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Real-Time Bidding (RTB) Explained

Real-time bidding (RTB) is the auction process that determines which ad a user sees when they load a webpage or open an app. Each available impression is put up for auction, multiple buyers submit bids simultaneously, and the winner’s ad is served, all within the time it takes a page to load. RTB is the foundational transaction mechanism of programmatic advertising.

Why RTB was developed

Before RTB, digital publishers sold unsold (remnant) inventory through ad networks at flat rates, with little transparency about where ads would appear or who would see them. Advertisers had no way to bid differently for a high-value user versus a low-value one, even if they were visiting the same page.

RTB solved this by making the price of each impression variable and determined per user, per page view, in real time. Advertisers could finally pay more for impressions they valued highly and less (or nothing) for those they did not.

How an RTB auction works

When a user triggers an ad request (by loading a page, opening an app, or starting a video), the following sequence unfolds:

  1. Ad request: the publisher’s ad server or header bidding wrapper sends a signal that an impression is available.
  2. Bid request: the SSP (supply-side platform) assembles contextual and audience data about the impression and broadcasts a bid request to connected DSPs and ad exchanges.
  3. Bid evaluation: each DSP receives the bid request and evaluates it against its active campaigns. It checks whether the impression matches targeting parameters (audience, geography, device, context) and calculates a bid price.
  4. Bid response: the DSP sends back its bid (or a no-bid) within the allotted time window, often 80-120 milliseconds.
  5. Auction settlement: the ad exchange collects all bids and applies the auction rules to determine the winner.
  6. Ad serving: the winning creative is returned to the publisher’s ad server and rendered in the user’s browser or app.
  7. Logging: both the DSP and SSP record the transaction for billing, reporting, and model updates.

The full sequence, from ad request to rendered ad, typically completes in under 200 milliseconds.

Auction types

First-price auction

The winning bidder pays exactly what they bid. This is now the dominant auction format in programmatic. Advertisers using first-price auctions must develop bid shading strategies, bidding slightly below their true valuation, to avoid overpaying for impressions.

Second-price auction (Vickrey auction)

The winner pays one cent (or one currency unit) above the second-highest bid, not their own bid. This format was common in early programmatic and still exists in some contexts. It theoretically encourages bidders to bid their true value.

What information is in a bid request

A bid request contains structured data that DSPs use to evaluate an impression. Typical fields include:

  • Impression data: ad slot size, format (banner, video, native), placement position on the page
  • Publisher data: site or app domain, content category, language
  • User data: anonymous identifiers, device type, operating system, inferred location
  • Audience data: any segments attached by the publisher’s data platform
  • Deal data: if the impression belongs to a private marketplace deal, the deal ID

The amount of user-level data in bid requests is decreasing as privacy regulations and browser policies restrict cookie-based tracking.

Latency and timeouts

Latency is a critical constraint in RTB. Publishers set a bid timeout, the maximum time they will wait for bids before the auction closes and a default or remnant ad is served. If a DSP’s response arrives after the timeout, the bid is ignored regardless of its value.

DSPs must balance the computational cost of evaluating millions of bid requests per second against the need to respond within tight time windows. Large DSPs may receive and process hundreds of thousands of bid requests per second.

RTB vs. direct and guaranteed deals

RTB competes with other transaction types for the same impressions. Publishers typically prioritize guaranteed deals (programmatic guaranteed, direct IO) over open auction RTB, meaning RTB often accesses inventory that was not sold through direct channels. However, header bidding has changed this dynamic by allowing RTB buyers to compete with direct deals simultaneously.

Brand safety and fraud in RTB

The open, automated nature of RTB creates risks. Advertisers must use controls to avoid:

  • Invalid traffic (IVT): bot traffic that generates fraudulent impressions
  • Domain spoofing: fraudulent sites misrepresenting themselves as premium publishers
  • Unsafe adjacency: ads appearing next to content that conflicts with brand guidelines

Standards from organizations like IAB Tech Lab (ads.txt, sellers.json, OpenRTB) and third-party measurement vendors help address these issues, but active campaign management is still required.

How Arago helps

Arago uses RTB to place local retail advertisers’ campaigns across a curated set of premium inventory sources in real time. Its bidding logic is designed for local-market objectives (reaching shoppers near specific locations, adjusting to seasonal demand, and optimizing toward in-store and online outcomes) so that every bid reflects the actual value of an impression for a local retailer.