What Is an SSP? How Does a Supply-Side Platform Work?

What Is an SSP?

An SSP (Supply-Side Platform) is sell-side software that enables websites, mobile apps, and digital publishers to achieve maximum revenue (Yield Optimization) by automatically offering their ad inventory to multiple Ad Exchanges, DSPs, and ad networks.

The SSP operates as the publisher’s financial and operational representative. Its core objective is to sell every individual ad impression at the highest possible eCPM while driving the share of unsold inventory as close to zero as possible (Fill Rate Optimization).

Core SSP Capabilities and Algorithms

1. Dynamic Floor Pricing:
   • Hard Floor: No bid below the defined price (e.g., $1.20 CPM) is accepted.
   • Soft Floor: Bids below the defined price are still admitted to the auction; however, a buyer that clears the soft floor threshold gains priority under first-price rules. Modern SSPs use machine learning to set dynamic floor prices based on hourly traffic quality and audience geography.
2. Header Bidding Integration (Client-Side & Prebid Server):
   • By running Prebid.js on the publisher’s page, it allows 10+ different SSPs to bid simultaneously before the page call ever reaches Google Ad Manager.
   • In server-to-server (S2S) setups, the entire auction is completed on the SSP’s server in ~50 milliseconds to reduce browser load.
3. Programmatic Deal Types:
   • Open Auction (Open Marketplace): A model in which all buyers compete freely above the floor price.
   • Private Marketplace (PMP): An invitation-only, prioritized auction with a guaranteed floor price, where a dedicated Deal ID is allocated to specific advertisers.
   • Preferred Deals: A model that grants first-look rights at a fixed price, without an auction.
   • Programmatic Guaranteed (PG): Automated delivery in which impression volume and price are fixed in advance, much like a traditional direct reservation.

Publisher Transparency Standards: ads.txt and sellers.json

To prevent counterfeit inventory (domain spoofing) and unauthorized resellers within the SSP ecosystem, IAB Tech Lab standards are applied:

• ads.txt: A text file the publisher places in the root directory of its own domain, listing only the SSP partners it has authorized (domain.com/ads.txt).
• sellers.json: A JSON file in which the SSP publicly verifies the identities and publisher IDs (seller_id) of the publishers selling through its platform. DSPs cross-check these two files to automatically filter out fraudulent impression bids.

SSP vs. DSP Comparison

Citation

Yalçın, K. (2026). What Is an SSP? How Does a Supply-Side Platform Work?. KorayYalcin.org Publications & Research.

Related Articles

• What Is Programmatic Advertising?
• What Is a DSP? Demand-Side Platform Guide
• DSP vs. SSP vs. Ad Exchange: Key Differences