Header Bidding

Header bidding is the technique of running multiple SSP auctions in parallel before the publisher's ad-server is called. On the web, it lives in JavaScript loaded in the page header (hence the name) — typically the open-source Prebid library. The SSPs each return a bid; the highest is passed to the ad-server, which decides whether to take it or run its own direct-sold campaign instead.

Header bidding was invented (~2015) to fix the unfairness of the waterfall — a sequential model where SSPs were called one at a time in a fixed order and the first one to fill won regardless of price. Header bidding turns that into a parallel auction, which routinely lifts publisher yield 10-30%.

On CTV, header bidding has matured in the last 3 years; FreeWheel's MRM, SpringServe, and Magnite ATD all support parallel SSP calls. On DOOH, it's earlier — most DOOH SSPs still run waterfall logic because the latency and per-screen QPS profile are different. The CTV/DOOH variant is sometimes called "unified auction" to distinguish from the web's header bidding origins.

The key win of header bidding isn't yield uplift per se; it's fairness. Every connected SSP gets the same impression opportunity at the same time, which makes SPO a coherent buy-side strategy. Without parallel auctions, "SPO" is meaningless because the path order is determined by the ad-server, not the buyer.