A supply path is the route one advertising impression travels from the publisher offering it to the demand-side platform (DSP) that decides whether to bid. Every company handling the request forms a link: a header bidding wrapper, a supply-side platform (SSP), an exchange, a reseller, sometimes a curation layer. Because one ad slot can be offered through several chains at once, an impression usually has more than one path available, and the route a buyer takes determines the fees deducted, the latency incurred and the signals that survive the trip.

The term exists because programmatic advertising is not a market of direct transactions. A publisher sells through intermediaries, those intermediaries sell through others, and money returns down the same line minus a cut at each stop. Before the route could be recorded, buyers saw what they paid and publishers saw what they received, with the gap between the two unexplained.

What a path is made of

Three files and one bid request field describe a path.

The first is ads.txt, published by the IAB Tech Lab in 2017. Hosted at the root of a publisher domain, it lists the advertising systems permitted to sell that inventory, each line carrying a seller domain, an account identifier, the word DIRECT or RESELLER and an optional certification authority identifier. Those two labels are the first fork in any path: DIRECT means a publisher contract, RESELLER means authorisation to sell inventory the system does not control.

The second is sellers.json, running the other way: an advertising system publishes the entities it represents, each record carrying a seller identifier, business name, domain and a seller type of PUBLISHER, INTERMEDIARY or BOTH. Sellers may elect confidential status, suppressing name and domain.

The third is the OpenRTB SupplyChain object, which records the route an individual request actually took. It carries a version, a completeness flag and an ordered array of nodes. Each node holds asi, the domain of the advertising system; sid, that system's identifier for the seller being paid; hp, set to 1 when the node sits in the payment flow; and optionally rid, name, domain and an extension field. The completeness flag reads 1 only when every intermediary in the payment chain has been declared. OpenRTB 2.5 carried the object in source.ext.schain; OpenRTB 2.6, released in April 2022, promoted it to source.schain.

Google's documentation shows how short a path can be. A publisher selling directly through Ad Manager, AdMob or AdSense generates one node, naming google.com and the seller identifier; Open Bidding generates two, the second being the yield partner. Prebid notes that the specification forbids SSPs from adding upstream nodes, so wrapper operators handling publisher payments must declare themselves.

Buyers act on those declarations through platform settings. Display & Video 360 made Authorized Sellers Only the default line item setting in August 2019, restricting purchases to the sellers named in a domain's ads.txt file. Seller-level control came later and carried a price: DV360 opened a seller identifier blocklist to all advertisers in April 2025 with a 1.5% fee on media costs.

Direct routes and resold routes

Available routes per impression outnumber what most media plans assume. Jounce Media found the average real-time bidding publisher directly integrated with 24.5 sell-side platforms as of March 2025, authorising 15.3 of them to initiate resold auctions, with rebroadcasting behind 37% of display auctions.

Duplication follows. A DataBeat sellers report of June 11, 2026 found that 46% of the publisher domains reachable through tier 1 SSPs could also be reached by a second route, at an average of 1.31 intermediaries per domain, with tier 2 platforms at 38% and tier 3 at 34%. The cause was established SSPs appearing as resellers on domains where a direct relationship already existed. A later DataBeat report found 56.9% of new supply relationships still forming through resellers, so routes per impression keep multiplying rather than consolidating.

Origin and evolution

Before header bidding, path choice barely existed as a buy-side question. Publishers ranked networks in the ad server by historical yield and offered the impression down the list, so the seller fixed the route. Header bidding, standardised through Prebid.js from 2015, let publishers solicit bids from every connected SSP at once, creating parallel routes and the possibility of choosing between them.

The commercial case arrived through publisher grievance. The Guardian bought its own inventory through exchanges in October 2016 and found roughly 30 pence in the pound reaching it, according to Digiday, before suing Rubicon Project in March 2017 over undisclosed buyer fees.

