Using Binary Delta Compression (BDC)
Technology to Update Windows XP and
Windows Server 2003
Microsoft Corporation
Published: June 2005
Author: Susan Potter
Editor: Bonnie Birger
Abstract
This document describes Binary Delta Compression (BDC) technology and its use in
deploying software updates for Microsoft® Windows® XP and Windows Server™ 2003
operating systems. BDC is a differential compression technology that reduces the
download size of software update packages. Express Installation updates that use BDC
technology also make use of Background Intelligent Transfer Service (BITS), a file
transfer technology that efficiently transfers files using idle network bandwidth.
The information contained in this document represents the current view of Microsoft
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Contents
Binary Delta Compression (BDC) Technology ................................................................... 5
Introduction to Binary Delta Compression ....................................................................... 5
Windows Update and BITS Technology Support ............................................................ 6
Express Installation Overview ......................................................................................... 7
Network Connectivity During Installation ......................................................................... 9
Compression Ratios for BDC ........................................................................................ 10
How Binary Delta Compression Works ......................................................................... 10
Selecting a Delta File to Download ................................................................................ 12
Self-Contained Installation vs. Express Installation ....................................................... 13
Summary ....................................................................................................................... 15
BDC
5
Binary Delta Compression (BDC)
Technology
This document describes how Binary Delta Compression (BDC) technology helps reduce
the download size of software update packages for Microsoft® Windows® XP and
Microsoft® Windows Server™ 2003 operating systems. Express Installation packages
use BDC for downloading only the differences between old files and new files, resulting in
smaller software update packages that require less time and network bandwidth to
deploy. Files downloaded from versions of Microsoft Windows Update later than version
4.0 also support the use of the Background Intelligent Transfer Service (BITS), which
efficiently transfers update files, preserves the responsiveness of other network
applications, and automatically resumes file transfers after restarts or network
disconnects. In addition, update installations that use the latest version of the Microsoft
Software Update Services (SUS) client no longer require Microsoft Internet Information
Services (IIS) server but can download updates from any server that supports HTTP 1.1,
thereby reducing server traffic. Together, BDC and BITS can help streamline the process
for downloading critical and security updates. (For information about updating Microsoft®
Windows® 2000 using BDC, see Binary Delta Compression at
http://go.microsoft.com/fwlink/?LinkId=44102.)
Introduction to Binary Delta Compression
Binary Delta Compression (BDC) is a differential compression technology that helps
reduce the total download size of software update packages for Microsoft Express
Installations, while ensuring that there is no loss of original data during the compression
and transfer process. This technology reduces the download size of software updates by
tailoring the download for each target computer, while producing exactly the same
installation results as conventional update packages. Express Installation packages
require less network bandwidth and help to reduce deployment costs.
Conventional “self-contained” update packages — which contain all of the new files in
compressed form — range from 500 kilobytes (KB) to several megabytes (MB), while
service packs can be 100 MB or more. This makes download size significant when
bandwidth is an issue. Conventional data compression techniques use a compressor that
accepts one file as input and produces a single compact version of that file as output. A
decompressor performs the inverse function, accepting the compact form as input and
reconstructing the original file for output on the target computer. Express Installation
BDC
6
packages that use BDC not only compress the required files but also reduce the size of
the download by downloading only the differences (deltas) between existing files on a
target computer and the desired new files where possible. The desired new files are then
synthesized from the existing files and the downloaded delta files. After the synthesized
delta files are verified as correct, the installation proceeds in the same manner as for selfcontained update packages.
Windows Update and BITS Technology
Support
Updates that use BITS technology can be downloaded from the latest versions of
Windows Update at http://go.microsoft.com/fwlink/?LinkId=18539. When you are running
Windows XP or Windows Server 2003 with Service Pack 1 (SP1) operating systems, you
automatically access the latest version of Windows Update (or Automatic Updates),
thereby making BITS technology available for your downloads. This includes downloads
of critical and security updates, such as Service Pack 2 (SP2) for Windows XP. (Note that
BITS technology is not currently available for downloads to Windows 2000.)
