Greetings,
Attached is the 2nd draft for the addition of an on-disk format to
pkg(5). Any and all feedback is greatly appreciated by the end of May
26th, 2010.
For those that saw the first draft, you can find a webrev with
differences here:
http://cr.opensolaris.org/~swalker/pkg-ondisk-2/
Cheers,
-Shawn
pkg(5): image packaging system
This information is Copyright (c) 2010, Oracle and/or its affiliates.
All rights reserved.
ON-DISK FORMAT PROPOSAL
1. Introduction
1.1. Date of This Document:
05/18/2010
1.2. Name of Document Author/Supplier:
Shawn Walker, Oracle,
on behalf of the pkg(5) project team
1.3. Acknowledgements:
This document is largely based on comments from the following
individuals to whom the author is exceedingly indebted to:
- Danek Duvall
- Mike Gerdts
- Stephen Hahn
- Krister Johansen
- Dan Price
- Brock Pytlik
- Bart Smaalders
- Peter Tribble
2. Project Summary
2.1. Project Description:
"...the repository can be archived up, put on a CD, memory
stick, 2D barcode, and protected by the Black Knight, fire
moats, komodo dragons, etc." - Danek Duvall
pkg(5) is primarily a network-oriented binary packaging system.
Although some of the tools it provides support filesystem-based
operations for publication, the primary expected use for package
operations (such as install, update, search, etc.) is between an
intelligent client and one or more servers that provide access
to a package repository and/or other interactive services.
This project seeks to define and establish an on-disk format
(and corresponding container format), for the pkg(5) system,
with the intent that it can enable the ubiquitous, transparent
use of package data from filesystem-based resources.
The changes proposed by this project are evolutionary, not
revolutionary, in nature. In particular, this project seeks
to refine and adopt the existing repository format used by the
pkg(5) depot server as the on-disk format. Supplementary to
that, it also seeks the addition of a container format to ease
provisioning of the on-disk format, and the unification of the
scheme used by the client and server to store package data.
2.2. Problem Area:
For some deployments, network-based package data access is not
possible or is undesirable. Concerns often cited in this area
include:
- lack of access control or ability to easily integrate with
existing access control systems,
- inability to rely on alternative (or existing) provisioning
arrangements (such as NFS-based file servers),
- environmental or procedural requirements that prohibit the
ability to or use of a network-based service,
- characteristics of network protocols (such as HTTP, etc.) that
artificially limit functionality or performance (as opposed to
iSCSI or other alternatives),
- ease of administration of filesystem-based resources, and
- ease of transferring package data.
3. Project Technical Description:
3.2. Details:
This project defines an on-disk format (and corresponding con-
tainer format) that is intended for the supplemental or complete
provisioning of package data at all stages of the package life-
cycle. That is, when package data is published, stored by the
client or server, or otherwise used during package operations.
The on-disk format (defined in detail later in this document)
is intended to be distributable in its raw form (a pre-defined
structure of directories and files) or within a container format
(such as a zip file, etc.).
Out of necessity, the use of filesystem-based resources (such as
those provided by the on-disk format) will sometimes limit the
operations that can be performed to a subset of those normally
available when interacting with a network-based repository. For
example, search and publisher configuration may not be possible,
and purely interactive services such as the BUI (Browser UI)
offered by the depot server for a repository, RSS feeds, and
others will not be available.
Because of the wide-ranging impact of the changes required to
implement this functionality, it is intended that the project
be implemented in the following sequence:
- Client Support for filesystem-based Repository Access
- Depot Storage, Client Transport and Publication Tool Update
- Client Storage and Image Format Update
- Client and Depot Support for On-Disk Archive Format
3.3. Bug/RFE Number(s):
As an example of the kinds of defects and RFEs intended to be
resolved by this project, see the following selection of
defect.opensolaris.org bug IDs:
2152 standalone package support needed (on-disk format)
166 depot doesn't set directory mode when creating directories
2086 validate that a repository is really a repository in pkg.depotd
6335 publisher repo with invalid certificate information shouldn't
prevent querying other repos
6576 pkg install/image-update support for temporary publisher origins
desired
6940 depot support for file:// URI desired
7213 ability to remove published packages
7273 manifests should be arranged in a hierarchy by publisher
7276 /var/pkg metadata needs reorg (looks busy)
8433 client and pull need to refer to refer to "repository" instead of
"server"
8722 advanced repository metadata store needed
8725 versioning information for depot and repository metadata needed
9571 CachedManifest should be named FactoredManifest
9572 CachedManifest should allow consumers to specify cache location
9872 publication api should use new transport subsystem
9933 ability to control repository creation behaviour or removal of it
10244 caching dictionaries as a class variable prevents multi-image and
repo search
11362 Image update dying when trying to talk to a disabled and offline
publisher
11740 publishers with installed packages should not be removable
12814 publisher prefixes should be forcibly lower-cased or case
insensitive
14802 ability to have separate read / write download caches
15320 pkgsend will traceback if unable to parse server error response
15371 repository property defaults opensolaris.org-specific
3.4. In Scope:
Filesystem-based data resourcing for package operations.
