Real-Time Bidding (RTB): How Programmatic Advertising Auctions Work

Real-Time Bidding (RTB) is a programmatic advertising process in which buyers compete for individual ad impressions through automated, real-time auctions. A publisher or its supply-side technology makes an ad opportunity available, eligible buyers evaluate the opportunity, DSPs submit bids, and the applicable marketplace rules determine the winning bid and how the transaction is completed.

The simplest RTB flow is:

Ad opportunity → Bid request → DSP evaluation → Bid response → Auction → Winner → Ad delivery

The important distinction is that RTB is part of programmatic advertising, while OpenRTB is a protocol used to communicate information between participants in many RTB implementations.

This guide explains how an RTB auction works, what a bid request contains, how DSPs decide what to bid, how first-price auctions differ from second-price models, what bid shading means, and which metrics matter to programmatic advertisers.

What Is Real-Time Bidding (RTB)?

Real-Time Bidding (RTB) is an automated buying and selling process in which an individual digital advertising impression can be offered to eligible buyers for bidding. A DSP evaluates the advertising opportunity and can return either a bid with a price and applicable creative information, or no bid. The auction marketplace then applies its own eligibility and auction rules to determine whether a bid wins.

RTB in one sentence: RTB allows advertisers to make automated buying decisions at the impression-opportunity level instead of manually negotiating every individual impression.

Example

A user opens a webpage containing an available display ad slot. The advertising system makes that impression available to buyers. A DSP evaluates the opportunity and determines that the impression matches an advertiser’s campaign. The DSP submits a bid. Other eligible buyers may also submit bids. The marketplace evaluates the eligible bids and determines the outcome according to its auction rules.

Is RTB the Same as Programmatic Advertising?

No. RTB is one transaction mechanism within the broader programmatic advertising ecosystem. Programmatic advertising describes the broader use of automated technology to buy and sell digital advertising. RTB specifically refers to real-time auction-based buying. Other programmatic transaction types can include:

• Open auctions
• Private marketplaces (PMPs)
• Preferred deals
• Programmatic guaranteed transactions

The buying mechanism therefore matters when describing how an impression is purchased.

RTB vs programmatic advertising

Term	Meaning
Programmatic advertising	Broad category of automated digital advertising buying and selling
RTB	Real-time auction-based transaction mechanism
Open auction	An auction environment where eligible buyers can compete
PMP	A restricted programmatic marketplace with defined buyers and/or inventory
Programmatic guaranteed	A programmatic transaction with pre-arranged inventory and terms

Key takeaway: every RTB transaction is programmatic, but not every programmatic transaction is an RTB auction.

How Does an RTB Auction Work?

An RTB auction follows a sequence in which an ad opportunity is created, information about the opportunity is sent to eligible buyers, DSPs decide whether and how much to bid, and the marketplace determines the auction outcome. A simplified sequence is:

1. An ad opportunity becomes available.
2. Supply-side technology sends a bid request to eligible buyers.
3. DSPs evaluate the opportunity.
4. DSPs return bids or no-bids.
5. The marketplace applies eligibility, floor and auction rules.
6. A winning bid is selected.
7. The winning creative or ad response is used for delivery.
8. Relevant win, billing or loss events can be recorded according to the implementation.

IAB’s OpenRTB documentation describes this general interaction as bid requests being sent to bidders, responses being evaluated under prevailing auction rules, and a winner being selected.

Step 1: An ad impression opportunity becomes available

RTB starts when an advertising opportunity becomes available for sale. For example, a user opens a news website and an available display placement needs an advertisement. The publisher wants to monetize that placement, so its advertising stack can make the opportunity available to connected demand sources. The available inventory may contain information about:

• Ad format
• Placement
• Size
• Website or app
• Device
• Context
• Geographic signals
• Applicable restrictions
• Bid floor
• Deal information

The exact information depends on the marketplace, implementation, media format and applicable privacy controls.

Step 2: A bid request is sent to buyers

A bid request is the structured information sent to a buyer so the buyer can decide whether to bid on an advertising opportunity. In OpenRTB, the request can contain information describing the request itself, the impression or item being offered, the site or app, device, user and applicable restrictions or regulations.

The OpenRTB specification defines a request structure containing information such as a request ID, auction parameters, timing information and the advertising item being offered. Google’s Authorized Buyers documentation similarly describes a bid request as information about an ad opportunity that allows a bidder to determine the appropriate creative and bid amount.