In addition, when you update Windows XP or Windows Server 2003 operating systems
from Windows Update for the first time, your download includes the latest Software
Update Services (SUS) client. This version of the SUS client brings significant
improvements to Express Installations, including:

Downloads managed using BITS technology
Files for Windows XP and Windows Server 2003 (with SP1 installed) downloaded
from Automatic Updates and Windows Update are managed using BITS technology.
BITS manages file transfers to preserve the responsiveness of other network
applications, and automatically resumes a transfer if the network connection is
disrupted or the computer is restarted. For more information about BITS, see
Background Intelligent Transfer Service in Windows Server 2003 at
http://go.microsoft.com/fwlink/?LinkId=41252.

Increased availability of Packages
Previously, the server for Express Installation package downloads could only be a
customized Internet Information Services (IIS) server. Clients could download
updates only from these customized Microsoft servers, which caused large volumes
of traffic on a limited number of update servers. With the latest version of Windows
Update, any server that supports HTTP 1.1 can host and cache Express Installation
packages and the deltas for download, which substantially increases the availability
of packages.
BDC
7
The following table provides a summary of operating system compatibility with the latest
versions of Windows Update, BDC, and BITS. Note that when you access Windows
Update, the version of Windows Update that you connect to is determined by the
operating system that you are running, and does not display to the user.
Operating System
Windows
Supports BDC
Supports BITS
Update version
Requires server
running IIS for
update
download
Windows 2000
4
Yes
No
Yes
Windows XP
5
Yes
Yes
No
Windows
4
Server 2003 without
SP1 installed
Yes
No
Yes
Windows
Server 2003 with
SP1 installed
Yes
Yes
No
5a
Express Installation Overview
Express Installation packages are used primarily for individual computers that have
Internet connectivity during installation; self-contained packages are used primarily for
corporate deployment and other scenarios where downloading over the Internet is not
possible or not desired. Beginning in 2003, Microsoft extended Express Installation
benefits to critical, security, and other Windows updates available from the Windows
Update Web site at http://go.microsoft.com/fwlink/?LinkId=18539. The package installer
(formerly called Update.exe) is used to install these software updates.
The following diagram illustrates the Express Installation process.
BDC
8
Figure 1: The Express Installation Process
The steps for an Express installation using the SUS client (which is included in your first
download from Windows Update 5.0) are as follows:
1. When updates become available to a target computer from Windows Update (version
5.0 or later), or through Automatic Updates, the SUS client on the computer uses
BITS technology to download the Express Installation package (which includes only
the installer files and installation instructions, but does not contain the new payload
files to be installed).
2. The Express Installation package is expanded into a sandbox (a restricted
environment) on the target computer.
3. Update.exe, which is one of the files contained inside the Express Installation
package, conducts an inventory of the target computer to determine:
a. Whether any of the files to be updated are available in an updated form in some
location on the computer.
b. Whether any older versions of the files to be updated are available on the
computer. Such files can be updated by downloading only delta files to update
the old files. This is explained in detail in the section "Selecting a Delta File to
Download" later in this white paper.
Note:
For more information about the package installer for Windows, see The
Package Installer (Formerly Called Update.exe) for Microsoft Windows
BDC
9
Operating Systems and Windows Components at
http://go.microsoft.com/fwlink/?LinkId=44441.
4. A request for files and delta files is submitted to the Windows Update Agent resident
on the target computer. This request will be for delta files when older versions of the
files being updated are available on the target computer. Otherwise, compressed
versions of the entire updated file will be requested.
5. The SUS client downloads the files and delta files requested by Update.exe. BITS
transfers files over unused bandwidth, and also maintains file transfers when a
network disconnection occurs or a computer needs to be restarted. When the
network connection is reestablished, BITS continues the download from where it left
off.
6. The downloaded files are copied into the sandbox. Deltas are applied to old versions
of the respective files to synthesize the most updated version.
7. The SUS client then invokes Update.exe to validate the downloaded delta files and to
generate new files. Update.exe verifies the integrity of the newly created files. If any
new files unexpectedly fail this check, the self-contained package is downloaded and
run instead.
8. Update.exe notifies the SUS client that all of the files required to install the update
are present.
