of top-level reserved file names, checksums for
   transfer validation, and optional small metadata blocks.  The name
   BagIt is inspired by the "enclose and deposit" method [ENCDEP],
   sometimes referred to as "bag it and tag it".

   In this document the word "directory" is used interchangeably with
   the word "folder" and all examples conform to Unix-based filesystem
   conventions which should tranlate easily to Windows conventions after
   substituting the path separator ('\' instead of '/').  The BagIt
   format itself places no limitations on file and path lengths, so
   implementors thinking about maximal interoperation may wish to
   consider the issues listed in the Interoperability section of this

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2.  BagIt Package Layout

   A "bag" consists of a base directory containing a sub-directory named
   "data/" that holds the payload and a set of top-level files
   comprising the "tag".  The base directory may have any name and the
   "data/" directory may contain an arbitrary file hierarchy.

           |    manifest-<algorithm>.txt
           |    bagit.txt
           |    [optional additional tag files]
           \--- data/
                 |    [optional file hierarchy]

   The "tag" consists of one or more files named "manifest-
   _algorithm_.txt", a file named "bagit.txt", and zero or more
   additional files.  In top-level text files with ".txt" extension,
   each line should be terminated by a newline (LF) or carriage return
   plus newline (CRLF); in practice cautious programmers will also
   accept a carriage return by itself (CR) as a line terminator.  In all
   such tag files, text is assumed to be Unicode encoded as UTF-8

   The "bagit.txt" file should consist of exactly two lines,

   BagIt-Version: M.N
   Tag-File-Character-Encoding: UTF-8

   where M.N identifies the BagIt major (M) and minor (N) version
   numbers, and UTF-8 identifies the character set encoding of tag

2.1.  File Manifest

   One or more manifest files must be present.  A manifest is a top-
   level file with a name of the form manifest-_algorithm_.txt, where
   _algorithm_ is a string specifying a cryptographic checksum
   algorithm, such as


   Implementors of tools that create and validate bags are strongly
   encouraged to support at least two widely implemented checksum
   algorithms: "md5" [RFC1321] and "sha1" [RFC3174].  A manifest
   contains a complete list of payload files that must be present in a
   fully constituted bag.  Each line of a file manifest-_algorithm_.txt
   has the form

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   where FILENAME is the pathname of a payload file relative to the base
   directory and CHECKSUM is a base64-encoded checksum calculated
   according to _algorithm_ over the file's contents.  As a result,
   every payload FILENAME listed begins "data/...".  Any tag (top-level)
   FILENAME may optionally appear in a manifest.  One or more linear
   whitespace characters (spaces or tabs) separate the CHECKSUM and

   Sometimes it is desirable to record a checksum for a tag file without
   listing it in a manifest, but using instead an accompanying _tag
   checksum file_.  Applications include recording a checksum for the
   file manifest itself or for a tag file added after the manifest was
   received.  The name of a tag checksum file for tag file _tfname_ has
   the form _tfname_._algorithm_.  For example, a tag checksum file
   using MD5 over "manifest-sha1.txt" would have the name


   A tag checksum file contains a single line having the same form
   (CHECKSUM FILENAME) and semantics as a file manifest.  In essence, it
   is a one-line manifest listing the base64-encoded checksum calculated
   according to _algorithm_ over the contents of _tfname_.

2.2.  Valid Bags and Complete Bags

   A bag is considered _valid_ if it is _complete_ and if each CHECKSUM
   in every manifest can be verified against the contents of its
   corresponding FILENAME.

   A bag is considered _complete_ if every manifest covers the same set
   of files and every file in the payload is listed in every manifest.
   This means that a bag is complete (a) if the set of files listed in
   any one manifest is identical to the set of files listed in every
   manifest, (b) if every payload and tag file listed in every manifest
   is present, and (c) if every file present in the payload is listed in
   every manifest.  Hence tag files do not need to be listed in the
   manifest(s), but in a complete bag any tag files appearing in one
   manifest must appear in all manifests.

   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.  An optional top-level file named
   "fetch.txt", if present, contains such a list.  Each line of
   "fetch.txt" has the form


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   where URL identifies the file to be fetched, LENGTH is the number of
   octets in the file (or "-" to leave it unspecified), and FILENAME
   identifies the corresponding payload file.  One or more linear
   whitespace characters (spaces or tabs) separate these three values,
   and any such characters in the URL must be hex-encoded.

