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)
http://www.ietf.org/internet-drafts/draft-kunze-bagit-00.txt
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Abstract
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
document.
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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.
<bag_dir>/
| 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
[RFC3629].
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
files.
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
manifest-md5.txt
manifest-sha1.txt
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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CHECKSUM FILENAME
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
FILENAME.
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
manifest-sha1.txt.md5
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
URL LENGTH FILENAME
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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
indefinitely.
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
Contact-Email: ej@spengler.edu
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
responsible.
External-Description A brief explanation of the contents and
provenance.
Delivery-Date Date (YYYY-MM-DD) that the content is being
transferred.
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 "mybag.zip" 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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mybag/
|
| 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: alfred@ucop.edu )
| (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
information.
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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,
<http://www.iwaw.net/05/papers/iwaw05-tabata.pdf>.
[GRABIT] NDIIPP/CDL, "The GrabIt Package Exchange Protocol", 2008,
<http://dot.ucop.edu/home/jak/grabitspec.html>.
[MSFNAM] Microsoft, "Naming a File", 2008,
<http://msdn2.microsoft.com/en-us/library/aa365247.aspx>.
[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-
drafts.
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.
Hetzner).
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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
content.
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
parallelism.
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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
USA
Fax: +1 202-707-1957
Email: aboy@loc.gov
John A. Kunze
California Digital Library
415 20th St, 4th Floor
Oakland, CA 94612
US
Fax: +1 510-893-5212
Email: jak@ucop.edu
Liz Madden
Library of Congress
101 Independence Avenue
SE Washington, DC 20540
USA
Fax: +1 202-707-1957
Email: emad@loc.gov
Justin Littman
Library of Congress
101 Independence Avenue
SE Washington, DC 20540
USA
Fax: +1 202-707-1957
Email: jlit@loc.gov
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