Measuring tape.

Attribution Reporting proposal updates in January 2022

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This content was initially posted in the Attribution Reporting API documentation. We're now keeping a running list of updates, with links out to more information.

The Attribution Reporting proposal has undergone a number of changes to address community feedback, from API mechanism changes to new functionality.


Who is this post for?

This post is for you:

  • If you already understand the API—for example, if you've been observing or participating in the discussions on the WICG repository and want to understand the batch of changes made to the proposal in January 2022.
  • If you're using the Attribution Reporting API in a demo or in an experiment in production.

If you're just getting started with this API and/or have not experimented with it yet, go directly to the introduction to the API instead.

Migration ahead

The proposed changes will be implemented in Chrome for experimentation.

Ecosystem feedback on these changes is greatly appreciated. Join the discussion by commenting on the links listed under Join the public discussion in this post, or by creating new issues. Learn more about participating.

This is not an API guide; details in the new proposal are subject to change. If you intend to experiment with the API: hold your migration until code is available in Chrome, and subscribe to the developer mailing list for updates.

Once these changes are implemented in Chrome: if you use event-level reports from the Attribution Reporting API in a demo or in an experiment in production (origin trial), you'll need to edit your code for the API to continue working. You may also consider using the new features.

This article also lists changes for aggregatable reports. However, these changes, if implemented, will not require any action or migration, as there's no browser implementation yet for aggregatable reports at the time of this writing.

Name changes

Summary reports and aggregatable reports

What you may have seen described as aggregate reports will now be referred to as summary reports.

Summary reports are the final output of the aggregation of multiple aggregatable reports, formerly called contributions or histogram contributions.

API mechanism changes

Header-based source registration (event-level reports)

What's changing, and why?

When the user views or clicks an ad, the browser—locally on the user's device—records this event, alongside parameters that are specific to attribution reporting (such as the attributionsourceeventid, attributiondestination, attributionexpiry and other parameters). The values of these parameters are set by the adtech.

The way these parameters are set is changing.

In the previous proposal, the parameters had to be included client-side: either in the anchor tags as HTML attributes, or as arguments of a JS-based call. Parameters had to be known at click or view time.

In the new proposal, the value of these parameters is defined on the adtech server instead.

Diagram of header-based source registration

This has a number of upsides, notably in terms of security: the header mechanism gives the reporting origin—typically, an adtech—direct control over whether an attribution source is registered in their scope. This partially mitigates fraud concerns, as with this change a genuine browser will never register a source without the reporting origin's opt-in.

How does source registration work?

  1. For a given ad, the adtech would now need to define a specific client-side attribute attributionsrc. The value of this attribute is a URL to which the browser will send a request; this request will include a new HTTP header Attribution-Reporting-Source-Info whose value, navigation or event,specifies whether the source was a click or a view respectively.

  2. Upon receiving this request, the click/view tracking server should respond with a HTTP header, Attribution-Reporting-Register-Source, that contains the desired attribution parameters.

  3. The origin that returns this header is now the reporting origin (formerly defined as attributionreportto).

    HTTP Response Header Attribution-Reporting-Register-Source:

    "source_event_id": "267630968326743374",
    "destination": "https://toasters.example",
    "expiry": "604800000"

Learn more in the technical explainer

Registering attribution sources

Join the public discussion

Issue #261

Header-based attribution trigger (event-level reports)

What's changing, and why?

Just like click or view registration, the new proposal changes the attribution trigger—when an adtech instructs the browser to record a conversion—to a header-based approach.
This mechanism is aligned with the header-based source registration, and is more conventional than the previously used redirect mechanism.

Additionally, in the new proposal, the attributionsrc attribute is needed on the conversion page.

The rationale is a matter of permissions: in the previous proposal, the trigger-side site—typically, an advertiser site—did have general control on the feature via a Permissions-Policy header, but did not have granular, element-level control on whether an element can send out a request to a party that would ultimately trigger an attribution. attributionsrc changes this: this mandatory marker gives the advertiser the ability to monitor and hence control which elements can trigger an attribution.

