Waterfall

Waterfall (also called "daisy chain") is the legacy sell-side technique where SSPs are called sequentially, SSP A first, then SSP B if A doesn't fill, then SSP C if B doesn't fill, and so on. The order is set by the publisher, typically by historical CPM (highest-paying first).

The problem with waterfall is structural unfairness: an SSP further down the chain may have buyers willing to pay more on a specific impression, but they never get the chance because the upstream SSP filled at a lower price. This led to publisher yield being systematically below market-clearing for a decade.

Header bidding (web, 2015) and the unified auction (CTV, ~2020) replaced the waterfall by running parallel auctions: all SSPs bid at the same time, and the publisher picks the highest. Industry-wide, parallel auctions lifted publisher RPM 10-30% in the web rollout and a similar range in CTV.

On DOOH, waterfall logic is still common in 2026, most DOOH SSPs run sequential demand calls because the per-screen QPS is low and latency budgets are looser. Trillboards' supply layer runs parallel auctions where partner integrations support it and falls back to ordered waterfall only for one-off direct integrations.

The word "waterfall" is sometimes used loosely to mean "ordered fallback list" in any context, but in programmatic, it specifically means sequential SSP calls.