A reservation system receives placement inventory data from publishers that define placements that the publishers are offering for reservation. Placement queries that include targeting criteria for advertisements are received from an advertiser. Available placements that satisfy the target query from the placements defined in the placement inventory data are identified as target placements. The advertiser can reserve the placements from the publisher, and modify the reservation during a reservation period. Advertisements belonging to a campaign of the advertiser can be provided to the publisher either by the advertiser directly in response to publisher requests to serve an advertisement for the reservation, or can be provided to the publisher by the reservation system or an advertising server in communication with the reservation system. Discrepancies of reported performance of advertisements in the placements reported by the advertisers and the publishers can be reconciled and attributed to either the advertiser or the publisher.