Real-Time Bidding Protocol

Cookie Matching

This guide explains how the Cookie Matching Service enables you to make more effective bidding choices.

The Cookie Matching Service enables a buyer to associate two kinds of data:

  1. the cookie that identifies a user within the buyer domain, and the cookie that identifies the user for Google. (We share a buyer-specific encrypted Google User ID for buyers to match on.)

With an RTB application, the buyer can bid on impressions where the user has a specific Google User ID, and can use information associated with the Google User ID as criteria in a bid for an ad impression. If Google hosts the match table, that may simplify integration, decrease latency, and enable future enhancements.


A browser cookie is typically set by the party that owns the domain to which the cookie belongs. The cookie identifies a user within that domain. The security model of the browser restricts one party from reading the cookie set by another party, even if both parties would otherwise agree to such an exchange.

The buyer typically identifies users with cookies that belong to the domain of a third-party ad network. The buyer may index a database of user information with those cookies.

For itself, Google identifies users with cookies that belong to the domain under which Google serves ads. For buyers, Google identifies users using a buyer-specific Google User ID which is an encrypted version of the cookie, derived from but not equal to that cookie. Google passes the Google User ID to the buyer (raw DoubleClick cookies are never sent).

When receiving a particular Google User ID for the first time, the buyer has no knowledge about the user associated with the Google User ID other than what the bid request reveals. The buyer can associate the Google User ID with a buyer cookie, and subsequently consider user information associated with the buyer cookie in making decisions about users identified by the Google User ID. This can be useful in remarketing (link is external) campaigns, and in refining targeting or bidding for impressions as they become available in real time.

The Cookie Matching Service provides the information that a buyer network needs to maintain an association between the buyer cookie and the Google User ID, in the form of a data structure called a match table. Additionally, the buyer can provide data for Google to store and add to future bid requests.

Benefits of hosted match tables

Buyers who choose to have Google host their match tables stand to gain the following benefits:

  • Less infrastructure support Mapping the Google User ID to a useful form no longer requires a table lookup During pre-targeting, there is the option to filter on whether or not a cookie match exists, which can reduce unwanted bid requests


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