Everything you ever wanted to know about compression but were afraid to ask…

Using HTTP Compression for Faster Downloads (IIS 6.0)

If your Web sites use large amounts of bandwidth or if you want to use bandwidth more effectively, consider enabling
HTTP compression, which provides faster transmission times between IIS and
compression-enabled browsers regardless of whether your content is served from
local storage or a UNC resource. If your network bandwidth is restricted, HTTP
compression can be beneficial unless your processor usage is already very high.

IIS provides the following
compression options:

•	Static files only.
•	Dynamic application responses only.
•	Both static files and dynamic application responses.

Dynamic processing can affect CPU
resources because IIS does not cache compressed versions of dynamic output. If
compression is enabled for dynamic responses and IIS receives a request for a
file that contains dynamic content, the response that IIS sends is compressed
every time it is requested. Because dynamic compression consumes considerable
CPU time and memory resources, use it only on servers that have slow network
connections but CPU time to spare.

Compressed static responses can be
cached and therefore do not affect CPU resources like dynamic responses do.

How HTTP Compression Works

When IIS receives a request, it
checks whether the browser that sent the request is compression-enabled. (Recent
versions of Microsoft® Internet Explorer and most other browsers typically send
the following header if they are compression-enabled: Accept-Encoding: gzip,
deflate.) IIS then determines whether the request is for a static file or for
dynamic content.

If the content of the file is
static, IIS checks whether the file has previously been requested and is already
stored in a compressed format in the temporary compression directory. If a
compressed version of the requested file is not found, IIS sends an uncompressed
version of the requested file to the client browser while a background thread
compresses the requested file. The newly compressed file is then stored in the
compression directory, and subsequent requests for that file are serviced
directly from the compression directory. In other words, an uncompressed version
of the file is returned to the client unless a compressed version of the file
already exists in the compression directory.

If the file contains dynamic
content, IIS compresses the response as it is generated and sends the compressed
response to the browser. No copy of the file is cached by the Web server.

The performance cost of compressing
a static file is modest and is typically incurred only once, because the file is
then stored in the temporary compression directory. The cost of compressing
dynamically generated files is somewhat higher because the files are not cached
and must be regenerated with each request. The cost of expanding the file at the
browser is minimal. Compressed files download faster, which makes them
particularly beneficial to the performance of any browser that uses a network
connection with restricted bandwidth (a modem connection, for example).

When you enable HTTP compression,
compressed files are given a default expiration date of Jan. 1, 1997. This
expiration date prevents proxy servers from serving cached copies of compressed
files to browsers that are not compression-enabled. This expiration date also
forces browsers to return to the server for a fresh copy of the file when the
user makes a new request instead of displaying a cached copy of the file. When
you enable HTTP compression, the default settings for the HcExpiresHeader,
HcCacheControlHeader, and HcSendCacheHeaders metabase properties
ensure that older clients and proxy servers do not attempt to cache compressed
files. Before you change these settings, see

Metabase Property Reference for information about these and related metabase
properties.

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Determining Whether HTTP Compression Will Improve
Performance

If your server generates a large volume of dynamic content, consider whether the additional processing cost of
HTTP compression is one that you can reasonably afford. If the % Processor Time
counter is already 80 percent or higher, enabling HTTP compression is not
recommended.

To evaluate how much of your
processor is typically being used, follow these steps:

1.	Establish a baseline for your processor usage by using System Monitor to log the following counters over several days. If you use Performance Logs and Alerts, you can log the data to a database and then query the data, examining the results in detail. • Processor\% Processor Time. This counter has a total instance and a separate instance for each processor in the system. If your server has more than one processor, you need to watch the individual processors as well as the total to discover any imbalance in the workload. • Network Interface\Bytes Sent/sec. Counters for the Network Interface performance object display the rate at which bytes are transmitted over a TCP/IP connection by monitoring the counters on the network adapter. For information about additional counters to monitor for this object, see Table 6.8.
2.	Enable compression, and continue to log the values for these counters for an extended period — preferably for several days — so you have a good basis for comparison. Collect a broad sample to determine how compression affects various aspects of performance. Conduct the following tests: • Enable static compression only, dynamic compressions only, and both. • Change the list of files that you use for compression testing for both static and dynamic content. • Vary the compression level. Try this on all content types.
3.	Compare the data from monitoring with and without compression, and with different types of compression, different compression levels, and different files.

