Key Takeaways

• SSPs aggregate publisher inventory and make it available for programmatic buying. They connect publishers to multiple demand sources at the same time.
• DSPs aggregate advertiser demand and evaluate bid opportunities. They decide which impressions to bid on and at what price.
• Exchanges handle the transaction between SSPs and DSPs. Some operate independently. Many SSPs also function as exchanges.
• The boundaries between these roles have blurred. Modern SSPs often include exchange functionality. Some DSPs connect directly to publishers. The labels matter less than the actual supply path.
• Every entity in the chain adds a node to the supply path. More intermediaries mean more verification points, more fee extraction, and more opportunities for the chain to break.

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How SSPs, DSPs, and Exchanges Connect in the Supply Chain

The programmatic supply chain looks simple in textbook diagrams. Publisher on one side, advertiser on the other, SSP and DSP in the middle with maybe an exchange connecting them.

In reality, the chain is messier. The roles overlap. The paths multiply. And how these entities connect directly affects your transparency profile, your supply chain verification, and ultimately your CPMs.

What Each Entity Does

Supply-Side Platforms (SSPs)

SSPs are the publisher's primary interface with programmatic demand. When a user visits a web page or opens an app and an ad impression becomes available, the publisher's ad server sends that opportunity to one or more SSPs.

The SSP's job:

• Receive ad requests from publishers
• Enrich the bid request with contextual data, user signals, and targeting information
• Send the bid request to connected DSPs and exchanges
• Run the auction when bids return
• Serve the winning ad and handle the creative delivery
• Collect payment from the buyer and pass revenue to the publisher (minus their fee)

SSPs maintain sellers.json files that list every publisher and reseller on their platform. Each publisher's ads.txt should include every SSP they work with.

Examples: Google AdX, Magnite (formerly Rubicon Project), PubMatic, Index Exchange, OpenX.

Demand-Side Platforms (DSPs)

DSPs are the advertiser's primary interface with programmatic supply. When a DSP receives a bid request from an SSP or exchange, it evaluates the opportunity against its advertisers' campaigns, budgets, and targeting criteria.

The DSP's job:

• Receive bid requests from SSPs and exchanges
• Evaluate the opportunity against active campaigns
• Check supply chain transparency using ads.txt, sellers.json, and schain
• Calculate bid price based on targeting value, supply quality, and fraud risk
• Submit bids for desirable impressions
• Track delivery and optimize campaign performance

DSPs are the entities that enforce supply chain transparency standards. They decide how to weight ads.txt compliance, sellers.json verification, and schain completeness in their bidding logic. Their enforcement is what gives these standards their revenue impact.

Examples: The Trade Desk, DV360 (Google), Amazon DSP, MediaMath, Xandr.

Ad Exchanges

Exchanges are platforms that handle the programmatic auction between supply and demand. They sit between SSPs and DSPs, aggregating bid requests from multiple SSPs and making them available to multiple DSPs.

The exchange's role:

• Aggregate supply from multiple SSPs
• Distribute bid requests to connected DSPs
• Run or facilitate auctions between buyers
• Handle transaction settlement between parties

In practice, the distinction between SSPs and exchanges has blurred. Most major SSPs also function as exchanges. Google AdX is both an SSP (accepting publisher inventory) and an exchange (running auctions with multiple DSPs). The same is true for most big platforms.

How They Connect: The Supply Path

The simplest supply path looks like this:

text
Publisher > SSP > DSP > Advertiser

The publisher sends an ad request to the SSP. The SSP sends a bid request to connected DSPs. A DSP wins the auction and serves the ad. Two intermediaries, one clear path.

But most publishers work with multiple SSPs through header bidding, creating parallel paths:

text
Publisher > SSP-A > DSP-1
         > SSP-B > DSP-1
         > SSP-C > DSP-2
         > AdX   > DSP-1, DSP-2, DSP-3

The same impression is offered through multiple SSPs at the same time. The same DSP might see the same impression from multiple SSPs. This multiplied exposure increases competition but also increases complexity.

Then add resellers and exchanges:

text
Publisher > SSP-A > Exchange-X > DSP-1
                  > DSP-2
         > SSP-B > DSP-1
         > Reseller-Y > SSP-C > DSP-3

Now the same impression might travel through two, three, or more intermediaries before reaching certain DSPs. Each intermediary is a node in the supply chain. Each takes a fee. Each must be verified.

Where Transparency Standards Apply

Each entity in the supply chain has specific transparency obligations.

Publishers maintain ads.txt (and app-ads.txt) listing every SSP and reseller authorized to sell their inventory. This includes direct SSP relationships and any resellers further down the chain.