### Important bid-request concepts

Signal / Object	Purpose
Request ID	Identifies the bid request
Impression / Item	Describes the advertising opportunity
Site / App	Identifies the publishing environment
Device	Provides permitted device-related information
User	Provides permitted audience/user information
Regulations	Communicates applicable regulatory information
Auction type	Indicates the applicable auction model where supported
Bid floor	Can establish a minimum acceptable bid
tmax	Defines the response-time limit in supported implementations

Not every bid request contains every possible field.

What Information Does an RTB Bid Request Contain?

An RTB bid request can contain information about the advertising opportunity, publishing environment, device, user, auction and transaction restrictions. Common categories include:

• Impression information: ad placement, format, size, video requirements, native requirements, floor price and deal information.
• Publisher information: website, app, publisher-related identifiers, and content or placement context.
• Device information: device type, operating system, browser or user-agent information and other permitted device signals.
• User and audience information: depending on the environment, privacy framework and implementation, the request can contain permitted user or audience signals.
• Regulatory information: OpenRTB can communicate regulatory and privacy-related information through applicable request fields.

The availability of user and identity signals is not universal. Privacy requirements, consent, browser/platform policies, publisher implementation and marketplace rules can affect what information is available to buyers.

Step 3: The DSP evaluates the impression

A Demand-Side Platform (DSP) evaluates the bid request against an advertiser’s campaign requirements and decides whether to bid and how much to bid. The DSP can evaluate factors such as:

• Campaign targeting
• Geography
• Audience signals
• Context
• Device
• Inventory quality
• Brand-safety requirements
• Frequency controls
• Budget
• Pacing
• Creative eligibility
• Deal eligibility
• Estimated value of the impression

A DSP does not need to bid on every impression. If the impression does not meet campaign requirements, the DSP can return a no-bid.

Why doesn’t a DSP bid on every impression?

A DSP skips an impression when its bidding logic determines that the opportunity is not valuable or eligible enough for the campaign. Possible reasons include:

• The audience does not match the campaign.
• The inventory is excluded.
• The bid floor is too high.
• The advertiser’s budget or pacing rules restrict bidding.
• The creative is not eligible.
• Frequency limits have been reached.
• The placement does not satisfy brand-safety requirements.
• The expected campaign value is too low.

OpenRTB supports a no-bid response, and Google documents bid filtering and response requirements for its RTB environment.

Step 4: The DSP calculates a bid

A DSP calculates a bid based on the estimated value of the advertising opportunity and the campaign’s bidding strategy. The DSP can combine: campaign objective + targeting + impression value + inventory quality + competition + budget + bidding strategy. The result is a bid price that the DSP is willing to submit.

Example. Suppose three DSPs evaluate the same impression:

Buyer	Bid
DSP A	₹120 CPM
DSP B	₹105 CPM
DSP C	No bid

These numbers are an illustrative example, not a market benchmark. The auction still needs to apply its own eligibility and pricing rules before determining the outcome.

Step 5: The bid response is returned

A bid response tells the marketplace that a buyer wants to compete for the impression and provides the bid information required by that implementation. An OpenRTB bid response can contain information such as:

• Request ID
• Bid price
• Impression/item reference
• Buyer seat
• Deal ID where applicable
• Creative or ad markup information
• Notification URLs
• Currency

The OpenRTB specification defines the bid response and bid objects used to communicate the buyer’s offer. A no-bid response can indicate that the buyer does not want to compete for the opportunity.

Step 6: The auction applies its rules

The marketplace does not simply compare every number without applying rules; bids must satisfy the applicable eligibility, floor, creative, deal and auction requirements. Possible checks include:

• Is the buyer eligible?
• Does the bid satisfy the applicable floor?
• Is the creative acceptable?
• Is the deal eligible?
• Does the bid satisfy publisher restrictions?
• What auction model applies?
• Are there other marketplace-specific rules?

This is why the simplified statement “the highest bid always wins” is incomplete. A better explanation is: the highest eligible bid can win when the applicable auction rules select it as the winning offer.

Google’s RTB documentation describes an example where the highest bid wins an open auction, while its OpenRTB documentation also defines auction-type fields and marketplace-specific behavior.