Note that on the source side—typically, a publisher site—a page-wide control via Permissions-Policy, as well an element-wide control via attributionsrc, are present.

How does attribution trigger work?

Upon receiving a pixel request and deciding that it should be categorized as a conversion, an adtech should respond with a new HTTP
header Attribution-Reporting-Register-Event-Trigger.

This header's value specifies how to treat the trigger event, as a JSON object. This is the same information that was defined as query parameters in the previous proposal.

HTTP Response Header Attribution-Reporting-Register-Event-Trigger:

trigger_data: (unsigned 3-bit integer),
trigger_priority: (signed 64-bit integer),
deduplication_key: (signed 64-bit integer)

Redirection (optional)

Optionally, the adtech server can make the response that contains Attribution-Reporting-Register-Event-Trigger a redirect response. With this, it enables third-parties to observe the conversion event and to instruct the browser to attribute it.

Redirection is optional; it's not needed when both an adtech and a third-party have pixels on the page.

More details in Third-pary reporting.

Learn more in the technical explainer

Triggering Attribution

Join the public discussion

Issue #91

No worklet (aggregatable reports)

What's changing, and why?

In the previous proposal for aggregatable reports, JavaScript access was required to invoke a worklet—a JavaScript-based mechanism—that would generate these reports.

In the new proposal, no worklet is required. Instead, an adtech would define declaratively—via HTTP headers—the rules the browser should use to generate aggregatable reports.

The new proposal offers several benefits:

  • Browser implementation: the new design, unlike the worklet design, is substantially simpler because it doesn't require a new execution environment in browsers.
  • Developer experience: the new design relies on headers, which are commonly used and widely known by developers—unlike worklets. It also aligns closely with the API surface for source registration, making the API easier to learn and use.
  • Adoption: the new design enables more existing measurement systems to use aggregatable reports. Many measurement solutions are HTTP-only: they rely on image requests—pixel requests—that don't require JavaScript access. But because the worklet approach did require JavaScript access, it may have been difficult to migrate to from some existing measurement systems.
  • Robustness: the new design helps mitigate data loss because it's easier to integrate with keepalive semantics, for example if a click or view is registered when a user is leaving a page.

How does the worklet-free mechanism work?

This declarative mechanism is based on HTTP headers—just like the event-level source registration and the attribution trigger header. More details on this in the next sections.

Join the public discussion

Issue #194

Header-based source registration (aggregatable reports)

A new mechanism is proposed to register a source for an aggregatable report; this mechanism is the same as the event-level source registration.

Only the header name is different: Attribution-Reporting-Register-Aggregatable-Source.

Learn more in the technical explainer

Attribution source registration

Header-based attribution trigger (aggregatable reports)

A new mechanism is proposed to register a source for an aggregatable report; this mechanism is the same as the event-level attribution trigger.

Only the header name is different: Attribution-Reporting-Register-Aggregatable-Trigger-Data.

Learn more in the technical explainer

Attribution trigger registration

New features

Third-party reporting (event-level reports and aggregatable reports)

What's changing, and why?

Two aspects of the new proposal help better support third-party reporting use cases:

  • Optionally, adtechs can redirect network requests to other adtechs servers, which allows those other adtechs to perform their own source and trigger registration. This is a common way third parties are configured today. This makes the API easier to adopt, among others in existing third-party reporting systems.
  • Reporting origins—typically, adtechs—no longer share most privacy limits. This supports use cases where multiple adtechs work with the same publishers or advertisers.

How does third-party reporting work?

In the new proposal, response-based source registration and trigger rely on HTTP headers. An adtech can leverage HTTP redirects for these requests.

If a click/view request on a publisher site (source registration) is subsequently redirected to multiple parties, each of these parties can register this view or click (source event).
Similarly, an adtech can redirect a specific attribution request made from an adivertiser site, allowing multiple other parties to register a conversion (attribution trigger).

Each party is able to access their separate reports, and to configure them with separate data.

To prevent abuse, explicit reporting limitations have been added. Learn more in Privacy protection changes.