Important

If you see signs of blocking or
bottlenecking during the preceding testing, promptly stop the test. A
significant drop in the value of either counter indicates that performance with
compression enabled has decreased relative to performance without compression
enabled.

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Choosing Compression Options

When you configure HTTP compression
by using IIS Manager, IIS 6.0 automatically applies the default settings shown
in Table 6.12. You can configure custom settings for each compression option by
editing the metabase or alternatively — in the case of the compression directory
— by using IIS Manager.

Table 6.12 Default Settings When HTTP Compression Is Enabled
Compression Options	File Type	Default Configuration	Configuration Method
File types compressed	Static	.txt, .htm, and .html	Metabase
	Dynamic	.exe, .dll, and .asp	Metabase
Compression schemes	Static	Both gzip and deflate	Metabase
	Dynamic	Both gzip and deflate	Metabase
Compression level	Static	10	Metabase
	Dynamic	0	Metabase
Compression directory	Static	Size: 95 MB Location1: %Windir%\IIS Temporary Compressed Files	Metabase or IIS Manager
	Dynamic	No directory2	N.A.

¹ The compression directory must reside on an NTFS file
system volume.

² When dynamic files are compressed, they are not cached.

If the default HTTP compression
configuration does not meet the needs of your organization, you can make the
following changes to the configuration:

•	Specify additional file types to compress. Edit the HcFileExtensions metabase property (for static files) and the HcScriptFileExtensions metabase property (for dynamic files) to apply compression to additional file types. Add only compressible files types to your compression scheme because trying to compress an already compressed file — such as .jpg, .mp3, or .zip files — wastes processing time.
•	Enable or disable the gzip or deflate compression scheme. When you use IIS Manager to enable compression, IIS enables both compression schemes (gzip and deflate), so that IIS can apply whichever compression scheme the client requests. You can edit the metabase to specify that only a particular compression scheme is used. If you disable the only compression scheme that a client supports, the server replies with an uncompressed file. Note Disabling either the gzip or deflate compression schemes can have unintended consequences, such as not compressing responses to a particular browser. For maximum compatibility, it is recommended that you leave both compression schemes enabled.
•	Change the level of compression for static or dynamic files. Compression levels range from 0 through 10. Higher compression levels produce smaller compressed files but use more CPU and memory. Lower compression levels produce slightly larger compressed files, but with less impact on CPU and memory usage. To configure a compression level other than 10 for static files and other than zero for dynamic files, edit the HcOnDemandCompLevel and HcDynamicCompressionLevel metabase properties. For dynamic compression, increasing the compression level can significantly increase CPU usage. The default compression level of zero uses the least amount of CPU resources and can increase performance if network bandwidth is adequate. Raise the dynamic compression level above zero only if you need more network bandwidth and have sufficient CPU capacity to handle the extra load.
•	Change the location or size of the compression directory. To change the location and size of the compression directory for static files, edit the HcCompressionDirectory, HcDoDiskSpaceLimiting, and HcMaxDiskSpaceUsage metabase properties.

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Compression Strategies

When the CPU of your server is not
heavily loaded, the simplest compression strategy is to enable static and
dynamic compression for all of the sites and site elements (directories and
files) on the server. This is known as global HTTP compression. However, when
the CPU load of your server is high, you might not want to enable compression
for all of the sites and site elements on the server.

Depending on CPU usage, there are
two strategies for fine tuning which sites and site elements are compressed:

•	Enable compression globally, and then disable compression selectively for specific sites or site elements.
•	Leave global compression disabled, and then enable compression selectively for specific sites or site elements.

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Enabling Global HTTP Compression

When you enable HTTP compression by
using IIS Manager, IIS applies compression according to the default settings
shown in Table 6.12. With the exception of the location and size of the
compression directory, you cannot change the default settings by using
IIS Manager. Instead, you must edit the metabase to change the default settings.

Important

After enabling HTTP compression
(either by using IIS Manager or by editing the metabase), you must restart IIS
for the change to take effect.

For more information about enabling
global HTTP compression by using IIS Manager and through Adsutil.vbs, see

Enabling HTTP Compression.

