| Network Working Group | A. Boyko |
| Internet-Draft | J. Kunze |
| Expires: October 13, 2013 | California Digital Library |
| J. Littman | |
| L. Madden | |
| Library of Congress | |
| B. Vargas | |
| April 11, 2013 |
The BagIt File Packaging Format (V0.97)
draft-kunze-bagit-09.txt
This document specifies BagIt, a hierarchical file packaging format for storage and transfer of arbitrary digital content. A "bag" has just enough structure to enclose descriptive "tags" and a "payload" but does not require knowledge of the payload's internal semantics. This BagIt format should be suitable for disk-based or network-based storage and transfer.
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 http://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 October 13, 2013.
Copyright (c) 2013 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. 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.
BagIt is a hierarchical file packaging format designed to support disk-based or network-based storage and transfer of arbitrary digital content. A bag consists of a "payload" and "tags". The content of the payload is the custodial focus of the bag and is treated as semantically opaque. The "tags" are metadata files intended to facilitate and document the storage and transfer of the bag. The name, BagIt, is inspired by the "enclose and deposit" method [ENCDEP], sometimes referred to as "bag it and tag it".
Implementors of BagIt tools should consider interoperability between different platforms, operating systems, toolsets, and languages. Differences in path separators, newline characters, reserved file names, and maximum path lengths are all possible barriers to moving bags between different systems. Discussion of these issues may be found in the Interoperability section of this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
An implementation is not compliant if it fails to satisfy one or more of the MUST or REQUIRED level requirements for the protocols it implements. An implementation that satisfies all the MUST or REQUIRED level and all the SHOULD level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST level requirements but not all the SHOULD level requirements for its protocols is said to be "conditionally compliant."
This specification uses a number of terms to describe BagIt, some of which are in common use, some of which are newly defined by this specification, and others which may have meanings obvious only to those in the community from which this spec arose. Terms defined in this section are intended to clarify any ambiguity.
A bag consists of a base directory containing (1) a set of required and optional tag files; (2) a sub-directory named "data", called the payload directory; and (3) a set of optional tag directories. The payload files in the payload directory are an arbitrary file hierarchy (see Section 2.1.2). The tag files in the base directory consist of one or more files named "manifest-algorithm.txt" (see Section 2.1.3), a file named "bagit.txt" (see Section 2.1.1), and zero or more additional tag files (see Section 2.2). The tag files in the optional tag directories are arbitrary file hierarchies and the tag directories MAY have any name that is not reserved for a file or directory in this specification.
The base directory MAY have any name.
<base directory>/
| bagit.txt
| manifest-<algorithm>.txt
| [optional additional tag files]
\--- data/
| [payload files]
\--- [optional tag directories]/
| [optional tag files]
BagIt-Version: M.N Tag-File-Character-Encoding: UTF-8
The "bagit.txt" tag file MUST consist of exactly two lines: [RFC3629]
The appropriate version for a bag that conforms to this version of the specification is "0.97".
The base directory MUST contain a sub-directory named "data", called the payload directory.
The payload directory contains the custodial content within the bag. The files under the payload directory are called payload files, or the payload. The payload is treated as octet streams for all purposes relating to this specification, and is not otherwise prescribed.
A payload manifest is a tag file that lists payload files and checksums for those payload files generated using a particular bag checksum algorithm. Every bag MUST contain one payload manifest file, and MAY contain more than one. A payload manifest file MUST have a name of the form manifest-algorithm.txt, where algorithm is a string specifying the bag checksum algorithm used in that manifest, such as:
manifest-md5.txt manifest-sha1.txt
A bag MUST NOT contain more than one payload manifest for a particular bag checksum algorithm.
Each line of a payload manifest file MUST be of the form:
CHECKSUM FILENAME
where FILENAME 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. The hex-encoded checksum MAY use uppercase and/or lowercase letters. The slash character ('/') MUST be used as a path separator in FILENAME. One or more linear whitespace characters (spaces or tabs) MUST separate CHECKSUM from FILENAME. An asterisk ('*') MAY preceed FILENAME for interoperability on some platforms (see Section 7.2.1). There is no limitation on the length of a pathname. The payload manifest MUST NOT reference files outside the payload directory.
