What is a supply-side platform?

Software publishers use to sell programmatic ad inventory. The SSP runs the auction, enforces floor prices, manages deal IDs, and connects publisher inventory to DSPs.

Which SSP is biggest?

Google Ad Manager + AdX has the largest publisher footprint. Among independents, PubMatic and Magnite (especially in CTV) are the largest by revenue.

Why do publishers use multiple SSPs?

To maximize fill rate and CPM. Different SSPs have different DSP connections; running multiple SSPs in a unified header-bidding auction usually adds 5-15% to revenue versus a single SSP.

What does an SSP charge?

Typically 10-20% of revenue, taken from the publisher's side. The fee is paid by the publisher; the buyer's bid is gross, the publisher's net is after the SSP take.

Is header bidding still relevant in 2026?

Yes. It's how publishers create unified auctions across multiple SSPs. The mechanics have moved server-side for most large publishers, but the principle (every SSP bids in parallel, highest wins) is unchanged.

How do I know my bid actually reached the publisher?

Verify the supply path with sellers.json and ads.txt. Both files are publicly accessible. A reputable buyer's tech stack runs supply-path verification automatically.

Supply-side platforms (SSPs): how publishers sell programmatically

A supply-side platform is the publisher's mirror image of the DSP. Where the DSP gives advertisers a seat to buy programmatic inventory, the SSP gives publishers a seat to sell it. The SSP runs the auction, enforces floor prices, manages deal IDs, integrates with the publisher's ad server, and reports on yield.

By David Schaefer · LinkedIn · Updated May 2026

What an SSP actually does

When a user loads a publisher page, the publisher's ad server requests an ad. The request goes to the SSP, which broadcasts a bid request to every DSP that's connected. DSPs submit bids in real time. The SSP picks the winning bid (highest, subject to floor and deal-priority rules), returns the winning ad to the publisher, and logs the transaction. All in under 200 milliseconds.

The SSP is doing yield management on the publisher's behalf: setting floor prices, deciding which buyers see which impressions, prioritizing deal IDs (PG > PMP > open exchange), and increasingly running unified-auction logic that fairly evaluates all bid sources at once.

The major SSPs in 2026

SSP	Strength	Notable
Google Ad Manager (GAM) + AdX	Largest publisher footprint, deep integration with Google Ads & DV360	Both ad server (GAM) and exchange (AdX). Most publishers use GAM as their primary ad server.
PubMatic	Independent, strong supply-path optimization, OpenWrap header bidding	Public company (NASDAQ: PUBM). Big in identity solutions and curated marketplaces.
Magnite	CTV leader (Magnite Streaming, formerly SpotX + Rubicon merger)	Largest independent CTV SSP. Strong with major streaming platforms.
OpenX	Independent, strong with mid-tier publishers	Notable for early header-bidding leadership.
Index Exchange	Independent, transparent fee model	Known for the cleanest data and reporting in the SSP space.
Yahoo SSP (formerly Yahoo Ad Tech)	Identity graph, broad inventory	Built on legacy Verizon Media stack.
FreeWheel	Linear-TV + CTV integration	Owned by Comcast. Bridges programmatic CTV with legacy linear stacks.

The fee question

SSP fees are taken from the publisher's side of the transaction. A buyer bidding $5.00 may have $4.50 reach the publisher after the SSP's 10% take. The ANA's 2023 supply-chain study made SSP fees a public-policy issue: many publishers had multiple SSPs running auctions on the same inventory, with combined fees taking 20-30% of revenue before any other deductions. Supply-path optimization (SPO) reduces this by routing demand through a smaller number of trusted SSPs.

Header bidding

Header bidding is the mechanism publishers use to expose inventory to multiple SSPs simultaneously, before the ad server's waterfall decision. The publisher's page runs a small JavaScript wrapper (Prebid is the standard open-source implementation) that asks every connected SSP to bid, picks the highest bid, then passes that bid into the ad server as a price floor. Header bidding covers the mechanics in depth.

How publishers think about SSPs

Publishers don't pick one SSP — they pick a stack. A typical premium publisher's stack: Google Ad Manager as the ad server, Google AdX as the primary exchange, 4-6 other SSPs (Magnite, PubMatic, Index, OpenX) connected via header bidding, plus direct deals trafficked through GAM. The goal is to maximize fill rate and CPM by exposing every impression to the broadest possible demand.

Sellers.json and supply-path verification

Sellers.json is a public file each SSP publishes that lists every account that's authorized to sell inventory through it. Combined with ads.txt (published by each publisher), buyers can verify the supply chain end-to-end: the impression was offered by publisher X, sold through reseller Y, auctioned by SSP Z. This is how reputable buyers detect and avoid spoofed inventory and resold impressions.

Are SSPs and ad servers the same thing?

No, though Google blurs the line by selling both as Google Ad Manager. The ad server is the publisher's decision engine — which ad to serve, in what order, with what priority. The SSP is the auction infrastructure that brings programmatic bids into that decision. Many publishers use Google Ad Manager as their ad server while connecting to multiple SSPs via header bidding.

Why does a publisher use multiple SSPs?

To maximize fill rate and CPM. Different SSPs have different DSP connections; running multiple SSPs in a unified auction means every DSP gets a chance to bid on every impression. The marginal demand from a fifth or sixth SSP usually still adds 2-5% to revenue.

What is supply-path optimization?

SPO is the buyer's exercise of reducing the number of SSP hops between their DSP and the publisher. If three SSPs each offer the same inventory, the DSP bids three separate times on the same impression — paying three SSP fees if their bid wins via the most expensive path. SPO picks the cheapest legitimate path and suppresses bids on the others.

How do publishers set floor prices?

Dynamic floor pricing uses ML to set floors per impression based on demand signals — user, page, time, device. Static floors are set per ad unit per geography. Most modern SSPs default to dynamic flooring. Setting floors too high suppresses demand; too low gives away revenue. The optimal floor is the second-highest bid that you can confidently expect.

What's the difference between AdX and GAM?

GAM (Google Ad Manager) is the publisher's ad server. AdX (AdExchange) is the Google-operated SSP. Many publishers use both, plus other SSPs in parallel. AdX is part of GAM's UI, which is why the products are often conflated.

Are SSP fees taken on top of DSP fees?

Yes, but from the other side of the transaction. The DSP fee is taken from the buyer's spend; the SSP fee is taken from the seller's revenue. Both fees compress the working-media ratio. Total fee load on an open-exchange impression is commonly 30-40% of the buyer's spend.

Operating checklist

Define the desired outcome before opening a platform UI.
Validate platform fit against budget, geo, and inventory access needs.
Set frequency caps, viewability floors, and IVT filters at line-item launch.
Run a controlled holdout to measure incrementality, not last-click.
Review pacing, viewability, and brand-safety reports weekly.
Reconcile delivery against publisher-side reporting monthly.
Document seat IDs, deal IDs, contacts, and lessons learned in a runbook.