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Configuring HTTP Compression for Individual Sites and Site
Elements

All of the compression
configurations discussed so far are located under W3SVC/Filters/Compression in
the metabase and are used to apply global HTTP compression, which is applied by
using IIS Manager or by using one of the metabase properties that are used for
global compression (beginning with Hc). IIS 6.0 offers two new metabase
properties, DoStaticCompression and DoDynamicCompression(beginning
with Do), which can be used to enable or disable compression at the individual
site and site element levels. Use these two new properties to apply compression
to individual namespaces. Hence, you can enable global compression by using IIS
Manager or the Hc global properties and then disable one of the Do properties
(or vice versa) to obtain compression at a specific location in the IIS
metabase.

For more information about enabling
compression for individual Sites and Site elements, see

Enabling HTTP Compression.

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Specifying File Types and Compression Schemes by Editing
the Metabase

After enabling HTTP compression,
you can use the Adsutil.vbs command-line utility (located in the systemroot\Inetpub\AdminScripts
folder) to edit the metabase, specifying which static or dynamic file types to
compress and which compression scheme to apply.

Using Adsutil.vbs, you can
customize HTTP compression in the following ways:

•	Apply the gzip or deflate compression scheme, or both. To apply both schemes, you must execute a separate set command for each compression scheme.
•	Customize the file types to which both static and dynamic compression are applied. You must execute a separate set command for each compression type. If you use IIS Manager to enable compression and you then use Adsutil.vbs to specify specific static or dynamic file types, you replace any default values that were applied by IIS Manager.

Note

For compression to be performed,
the HcDoStaticCompression metabase property must be set to true
(for static compression) and the HcDoDynamicCompression metabase property
must be set to true (for dynamic compression). You can set these by using
IIS Manager or Adsutil.vbs. For instructions, see the procedures in the
preceding section.

The following syntax enables
compression at the W3SVC level (which means it is applied to all Web sites and
virtual directories on the Web server):

cscript.exe adsutil.vbs set W3svc/Filters/Compression/{GZIP|DEFLATE}/

{HcFileExtensions "StaticFileType" ...}|{HcScriptFileExtensions
"DynamicFileType" ...}

You must restart the WWW service
before any changes take effect. For information about restarting a service, see


Common Administrative Tasks. Table 6.13describes the parameters for customizing file
types and compression schemes.

Table 6.13 Parameters for Setting Compression Schemes and File Types for HTTP Compression
Parameter	Description
	Specifies which compression scheme (either gzip or deflate) to apply to the specified file types. To enable both compression schemes, you must run this command once for each compression scheme. For maximum compatibility, leave both compression schemes enabled.
	Specifies which static file types to apply the compression scheme to — for example, .txt, .js, .css, .doc, or .xls files. To specify a file type, enclose the file name extension (without the leading period) in quotation marks. Separate multiple file types with a space. This parameter cannot be executed together with the HcScriptFileExtensions "DynamicFileType" parameter.
	Specifies which dynamic file types to apply the compression scheme to — for example, .exe, .dll, or .asp files. To specify a file type, enclose the file name extension (without the leading period) in quotation marks. Separate multiple file types with a space. This parameter cannot be executed together with theHcFileExtensions "StaticFileType" parameter. Important Setting the HcScriptFileExtensions parameter to an empty string can adversely affect your servers performance. If this parameter is empty, all dynamic responses are sent compressed. In addition, any static file type not specified in HcFileExtensions is dynamically compressed and therefore not cached.

For example, the following command
uses a deflate compression scheme to apply static compression at the W3SVC level
to text files (.txt) and cascading style sheets (.css):

cscript.exe adsutil.vbs set w3svc/Filters/Compression/DEFLATE/HcFileExtensions
"txt" "css"

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Specifying the Location and Size of the Compression
Directory for Static Compression

Three metabase properties enable
you to change the location of the default directory for static compression and
to limit its size.

Changing the location of the compression directory

The HcCompressionDirectory
metabase property specifies the folder where compressed versions of static files
are temporarily cached. By default, the compression directory is at %Windir%\IIS
Temporary Compressed Files.