Step 7: The winning bid is selected

The auction selects a winning bid according to the rules of the applicable marketplace or transaction. The result depends on factors such as:

• Bid price
• Eligibility
• Bid floor
• Auction type
• Deal rules
• Publisher restrictions
• Creative eligibility

The winning bid is then used for the next stage of the transaction. The actual commercial flow can involve platform fees, intermediary economics, billing rules and marketplace-specific pricing mechanics. For content about auction mechanics, distinguish:

• Bid price: what the buyer submits.
• Winning bid: the eligible bid selected by the auction.
• Clearing price: the price determined by the applicable auction/pricing rules.
• Advertiser cost: the amount ultimately charged under the relevant buying arrangement.

First-Price vs Second-Price Auctions

First-price and second-price auctions differ mainly in how the winning price is determined.

What is a first-price auction?

In a first-price auction, the winning buyer generally pays the amount of its winning bid, subject to the marketplace’s pricing and billing rules.

Buyer	Bid
DSP A	₹120 CPM
DSP B	₹105 CPM
DSP C	₹90 CPM

If all three bids are eligible and the auction is a simple first-price auction, DSP A wins with a ₹120 CPM bid. The advertiser’s actual cost can still depend on the transaction’s broader commercial structure.

What is a second-price auction?

In a second-price auction, the winning buyer’s price is determined using the next-highest eligible bid and the auction’s pricing rules rather than simply charging the winner its full submitted bid. In the same simplified example, DSP A wins, and a simplified second-price model would calculate the winning price from the second-highest eligible bid, potentially with an applicable increment. The exact pricing formula is marketplace-specific.

OpenRTB identifies auction types including first-price and second-price-plus models in its specification.

Are RTB auctions always first-price?

No. Auction mechanics depend on the marketplace and transaction environment. However, first-price auctions are highly important in modern programmatic advertising. Google’s Authorized Buyers platform transitioned Ad Exchange inventory to a unified first-price auction in 2019. The broader RTB ecosystem can still contain different auction and transaction structures, so marketers should verify the pricing model for the specific platform they are using rather than assuming one auction model applies everywhere.

What Is Bid Shading?

Bid shading is a bidding technique designed to reduce the price paid in first-price auctions while preserving a reasonable probability of winning. In a first-price auction, submitting a very high bid can increase the amount paid if that bid wins. A bid-shading algorithm can estimate a lower bid based on available market and auction signals.

Simplified example. An advertiser’s estimated maximum value is ₹120 CPM. A possible shaded bid is ₹102 CPM. If ₹102 is sufficient to win the opportunity, the advertiser may avoid bidding the full ₹120. The actual bid-shading calculation depends on the DSP and available auction information.

Academic research published in 2026 describes bid shading as a strategy used in first-price RTB environments to balance winning probability against the risk of overpaying.

What Is OpenRTB?

OpenRTB is a standardized protocol for communicating real-time programmatic bid requests and bid responses. RTB and OpenRTB are therefore related but different concepts:

Concept	Meaning
RTB	Real-time auction-based advertising transaction process
OpenRTB	Protocol used to standardize communication in RTB transactions
Bid Request	Information sent to the buyer about an opportunity
Bid Response	Buyer’s response containing bid information
Auction	Process that evaluates eligible bids according to applicable rules

IAB Tech Lab maintains the OpenRTB specifications. Its current standards information lists OpenRTB 2.6.x as the programmatic real-time bidding protocol, while OpenRTB 3.0 is a separate finalized specification architecture.

Why OpenRTB matters

Without a common protocol, every exchange and bidder could require a completely different communication structure. OpenRTB provides standardized structures for information such as:

• Bid requests
• Impression opportunities
• Site/app context
• Device information
• User information
• Bid responses
• Deals
• Auction parameters

Google’s Authorized Buyers platform supports OpenRTB JSON and Protobuf formats for its RTB integrations.

How Fast Does an RTB Auction Happen?

An RTB bidder must respond within the time limit specified by the applicable marketplace because a late response can be excluded from the auction. For example, Google’s Authorized Buyers documentation states that bidder response time can vary by format and auction type and that the tmax field defines the applicable response deadline. OpenRTB also defines tmax as the maximum time allowed for bids to be received, including relevant network latency.

The exact deadline therefore depends on the platform and transaction. Do not assume that every RTB auction always takes one fixed number of milliseconds.