3.5. Out of Scope:
Package signing and fine-grained access control for package
repositories.
4. On-Disk Format Technical Description:
4.1. Overview:
The on-disk format is intended to exist both in a raw format as
a pre-defined structure of directories and files, and in an
archive format which is primarily a simple container for
the raw format.
4.2. Raw Format:
4.2.1. Goals:
The goals for the raw on-disk format include:
- unification of client and server package data storage
for data common to both,
- transparent usage of package data regardless of operation
or use by client or server,
- ease in composition and decomposition of package data
stored within by publisher or package,
- re-use of existing publication tools for on-disk format,
- enablement of future publication tools to automatically
be able to manipulate or use on-disk format, and
- ease of provisioning.
4.2.2. Raw Format specification:
The pkg(5) repository format is a set of directories and
files that conform to a pre-defined structure.
For a version 1 repository (the current format), the
structure is as follows:
<REPO_ROOT>/
catalog/
<catalog v1 files>
index/
<index files>
file/
<first two letters of file hash>/
<file-named-by-hash>
pkg/
<stem>/
<manifest-file>
trans/
<in-flight transaction files>
cfg_cache (optional repository configuration file)
Version 2 of the repository format is primarily a variation
of the Version 1 format that stores package data grouped by
publisher. In particular, it eliminates the possibility of
unintended collisions between publisher-specific package
metadata and simplifies composition and decomposition of
repository content. Its structure can be defined as
follows:
<REPO_ROOT>/
file/ (optional)
publisher/ (optional)
<prefix>/ (optional)
catalog/ (optional)
<catalog v1 files>
file/ (optional)
<first two letters of file hash>/
<file-named-by-hash>
index/ (optional)
pkg/ (optional)
<stem>/
<manifest-file-for-pkg-version>
trans/ (optional)
<in-flight transaction files> (optional)
pub.p5i (optional)
cfg_cache (optional repo configuration file)
By default, repository operations will store data in the
publisher-specific location found under publisher/<prefix>
for new repositories.
In the case that the top-level file/ directory is
populated, automatic decomposition of contents into its
publisher-specific components will not be possible unless
corresponding package manifests are also available.
To support easy composition, filtering, and creating of
package archives, directories above marked with the text
'(optional)' must not be required for <REPO_ROOT> to be
considered a valid repository. The behaviour of consumers
accessing the contents of the repository should be as
follows:
- file/
This optional directory serves as a place to store file
data for more than one publisher. Package files are
stored in gzip format using a sha1sum of the file as the
filename, and then the first two letters of the filename
as the parent directory's name.
- publisher/
If present, the repository is assumed to be in version 2
format. If absent, the format of the repository will be
determined by the presence of the other top-level
directories and/or files.
- publisher/<prefix>/catalog/
If absent, consumers should determine the list of
packages available based on the manifest files present
in the publisher/ subdirectory. If present, consumers
should expect v1 (or newer) catalog files, or none at
all, to be contained within.
- publisher/<prefix>/file/
Consumers should always check this subdirectory first
(if present) when retrieving package file data if the
publisher is known. Package files are stored in gzip
format using a sha1sum of the file as the filename, and
then the first two letters of the filename as the parent
directory's name.
- publisher/<prefix>/index/
If absent, search functionality should be disabled for
this publisher, or a fallback to 'slow manifest-based
search' performed. If present, consumers should expect
v1 (or newer) search files, or none at all, to be con-
tained within.
- publisher/<prefix>/pkg/
If absent, search must be disabled for this publisher
even if index is present. If present, manifests are
stored in pkg(5) manifest format using the uri-encoded
version of the package FMRI as the filename, and using
the uri-encoded package FMRI stem (name) as the parent
directory's name.
