Compression Policies

HTTP Compression is a publicly defined way to compress content (mostly textual) transferred from Web servers across the world wide Web to browsers. The impact of compression is that the number of transmitted bytes is reduced and thus a higher performance is gained. HTTP Compression uses public domain compression algorithms to encode HTML, XML, JavaScript, CSS and other file formats at the server-side.

AppDirector Compression policy includes 3 levels of optional exceptions. The First level is a list of known problems (bugs) in commonly used browsers that cause them to mishandle specific types of compressed content and thus predefined expectations exists to avoid these problems. This list can be viewed as the in the Browsers Exceptions Rule-List screen as the “Predefined Browser Limitation Rule-List”. The “Predefined Browser Limitation Rule-List” can be customized by copying it using the Copy Rule-Lists into a new Browser Exceptions rule list. The second level of exceptions provided in the form a customized Browser exceptions Rule-List that can define compression behavior in term of browser type (User-Agent) and Content-Type (File type as it displays in Object headers). The third level of exceptions allows defining very specific compression behavior per URI, similarly to Cache URL exception Rule-List. Exceptions are evaluated from most granular (URL Exceptions) through to more gross ones (Browser Exceptions) to the most general ones (Predefined Browser Limitations), to enable the user to use them as a narrowing series of filters.

You can configure compression of HTTP content from AppDirector to the client. AppDirector supports either Gzip or Deflate algorithms. AppDirector has a default compression by software service. The compression level indicates how much the file is compressed. It does not indicate the compressed file size, this depends on the file content itself.

To configure an AppDirector compression policy

1.	From the AppDirector menu, select Layer 4 Traffic Redirection > Compression Policies. The AppDirector Compression Policy pane displays.

2.	Set the parameters.

 

Parameter	Description
Policy Name (Mandatory)	Name of Policy. Default: Empty
Algorithm	Used for defining the preferred compression algorithm that is used by AppDirector if client can receive both Gzip and Deflate compressed content. Values: Gzip, Deflate Default: Gzip
Engine	Defines the type of compression engine used and lets you choose software compression or hardware compression. Default: Software (if the hardware card is installed, the default becomes Hardware. If the hardware card is not installed, Hardware is not allowed).
Minimum Content Length (Bytes)	Minimum file size to be compressed to avoid wasting resources on files that are already small. Values: 1 – 65535 bytes (~64KB) Default: 10240 bytes Note: If the content length header is not specified, the limit does not apply.
Maximum Content Length (Bytes)	Maximum value of content length header of the object to be compressed. Values: 0 – 2147483647 (~20GB) Default: 10485760 bytes Notes:   If the content length header is not specified, the limit does not apply. The value is 0 (unlimited) after upgrading of compression policies.
Compression Level	Used for setting the Gzip compression level. Values: Low <1 > 9 High (Gzip software compression only) Default: 1 Note: Changing the compression level to > 1 has a severe impact on throughput and only a minor reduction in compressed file size.
Allow Compression By Server	This deletes the Accept Encoding Header from requests sent to the server to prevent it from performing compression. Values: Disabled, enabled Default: Disabled
Use Predefined Browser Exceptions	Enable or Disable workarounds for browser limitations for compression according to their type. Values: Predefined Browser Limitations List None Default: Predefined Browser Limitations List Note: Predefined Browser Exceptions are defined in the Predefined Browser Limitations Rule-List that can be found in the Compression Browsers Exception Rule Lists.
Browser Exceptions Rule List	Helps to skip compression of certain objects that create a problem when uncompressed or require too much resource with little benefit (for example, PDFs and PPT folders). Values: None, Specific URL Default: None
URL Exceptions Rule List	Define specific URLs (files/folders) for which all other settings should be overridden and compression or no-compression should be performed. Values: None, Specific URL Default: None Notes:   You can define Domain without anything in the URL field to support use-cases of different behavior per domain. However, if the Domain is not defined (Domain_type=none), then the URL field is mandatory.

3.	Click Set.

4.	Go to Layer 4 Policies.

5.	In the required Layer 4 policy, select the compression policy that you configured