The present disclosure is directed to methods for identifying a user by a demand side platform (DSP) across advertiser exchanges. The method includes establishing, by a DSP, a cookie mapping for a user. The cookie mapping includes a mapping of user identifiers for the user from advertisement exchanges to a user identifier assigned by the DSP for the user. The DSP stores to the cookie mapping a first mapping to the user identifier of the DSP, comprising a first user id received by a bidder from a first exchange and a first exchange id for the first exchange. A bidder inserts a pixel into a bid for an impression opportunity to a second exchange. The pixel includes a key to the cookie mapping and a second user id for the user and a second exchange id. The second user id is received by the bidder from a second exchange.