Chromium is able to detect some ads and the resources they load in the browser. This enables the browser to measure the size, performance, and count of ads displayed to our users. It also allows the browser to intervene on the user’s behalf when ads run counter to the user’s interest (e.g., by using excessive resources or by engaging in abusive behavior.
The ad detection infrastructure is called Ad Tagging. Ad Tagging works by matching resource requests against a filter list (see how the list is generated) to determine if they’re ad requests.
The current filter list and historical versions can be found here.
Any requests matching the filter are tagged as ads. Further, requests (and some DOM elements such as iframes) made on behalf of previously tagged scripts are also tagged as ads by the AdTracker. An iframe will be marked as an ad iframe if its url matches the filter list, if tagged script is involved in the creation of the iframe, or if its parent frame is an ad iframe. The main frame of a page will never be tagged as an ad. Any request made within an ad iframe is considered an ad resource request.
The Subresource Filter loads the filter list and matches urls against it. The list is distributed via the component updater. This same list and component is used for blocking ads on abusive sites and those that violate the Better Ads Standard.
Each subresource request in the render process is processed by the subresource filter before the request is sent. If it matches the list, the ResourceRequest is tagged as an ad.
Each subframe navigation request is processed in the browser process by the SubresourceFilter and if the final URL in the navigation matches the list then the render process is told (via the SubresourceFilterAgent) and the iframe is marked as an ad iframe.
The AdTracker keeps track of each script subresource that is considered an ad. Any time that a new resource is requested or an iframe is created, V8's stack is scanned and if any of the known ad scripts are in the stack then the resource request or iframe is considered an ad.