9. If Automatic Updates is configured to require confirmation, the SUS client prompts
the computer administrator that an update is ready to install, and invokes Update.exe
when the administrator chooses to accept installation. If Automatic Updates is set to
install updates automatically, or if the SUS client performed the previous steps
because the administrator chose to install the updates by using the Windows Update
Web site, the installation continues automatically.
Network Connectivity During Installation
Unlike a self-contained package, an Express Installation package performs an additional
download while it is running. In the past, Express Installations required a consistent
network or Internet connection to the server hosting the updates while the installation ran.
However, this requirement for network connectivity made Express Installations unsuitable
when network or Internet connectivity to the server was not consistently available during
update installation. Yet a previously downloaded self-contained package could be
installed at a later time, even when a dial-up Internet connection was not available. But
for installations that support BITS technology, it is now possible for installations to stop
downloading when network connectivity is interrupted and then resume a download as
BDC 10
network connectivity becomes available. This improvement allows updates to be
deployed in a more timely fashion on installations that support BITS technology, when
using either Automatic Updates or Windows Update.
Compression Ratios for BDC
The size of files downloaded using BDC is dependant on the total amount of data
required to bring a file up-to-date. When delta files can be used, the total size of the
download can be significantly reduced.
While a compression ratio for conventional compression of executable files might be
around 3:1, the compression ratio for BDC can be from 10:1 to 1,000:1 or higher,
depending on the size of the original file and the number of differences between the
original files and the new files. For example, if the code change is to fix a single buffer
overrun, the delta file might be as small as a few hundred bytes.
Using Express Installation with BDC, a service pack download is considerably smaller
than the equivalent self-contained package. This makes it much more convenient to
download service packs over low-bandwidth Internet connections. For example, the
Microsoft® Windows® 2000 Service Pack 4 self-contained package is 135 MB, which
would be impractical to download over a dial-up modem. However, the Express
Installation package for this service pack typically downloads only 15 MB for a computer
running Windows 2000 Server with Service Pack 3 installed. For computers that do not
have the latest service pack installed, there will be a bigger difference in file size between
the currently installed files and the required files, resulting in larger downloads.
How Binary Delta Compression Works
Binary Delta Compression is a technique for compressing files that differs somewhat from
conventional compression. Conventional data compression techniques for file delivery
use a compressor that accepts one file as input and produces a single compact version
of that file as output. A decompressor performs the inverse function, accepting the
compact form as input and reconstructing the original file for output on the target
computer. This process is summarized in the following diagram.
BDC 11
Figure 2: Conventional File Compression and Decompression
In contrast, for each file to be delivered, BDC takes two files as input: the new file for
delivery (Fnew) and an old file (Fold) from the target computer, which is usually an older
version of the new file. The BDC compressor (Binary Delta Compression Creator)
determines the differences between the old file and the new file and creates a compact
delta file (∆F) as output. On the target computer, the BDC decompressor (Binary Delta
Compression Applicator) takes the existing old file as a reference and the compact delta
file as input and creates the new file as output. If the old file and the new file are very
similar, the size of the delta file will be very small, generally much smaller than the file
that results from simply compressing the new file conventionally. The size of the delta file
is proportional to the number of differences between the old file and the new file.
Binary Delta Compression Creator ( Fold , Fnew )→∆F
Binary Delta Compression Applicator ( Fold , ∆F )→Fnew
If the old file is known to exist on the target computer, only the compact delta file needs to
be transmitted to the target computer to construct the new file.
The Binary Delta Compression process is illustrated in the following Input/Output
diagram.
BDC 12
Figure 3: Binary Delta Compression and Decompression
Selecting a Delta File to Download
BDC is effective in reducing the download size of software update packages because
most software updates contain only small modifications relative to the files that already
exist on the target computer. However, a target computer might have many different
versions of a file at one time. Therefore, the package installer for Windows must select
which file is most similar to the target file and will yield the smallest delta file.
For BDC to be effective, the appropriate delta file must be selected for download based
on which old files are available on the target computer.
When Automatic Updates or Windows Update downloads an Express Installation
package onto the target computer, the package initially contains only the installation files
(including Update.exe) and instructions, but does not contain the new files to be installed.