Payload manifests only include the pathnames of files. Because of this, a payload manifest cannot reference empty directories. To account for an empty directory, a bag creator may wish to include at least one file in that directory; it suffices, for example, to include a zero-length file named ".keep".
A tag manifest is a tag file that lists other tag files and checksums for those tag files generated using a particular bag checksum algorithm. A bag MAY contain one or more tag manifests. A tag manifest file MUST have a name of the form "tagmanifest-algorithm.txt", where algorithm is a string specifying the bag checksum algorithm used in that manifest, such as:
tagmanifest-md5.txt tagmanifest-sha1.txt
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 FILENAME listed in a tag manifest begins "data/".
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 readability. All metadata elements are optional and MAY be repeated. Implementations SHOULD assume that the ordering is significant and provide access to the metadata elements in the order they are given in the "bag-info.txt" file.
A metadata element MUST consist of a label, a colon, and a value, each separated by optional whitespace. It is RECOMMENDED that lines not exceed 79 characters in length. Long values may be continued onto the next line by inserting a newline (LF), a carriage return (CR), or carriage return plus newline (CRLF) and indenting the next line with linear white space (spaces or tabs).
Reserved metadata element names are case-insensitive and defined as follows.
In addition to these metadata elements, other arbitrary metadata elements may also be present.
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...
Bagging-Date: 2008-01-15
External-Identifier: spengler_yoshimuri_001
Bag-Size: 260 GB
Payload-Oxum: 279164409832.1198
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...
Here is an example "bag-info.txt" file.
URL LENGTH FILENAME
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 tag file named "fetch.txt" contains such a list. Each line of "fetch.txt" has the form [RFC3986]. There is no limitation on the length of any of the fields in the "fetch.txt".
The "fetch.txt" file allows a bag to be transmitted with "holes" in it, which can be practical for several reasons. For example, it obviates the need for the sender to stage a large serialized copy of the content while the bag is transferred to the receiver. Also, this method allows a sender to construct a bag from components that are either a subset of logically related components (e.g., the localized logical object could be much larger than what is intended for export) or assembled from logically distributed sources (e.g., the object components for export are not stored locally under one filesystem tree).
A bag MAY contain other tag files that are not defined by this specification. Implementations SHOULD ignore the content of any unexpected tag files, except when they are listed in a tag manifest. When unexpected tag files are listed in a tag manifest, implementations MUST only treat the content of those tag files as octet streams for the purpose of checksum verification.
All tag files specifically described in this specification 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 newline (LF), a carriage return (CR), or carriage return plus newline (CRLF). Text tag files MUST end in the extension ".txt".
In all text tag files except for the bag declaration file, text MUST be encoded in the character encoding specified in the "bagit.txt" bag declaration file. Text tag files except for the bag declaration file MAY include a byte-order mark (BOM) only if the specified encoding requires it for proper decoding. (Note that UTF-8 does not.)
As specified in Section 2.1.1, the bag declaration file must be encoded in UTF-8 and must not include a byte-order mark.
The payload manifest and tag manifests assert integrity of the payload and tags in a bag using checksum algorithms. The operation of those algorithms, and the formatting of their output within a manifest file, are generally beyond the scope of this specification, except that the output format MUST be able to fit in the manifest format specified in Section 2.1.3.
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.
Implementors of tools that create and validate bags SHOULD support at least two widely implemented checksum algorithms: "md5" [RFC1321] and "sha1" [RFC3174].
A complete bag MUST have the following attributes:
A bag is incomplete when it exhibits any of the following exceptions to the attributes of a complete bag:
A valid bag must have the following attributes:
If a bag is neither valid, complete, nor incomplete, it is invalid. Definitions for the various ways a bag may be invalid are not covered by this specification.
Tag files that do not appear in a tag manifest can be modified, added to, or removed from a bag without impacting the completeness or validity of the bag.
