Real-time bidding

Real-time bidding (RTB) is the auction method in which each ad impression is offered to bidders through a bid request and sold to the winning bid while the page, app or stream is still loading.

RTB is the most common way programmatic impressions change hands. When a user opens a page, the publisher's SSP or exchange builds a bid request. It describes the ad slot, the device, the content and any consented identifiers. The exchange sends the request to several DSPs at once. Each DSP decides whether the impression fits a campaign, prices it, and answers with a bid response or a no-bid.

The exchange sets a deadline. In OpenRTB 2.6 that deadline is the tmax field, the maximum time in milliseconds the exchange allows for bids to arrive, including internet latency (IAB Tech Lab, OpenRTB 2.6). Late bids are dropped. The at field states the auction type: 1 for first price, 2 for second price plus. Exchanges began with second-price auctions. Google's exchange moved to first price in 2019, and most auctions now clear at first price (CMA, Appendix M, paragraph 38, 1 July 2020).

After the auction, the winning creative is served and the exchange fires win and billing notices. Because the loop must finish before the page renders, bidder latency and queries per second decide who can compete.

Example: An exchange sends a bid request with "tmax": 120 and "at": 1. Two DSPs bid $3.10 and $2.75 CPM within 80 milliseconds. A third answers after 140 milliseconds and is ignored. The $3.10 bid wins and pays $3.10, because the auction is first price.

Related terms: bid request, bid response, OpenRTB, timeout, first-price auction, QPS.

Authoritative source: IAB Tech Lab, OpenRTB 2.6 specification, sections 3.2.1 and 4.4 (accessed 9 November 2026).

Read the full guide: How real-time bidding works, step by step.