Update.exe takes an inventory of the available old files on the target computer and then
requests the SUS client to selectively download the set of delta files necessary to
produce the set of new files required for the installation. The set of delta files required is
dependent on the configuration of the target computer, so different target computers
might download different sets of delta files to produce the same set of new files for
installation. When delta files are downloaded using BITS technology, the download is
robust enough to persist across restarts and network disconnections.
The sets of delta files for any given software update package are stored in a patch
storage file (PSF) that is hosted on an HTTP 1.1 server. In addition to delta files based on
BDC 13
any number of older, previously released versions of the new files, the PSF also
contains compressed copies of the updated files. If a given target computer does not
have an old file that matches any of the delta files contained in the PSF, a compressed
copy of the updated file is downloaded instead of a delta file. This provides a seamless,
fault-tolerant mechanism to ensure that all of the new files can be produced on the target
computer regardless of its existing configuration. Because each PSF contains all of the
compressed new files and many delta files for each new file, the patch storage files are
often quite large. However, because each individual installation downloads only the
required set of delta files necessary for that target computer, each installation will
download only a small fraction of the entire contents of a PSF.
The following scenario illustrates a typical express download and installation sequence
using Update.exe to update the file Webcheck.dll to version 5.2. The functionality of
Update.exe is described in detail in The Package Installer (Formerly Called Update.exe)
for Microsoft Windows Operating Systems and Windows Components at
http://go.microsoft.com/fwlink/?LinkId=44441.
1. The Express Installation package is downloaded and run. This package includes
installation instructions, the package installer for Windows, and information about the
new Webcheck.dll file, but does not contain the new Webcheck.dll version 5.2 file
itself.
2. Update.exe takes inventory and discovers an existing Webcheck.dll version 4.9 file
on the target computer.
3. Update.exe downloads the Webcheck.dll version 4.9-to-5.2 delta file from the PSF
and then creates the new Webcheck.dll version 5.2 from the existing Webcheck.dll
version 4.9 and the delta file.
4. Update.exe verifies the integrity of the new Webcheck.dll version 5.2 from its
cryptographic signature.
5. Installation proceeds in the same manner as if Webcheck.dll version 5.2 was
included in the original package, replacing the 4.9 version with the new 5.2 version.
Self-Contained Installation vs. Express
Installation
A self-contained package contains installer files and new files to be installed within a
single self-extracting compressed file. An Express Installation package contains only
installer files within a single compressed file; it does not include the new files to be
installed. A typical Express Installation does include some additional steps but the
installation process is otherwise the same.
BDC 14
This table lists the steps that are completed during a self-contained installation versus
Express Installation.
Sequence
Steps
Typical self-
Typical Express
contained
Installation
installation
1
Update.exe interprets installation
instructions (from Update.inf) that
define which files should be
installed to which locations on the
target computer.
Yes
Yes
Update.exe also interprets rules
that describe when file
replacements are dependent on file
version checks or hardware
compatibility.
2
Update.exe computes a sequence
of file copy operations to be
performed from the instructions.
Yes
Yes
3
Update.exe archives existing files
that will be replaced. This provides
the capability to uninstall the
update.
Yes
Yes
4
Update.exe determines which new
files are required for installation
and are not present.
No
Yes
5
Update.exe inventories the existing
versions of the files that will be
replaced by the new files and
determines which delta files — or
in some cases compressed copies
of the updated files— should be
downloaded to produce the
required new files from the existing
files.
No
Yes
BDC 15
Sequence
Steps
Typical self-
Typical Express
contained
Installation
installation
6
Update.exe requests the delta file
download from the SUS client.
No
Yes
7
The SUS client then invokes
Update.exe to validate the
downloaded delta files and
generates new files. Update.exe
notifies the SUS client that all the
files required to install the update
are present, or requests some files
as described in Step 6.
No
Yes
8
Update.exe copies the new files to
the locations specified in
Update.inf.
Yes
Yes
Summary
The package installer for Windows can conduct Express Installations using Binary Delta
Compression (BDC) technology to reduce the download size of software update
packages. This technology can significantly decrease total download size and time for
computers with slower network connections. For Windows XP and Windows
Server 2003 SP1, these Express Installations use BITS to conduct file downloads so that
the process is managed to preserve the responsiveness of other network applications,
and installation will automatically resume in case the computer is restarted or if network
connectivity is disrupted for any other reason.