In some scenarios, it may be convenient to serialize the bag's filesystem hierarchy (i.e., the base directory) into a single-file archive format such as TAR or ZIP (the serialization) and then later deserialize the serialization to recreate the filesystem hierarchy. Several rules govern the serialization of a bag and apply equally to all types of archive files:
When serializing a bag, care must be taken to ensure that the archive format's restrictions on file naming, such as allowable characters, length, or character encoding, will support the requirements of the systems on which it will be used. See Section 7.2.
myfirstbag/
|
| manifest-md5.txt
| (49afbd86a1ca9f34b677a3f09655eae9 data/27613-h/images/q172.png)
| (408ad21d50cef31da4df6d9ed81b01a7 data/27613-h/images/q172.txt)
|
| bagit.txt
| (BagIt-version: 0.96 )
| (Tag-File-Character-Encoding: UTF-8 )
|
\--- data/
|
| 27613-h/images/q172.png
| (... image bytes ... )
|
| 27613-h/images/q172.txt
| (... OCR text ... )
....
This is the layout of a basic bag containing an image and a companion OCR file. Lines of file content are shown in parentheses beneath the file name.
mysecondbag/
|
| manifest-md5.txt
| (93c53193ef96732c76e00b3fdd8f9dd3 data/Collection Overview.txt )
| (e9c5753d65b1ef5aeb281c0bb880c6c8 data/Seed List.txt )
| (61c96810788283dc7be157b340e4eff4 data/gov-20060601-050019.arc.gz)
| (55c7c80c6635d5a4c8fe76a940bf353e data/gov-20060601-100002.arc.gz)
|
| fetch.txt
| (http://WB20.Stanford.Edu/gov-06-2006/gov-20060601-050019.arc.gz
| 26583985 data/gov-20060601-050019.arc.gz )
| (http://WB20.Stanford.Edu/gov-06-2006/gov-20060601-100002.arc.gz
| 99509720 data/gov-20060601-100002.arc.gz )
| ( ...............................................................)
|
| bag-info.txt
| (Source-organization: California Digital Library )
| (Organization-address: 415 20th St, 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. )
| (Bag-date: 2008.04.15 )
| (External-identifier: ark:/13030/fk4jm2bcp )
| (Bag-size: about 22Gb )
| (Payload-Oxum: 21836794142.831 )
| (Internal-sender-identifier: UCDL )
| (Internal-sender-description: UC Davis Libraries )
|
| bagit.txt
| (BagIt-version: 0.96 )
| (Tag-File-Character-Encoding: UTF-8 )
|
\--- data/
|
| Collection Overview.txt
| (... narrative description ... )
|
| Seed List.txt
| (... list of crawler starting point URLs ... )
....
The following example bag contains content from a web crawler. As before, lines of file content are shown in parentheses beneath the file name, with long lines continued indented on subsequent lines. This bag is not complete until every component listed in the "fetch.txt" file is retrieved.
The paths specified in the payload manifest, tag manifest, and "fetch.txt" file do not prohibit special directory characters which might be significant on implementing systems. Implementors SHOULD take care that files outside the bag directory structure are not accessed when reading or writing files based on paths specified in a bag.
For example, path characters such as ".." or "~" in a maliciously crafted "fetch.txt" file might cause a naive implementation to overwrite critical system files.
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. Checksums are intended as a reasonable guarantee against corruption during transit, not a strong cryptographic protection against intentional spoofing.
The size of files, as optionally reported in the "fetch.txt" file, cannot be guaranteed to match the actual file size to be downloaded. Implementors SHOULD take care to appropriately handle cases where the actual file size does not match the file size reported in the fetch.txt. Implementors SHOULD NOT use the file size in the "fetch.txt" file for critical resource allocation, such as buffer sizing or storage requisitioning.
When creating a bag on physical media (such as hard disk, CD-ROM, or DVD) for transfer to another organization, the sender should select and format the media in a manner compatible with both the content requirements (e.g., file names and sizes) and the receiver's technical infrastructure. If the receiver's infrastructure is not known or the media needs to be compatible with a range of potential receivers, consideration should be given to portability and common usage. For example, a "lowest common denominator" for some potential receivers could be USB disk drives formatted with the FAT32 filesystem.
Although overall bag size is unlimited in principle, network-based transfers may involve constraints on the amount of bag data that a receiver can receive at one time. It may be practical to split a large bag into several smaller bags.