Standards followed the complaint. IAB Tech Lab released sellers.json and the SupplyChain object for public comment on April 11, 2019, finalising both on July 31 that year. Index Exchange implemented the pair early, and buy-side mandates began: Centro told exchanges and networks it would block intermediaries that did not comply, with Ian Trider, then its director of RTB operations, describing the strategy as "minimizing the number of partners it takes to deliver an ad to a publisher".

Measurement caught up next. The ISBA and PwC Programmatic Supply Chain Transparency Study of May 6, 2020 tracked 267 million impressions, matched only 31 million from DSP to SSP and detected more than 1,000 distinct supply chains across 15 advertisers. Roughly half of advertiser spend reached publishers; 15% could not be attributed at all. A second study in January 2023 reported a 58% match rate, a 3% unattributed share and 65% of spend reaching media owners.

Later benchmarks measured quality rather than accounting. The ANA study released in December 2023 found 36% of post-transaction budget landing on valid, viewable, measurable impressions that were not made-for-advertising. IAB Spain's first SSP guide of April 15, 2026 put working media at 41% of programmatic spend and priced the difference between routes: direct SSP connections deliver 70% to 80% of an advertiser's money to publishers, indirect paths 40% to 50%.

What the route costs

Fees are the visible cost. Research cited by Integral Ad Science when it automated supply path selection on January 14, 2026 put the share of advertising money failing to reach publishers at 42% to 49%. Digitas reported its quality spend rate moving from 88% to 97% within a month, a figure supplied by the vendor.

Signal loss is the less visible cost. Equativ's October 2024 analysis put match rate losses between platforms at 40% to 70%, each identifier sync compounding the attrition, so a shorter path preserves more addressability than a longer one.

Access to a shorter route is itself priced. The Trade Desk charges publishers a flat 4.5% fee for OpenPath, its direct publisher integration, which chief executive Jeff Green told investors in February 2026 was meant to run close to breakeven. Viant launched a rival portal at no cost to publishers on June 11, 2026, with 85% of its connected television spend already flowing through its own direct route.

Limitations and disputes

Declaration is not verification. Every element of a recorded path is self-asserted: ads.txt states who may sell, sellers.json who is represented, the SupplyChain object who handled the request. Nothing in the stack proves any of it cryptographically, and the confidentiality option in sellers.json lets a seller withhold its name while remaining in the chain.

The word direct is contested. An SSP holding a publisher contract and an SSP reselling that publisher both appear in a bid stream, and the 46% duplication figure suggests the same platform often occupies both roles at once. Whether a DSP-operated route is structurally cleaner is disputed too: Dentsu and WPP exited OpenPath citing transparency concerns and hidden fees, according to a February 2026 Adweek report.

Identifying duplicate routes became harder in 2025. Prebid.org made transaction identifiers bidder-specific on August 27, 2025, removing the field buyers used to recognise one impression arriving through several exchanges. The Trade Desk answered by launching OpenAds on October 2, 2025 with a forked Prebid codebase preserving the original behaviour, a remedy controlled by one buyer rather than a standards body.

Much of the evidence is commercial. Duplication rates come from a vendor partner network rather than a census, and performance claims for direct routes from the platforms selling them.

Not the same as

Supply path optimisation is the buy-side practice of ranking and pruning paths. The path is the object, the optimisation the activity performed on it.

Supply chain describes those commercial relationships in aggregate. A supply path is one request travelling through part of that structure.

Traffic shaping, covered in the explainer on shaping, decides whether a request is sent at all. Path selection decides which of the sent requests a buyer transacts through.

Curation packages inventory with data before it reaches a buying platform. Germany's BVDW placed curation technically within the SSP layer on February 26, 2026, which makes a curated deal a path with an extra enrichment step, not a shorter one.

