Real-time bidding (RTB) is the auction that sells most digital ad impressions. When a page or app loads, the publisher's ad technology sends a description of the impression to many buyers at once, each buyer's DSP decides in milliseconds whether and how much to bid, and the highest valid bid wins. The whole process usually happens in roughly 100–200 milliseconds, before the ad appears.
The players
| Player | Side | What it does |
|---|---|---|
| Advertiser / agency | Buy | Sets goals, budgets and creatives |
| DSP (demand-side platform) | Buy | Evaluates each impression and bids for the advertiser |
| Ad exchange | Middle | Runs the auction between buyers and sellers |
| SSP (supply-side platform) | Sell | Packages the publisher's inventory and connects it to exchanges and DSPs |
| Publisher / app | Sell | Owns the website or app where the ad appears |
| Data providers, verification | Both | Add audience data, measure viewability and fraud |
Read more on each side in our guides to choosing a DSP and choosing an SSP.
One auction, step by step
- A user opens a page or app. The ad slot's code or SDK calls the publisher's ad server or SSP.
- The SSP builds a bid request — usually in the OpenRTB format — describing the impression: ad size or format, site or app, device, OS, approximate location, available user identifiers (subject to consent), floor price and the supply chain.
- The bid request goes to exchanges and DSPs. In web header bidding, several SSPs are called in parallel from the page.
- Each DSP evaluates the impression against its campaigns: targeting, budget, frequency caps, predicted click or conversion probability, brand-safety rules. It returns a bid and a creative — or no bid.
- The auction runs. Most exchanges now use first-price auctions: the winner pays what it bid (minus any fees), not the second-highest price.
- The winning ad is served, the impression is logged and tracking pixels fire.
- Later events — clicks, installs, purchases — are attributed back to the impression by the advertiser's analytics or MMP.
OpenRTB in one paragraph
OpenRTB is the IAB Tech Lab standard that defines the format of bid requests and responses, so any SSP can talk to any DSP. The current 2.x line (2.6 and later) supports display, video, audio, native and CTV, plus fields for privacy signals and the supply chain object that lists every company that handled the impression.
Transparency standards that protect buyers
- ads.txt / app-ads.txt: a public file on the publisher's domain listing which sellers are authorised to sell its inventory. DSPs check it to avoid spoofed inventory.
- sellers.json: a file published by SSPs and exchanges listing the sellers they represent.
- SupplyChain object (schain): in the bid request, a list of every intermediary in the path.
Together, they let buyers verify that an impression really comes from the publisher it claims and see how many hands it passed through.
Where the money goes
Between the advertiser's budget and the publisher's revenue sit DSP fees, data fees, verification fees, exchange fees and SSP fees. Industry studies have repeatedly found that a significant share of spend does not reach publishers. Buyers reduce this through supply path optimisation (SPO): buying through fewer, more direct paths and preferring sellers listed as direct in ads.txt and sellers.json.
RTB vs other programmatic deals
| Deal type | How it works | Good for |
|---|---|---|
| Open auction (RTB) | Anyone can bid | Scale and price efficiency |
| Private marketplace (PMP) | Invited buyers bid on selected inventory | Premium inventory, better control |
| Preferred deal | Fixed price, first look, no guarantee | Priority access |
| Programmatic guaranteed | Fixed price and volume, automated delivery | Reserved campaigns |
RTB for performance marketers
For performance campaigns, RTB works when the DSP optimises on the events that matter — installs, registrations, purchases — not just clicks. That needs:
- conversion data flowing back from your MMP or tracker,
- placement-level reporting, so you can block poor apps and sites,
- fraud filtering, and
- frequency caps to avoid showing the same user too many ads.
ClickSpero's programmatic service is built on exactly these controls.
FAQ
What is the difference between RTB and programmatic?
Programmatic means buying ads with software instead of manual insertion orders. RTB is one form of programmatic — the real-time open or private auction. Programmatic guaranteed deals are programmatic but not auction-based.
What is a first-price auction?
The winning bidder pays its own bid. Most of the industry moved from second-price to first-price auctions around 2019, which is why DSPs use bid shading to avoid overpaying.
What is a bid floor?
The minimum price a publisher will accept for an impression. Bids below the floor are rejected.
Why does ads.txt matter?
It lets buyers check that the company selling an impression is authorised by the publisher, which reduces domain spoofing fraud.
Sources
- IAB Tech Lab — OpenRTB (opens in a new tab)
- IAB Tech Lab — ads.txt, app-ads.txt and sellers.json (opens in a new tab)
Published 25 October 2026 by the ClickSpero Editorial Team. Platform features, fees and policies change often — check the provider’s current terms before you rely on a number. Third-party names are trademarks of their owners; no endorsement is implied.