Browser/proxy server caching

To speed up user experience on the web, most browsers implement a technology called caching. Caching allows information such as webpages, images, and so on, to be saved on a user s computer. If the user calls for a previously requested webpage, the browser is able to access the information more quickly by recalling it from a cache, rather than making another request to the site itself.

For advertisers using DoubleClick for Advertisers, ad images and clickthrough URLs are stored on DoubleClick servers. These images and URLs are called by image redirect tags that are placed in ad slots on websites. The image tags are sent from the websites to users browsers in place of images and clickthrough URLs. When the browser processes these image tags, it makes a request to the DoubleClick servers to retrieve the ad image and clickthrough URL, which are then returned to the browser. When the servers send an ad image, DFA records an impression; when a click occurs, DFA records a click. However, with caching, the browser or proxy server stores the ad image in a cache and retrieves it from that cache, rather than from the DoubleClick servers. As a result, DFA does not record a new impression.

Typically, when a browser sees another tag with the same name, it pulls the ad from its cache. You can prevent browser caching by dynamically creating unique tags for each ad image served. This ensures that when users navigate from page to page, they receive new ad images from DoubleClick servers for each request, rather than receiving an image delivered from the browser s cache. With cache busting, a different random number is added to the image tags that reference the ad image.

What's random number generation (RNG)?

RNG is the process of inserting a random number in HTML tags to prevent a browser or proxy server from caching an ad image. Random numbers change every time the tags are requested and sent. You can use any random string for cache busting; however, this string is most commonly a number. You must change the ord= attribute in the DFA ad tag. Tags with different ord= values defeat caching, because the different value forces the browser to retrieve a new banner from the ad server.

If a site doesn't use DoubleClick for Publishers as its ad serving solution, do they still need to implement RNG in the tags?

Yes. Either their ad serving solution already has a way of generating a random number, or they need to incorporate a form of JavaScript code in the tags. We recommend that sites continue to use the technology they are currently using for generating their webpages in order to generate the random number. If they use one of the following ad serving solutions, they can replace the [timestamp] component of the DoubleClick tags with the appropriate variable:

Ad serving solution Timestamp variable
DART Enterprise $gmt$
RealMedia %GMTTIME%

In order for the click-through URL and the image to work correctly, you must place the variable as the value of the ord= key in both the <HREF> and the <IMG SRC> tags.

These variables are guidelines only. Please verify the timestamp for your ad serving solution with your contact at the respective company, as these variables vary according to the third-party ad server version.

What happens if I don't use RNG to defeat caching?

Once a browser or proxy server receives a creative asset from DFA, it stores it in the browser cache. The browser draws any additional calls for the asset from the cache, rather than from the DFA ad servers. DFA therefore doesn't record additional impressions, which causes discrepancies.