Register multiple triggers without redirects

It's also possible to register multiple attribution triggers without using redirects, by adding multiple pixel elements on the conversion side (one per trigger).

Join the public discussion

Issue #91 Issue #261

View-through measurement (event-level reports and aggregatable reports)

What's changing, and why?

In the new proposal, view-through measurement and click-through measurement work in a unified way:

  • registerattributionsrc, the view-specific attribute that instructed the browser to record views alongside clicks, is no longer part of the proposal.
  • The privacy mechanisms are now unified across click and view. On this, see details in Noise and transparency.

This change is proposed to align with the new header-based registration mechanism. It also simplifies developer experience when intending to support both click- and view-through measurement.

How does view-through measurement work?

View-through measurement and click-through measurement both rely on header-based registration.

Learn more in the technical explainer

Event-level reports (for both clicks and views)

Join the public discussion

Issue #261

Debugging / Performance analysis (event-level reports and aggregatable reports)

What's changing, and why?

A debugging mechanism has been added to the proposal to help developers detect bugs, as well as compare the performance of Attribution Reporting to existing cookie-based measurement solutions.

Diagram of the new cookie-based debugging system

How does debugging work?

The Attribution Reporting API explicitly prevents the linking of detailed source events (ad clicks or views on a publisher site) and detailed trigger events (detailed conversions on an advertiser site). This prevents cross-site tracking enabled by third-party cookie-based solutions.

During the experimentation phase, the debugging functionality proposal does provide a link between a detailed source event and a detailed trigger event. However, the debugging functionality intentionally requires the ability to already set a third-party cookie. This means that the debugging functionality will not be available if third-party cookies are blocked. This also means it will naturally no longer be available after support for third-party cookies is phased out.

Both source and trigger registration will accept a new parameter debug_key, a 64-bit unsigned integer (that is, a large number).

If a report is created with source and trigger debug keys and if a Samesite=None ar_debug=1 cookie is present in the reporting origin's cookie jar at source and trigger registration time, a debug report (JSON) will be sent to a .well-known/attribution-reporting/debug endpoint:

"source_debug_key": 1234567890987,
"trigger_debug_key": 4567654345028

Event-level and aggregatable reports will also include these two new parameters, so that they can be associated with the correct debug report.

Learn more in the technical explainer

Optional: extended debugging reports

Join the public discussion

Issue #174

Filtering capabilities (event-level reports and aggregatable reports)

This is a new feature—more precisely, it was already part of the aggregatable reports proposal and has now been added to the event-level reports proposal.

What's changing, and why?

Because they support important use cases in today's advertising ecosystem, a number of use cases will now be supported in both event-level and aggregatable reports:

  • Conversion filtering: filter a conversion based on source-side information. For example, select different trigger data (conversion data) for ad clicks and views.
  • Attribution mismatching: filter conversions that have been misattributed; this is a specific type of conversion filtering. For example, filter out conversions that get matched to the wrong ad click/view due to the etld+1 destination scope in the API.

How do filtering capabilities work? (for event-level reports)

An optional source_data field in the source-side JSON object can define items that will be subsequently used by the browser at conversion time to apply filtering logic.

source_event_id: "267630968326743374",
destination: "https://toasters.example",
expiry: "604800000"
source_data: {
conversion_subdomain: ["electronics.megastore"
product: "198764",
// Note that "source_type" will be automatically generated as one of {"navigation", "event"}

Trigger registration will now accept an optional header Attribution-Reporting-Filters.

HTTP Response header Attribution-Reporting-Filters:

"conversion_subdomain": "electronics.megastore",
"directory": "/store/electronics"

Alternatively, the Attribution-Reporting-Register-Event-Trigger header can be extended with a filters field to do selective filtering to set trigger_data based on source_data.

If keys in the filters JSON match keys in source_data, the trigger is
completely ignored if the intersection is empty.

Learn more in the technical explainer

Optional attribution filters

Join the public discussion

Issue #194
Issue #201

Privacy protection changes

Noise and transparency (event-level reports and aggregatable reports)

What's changing, and why?