   Because "fetch.txt" lists files that are absent from a sent bag,
   receivers that are storing completed bags will want some way to
   record that the bag no longer needs completing, such as renaming this
   file (e.g., to "fetch-orig.txt") or changing a database flag.
   Receipt of a bag is not final until all such files are fetched.  The
   receiver of a bag with a "fetch.txt" tag file is expected promptly to
   complete the bag by fetching all URL-identified components as the
   sender is not bound to make the absent components available

   It is often practical to transmit a bag with "holes", that is, with a
   "fetch.txt" file, since it obviates the need for the sender to create
   a large serialized copy of the content and stage that content until
   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).

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3.  Other BagIt Metadata: package-info.txt

   Any other tag files are considered to be package information separate
   from the payload content.  The "data/" directory is the custodial
   focus of a bag, and the top-level files comprising the tag are
   intended to facilitate and document the transfer.  The tag could also
   be used to help in returning the bag to its sender at some point in
   the future.

   Tag information is optional.  If present, tag information at a
   minimum consists of a package-info.txt file.  This is a text file
   intended primarily for human readability using email-style headers
   [RFC2822].  It is recommended that lines not exceed 79 characters in
   length.  As mentioned earlier, text is assumed to be Unicode encoded
   as UTF-8.

   The package-info.txt file contains metadata elements describing the
   overall package.  It looks like this.

    Source-Organization: Spengler University
    Organization-Address: 1400 Elm St., Cupertino, California, 95014
    Contact-Name: Edna Janssen
    Contact-Phone: +1 408-555-1212
    External-Description: Uncompressed greyscale TIFF images from the
         Yoshimuri papers colle...
    Delivery-Date: 2008-01-15
    External-Identifier: spengler_yoshimuri_001
    Package-Size: 260 GB
    Bag-Group-Identifier: spengler_yoshimuri
    Bag-Count: 1 of 15
    Internal-Sender-Identifier: /storage/images/yoshimuri
    Internal-Sender-Description: Uncompressed greyscale TIFFs created from
         microfilm and are...

   All elements are provided as clues to ease handling on the sender and
   receiver ends.  No particular relationship between the sender
   organization and the payload content is assumed; for example, the
   sender may be a content aggregator, redistributor, collector,
   curator, or producer.

   Reserved element names are case-insensitive and defined as follows.

   Source-Organization  Organization transferring the content.

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   Organization-Address  Mailing address of the 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

   Delivery-Date  Date (YYYY-MM-DD) that the content is being

   External-Identifier  A sender-supplied identifier for the package.
      This identifier must 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.

   Package-Size  Size or approximate size of the package being
      transferred, followed by an abbreviation such as MB (megabytes),
      GB, or TB; for example, 42600 MB, 42.6 GB, or .043 TB.

   Bag-Group-Identifier  (optional) A sender-supplied identifier for the
      set, if any, of bags to which it logically belongs.  This
      identifier must 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.

   Bag-Count  (optional) 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.

   Internal-Sender-Identifier  (optional) An alternate sender-specific
      identifier for the content and/or package.  This value may be
      useful to senders who may retrieve the content in the future.  For
      instance, it might contain values that are relevant to the re-use
      of the content at the sender's organization.

   Internal-Sender-Description  (optional) A sender-local prose
      description of the contents of the package, to assist in later use
      if returned to the sender.

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   Arbitrary other package metadata elements may follow these elements.
   Such elements could be used to describe the payload in ways intended
   for the sender in case of bag return.

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4.  Network Transfer and Serialization

   When sending a bag over a network, in some scenarios it is convenient
   for the sender first to serialize the filesystem hierarchy
   representing the bag (the outermost base directory) into a single-
   file archive format such as TAR or ZIP.  After receiving the
   resulting aggregate file, which we will call a _serialization_, the
   receiver deserializes it to recreate the filesystem hierarchy.
   Several rules govern the serialization of a BagIt bag and apply
   equally to TAR or ZIP archive files:

   1.  One and only one bag is contained in one serialization.

   2.  The serialization has the same name as the bag's base directory,
       but with an extension added to identify the format; for example,
       the receiver of "mybag.tar.gz" expects the corresponding base
       directory to be created as "mybag".

   3.  A bag is never serialized from within its base directory, but
       from the parent of the base directory (where the base directory
       appears as an entry).  Thus, after a bag is deserialized in an
       empty directory, a listing of that directory shows exactly one
       entry.  For example, deserializing "" in an empty
       directory causes the creation of the base directory "mybag" with
       the payload and all the tag files deserialized beneath it.

   4.  One un-archiving (deserialization) step produces a single base
       directory bag with the top-level structure as described in this
       document without requiring an additional un-archiving step.  For
       example, after one un-archiving step it would be an error for the
       "data/" directory to appear as "data.tar.gz".  TAR and ZIP files
       may appear inside the payload beneath the "data/" directory,
       where they would be treated opaquely along with any other payload
       file or directory.