RTB Example: How One Impression Is Auctioned

Imagine an advertiser running a programmatic campaign targeting eligible audiences in India. A user in Delhi opens a publisher’s website. The simplified transaction could look like this:

1. Ad opportunity. The webpage has an available advertising placement.
2. Bid request. The supply-side system sends information about the opportunity to eligible buyers.
3. DSP evaluation. DSP A determines that the impression matches its advertiser’s targeting. DSP B also evaluates the opportunity. DSP C decides not to bid.
4. Bid responses. DSP A submits ₹120 CPM. DSP B submits ₹105 CPM.
5. Auction. The marketplace checks the applicable rules. Both bids are eligible.
6. Winner. DSP A wins the auction in this simplified first-price example.
7. Ad delivery. The winning ad is used for the impression according to the marketplace and publisher implementation.
8. Measurement. Relevant impression, win, billing, viewability, click or conversion signals can be recorded depending on the implementation and measurement setup.

This example explains the auction logic, not a guaranteed implementation for every publisher, SSP, exchange or DSP.

What Happens If No Buyer Bids?

If no eligible buyer submits a successful bid, the auction does not produce a winning buyer for that opportunity. A DSP can return no-bid. A bid can also fail to qualify because of:

• Bid-floor requirements
• Buyer restrictions
• Creative restrictions
• Deal requirements
• Publisher rules
• Technical validation
• Other marketplace conditions

OpenRTB defines no-bid signaling, while Google documents multiple reasons why bids can be filtered or excluded. The publisher’s ad-serving system can then follow its own fallback or alternative demand logic.

What Is the Role of the DSP in RTB?

A DSP is the buyer-side technology that evaluates advertising opportunities and helps advertisers decide whether and how much to bid. The DSP can perform functions such as:

• Targeting evaluation
• Bid calculation
• Budget management
• Pacing
• Frequency management
• Creative selection
• Optimization
• Reporting
• Audience activation where supported

The DSP is therefore not simply a “button that places the bid.” It is the decision-making layer between the advertiser’s campaign strategy and the individual auction opportunity.

What Is the Role of the SSP in RTB?

An SSP is supply-side technology used by publishers to manage and monetize advertising inventory through connected demand sources. An SSP can help with:

• Inventory management
• Demand connections
• Auction participation
• Floor-price controls
• Publisher rules
• Deal management
• Reporting
• Supply optimization

The exact responsibilities vary by platform. An SSP does not necessarily mean that every impression follows one fixed path through one exchange.

What Is the Role of an Ad Exchange?

An ad exchange is a marketplace or transaction layer that facilitates buying and selling between demand and supply participants. Depending on the implementation, the exchange can:

• Receive advertising opportunities
• Send bid requests to eligible buyers
• Receive bid responses
• Apply auction rules
• Select or facilitate the winning bid
• Send win/loss notifications
• Support billing or transaction notifications

OpenRTB documentation describes exchanges broadcasting bid requests to bidders, evaluating responses under auction rules and communicating auction outcomes.

DSP vs SSP vs Ad Exchange

Component	Primary side	Main function
Advertiser	Demand	Defines campaign objective and budget
DSP	Demand	Evaluates opportunities and submits bids
Ad Exchange	Marketplace	Facilitates the auction/transaction
SSP	Supply	Helps publishers sell advertising inventory
Publisher	Supply	Provides advertising inventory

The exact architecture can vary because programmatic supply chains can contain different platforms, intermediaries and transaction paths.

RTB vs Header Bidding

RTB and header bidding are not the same technology, although they can participate in the same programmatic transaction environment. RTB describes real-time auction-based buying. Header bidding is a publisher-side technique that can solicit bids from multiple demand partners before an ad-server decision. A simplified header-bidding flow might look like: Publisher page → Header bidding wrapper → Multiple demand partners → Bids → Ad server decision. An RTB transaction can occur within that broader process. Therefore:

• RTB = buying/auction mechanism
• Header bidding = publisher-side demand orchestration technique

The two concepts should not be used as synonyms.

What Determines Whether an Advertiser Wins an RTB Auction?

Bid price is important, but auction eligibility and marketplace rules also determine whether a bid can win. Important factors can include:

• Bid price
• Bid floor
• Buyer eligibility
• Campaign targeting
• Creative eligibility
• Deal eligibility
• Publisher restrictions
• Auction type
• Budget and pacing
• Frequency controls
• Inventory requirements
• Marketplace-specific rules

The practical lesson is: winning more auctions is not automatically the same as running a better campaign.

RTB Metrics Programmatic Marketers Should Understand

RTB performance should be evaluated using both auction metrics and campaign-outcome metrics.