In the new proposal, one of the privacy mechanisms for reports has been improved: reports are subject to randomized response.
This means that some real conversions will be reported correctly; and a certain percentage of the time, some real conversions will be suppressed or some fake conversions will be added.

This new technique has a few benefits:

  • It unifies the privacy mechanism for clicks and views.
  • It's simpler to reason about than a mechanism where trigger data (conversion data) and trigger-source link noise would be separated.
  • It sets up a privacy framework which could, with the right noise settings, ensure that no party can rely on the API to know with certainty that an individual user has converted (or not) for a certain ad.

This new mechanism replaces the previous mechanism where 5% of the time, trigger data (conversion data) was replaced with a random value.

Additionally, the randomized response probability value has been added to the report body (randomized_trigger_rate field). This field specifies the probability (0 to 1) that a source is subject to randomized response.

This has two main benefits:

  • It makes the underlying browser behavior transparent to the parties that will receive the reports (typically, adtechs).
  • It's helpful for a future where the API would be supported across browsers: different browsers may decide to apply different levels of noise depending on their privacy goals, and the parties that will handle the report will need visibility into this.

How does noise work?

In the new proposal, at the time a source is registered (i.e. an ad click or view is recorded), the browser randomly decides whether it will truthfully attribute conversions and send reports for this ad click/view, or whether it will generate a fake output instead.

The fake output can be:

  • No report at all—regardless of whether the user converts;
  • One or several fake reports—regardless of whether the user converts.

In fake reports, the trigger data (conversion data) is random: a random 3-bit value for clicks (any number between 0 and 7) and a random 1-bit value for views (0 or 1).

Like real reports, fake reports are not sent immediately after the user converts. They're sent at the end of a random reporting window.

Key Term

After an initial ad click, a schedule of reporting windows begins. Each reporting window has a deadline. Reports for conversions that are attributed before that deadline will be sent at the end of that window.

There are three reporting windows for clicks (2 days, 7 days or 30 days after click). Each fake report is randomly assigned to one of the reporting windows.

Separately, as the previous proposal already stated, ordering of reports within a window is random.

Fake reports for views are not subject to a random reporting window, because there is one single reporting window for views.

Learn more in the technical explainer

Noisy fake conversions examples

Join the public discussion

Issue #84
Issue #273

Reporting limitations (event-level reports and aggregatable reports)

Reporting origin limits

What's changing, and why?

The new proposal explicitly limits how many parties can measure events between two sites.

  • The maximum number of unique reporting origins (typically adtechs) that can register sources per {publisher, advertiser} is proposed to be capped to 100 per 30 days. This counter will be incremented for each ad click or view (source event), even those that are not attributed.
  • The maximum number of unique reporting origins (typically adtechs) that can send reports per {publisher, advertiser} is proposed to be capped to 10 per 30 days. This counter will be incremented for every attributed conversion.

These limits are intended to be high enough that they don't limit any actor's ability to measure conversions, but low enough that they help mitigate some forms of API abuse.

Reporting cooldown / rate limits

What's changing, and why?

Reporting cooldown is a privacy mechanism that throttles the amount of total information sent through this API in a given time period for a user.

In the new proposal, 100 reports per {source site, destination, reporting origin} (typically {publisher, advertiser, adtech}) can be scheduled over 30 days.

Beyond this limit, the browser will stop scheduling reports that match this given {source site, destination, reporting origin} (typically {publisher, advertiser, adtech})—until the rolling 30-day report count falls below 100 for that {source site, destination, reporting origin}.

Learn more in the technical explainer

Reporting cooldown / rate limits

Destination capping (event-level reports only)

The new proposal subjects both clicks and views to destination capping; note that the previous proposal already mentioned destination capping for views.

What's changing, and why?

Destination capping is modified to include the reporting origin (typically, an adtech) in the scope: 100 unique pending destinations (typically, advertiser sites, or sites where conversions are expected to take place) are allowed per {publisher, adtech}.

This is a privacy protection to limit browsing history reconstruction.

Learn more in the technical explainer

Limiting the number of unique destinations covered by pending sources

All resources

The header image is from Diana Polekhina on Unsplash.

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