SSPs and Exchanges maintain sellers.json listing every seller on their platform. This includes direct publishers and intermediaries (resellers who bring inventory to the platform).

All entities in the path contribute to the SupplyChain object (schain). As the bid request passes from entity to entity, each adds its own node documenting its participation.

DSPs consume and verify all three standards. They check ads.txt for authorization, sellers.json for identity, and schain for path completeness.

The verification chain is only as strong as its weakest link. If one SSP in the path doesn't maintain sellers.json, that node can't be verified. If one intermediary doesn't add its schain node, the chain becomes incomplete.

The Fee Stack

Every entity in the supply chain takes a fee. Understanding this is important because it directly affects how much of the advertiser's spend reaches you.

A typical supply path fee structure:

| Entity | Fee Range | What They Take |

|--------|-----------|----------------|

| DSP | 10-20% | Deducted from advertiser budget |

| Exchange | 5-15% | Deducted from the bid price |

| SSP | 10-20% | Deducted before paying publisher |

In a simple two-intermediary path (SSP + DSP), you might receive 60-80% of the advertiser's spend. In a longer path with exchanges and resellers, that number can drop to 40-50%.

This is why supply path optimization (SPO) matters. DSPs actively analyze supply paths to find the shortest, cheapest, most transparent route to publisher inventory. They prefer paths with fewer intermediaries, lower fees, and full transparency verification.

For publishers, this means the SSPs and paths you use directly affect not just your transparency profile but also the net revenue from each impression.

How Header Bidding Changed the Landscape

Header bidding (introduced around 2016) changed everything about how SSPs, DSPs, and exchanges connect. Before header bidding, most publishers used a waterfall model where they offered inventory to one SSP at a time in priority order.

Header bidding lets publishers offer inventory to multiple SSPs at the same time, letting them compete in a unified auction. This increased competition for publisher inventory but also multiplied the number of supply paths.

The result: a single impression might reach the same DSP through five different SSPs. The DSP sees five bid requests for the same impression at slightly different prices (due to different SSP fees). The DSP must decide which path to bid through.

This is where transparency becomes a competitive advantage. DSPs use ads.txt, sellers.json, and schain data to evaluate which supply path is most trustworthy and cost-efficient. The SSP with complete transparency verification, the shortest path, and the lowest fees wins the bid.

Supply Path Optimization (SPO)

SPO is the DSP practice of analyzing and selecting the optimal supply path to a publisher. It considers:

• Transparency: Is the full path verified through ads.txt, sellers.json, and schain?
• Directness: How many intermediaries are in the path?
• Cost: What's the total fee stack?
• Quality: What's the historical performance of this path (viewability, completion rate, fraud rate)?

DSPs that practice SPO actively prune supply paths that don't meet their criteria. If your inventory reaches a DSP through three paths but only one is fully verified, the DSP may exclusively use the verified path and ignore the other two.

This means publishers should care about not just which SSPs they work with, but how those SSPs connect to DSPs. A direct SSP-to-DSP connection with full transparency verification is worth more than an indirect path through multiple intermediaries, even if the indirect path theoretically reaches more buyers.

What Publishers Should Do

Map your supply paths. Understand which SSPs you work with directly and how they connect to DSPs. Know whether your inventory is being resold further down the chain.

Prioritize direct relationships. Where possible, work directly with SSPs that have strong DSP connections rather than through layers of intermediaries.

Make sure transparency works at every hop. Every SSP in your ads.txt should have you listed in their sellers.json. Use BeamFlow's scanner to verify the full chain.

Review your ads.txt for path efficiency. If you have reseller entries that add intermediaries without adding meaningful demand, consider whether they're worth the added complexity and fee extraction.

Frequently Asked Questions

Do I need to understand the full supply chain to optimize revenue?

You don't need to understand every technical detail, but you should know which SSPs you work with directly, whether they maintain sellers.json properly, and whether your ads.txt accurately reflects your actual partnerships. These basics cover most of the transparency and optimization opportunity.

Can I control which supply path DSPs use?

Indirectly. You control which SSPs are in your ads.txt and which SSPs your ad server connects to. By choosing SSPs with strong transparency practices and direct DSP relationships, you influence the paths available to buyers. But the DSP ultimately decides which path to bid through.

Why do DSPs care about supply path length?

Longer paths mean more intermediaries, more fees, more verification complexity, and more opportunities for fraud or data leakage. Shorter, verified paths deliver higher confidence and better economics for both the DSP and the publisher.