Transmitting a whole bag in serialized form as a single file will tend to be the most straightforward mode of transfer. When throughput is a priority, use of "fetch.txt" lends itself to an easy, application-level parallelism in which the list of URL-addressed items to fetch is divided among multiple processes. The mechanics of sending and receiving bags over networks is otherwise out of scope of the present document and may be facilitated by protocols such as [GRABIT] and [SWORD].
This section is not part of the BagIt specification. It describes some practical considerations for bag creators and receivers circa 2010.
Some cautions regarding bag interchange arise in regard to the commonly available checksum tools distributed with the GNU Coreutils package (md5sum, sha1sum, etc.), collectively referred to here as "md5sum". First, md5sum can be run in binary or text mode; text mode sometimes normalizes line-endings. While these modes appear to produce the same checksums under Unix-like systems, they can produce different checksums under Windows. When using md5sum, it may be safest to run it in binary mode, with one caveat: a side-effect of binary mode is that md5sum requires a space and an asterisk ('*'), compared to two spaces in text mode, between the CHECKSUM and FILENAME in its manifest format.
Due to the widespread use of md5sum (and its relatives), it is not unexpected for bag receivers to see manifests in which CHECKSUM and FILENAME are separated by a space followed by an asterisk. Implementors creating or processing bags with md5sum should be aware of these subtle differences, and ensure compliance with the manifest specification in this document. Implementors creating and processing bags with other tools may wish to be tolerant of asterisks found in the manifests.
A final note about md5sum-generated manifests is that for a FILENAME containing a backslash ('\'), the manifest line will have a backslash inserted in front of the CHECKSUM and, under Windows, the backslashes inside FILENAME may be doubled.
< > : " / | ? *
As specified above, only the Unix-based path separator ('/') may be used inside filenames listed in BagIt manifests and "fetch.txt" files. When bags are exchanged between Windows and Unix platforms, care should be taken to translate the path separator as needed. Receivers of bags on physical media should be prepared for filesystems created under either Windows or Unix. Besides the fundamental difference between path separators ('\' and '/'), generally, Windows filesystems have more limitations than Unix filesystems. Windows path names have a maximum of 255 characters, and none of these characters may be used in a path component: [MSFNAM] for more information.
BagIt owes much to many thoughtful contributers and reviewers, including Stephen Abrams, Mike Ashenfelder, Dan Chudnov, Brad Hards, Scott Fisher, Keith Johnson, Erik Hetzner, Leslie Johnston, David Loy, Mark Phillips, Tracy Seneca, Brian Tingle, Adam Turoff, and Jim Tuttle.
| [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 File Exchange Protocol", 2008. |
| [MSFNAM] | Microsoft, , "Naming a File", 2008. |
| [RFC1321] | Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [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. |
| [RFC3986] | Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. |
| [SWORD] | UKOLN/JISC CETIS, , "Simple Web-service Offering Repository Deposit (SWORD)", 2008. |
(This appendix to be removed in the final draft.)
Allowing tag directories.
Fixed definition of valid.
Clarified that tag files do not need to be text files.
Clarified that repeatability and ordering of metadata elements in bag-info.txt.
Clarified case of hex-encoding in manifests.
Re-replaced entity reference for current version number in artwork, where it doesn't appear to work (xml2rfc bug?). Updated to latest IETF Trust Legal Provisions 200902. (jak)
Re-wording Tag File Format section.
Adding new section for Other Tag Files.
Minor clarification on the Fetch File description.
Synchronized the language between the Payload Manifest and the Tag Manifest sections.
Minor grammatical corrections and clarifications to the Payload Manifest section.
Re-worded and re-ordered payload section and structure intro. Except for the base directory naming, the structure intro is strictly explanatory.
Replaced current version number with entity reference.
Move checksum algorithm information into its own section.
Major re-wording of section on validity and completeness to provide explicit, enumerated definitions for "valid", "complete", and "incomplete" bags.
Added explicit wording about byte order marks (BOM) in UTF-8.
Re-named section titles for better clarity.
Re-wording security consideration on checksum purposes to more accurately reflect the real purposes of the checksums.