- publisher/<prefix>/trans/
If absent, this directory will be created during
publication operations. If present, in progress
transaction data is stored in a directory named
by the open time of the transaction as a UTC UNIX
timestamp plus an '_' and the URI-encoded package
FRMI. As an example:
1245176111_pkg%3A%2FBRCMbnx%400.5.11%2C5.11-0.116
%3A20090616T181511Z
- publisher/<prefix>/pub.p5i
This pkg(5) information (p5i) file should contain
suggested configuration information for clients
such as origins, mirrors, alias, etc. Consumers
can use this to provide clients with initial or
suggested configuration information for a given
publisher. If not present, the information found
in the repository configuration file should be
used as applicable.
An example version 2 repository might be structured as
follows:
<REPO_ROOT>/
publisher/
example.com/
catalog/
catalog.attrs
catalog.base.C
file/
ff/
fffff277f5a8fb63e57670afc178415c2c5e706d
index/
__at_depend
...
pkg/
package%2Fpkg/
0.5.11%2C5.11-0.136%3A20100327T063139Z
trans/
1245176111_pkg%3A%2FBRCMbnx%400.5.11%2C5.11-0.116
%3A20090616T181511Z
example.net/
catalog/
catalog.attrs
catalog.base.C
file/
af/
affff277f5a8fb63e57670afc178415c2c5e706d
index/
__at_depend
...
pkg/
package%2Fpkg/
0.5.11%2C5.11-0.133%3A20090327T062137Z
trans/
1245176111_pkg%3A%2FFAAMbnx%400.5.11%2C5.11-0.139
%3A20100616T181511Z
cfg_cache
4.3. Archive Format:
4.3.1. Requirements:
The requirements for the on-disk archive format include:
- support for archives greater than 8GB in size,
- support for files in archive greater than 4GB in size,
- support for storage of hard links,
- support for pathnames > 255 characters in length,
- core Python bindings exist or can be easily created using
existing library,
- can be a container of compressed files, as opposed to a
compressed container of uncompressed files,
- open, royalty-free, well-documented format with wide
platform support and acceptance,
- multi-threaded decompression and compression possible,
- simple composition and filtering of its content should be
possible, and
- random access to the archive contents must be possible
without reading the entire archive file.
4.3.2. Candidates:
A number of potential archive formats have been considered
for use, including:
- 7z (7-Zip)
- cpio
- pax (portable archive exchange format)
- ZIP
The evaluations provided for each format here are not in-
tended to be exhaustive; rather they focus on the specific
requirements of this project. For more information about
these formats, and the documents used to evaluate them,
please refer to section 6 of this proposal.
4.3.3. 7z Evaluation:
The 7z format was rejected for the following reasons:
- Does not permit random access to archive contents or
requires the entire archive file to access the contents
and adding this would require a custom variation of 7z.
- Although the 7z format supports compression methods other
than LZMA, a primary motivator for using 7z would be the
ability to use LZMA natively as part of the conatiner
format. However, the tradeoffs in terms of CPU and memory
footprint currently make LZMA unsuitable for pkg(5) when
compared to other compression algorithms such as those
used by gzip(1).
- Use of the 7z format would require integration of the LZMA
SDK (which also provides a basic 7z API in C) and the cre-
ation of python bindings or the integration of a third
party's (such as pylzma).
- No native support for extended attributes or UNIX owner/
group permissions.
4.3.4. cpio Evaluation:
The cpio format doesn't natively support random access to
archive contents, but the format itself doesn't prevent
this. An index could be added first file in the archive
with the information needed to provide fast, random access
to the archive contents.
The cpio format was rejected for the following reasons:
- The size of files in portable cpio archives is limited to
(8 * 2^30) bytes.
4.3.4. PAX Evaluation:
The PAX format meets all of the requirements except that of
random access to archive contents. However, the format
itself doesn't prevent this. A table of contents file could
be supplied as the first file in the archive with the info-
rmation needed to provide fast, random access to the con-
tainer contents.
4.3.5. ZIP Evaluation:
The ZIP format meets all of the requirements listed above
(assuming that ZIP64 extensions are used), with the ex-
ceptions listed below for which it was rejected:
- The use or implementation of some of the functionality
documented in the .ZIP file format requires a license from
PKWARE.