Due to a number of differences
between NTFS and FAT volumes — such as differences in time stamping and access
control mechanisms, and the efficiency with which NTFS handles directories that
contain large numbers of files — the compression directory must reside on an
uncompressed and unshared NTFS volume. In addition, the access control lists
(ACLs) for the folder must include Full Control access to the identity of the
application pool or to the IIS_WPG group. If a requested file resides on an NTFS
volume and the compression directory is on a FAT volume, IIS does not perform
HTTP compression on that file.

Setting a size limit for the compression directory

In addition, you can configure the
HcDoDiskSpaceLimiting and HcMaxDiskSpaceUsage metabase properties
to limit the amount of disk space that all files in the compression directory
can occupy. HcDoDiskSpaceLimiting enables disk space limiting. If it is
set to true, IIS limits compressed files to no more than the number of
bytes specified by HcMaxDiskSpaceUsage. After the limit is reached,
compressed files are removed from the compression directory on a least recently
used basis. If HcDoDiskSpaceLimiting is set to false, no disk
space limit is enforced for the compression directory.

HcDoDiskSpaceLimiting is relevant only if you have set the property
HcDoOnDemandCompression to true. For more information about these
metabase properties, see

Metabase Property Reference.

Important

Limiting the use of disk space has
a significant impact on performance because additional overhead is required to
check for the limit. Use this feature only when you lack adequate disk space to
cache all content files on your Web server.

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Disabling HTTP Compression for Requests That Come Through
Certain Proxy Servers

Certain HTTP proxy servers,
including some advertised as HTTP 1.1–compliant, do not handle the caching of
compressed objects correctly. The HcNoCompressionForProxies metabase
property allows you to disable the HTTP 1.1 response for compression requests
that come through proxy servers.

In addition, you can use the
HcNoCompressionForHttp10 metabase property to disable compression for
requests that contain an HTTP 1.0 version number. HTTP 1.0, as described in
RFC 1945, Hypertext Transfer Protocol — HTTP/1.0, provides a minimal level of
support for certain types of compression. However, some confusion exists
concerning HTTP 1.0 compression, especially with regard to proxy servers. To
minimize the chance of inappropriately returning a compressed file to a client
that cannot decompress it, you can use the HcNoCompressionForHttp10
metabase property to disable compression in questionable scenarios.

For more information about the many
metabase properties that you can set when configuring HTTP 1.1 compression,
including a complete list of all the properties you can set, see
IIsCompressionSchemes (ADSI) in the

IIS 6.0 SDK.

Enabling HTTP Compression (IIS 6.0)

You can enable HTTP compression
server-wide or on a specific directory. HTTP compression improves bandwidth
utilization and speeds up Web site performance. In addition, you can compress
static or dynamic responses.

Important

You must be a member of the
Administrators group on the local computer to perform the following procedure or
procedures. As a security best practice, log on to your computer by using an
account that is not in the Administrators group, and then use the runas
command to run IIS Manager as an administrator. At a command prompt, type
runas /user:Administrative_AccountName"mmc
%systemroot%\system32\inetsrv\iis.msc".

Procedures

To enable global HTTP compression by using IIS
Manager

1.	In IIS Manager, double-click the local computer, right-click the Web Sites folder, and then click Properties.
2.	Click theService tab, and in the HTTP compression section, select the Compress application files check box to enable compression for dynamic files.
3.	Select the Compress static files check box to enable compression for static files.
4.	In the Temporary directory box, type the path to a local directory or click Browse to locate a directory. Once a static file is compressed, it is cached in this temporary directory until it expires, or the content changes. The directory must be on the local drive of an NTFS–formatted partition. The directory cannot be compressed or shared, and the access control lists (ACLs) for the directory must include Full Control access to the identity of the application pool or to the IIS_WPG group.
5.	Under Maximum temporary directory size, click a folder size option. If you specify a maximum size under Limited to (in megabytes) (the default setting is 95 MB), then when the limit is reached, IIS automatically cleans up the temporary directory by applying the "least recently used" rule.
6.	Click Apply, and then click OK.

Important

You must be a member of the
Administrators group on the local computer to run scripts and executables. As a
security best practice, log on to your computer by using an account that is not
in the Administrators group, and then use the runas command to run your
script or executable as an administrator. At a command prompt, type runas
/profile /user:MyComputer\Administratorcmd to
open a command window with administrator rights and then type cscript.exeScriptName
(include the script's full path and any parameters).