Metric	What it measures
Bid rate	Percentage of eligible opportunities for which the buyer submits bids
Win rate	Percentage of submitted bids that win
Bid CPM	CPM value submitted by the buyer
Clearing CPM	Price determined under the applicable auction/pricing rules
Impressions	Ads delivered
Viewability	Whether delivered ads met the applicable viewability definition
CTR	Click-through rate
Conversion rate	Percentage of measured users/actions resulting in conversions
CPA	Cost per acquisition
Frequency	Average exposure level under the applicable measurement definition
Invalid traffic rate	Measured or estimated invalid traffic under the applicable methodology

Why bid metrics should not be viewed alone

A campaign with a high win rate can still perform poorly if it is buying low-value inventory. A campaign with a low win rate can still perform well if the DSP is selectively bidding on valuable impressions. The optimization question is therefore: are we winning the right impressions at an economically sensible price?

How Should a Programmatic Marketer Optimize RTB?

1. Analyze win rate. Win rate shows how often submitted bids become winning bids. A low win rate can indicate strong competition, low bids or restrictive targeting. A high win rate is not automatically positive because aggressive bidding can increase costs.
2. Compare bid CPM and clearing CPM. Comparing submitted bid prices with actual clearing prices helps marketers understand auction competitiveness and bidding efficiency. Large differences can provide useful information about the market and bidding strategy, depending on the platform’s reporting.
3. Analyze inventory quality. Break performance down by domain, app, placement, device, geography, audience, format, exchange/SSP, deal and time period. This helps identify where performance is actually coming from.
4. Review supply paths. Supply-path analysis helps advertisers understand how inventory reaches the buyer and whether multiple routes provide overlapping access to the same inventory. Relevant transparency standards include IAB Tech Lab’s sellers.json and SupplyChain Object, which are designed to improve visibility into programmatic supply relationships.
5. Apply brand-safety and suitability controls. Brand-safety controls help advertisers restrict inventory that does not meet campaign requirements. The exact controls depend on the DSP, verification provider and campaign.
6. Evaluate campaign outcomes. The final optimization target should match the campaign objective. For awareness: reach, frequency, viewability and completed views where relevant. For consideration: CTR, landing-page engagement and video engagement. For performance: conversions, CPA, ROAS and revenue. A low CPM is not automatically a successful outcome.

Common RTB Mistakes

1. Treating RTB and programmatic as synonyms. RTB is part of programmatic advertising, not the entire programmatic ecosystem.
2. Saying the highest bid always wins. Only an eligible bid can compete, and the auction outcome depends on the applicable marketplace rules.
3. Calling OpenRTB an auction. OpenRTB is a communication protocol; it does not mean “the auction” itself.
4. Assuming every auction is second-price. Auction pricing models vary, and first-price auctions are widely used in modern programmatic environments.
5. Assuming bid price always equals advertiser cost. Bid price, clearing price and final advertiser cost are related but are not universally identical concepts.
6. Optimizing only for CPM. A cheaper impression is not automatically a more valuable impression.
7. Treating every user signal as universally available. User and identity signals depend on the implementation, privacy requirements, consent, browser/platform policies and marketplace rules.

Frequently Asked Questions About RTB

RTB Auction Flow: The Short Version

Remember this sequence:

1. Ad opportunity becomes available
2. Bid request is created
3. Eligible DSPs receive the opportunity
4. DSPs evaluate targeting, inventory and campaign rules
5. DSPs return bids or no-bids
6. Marketplace applies auction and eligibility rules
7. Winning bid is selected
8. Winning ad is used for delivery
9. Win, billing, impression and performance signals are recorded according to the implementation

Final Takeaway

RTB is the real-time auction-based buying process that allows programmatic buyers to compete for individual digital advertising opportunities. The core relationship is: Ad opportunity → Bid request → DSP decision → Bid response → Auction → Winning bid → Ad delivery. The most important concepts to understand are:

• DSP = evaluates and buys advertising opportunities
• SSP = helps publishers sell advertising inventory
• Ad exchange = facilitates marketplace transactions and auctions
• Bid request = describes an advertising opportunity
• Bid response = communicates the buyer’s bid
• First-price auction = winning bid generally determines the auction price
• Second-price auction = pricing uses the next-highest eligible bid under applicable rules
• Bid shading = attempts to reduce first-price auction costs while preserving win probability
• OpenRTB = protocol for communicating RTB transaction information

The biggest practical lesson is simple: programmatic success is not about winning the maximum number of auctions. It is about identifying valuable impressions, bidding appropriately, paying an efficient price and improving the campaign outcome.