   When packaging a bag in an archive file format, care must be taken to
   ensure that the format's restrictions on file naming, such as
   allowable characters, length, or character encoding, will support the
   receiver requirements of the bag being packaged.

   The mechanics of sending and receiving of bags over networks is out
   of scope of the present document and may be facilitated by protocols
   such as [GRABIT].

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5.  Example Bag

   Here's a bag of material resulting from a hypothetical web harvest.
   Lines of file content are shown in parentheses beneath the file name,
   with long lines continued indented on subsequent lines.  This bag is
   not completely retrieved, of course, until every component listed in
   the fetch.txt file is retrieved.

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|   manifest-md5.txt
|    (93c53193ef96732c76e00b3fdd8f9dd3 data/Collection Overview.txt        )
|    (e9c5753d65b1ef5aeb281c0bb880c6c8 data/Seed List.txt                  )
|   fetch.txt
|    (http://WB20.Stanford.Edu/gov-06-2006-ARC/gov-20060601-oth-050019.arc.gz
|        26583985 gov-20060601-oth-050019.arc.gz                             )
|    (http://WB20.Stanford.Edu/gov-06-2006-ARC/gov-20060601-img-100002.arc.gz
|        99509720 gov-20060601-img-100002.arc.gz                             )
|    ( ..................................................................... )
|   package-info.txt
|    (Source-organization: California Digital Library                      )
|    (Organization-address: 415 20th Street, 4th Floor, Oakland, CA. 94612 )
|    (Contact-name: A. E. Newman                                           )
|    (Contact-phone: +1 510-555-1234                                       )
|    (Contact-email:                                       )
|    (External-Description: The collection "Local Davis Flood Control      )
|      Collection" includes captured California State and local websites   )
|      containing information on flood control resources for the Davis and )
|      Sacramento area.  Sites were captured by UC Davis curator Wrigley   )
|      Spyder using the Web Archiving Service in February 2007 and         )
|      October 2007.                                                       )
|    (Delivery-date: 2008.04.15                                            )
|    (External-identifier: ark:/13030/fk4jm2bcp                            )
|    (Package-size: about 22Gb                                             )
|    (Internal-sender-identifier: UCDL                                     )
|    (Internal-sender-description: University of California Davis Libraries)
|   BagIt.txt
|    (BagIt-version: 0.9                                                   )
|    (Tag-File-Character-Encoding: UTF-8                                   )
\--- data/
     |   Collection Overview.txt
     |    (... narrative description ...                                   )
     |   Seed List.txt
     |    (... list of crawler starting point URLs ...                     )

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6.  Interoperability: Windows and Unix File Naming

   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: CON, PRN, AUX, NUL, COM1,
   COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3,
   LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9.  See [MSFNAM] for more

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7.  Security Considerations

   The BagIt package format poses no direct risk to computers and
   networks.  Implementors of tools that complete bags by retrieving
   URLs listed in a "fetch.txt" file need to be aware that some of those
   URLs may point to hosts, intentionally or unintentionally, that are
   not under control of the bag's sender.  Checksum algorithms are
   designed to protect against corruption and spoofing in bag transfer,
   but they are not a guarantee.

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8.  References

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

   [GRABIT]   NDIIPP/CDL, "The GrabIt Package Exchange Protocol", 2008,

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

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

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

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Appendix A.  Change History

   (This appendix to be removed in the final draft.)

A.1.  Changes from V0.91 Draft, 2008.03.14

   Added a "Security Considerations" section, as required for internet-

   Per Andy, added "tag checksum file" concept for files that aren't in
   the manifest (eg, the manifest itself, or files added after manifest
   was received and you don't want to re-build the manifest).

   Added Andy's MD5 and SHA-1 references.  Decided for now to leave out
   the TAR reference to avoid POSIX/GNU religious wars, and for symmetry
   left off the ZIP reference too.  Also, softened encouragement to
   serialize bags.

   Tightened wording in metadata section to split elements into
   recommended and optional.  It is now recommended that lines not
   exceed 79 characters in length.  Dates should use YYYY-MM-DD format.
   Added country code to phone number in example.

   Small edits to Abstract.  Formatting changes to this Appendix.

A.2.  Changes from V0.9 Draft, 2008.03.14

   Any run of one or more horizontal whitespace characters can separate
   values in manifest.txt and fetch.txt.

   Reduced protocol discussion to a one-sentence reference and changed
   the enclosing section name to Network Transfer and Serialization.

   Removed second method of conveying checksums (for tag files) based on
   preliminary acceptance per call of 3/14.  Andy will try to confirm
   that this preserves the group consensus from call of 3/12.

   Miscellaneous small edits.

