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Manual Chapter: Reducing Conditional GET Requests with Intelligent Browser Referencing
Manual Chapter
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Overview: Reducing conditional GET requests with Intelligent Browser Referencing

When an origin web server sends a response, the client's browser stores the response in its local cache. If the object has no expiration time, the browser makes subsequent requests for that content using a conditional GET request in the form of an extra request header field, such as If-Modified-Since. If the requested object is different than the content that the browser has cached, the origin web server sends a fresh copy of the object to the browser. Otherwise, the browser uses the object that is cached locally.

Although it is faster than serving the entire object each time the browser requests it, conditional GET requests can add up to a significant amount of traffic for your site. For example, if your site has several images for each page, clients might perceive a slow response time because of the large number of conditional GET requests for the image objects.

You can increase the efficiency of the client's web browser's local cache and improve perceived access to your site by enabling the Intelligent Browser Referencing (IBR) feature, which reduces or eliminates requests to your site for relatively static content, such as images and style sheet (CSS) files. When enabled, the Intelligent Browser Referencing feature prompts the WebAccelerator™ system to manage the web browser's cache in two ways:
  • By serving qualifying content with the expiration time set long enough (180 days) that it is unlikely that the browser re-requests the content in the near future.
  • By using an IBR tag (such as wa) to prepend a unique value to qualifying links or URLs embedded in your web pages. This value is a hash of the object and, as such, is guaranteed to uniquely identify the corresponding content stored in the system's cache.
The WebAccelerator system applies the Intelligent Browser Referencing feature only to the following:
  • Image tags: <img src="...">
  • Script tags: <script src="...">
  • Link tags: <link href="...">
  • Forms whose input type is an image: <form><input type="image” src="..."></form>

Intelligent Browser Referencing example

For this Intelligent Browser Referencing (IBR) example, you have three top-level nodes on the Policy Tree as follows:

  • Home. This branch node specifies the rules related to the home page.
  • Applications. This branch node specifies the rules related to the applications for the site, with the following leaf nodes:
    • Default. This leaf node specifies the rules related to non-search related applications.
    • Search. This leaf node specifies the rules related to your site’s search application.
  • Images. This branch node specifies the rules related to graphics images.

For this example, your site serves a simple page that consists of two image files that appear as follows:

<html> <head><title>example page</title></head> <body> <p>The images that your site serves:</p> <p><img src="myImages/image1.jpeg"></p> <p><img src="myImages/image2.jpeg"></p> </body> </html>

When the IBR tag (in this example, wa) is enabled, the WebAccelerator™ system modifies the page as follows:

<html> <head><title>example page</title></head> <body> <p>The images that your site serves:</p> <p><img src="myImages/image1.jpeg;waRG2076ND"></p> <p><img src="myImages/image2.jpeg;wa7YW905BV"></p> </body> </html>

The IBR tag that the WebAccelerator system prepends to each image source URL, is a hash of the image that is stored in cache. In addition, the browser receives a long expiration time for each of the image files.

As a result, the client browser conducts subsequent requests for the page by:

  • Performing a conditional GET request for the base page.
  • Obtaining the embedded images directly from cache if the IBR tag matches.
  • Requesting new images from the WebAccelerator system.

If an image on the page is modified, the WebAccelerator system changes the IBR tag for the image and informs the client of the change. When the client performs a subsequent conditional GET request for the base page and receives the refreshed page, it compares the image, and notes the difference between image1.jpeg;wa4RR87M90 and image1.jpeg;waRG2076ND. This difference prompts the client to re-request the image from the WebAccelerator system.

Advanced IBR settings for General Options

For the General Options list, this table describes the Advanced settings and strings for IBR Options.

Advanced control Default Description
IBR Prefix ;wa Specifies a string that the WebAccelerator™ prepends to a unique value for qualifying links or URLs embedded in your web pages.
Note: If you change the IBR prefix, F5 Networks® recommends thorough testing to ensure that your application functions properly.
IBR Default Lifetime 182 Days Specifies the lifetime for an Intelligent Browser Referencing (IBR) link or URL. Units of time range from Seconds through Months.
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