Ad exchanges: the marketplaces where DSPs and SSPs transact

What an exchange technically is

Strictly, an ad exchange is the auction-running infrastructure: receives bid requests from SSPs, receives bid responses from DSPs, picks the winner, returns the winning creative, and logs the transaction. In the original ecosystem (2008-2014), this was a separate layer: SSPs handled publisher relationships, exchanges handled auctions, DSPs handled buyer relationships.

That separation has eroded. PubMatic, Magnite, OpenX, Index Exchange, and Google AdX all run both SSP and exchange functionality. The terms are now used interchangeably in most operator conversations. The technically correct usage: "exchange" describes the auction venue; "SSP" describes the seller's-side tooling. Most platforms do both.

The big-three ad exchanges

Exchange	Affiliation	Notable
Google AdX	Google / Alphabet	Largest by revenue. Part of Google Ad Manager. Subject to ongoing antitrust litigation; potential structural changes coming.
Xandr Curate (formerly Xandr Marketplace)	Microsoft	Acquired from AT&T in 2022. Used for Microsoft Advertising's programmatic and CTV.
OpenX, PubMatic, Magnite, Index Exchange	Independent	The independent layer of the ecosystem. Each operates both SSP and exchange.

How auctions actually run

Originally, exchanges used second-price auctions: highest bidder won, paid the second-highest bid's price plus one cent. This was the classic Vickrey-style auction, designed to incentivize honest bidding. Around 2017-2019, Google and most major exchanges shifted to first-price auctions: highest bidder wins, pays their bid price. The shift was driven by header bidding making second-price unmanageable across multiple simultaneous auctions.

First-price auctions changed bidder behavior. DSPs now need to shade their bids — bid below the buyer's true willingness to pay, to avoid overpaying when there's no second-price discount. This bid-shading is itself algorithmic; modern DSPs do it automatically.

For the underlying auction theory, see Vickrey auction in Google Ads — the original second-price design Google Ads still uses for search, and the comparison to how display auctions work.

Header bidding's impact on exchanges

Before header bidding, the publisher's ad server (usually Google Ad Manager) ran a waterfall: try direct demand first, then SSPs in priority order, accept the first bid that cleared the floor. This systematically favored the SSP that was first in the waterfall (usually Google AdX, by virtue of being in the same product). Header bidding broke this by running all SSPs in parallel before the ad server got involved. The winning bid from header bidding entered the ad server's decision as a single floor price, competing against direct deals.

The result: exchanges that weren't first in the waterfall got more impressions. Independent exchanges grew. Google's exchange share remained large but no longer dominant in the way it was pre-2016.

Curated marketplaces inside exchanges

A curated marketplace is an exchange-hosted package of inventory: themed by audience, content category, or measurement criteria. PubMatic Connect, Magnite Curate, Index Exchange's Curation Stack — each major exchange has a curation layer. Buyers can transact with the curator (often an SSP, agency, or data partner) on packaged audiences without negotiating with each underlying publisher.

What's the difference between an exchange and an SSP?

Functionally minimal in 2026. Most SSPs operate exchanges; the auction layer and the publisher-side tooling are bundled. Use "exchange" when referring to the auction venue; use "SSP" when referring to the publisher's seat in that venue.

How many exchanges does my bid actually touch?

For an open-exchange impression with no SPO, your DSP may bid on the same impression through 4-8 different exchanges via different SSP paths. SPO reduces this to 2-3 trusted paths.

Does Google AdX still favor Google AdX?

The unified-auction reforms (Open Bidding, EBDA before it) removed the structural advantage AdX had in the waterfall. Recent antitrust litigation has scrutinized whether AdX still gets preferential treatment in the Google Ad Manager stack. Independent measurement is the working defense — if the publisher's data shows AdX clearing the auction at a rate disproportionate to its bid density, that's a problem worth surfacing.

Are first-price auctions better for buyers or sellers?

Sellers benefit short-term: first-price clears at the top bid rather than the second. Buyers eventually adjust via bid-shading; the equilibrium ends up close to the second-price equivalent. The reason first-price is now standard is operational, not economic — running second-price across multiple simultaneous auctions creates non-convergent gaming opportunities that hurt everyone.

What is OpenRTB?

OpenRTB is the open-standard protocol for real-time bidding requests and responses. It defines the JSON schema for a bid request (page URL, user identifiers, ad slot, geo, device, etc.) and the bid response (price, creative, deal ID). All major exchanges and DSPs implement OpenRTB, currently at version 2.6. The protocol is maintained by IAB Tech Lab.

Why is Google AdX in antitrust trouble?

The DOJ's 2023 case alleges that Google's combined ownership of the publisher ad server (GAM), the dominant exchange (AdX), and a dominant DSP (Google Ads / DV360) constitutes a self-dealing structure that harms competition. The case is ongoing; possible remedies include forced divestiture of one or more components. Buyers and publishers are watching closely; the structural changes could meaningfully reorganize the supply chain.

Operating checklist

1. Validate exchange / network access before negotiating deal terms.
2. Document the supply path end-to-end (publisher → SSP → exchange → DSP).
3. Set pre-bid filters for IVT, viewability, and brand safety at launch.
4. Run inclusion lists, not just exclusion lists.
5. Reconcile spend across DSP and SSP reports weekly.
6. Renegotiate fee structure annually against benchmark.
7. Document every deal ID, seat ID, and contact in a runbook.