SSP (supply-side platform)

A supply-side platform is software that sells a publisher's advertising impressions by packaging each impression into a bid request, sending it to many demand sources, running or forwarding an auction, and returning the winning ad to the page, app or stream.

An SSP sits between the publisher's ad server and the DSPs. It attaches the publisher's identifiers, consent signals and floor prices to the request, applies the publisher's rules (blocked categories, deal IDs, private auctions), and takes a fee from the clearing price. OpenRTB calls the SSP side the "exchange" and the DSP side the "bidder"; in practice most SSPs run an exchange and the two words are used together (OpenRTB 2.6).

SSP fees are the least visible in the chain. Adalytics found one SSP charging a single publisher between 7% and 42% on the same site with the same buyer and DSP, with no discernible pattern (ExchangeWire, 16 September 2026). Transparency standards aim at this: ads.txt declares which SSPs may sell a publisher's inventory, sellers.json declares who each SSP sells for, and the SupplyChain object records every node in the path (IAB Tech Lab, sellers.json).

Example: a publisher's Prebid Server instance sends the same impression to six SSPs; each returns a bid; the publisher's ad server picks the highest net of fees. The second part of that sentence, "net of fees," is what SPO and DPO are about.

Related: DSP, ad exchange, header bidding, sellers.json, SPO.

Source: IAB Tech Lab, OpenRTB 2.6 (exchange role); UK CMA market study (July 2020).

1. OpenRTB 2.6
2. ExchangeWire, 16 September 2026
3. IAB Tech Lab, sellers.json