A.3.  Changes from 2008.03.12 Draft

   Added Bill LeFurgy's edits (removing "disposable" and "archival"
   words), and a statement that BagIt can provide a way to store
   exchanged content.

   To network transfer section added requirements of serialization (is
   that the right word) for single-archive format files (per E.

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   Added back to bagit.txt a clarified Tag-File-Character-Encoding
   statement, and suggested that defensive programmers accept an
   isolated CR as line terminator (per S. Abrams).

   Wrestled with tag file checksums as per call of 3/12 and Justin's
   suggestions, and think there's now a far simpler approach given that
   the manifest lists files relative to the base directory.

   Left in but softened the idea of altering the name "fetch.txt" after
   files have been fetched.  This is important because the presence of
   the filename is essentially an instruction.

   More tightening of archive file guidelines.

A.4.  Changes from 2008.03.04 Draft

   According to the spirit of Justin's version, tightened wording and
   definitions, returned to a manifest that doesn't straddle a list of
   files and URLs, defined concepts of "complete" and "valid", removed
   the dubious Character-Encoding from bagit.txt.

   BagIt is clearly for exchange between storage systems A and B, but I
   think it's too much for BagIt to be about how something is stored on
   system A or B. I think it suffices to say that BagIt is designed to
   facilitate the possibility of eventual safe return of a received bag.
   With that in mind, BagIt clearly provides a natural way to store a
   received bag, but trying to dictate local storage layout doesn't buy
   us anything, may discourage potential adopters, and is impossible to
   police.  And as validation goes, BagIt isn't more useful for long-
   term ongoing validation than any other self-contained structure,
   because trusted checksums are generally not stored in the same
   structure (or bag) as the stuff they're validating (ie, they should
   be held distant from the content to be validated).

   Reworded to reduce formalisms, eg, "profiles", "conformance", heavily
   layered and numbered sections.  Also, I'd like to keep extensibility
   less explicit than profiles suggest, which is an invitation to
   complexity and non-interoperation in my experience.

   Per 3/7 phone call, eliminated 3 metadata elements: Access-Level,
   Metadata-Included-In-Package, and Metadata-Description.

   Per Erik Hetzner's feedback, tightened archive file guidelines.

   Miscellaneous wording improvements and typos.

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A.5.  Changes from 2008.02.28 Draft

   Per phone call of 2/29, changed holes.txt to urls-md5.txt.  Tried
   tightening language around the concept of a "manifest", which is
   really the sum of the files list and the URLs list.  A bag isn't
   completely received until all the URLs (and files) are accounted for.
   Propose changing manifest.txt to files-md5.txt, and declaring that
   "the manifest" = files-md5.txt + urls-md5.txt.

   Please review the network transfer considerations section.  I tried
   to consolidate and shorten some discussion that may have been vague
   or redundant, but I may have removed something that was important.

   Per Stephen Abrams' suggestion, there's now a bagit.txt file that
   specifies the BagIt version number and UTF-8 encoding.

   Metadata elements to not contain spaces (more RFC822-compliant, per
   Stephen); declared them to be case-insensitive.

A.6.  Changes from 2008.02.20 Draft

   Based on 2/22 phone call, looked for new way of expressing file
   manifest separate from URL manifest.  The proposal here is to have
   the file manifest revert to the original format, and add a new
   optional manifest file that specifies the "holes".  A bag with
   "holes" isn't complete until the "holes are filled".  To help fill
   the holes, each manifest line has a checksum, length, filename, and
   URL.  The "filename" in this case is necessary in case a bag is
   returned (so the original sender knows what component is what), but
   the "filename" does not specify where the receiver must store the

   Added language to the definition of External Identifier to allow it
   to suggest a globally unique reference: "if the identifier is
   recognizable from a globally unique scheme, the receiver should make
   an effort to permit the package to be referenced by this identifier."

   Added an Example section.

   Added a concept of Bag Group and Bag Count (within a group) as per
   2/22 phone call and comments from Mark Phillips.

   As per conference call, stopped using the terms "packager" or
   "producer" for the metadata, using instead "source organization" and
   "internal sender identifier" and "internal sender description".

   Incorporated Andy's prose on archive formats and transfer

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   Miscellaneous edits arising from Mark Phillips' comments.

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Authors' Addresses

   Andy Boyko
   Library of Congress
   101 Independence Avenue
   SE Washington, DC  20540

   Fax:   +1 202-707-1957

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

   Fax:   +1 510-893-5212

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

   Fax:   +1 202-707-1957

   Justin Littman
   Library of Congress
   101 Independence Avenue
   SE Washington, DC  20540

   Fax:   +1 202-707-1957

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