- While random archive content access is possible, the ZIP
file format stores the index for the archive at the end of
the archive (as opposed to the beginning). This increases
the number of round trips that would be required for
potential remote random content access. It also means
that extraction requires multiple seeks to the end of the
file before any content can be extracted from the archive,
which can be detrimental to performance for some media
types (optical, etc.).
4.3.6. Evaluation Conclusion:
Based on the requirements set forth in section 4.3.1, the
PAX format was selected as the on-disk archive format
for pkg(5) packages. However, to enable efficient access
to the archive contents, an index file needs to be present
as the first file in the archive.
Early evaluations of an unoptimised prototype were performed
using a repository containing all packages for build 136 and
unbundleds. The on-disk size of the repository was appox-
imately 4.98G. The resulting archive was 5.0G in size, with
an archive index file 9.7M in size (when the index was comp-
ressed using gzip).
First time access to the prototype archive for extraction of
a single file after creation yielded a total time of approx-
imately 5 seconds compared to approximately 36-42 seconds
for utilities such as pax(1), tar(1), or gtar(1).
Creation of the archive took 7 minutes, 35 seconds on a
custom-built Intel Core 2 DUO E8400, with 8GB Memory,
and a 1TB 7200 RPM SATA Drive w/ 64MB Cache.
4.3.7. Package Archive Specification:
pkg(5) archive files will have an extension of 'p5p' which
will stand for 'pkg(5) package'. The format of these
archives matches that defined by IEEE Std 1003.1, 2004 for
the pax Interchange Format, with the exception that the
first archive entry must not use the optional pax headers
allowed by the format, and must contain the index file
for the package archive. The layout can be visualised as
follows:
.--------------------------------------------------------.
| ustar header for package archive index file |
.--------------------------------------------------------.
| file data for package archive index file |
.--------------------------------------------------------.
| remaining archive data |
.________________________________________________________.
The reason for this limitation is to ensure that clients
performing selective archive extraction can be guaranteed
to find the location and size of the package archive index
file without knowing the size of the header for the index
file in advance (ustar headers are always 512 bytes in
size).
In addition, pkg(5) archives in this format make remote,
selective archive access possible. For example, a client
could request the first 512 bytes of a pkg(5) archive file
from a remote repository, then retrieve the archive index
file. Once it has the archive index file, it can then
perform a HTTP/1.1 byte-ranges request to selectively
transfer the data for a set of specific files from the
archive.
The index file must be named using the following template
and be compressed using the gzip format described by RFCs
1951 and 1952, and formatted according to section 4.3.8:
p5p.index.<index_file_number>.<index_version>.gz
<index_file_number> is an integer in string form that
indicates which index file this is. The number only
exists so that each index file can remain unique in
the archive. An archive may contain multiple index
files to support fast archive additions.
<index_version> is an integer in string form that
indicates the version of the index file. The initial
version for this proposal will be '0'.
If the first file in the archive is found not to be in the
layout or format shown above, or any of the index files in
the archive are found to not be in a format supported by
the client (version too old or too new), the archive will
be treated as a standard pax archive and some operations
may not be possible or experience degraded performance.
The same is also true if the index file is found to not
match the archive contents.
When creating the archive, or adding to an existing archive,
new index gzip files should be zero-padded with an extra 256
bytes at the end. This reserved space is used for fast
additions to existing package archives by updating the
previous index file with an entry for the new index file.
For example, the first index file's last entry should
contain the name and offset of the second index file,
and so on.
All pathnames after the first in the archive (if the first
file is the archive index file) must conform to the repo-
sitory layout specified in section 4.2.2 of this proposal.
Since a pkg(5) repository can contain one or more packages,
pkg(5) archive files can also contain the data for one or
more packages. This allows easy redistribution of a single
package and all of its dependencies in a single file.
Finally, it should be noted that only ascii character path-
names are expected in the archive as the raw repository
format does not use or support unicode pathnames.
4.3.8. Package Archive Index Specification:
The pkg(5) archive index file enables fast, efficient access
to the contents of an archive. It contains an entry for all
files in the archive excluding the index file itself in the
following format (also referred to as index format version
0):
<name>NUL<offset>NUL<entry_size>NUL<size>NUL<typeflag>NL
<name> is a string containing the pathname of the file
in the archive using only ascii characters. It can be
up to 65,535 bytes in length.
