Supply side platform (SSP) explained for website publishers

A supply side platform (SSP) is software that sells a publisher's ad impressions to many buyers at once. Each time a page loads, the SSP sends the ad slot out as a bid request, collects bids from advertisers' platforms, applies the publisher's price floors and rules, and passes the best bid back to the ad server.

Here's why that matters to you. Every SSP brings its own buyers, its own fees and its own settings, and most publishers add them because a partner suggested it, not because they tested what each one adds. That's how you end up with a slow page, a messy ads.txt file and no idea which partner is actually paying.

This guide covers what an SSP does, how it sells one impression, the controls inside it that change your revenue, how it plugs into your site, and how to choose one. If you'd rather have the auction side set up for you, our header bidding service does exactly that.

What is a supply side platform?

A supply side platform is the seller's tool in programmatic advertising: it offers your ad space to as many buyers as possible and tries to get the highest price for each impression. "Supply" is your inventory. The SSP works for you, the publisher.

So what is an SSP in one line? It's the auctioneer on your side of the table. Advertisers sit on the other side with demand-side platforms (DSPs), which bid on impressions for them. The two meet through IAB Tech Lab's OpenRTB standard, which "allows an individual ad impression to be put up for bid in real-time."

Some pages on this topic say SSPs serve advertisers. They don't. An advertiser never logs in to your SSP. You do, or your monetization partner does on your behalf.

For the buyer's half of this picture, our DSP vs SSP guide follows one impression through both platforms and shows where each fee comes out.

How does an SSP work?

An SSP works by turning each ad slot on your page into a bid request, sending it to buyers, and returning the winning bid to your ad server, all in the time it takes the page to load. Here's the order, from your page outwards.

1. A reader opens your page. Your ad code runs: a header bidding wrapper such as Prebid.js, Google Publisher Tag, or the SSP's own tag.
2. The SSP builds a bid request. It adds what it may share: page URL, slot size, device, country, any deals and, where the reader has consented, user IDs.
3. Buyers get the request. The SSP sends it to the DSPs and exchanges it's connected to. Each buyer checks it against live campaigns.
4. Buyers bid. Bids that fall below your floor are thrown out. Blocked advertisers and categories are filtered out too.
5. The SSP picks its best bid. That bid goes back to your wrapper or ad server.
6. Your ad server makes the final call. In Ad Manager, the SSP's bid competes with Ad Exchange, Open Bidding partners and your own line items. The ad loads and the impression is counted.

Today most of these auctions are first price. Google announced in September 2019 that it was "transitioning to unified first price auctions for Google Ad Manager." Its auction model page now says "Ad Manager determines the winning bidder based on the highest net bid submitted," and sellers are paid the closing price "net of Google's revenue share."

What an SSP does for publishers

An SSP's real value is the controls it gives you over who can buy your inventory and at what price. Reaching buyers is the start. The settings below are where SSP advertising either earns more or quietly leaks money.

Price floors

A price floor is the lowest bid you'll accept for an impression. Google calls its version unified pricing rules, which "centralize the management of auction floor prices for your non-guaranteed demand," per the pricing rules help page. Other SSPs have their own floor settings in their dashboards.

Set floors too high and good bids get thrown away. Set them too low and buyers learn they can win cheaply. We'd start low, by country and device, and raise them slowly while you watch fill rate and eCPM together. Our eCPM guide shows how to read those two numbers side by side.

Deals and private auctions

Deals let you sell to chosen buyers on set terms. Google describes private auctions as "a way to control the sale of their non-guaranteed inventory by inviting specific buyers" on its private auctions page. Most SSPs can carry a deal ID in the bid request, so a buyer who agreed a deal with you can bid through that SSP.

Blocks and brand controls

You can block advertiser domains, ad categories and some ad formats. Use this sparingly. Each block removes a buyer, so only block what really hurts your readers or breaks your site's rules.

Reporting

Every SSP reports impressions, bids, wins, eCPM and revenue, usually split by domain, ad unit, country and device. Look for net revenue, after the SSP's share. That's the number to compare across partners.

Supply chain data

Good SSPs pass on who's selling at each step. The OpenRTB SupplyChain object "enables buyers to see all parties who are selling or reselling a given bid request." Buyers use it to check that the path from your site to them is real.

SSP vs ad exchange, DSP and ad network

An SSP sells for the publisher, a DSP buys for the advertiser, an ad exchange is the marketplace where they meet, and an ad network bundles inventory and resells it. In practice the lines blur, since most big SSPs run their own exchange.

The SSP vs ad exchange question comes up most. A modern SSP usually includes an exchange, so you'll hear one company called both. What matters for you is the line in your ads.txt and whether that seller is listed as DIRECT or RESELLER. Our guide to programmatic advertising for publishers puts all these pieces in one chain.

Supply side platform examples

The supply side platform examples you'll meet most are the large sell-side companies that show up as bidders in header bidding wrappers and as lines in ads.txt files. This isn't a ranking, and the right mix depends on your traffic.

Check each one's current Prebid adapter and Open Bidding status before you plan around it. Partner lists change, and you can see Google's live list inside your Ad Manager account.

How an SSP connects to your site

An SSP connects to your site in one of three ways: as a bidder in a header bidding wrapper, as a server-side partner through Open Bidding, or through its own ad tag. Whichever you pick, it also needs a matching line in ads.txt and an entry in its sellers.json.