To enable global HTTP compression by using Adsutil.vbs

1.	Open a command prompt.
2.	To enable dynamic compression, type the following at the command prompt and then press ENTER: cscript adsutil.vbs set w3svc/filters/compression/parameters/HcDoDynamicCompression true
3.	To enable static compression, type the following at the command prompt and then press ENTER: cscript adsutil.vbs set w3svc/filters/compression/parameters/HcDoStaticCompression true

To enable HTTP Compression for Individual Sites and Site Elements

1.	Disable global static compression by executing the following command at a command prompt: adsutil set w3svc/filters/compression/parameters/HcDoStaticCompression false
2.	Enable static compression at this directory by executing the following command at a command prompt: adsutil set w3svc/1/root/Home/StyleSheets/DoStaticCompression true

To disable static compression for
only a single directory, first enable global static compression (if it is
disabled) and then disable static compression at that directory. For example, to
enable static compression for a directory at
http://www.contoso.com/Home/StyleSheets, perform the following steps:

1.	Disable global static compression by executing the following command at a command prompt: adsutil set w3svc/filters/compression/parameters/HcDoStaticCompression true
2.	Enable static compression at this directory by executing the following command at a command prompt: adsutil set w3svc/1/root/Home/StyleSheets/DoStaticCompression false

An example script
is on 192.168.69.198 C:\Inetpub\AdminScripts\CompressionScript.bat

Note! To compress an individual site
you need to know its number. Use
adsutil

Using the Adsutil.vbs Administration Script

Updated: August 22, 2005

Adsutil.vbs is an IIS
administration utility that uses Microsoft Visual Basic Scripting Edition
(VBScript) with Active Directory Service Interfaces (ADSI) to manipulate the IIS
configuration. This script should be run using CScript, which is installed with
Windows Script Host.

Important

You must be a member of the Administrators group on the local computer to run scripts and executables. As a security best practice, log on to your computer using an account that is not in the Administrators group, and then use the runas command to run your script or executable as an administrator. From the command prompt, type runas /profile /user:mymachine\administrator cmd to open a command window with administrator rights and then type cscript.exe ScriptName (include the script's full path and any parameters).

Usage

Cscript.exe adsutil.vbs COMMAND
<path> [<param>...]

Cscript.exe adsutil.vbs COMMAND
[<path> [<parameters>...]]

Command	Description
GET Path	Display chosen parameter.
SET Path Value	Assign a new value.
ENUM Path "/P"	Enumerate all parameters for the path. /P Enumerate the paths only (no data).
ENUM_ALL "/P"	Enumerate all parameters. /P Enumerate the paths only (no data).
DELETE Path	Delete the path or parameter.
CREATE Path [KeyType]	Create the path and assign it the KeyType.
APPCREATEINPROC Path	Create an in process application.
APPCREATEOUTPROC Path	Create an out-of-process application.
APPDELETE Path	Delete the application (if present).
APPUNLOAD Path	Unload an out-of-process application.
APPGETSTATUS Path	Get status of the application.
FIND Path	Find the paths where a parameter is set.
START_SERVER Path	Start the Web site.
STOP_SERVER Path	Stop the Web site.
PAUSE_SERVER Path	Pause the Web site.
CONTINUE_SERVER Path	Unpause the Web site.
HELP	Print all available commands.

Notes

• 
  <Path> is the path of the node for which you are setting
  the property, combined with the name of the property that you are setting. For
  example, if you want to set the ServerComment for Web server number 1,
  the command would be as follows:
  
  
  adsutil
  SET w3svc/1/ServerComment "Web Server Number 1"
  
  The next time
  
  IIS Manager is opened, it will reflect the name of the Web server as
  "Web Server Number 1."


• 
  The switch "-s:server name" can be used after any
  command to execute the command on a remote computer. (See first example below.)
  

Examples

• 
  Cscript.exe adsutil.vbs GET W3SVC/1/ServerBindings
  -s:remotecomputer1


• 
  Cscript.exe adsutil.vbs SET W3SVC/1/ServerBindings ":81:"


• 
  Cscript.exe adsutil.vbs CREATE W3SVC/1/Root/MyVdir
  "IIsWebVirtualDir"


• 
  Cscript.exe adsutil.vbs START_SERVER W3SVC/1


• 
  Cscript.exe adsutil.vbs ENUM /P W3SVC