<offset> is an unsigned long long integer in string form
containing the relative offset in bytes of the first
header block for the file in the archive. The offset is
relative to the end of the last block of the index file
in the archive they are listed in.
<entry_size> is an unsigned long long integer in string
form containing the size of the file's entry in bytes
in the archive (including archive headers and trailers
for the entry).
<size> is an unsigned long long integer in string form
containing the size of the file in bytes in the archive.
<typeflag> is a single character representing the type
of the file in the archive. Possible values are:
0 Regular File
1 Hard Link
2 Symbolic Link
5 Directory or subdirectory
All values not listed above are reserved for future
use. Unrecognised values should be treated as a
regular file.
An example set of entries would appear as follows:
cfg_cacheNUL0NUL546NUL2560NUL0
pkgNUL2560NUL0NUL1536NUL5
pkg/service%2Ffault-managementNUL4096NUL0NUL1536NUL5
It should be noted that other possible formats were
evaluated for the index file, including those based
on: JSON, XDR, and python's pack. However, all other
formats were found to be deficient for one or more
of the following reasons:
- larger in size
- no streaming support (required entire index file be
loaded into memory)
- significantly greater parsing times
5. Proposed Changes:
5.1. Client Support for filesystem-based Repository Access:
The pkg.client.api provided by pkg(5) will be updated to allow
access to repositories via the filesystem. All functionality
normally offered by pkg.depotd will be supported.
pkg(1) and packagemanager(1) will be modified to support the
use of URIs using the 'file' scheme. No user visible changes
will be made to any existing subcommands or options except
that URIs using the 'file' scheme will be allowed.
When accessing repositories using the 'file' scheme, clients
by default will not copy package file data into the client's
cache (e.g. /var/pkg/download). Instead, the transport system
will treat configured repositories as an additional read-only
cache.
5.2. Depot Storage, Client Transport and Publication Tool Update:
The pkg.server.repository module will be updated to support
the new repository format outlined in section 4.2.2. Existing
repositories will not automatically be upgraded, while new
repositories will use the new format. An --upgrade option
will be added to pkg.depotd to upgrade existing repositories
to the new format.
These changes will automatically allow the client to access
repositories in the new format when using filesystem-based
access. Older clients will remain unable to access repo-
sitories in the new format.
The client transport system will be updated to support all
publication operations and the publication tools and project
private APIs will be changed to use the client transport
system.
The '-d' option of pkgrecv(1) will be changed such that if
the name of a file with a '.p5p' extension is specified,
and that file does not already exist, a pkg(5) archive
file will be created containing the specified packages.
If the file already exists, it will exit with an error.
When pkgrecv(1) creates pkg(5) archive files, it will omit
catalog and index data.
Due to the transport changes above, pkgrecv(1) will also
be able to use pkg(5) archive files as a source of package
data. pkgsend(1) will not support the use of pkg(5)
archive files as a destination due to the publication
model it currently uses.
To support the expanded multiple publisher version 2 format
of repositories, the depot server will be updated to respond
to requests as follows:
- If clients include the publisher prefix as part of the request
path, then responses will be for that specific publisher's
data. For example:
http://localhost/dev/opensolaris.org/manifest/
0/opensolaris.org/backup%2Fareca/7.1%2C5.11-0.134
%3A20100302T005731Z
http://localhost/dev/file/0/opensolaris.org/
2ce6c746c85cd7ac44571d094b53c5fe1bfc32c8
- The default publisher specified in the repository cfg_cache
will be used when responding to requests for operations that
do not include the publisher prefix. For example:
http://localhost/dev/manifest/0/
backup%2Fareca/7.1%2C5.11-0.134%3A20100302T005731Z
...provides a response identical to the first case where the
publisher prefix was provided as part of the request. Those
expecting to maintain a large population of older clients
should reassign publisher URLs down a level, to include the
publisher explicitly although this is not required for
correct operation.
5.3. Client Storage and Image Format Update:
To simplify and unify the storage format used by the client,
and pkg(5) repositories, the format of the client image
will be changed to use the structure described below.
For a version 4 image (the proposed format), the structure is
as follows:
<IMG_ROOT>
cache/
<publisher_prefix>/
certs/
<publisher signing certificates>
pkg/
<stem>/
<manifest-cache-files>
publisher/
<as described in section 4.2.2>
index/
<client search index files>
ssl/
client ssl certificates>
history/
<client history files>
pm_cache/
<package manager data files>
state/
installed/
<image catalog files>
known/
<image catalog files>
A new property named 'version' will be added to the image
and will be readonly (cannot be set using the set-property
subcommand of pkg(1)).