Major restructuring of the document for brevity and precision.
Added RFC 2119 language.
Added terminology section.
Cleaning up example artwork so that parenthesis are more consistently used.
Explicitly stated version number required for comforming to the current version of the specification.
Various minor tweaks to grammar and wording.
Re-worded interoperability statement in the Introduction. (Justin)
Added statements regarding no limitations on various paths, URI, and other lengths.
Clarified that the bag directory may not contain any other directories except for the "data" directory.
A soel carriage return character is now explicitly allowed as a valid line separator.
Tag file encoding requirements are now required to be as-stated in the "bagit.txt". The "bagit.txt" file is explicitly required to be in UTF-8.
Wording cleanup, clarifying payload file manifests and tag file manifests.
Tags in "bag-info.txt" no longer have any ordering requirement.
Tag formatting now explicitly states where significant whitespace begins in the tag.
After some consideration, added some security considerations.
Made it clear that a bag may contain other bags, re: serialization.
Re-worded interoperabiilty to concerns to require creators to be spec-compliant, and readers to be tolerant of known potential issues.
Specificity to the FILENAME element in "fetch.txt" is relative to the bag root, and to make sure to treat leading slashes as relative.
Updated acknowledgements.
Various other minor edits for clarity and readibility.
Added language to require the slash ('/') as path separator, regardless of the platform where the bag was created. Added an extra co-author and an Acknowledgements section.
Deleted the unnecessary "(optional)" from four of the metadata elements, since all metadata elements are optional. Softened the equivalence of the serialization name and name of the contained bag base directory. Replaced the reference to RFC2822 with an inline description of the simpler bag-info.txt format.
Changed to a variable linear whitespace separator in the description of manifest layout and in manifest examples. Added two paragraphs under a new "Checksum tools" subsection of the Interoperability section to describe some of the peculiarities of dealing with the widely used GNU Coreutils checksum tools.
With the new version, 0.96, there is an important and incompatible change of file name (package-info.txt -> bag-info.txt), metadata element names (Package-Size -> Bag-Size, Packing-Date -> Bagging-Date), and descriptive language to replace the noun "package" with "bag" throughout the spec. This was to reduce unnecessary synonymy and free up the noun "package" to name the physical container (e.g., a mailing carton) used to transfer hard disks.
In section 7, another important change is the introduction of the Payload-Oxum ("octetstream sum") metadata element to convey precise, machine-readable payload size information for capacity planning (especially useful when preparing to receive files listed in fetch.txt). The Bag-size definition was adjusted to steer it more towards human consumption.
In section 2.2 the spec now requires exactly two spaces between checksum and filename in manifests. This results from the experience that as of 2008, not all widely available validation tools are flexible in the kind of separating whitespace recognized. The examples have been updated to include use the two-space form as well.
Comment added that while overall bag size is unlimited, practical limitations on the amount of data that a receiver can stage may warrant splitting a large bag into several smaller bags.
Added a reference to the SWORD protocol.
Minor edits for scanning and reformatting to cut down line length for some figures that exceeded 72 chars (limit for Internet-Drafts).
Added mention of preserving empty directories.
Simplified function of "tag checksum file" to "tag manifest", having same format as payload manifest. The tag manifest is optional and need not include every tag file.
Loosened interpretation of payload manifest to "union" concept: every payload file must be listed in at least one manifest but need not be listed in every manifest.
Shortened the Introduction's first paragraph to be less duplicative of text in the Abstract.
Changed Delivery-Date to Packing-Date.
Correctly sorted the author list and clarification of deserialization wording.
Author address corrections and miscellaneous stylistic edits.
Added some mention of physical media-based transfers, preferred characteristics of transfer filesystems, and network transfer issues.
Added basic bag example early and changed the narrative to more clearly delineate component files.
Wording changes under fetch.txt, and note that fetch.txt will need to be modified before bag return.
Fixed checksum encoding reference to base64 rather than hex. (B. Vargas)
Described simple normalization approach for checksum algorithm names. (B. Vargas)
In the example bag, add the ARC files found in the fetch.txt to the manifest as well (A. Turoff)