Recent developments

The shortest routes now being built remove the network hop entirely. Bedrock Platform ran a containerised DSP bidder inside Index Exchange infrastructure on April 21, 2026, placing decisioning logic inside the exchange rather than on external cloud servers. Index Exchange said on September 10, 2026 that the agency PMG now runs its buying inside the exchange through the same architecture, reaching 3,000 media owners with streaming television first in scope.

The declaration layer is under revision too. Brian O'Kelley argued in April 2026 that ads.txt cannot express commercial reality in connected television, proposing adagents.json to add placement identifiers, delegation types, country scoping and date windows. Neal Richter of Amazon acknowledged the older file's limits while defending its design. Nine years on, the industry still disagrees about how precisely a path should be described.

Timeline

  • 2015: Prebid.js establishes header bidding, creating parallel routes to the same impression
  • October 2016: The Guardian buys its own inventory through exchanges and finds roughly 30 pence in the pound reaching the publisher
  • March 2017: The Guardian sues Rubicon Project over undisclosed buyer fees
  • 2017: IAB Tech Lab publishes ads.txt, introducing the DIRECT and RESELLER labels
  • April 11, 2019: sellers.json and the SupplyChain object open for public comment
  • July 31, 2019: Both specifications are finalised
  • August 2019: Display & Video 360 makes Authorized Sellers Only the default line item setting
  • May 6, 2020: ISBA and PwC publish a study finding more than 1,000 supply chains across 15 advertisers, a 12% impression match rate and a 15% unattributed delta
  • April 2022: OpenRTB 2.6 moves the SupplyChain object to source.schain
  • January 2023: The second ISBA study reports a 58% match rate, a 3% delta and 65% of spend reaching media owners
  • December 2023: The ANA study finds 36% of post-transaction budget on valid, viewable, non-made-for-advertising impressions
  • October 2024: Equativ research puts cross-platform match rate losses at 40% to 70%
  • March 2025: Jounce Media reports 24.5 direct SSP integrations per publisher and rebroadcasting behind 37% of display auctions
  • April 2025: DV360 opens seller identifier blocking to all advertisers at a 1.5% fee
  • August 27, 2025: Prebid.org makes transaction identifiers bidder-specific
  • October 2, 2025: The Trade Desk announces OpenAds
  • February 26, 2026: BVDW publishes a curation whitepaper placing curation in the SSP layer
  • April 15, 2026: IAB Spain reports 41% working media and the direct versus indirect revenue split
  • April 21, 2026: Bedrock Platform runs a containerised bidder inside Index Exchange
  • June 11, 2026: DataBeat reports 46% duplicated domains at tier 1 SSPs; Viant launches SupplyIQ
  • September 10, 2026: Index Exchange says PMG is running its buying inside the exchange

Summary

Who. Publishers and their wrapper operators originate paths; SSPs, exchanges, resellers and curators occupy the intermediate nodes; DSPs and the agencies operating them choose which paths to transact through. IAB Tech Lab governs the files and fields that describe them, and Prebid, Google, The Trade Desk, Viant and Index Exchange operate the largest implementations.

What. A supply path is the sequence of intermediaries a bid request passes through between a publisher's ad slot and a bidding platform, recorded per request in the OpenRTB SupplyChain object and constrained by the ads.txt and sellers.json declarations of who may sell what.

When. Path choice became a buy-side variable after header bidding arrived in 2015, acquired standards in 2019 with sellers.json and the SupplyChain object, was measured in the ISBA studies of 2020 and 2023, and is now being shortened architecturally through containerised bidding from 2026.

Where. Paths exist in web, mobile app, connected television, audio and digital out-of-home inventory anywhere OpenRTB is used, with route length varying sharply by channel and by whether a publisher sells directly or through resellers.

Why. Each additional node takes a fee, adds latency and degrades identity matching, so two routes to the same impression can deliver materially different amounts of money to the publisher and materially different performance to the advertiser. Trade body measurements put working media between 36% and 41% of programmatic spend, and the difference between direct and resold routes is one of the largest single explanations for that gap.