Existing images will not automatically be upgraded to the new
format. A new subcommand named 'pkg upgrade-image' will be
added instead to allow an image to be upgraded to the newest
format. New images will automatically be created using the
new format.
5.4. Client and Depot Support for On-Disk Archive Format:
The pkg.server.repository module will be updated to support
the serving of a repository in readonly mode using a pkg(5)
archive file.
The pkg.client.api transport system will be updated to support
the usage of a pkg(5) archive file as an origin for package
data. When multiple origins are present for a publisher, they
will be checked in this order of preference for package data:
- local client cache
- pkg(5) archive files
- remaining origins and mirrors
To support the specification of temporary origins, the install
and image-update subcommands will be modified by adding a '-g'
option to specify additional temporary package origin URIs or
the path to a pkg(5) archive file. The '-g' option may be
specified multiple times. As an example:
$ pkg install -g /path/to/foo.p5p \
-g http://mytemprepo:10000/ \
-g file:/path/to/bar.p5p \
foo bar localpkg
$ pkg image-update -g /path/to/foo.p5p
The pkg list and pkg info commands will also be updated by
adding the '-g' option described above, with the exception
that the '-g' option may only be specified once, and only
the source named will be used for the operation. As an
example, to list the packages found in a pkg(5) archive
file or repository:
$ pkg list -g /path/to/foo.p5p
NAME (PUBLISHER) VERSION STATE UFOXI
foo 1.0-0.133 known -----
$ pkg list -g file:/path/to/repo
NAME (PUBLISHER) VERSION STATE UFOXI
repo-package-1 2.0-0.133 known -----
$ pkg list -g http://mytemprepo:10000
NAME (PUBLISHER) VERSION STATE UFOXI
localpkg 3.0-0.133 known -----
'-g' was chosen for the option usage described above to match
the '-g' already used by set-publisher and image-create for
origins, and due to the unfortunate existing usage of '-s'
by the 'pkg list' subcommand.
6. Reference Documents:
Project team members and community members have provided a number of
informal comments that served as the basis for the goals of this
project:
- "new on-disk format?", 18 Jan. 2008:
http://markmail.org/thread/2kg6w5bfwp4x3knc
- "reorganising the repository and client metadata", 23. Sep. 2009:
http://markmail.org/thread/stfrosvx3v6if2fi
- "ZAP - Zip Archive Packaging", Sep. 2007:
http://markmail.org/thread/ijyq3mlrhaofccgx
In addition, the following materials were referenced when writing
this proposal:
- "7z", 12 Apr. 2010:
http://en.wikipedia.org/wiki/7z
- "RFC2616: HTTP/1.1 Header Field Definitions", 01 Sep. 2004:
http://www.w3.org/Protocols/rfc2616/
rfc2616-sec14.html#sec14.35.1
- "cpio", 21 Mar. 2010:
http://en.wikipedia.org/wiki/Cpio
- "copy file archives in and out", 26 Mar. 2007:
http://heirloom.sourceforge.net/man/cpio.1.html
- "The gzip file format", Date Unknown:
http://www.gzip.org/format.txt
- "DragonFly File Formats Manual, cpio -- format of cpio archive
files"
http://leaf.dragonflybsd.org/cgi/web-man?command=cpio§ion=5
- "A Quick Benchmark: Gzip vs. Bzip2 vs. LZMA", 31 May. 2005:
http://tukaani.org/lzma/benchmarks.html
- "Lempel Ziv Markov Algorithm and 7-Zip", 7 Feb. 2008:
http://blogs.sun.com/clayb/entry/lempel_ziv_markov_algorithm_and
- "The Open Group Base Specifications Issue 6: pax Interchange
Format, IEEE Std 1003.1, 2004 Edition"
http://www.opengroup.org/onlinepubs/009695399/utilities/
pax.html#tag_04_100_13_01
- ".ZIP File Format Specification", 28 Sep. 2007:
http://www.pkware.com/documents/casestudies/APPNOTE.TXT
- "ZIP (file format)", 17 Apr. 2010:
http://en.wikipedia.org/wiki/ZIP_%28file_format%29
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