RTB (Real-Time Bidding)

Real-time bidding (RTB) is the protocol that prices and clears ad impressions in real time. The atomic flow is: an impression opportunity arises (a page load, a CTV ad pod, a DOOH ad slot starting), an SSP emits a bid request to multiple DSPs via an ad exchange, each DSP returns a bid or a no-bid within ~80-100ms, the exchange picks the winner, and the winning creative renders. The total wall-clock budget is ~120ms.

RTB was the technology innovation that turned digital advertising from insertion-order-based to impression-by-impression. Before RTB, a buyer committed to a fixed CPM for a fixed period; after RTB, every impression is priced independently against current demand. The trade-off is operational complexity: an RTB stack has to handle hundreds of thousands of QPS, sub-100ms latency budgets, and global geographic distribution.

The protocol spec is OpenRTB, maintained by the IAB Tech Lab. Version 2.6 (2022) is the current production-deployed spec; it added native support for DOOH bid requests, CTV pod-level requests, and a richer ID-less identity framework. Version 3.0 has been in late draft for years and remains experimental.

In DOOH, RTB is the bid-request → winning-creative loop for one ad slot. A 15-second slot every 90 seconds means each screen runs ~40 RTB auctions per hour.