Header bidding with Prebid.js

Prebid is the most common route. Prebid's introduction says its adapters "communicate with the bidders (SSPs, DSPs, and exchanges)," and Prebid is "free and fully open source." Each SSP gives you IDs for its adapter. Our Prebid.js setup guide walks through the full install with Google Ad Manager.

Open Bidding in Google Ad Manager

Open Bidding runs the SSP auction on Google's servers instead of in the reader's browser. Google's Open Bidding page says it "allows you to invite third-party SSPs (supply-side platforms) to compete for your inventory." No extra code goes on the page, which helps speed. The trade-off is less control over each bidder.

The ads.txt line

Every SSP that sells your inventory needs a line in your ads.txt file. IAB Tech Lab's ads.txt page says the file lets publishers "publicly declare the companies they authorize to sell their digital inventory." A line looks like this:

examplessp.com, 123456, DIRECT, abc123def456
google.com, pub-0000000000000000, DIRECT, f08c47fec0942fa0

The four fields are the SSP's domain, your account ID with that SSP, DIRECT or RESELLER, and the SSP's certification authority ID if it has one. Copy the line your SSP gives you exactly. One wrong digit and buyers treat that path as unauthorised. Our ads.txt guide covers the common mistakes.

The sellers.json entry

sellers.json is the SSP's side of the same check. IAB Tech Lab's sellers.json page says it lets buyers "discover who the entities are that are either direct sellers of or intermediaries" in a sale. Your seller ID in the SSP's file should match the ID in your ads.txt line, and its seller type should match DIRECT or RESELLER. Our sellers.json guide shows how to look yourself up.

How to choose an SSP (and how many to run)

Choose an SSP by the demand it adds that your current partners don't, then prove it with a test. A long partner list in your wrapper isn't a strategy. Each extra bidder adds requests, can slow the page and spreads the same buyers across more pipes.

Here's the check we run before adding a new SSP:

• Demand fit. Does it have buyers for your main countries, devices and formats (display, video, app, CTV)?
• Connection. Does it have a maintained Prebid adapter, or is it an Open Bidding partner?
• Clean supply path. Will you be listed as DIRECT in its sellers.json, with an ads.txt line to match?
• Reporting. Can you see net revenue by domain, ad unit, country and device?
• Terms. What are its revenue share, payment terms and minimum payout? Get these in the contract.
• Entry bar. Some SSPs only take publishers above a traffic level or through a partner. Ask before you spend time on setup.

Test one SSP at a time

1. Write down your baseline: net revenue per 1,000 page views, fill rate and page load time.
2. Add one SSP, on all the ad units it can serve, with the same timeout as your other bidders.
3. Run it for a few weeks so you see weekdays, weekends and the start and end of a month.
4. Compare total net revenue per 1,000 page views, not the new SSP's own revenue. If it only took impressions another partner would have won at a similar price, it added nothing.
5. Keep it, or remove the bidder and its ads.txt line. Then test the next one.

Step 4 is where most publishers go wrong. A new SSP always shows revenue in its own dashboard. The question is whether your total went up.

Do small publishers need an SSP?

Small publishers don't need to sign with SSPs directly to earn from programmatic ads. AdSense and most ad networks already sell your inventory through exchanges, so you're in the auction without managing an SSP yourself.

Direct SSP accounts start to pay off once you have steady traffic, an ad server like Google Ad Manager and time to test. Many SSPs also accept sites through a monetization partner, which handles the contracts, the ads.txt lines and the setup. If Google's exchange is your goal, our guide on how to get Google AdX access explains the routes in.

Frequently asked questions

What is an SSP in simple terms?

An SSP is the tool that sells your ad space for you. It shows each ad slot to lots of buyers at once, keeps out bids below your floor and hands the best bid to your ad server.

Is Google Ad Manager an SSP?

Google Ad Manager is mainly an ad server, with Google's Ad Exchange built in. Ad Exchange works as Google's exchange and sell-side auction, and Open Bidding lets other SSPs compete inside Ad Manager too.

Is an SSP the same as an ad exchange?

Not quite. An SSP works for the seller and an ad exchange is the marketplace where bids meet. Most large SSPs run their own exchange, though, so one company is often both.

How do SSPs make money?

SSPs usually keep a share of each winning bid before paying you. The share depends on the SSP and your contract, so check gross and net revenue in its reports and read the terms before you sign.

Can I use more than one SSP?

Yes. Header bidding and Open Bidding exist so several SSPs can bid on the same impression. Add them one at a time and keep only the ones that raise your total net revenue.

Do more SSPs slow down my site?

They can. Each bidder in a browser-side wrapper adds network requests. Keep timeouts tight, load ad code asynchronously, and move some bidders server-side if page speed drops.

Will an SSP guarantee higher revenue?

No. An SSP only adds revenue if it brings buyers who pay more than your current partners. That depends on your traffic, content and setup, which is why you test before you commit.

Next step for your SSP setup

Start with what you already have. List every SSP in your wrapper and ads.txt, check each one's seller ID against its sellers.json, and look at each partner's effect on total net revenue. You'll often find one or two you can drop and one setting, usually floors, worth fixing first.

Want a second pair of eyes? Our header bidding team reviews SSP setups and runs the tests for you. Tell us about your site and we'll tell you what we'd change first.