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Enforcing HTTP/3 (QUIC) in Chrome Tests

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Overview

By default, browsers do not immediately establish connections using HTTP/3 (h3). Instead, the initial connection is typically made using HTTP/2 (h2), after which the browser learns that the origin supports HTTP/3 and may use it for subsequent connections.

In a normal browsing environment, this information is retained by the browser and reused for future sessions. However, Catchpoint synthetic tests start with a fresh browser session on every execution and do not retain protocol discovery information.

As a result, each test run must rediscover HTTP/3 support, causing initial requests to use HTTP/2 before transitioning to HTTP/3.

Default Behavior

Without HTTP/3 enforcement enabled, test executions follow the browser's standard protocol discovery process:

  • Initial requests are made over HTTP/2
  • The browser discovers that the origin supports HTTP/3
  • Subsequent requests transition to HTTP/3

Default protocol discovery behavior without HTTP/3 enforcement

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Enforcing HTTP/3

You can force Chrome to establish connections using HTTP/3 from the beginning of the test by configuring the Cp-QuicForcedOrigins custom header.

This header allows you to specify one or more domains that should immediately use HTTP/3.

If HTTP/3 is unavailable, Chrome automatically falls back to the next available protocol.

Custom Header

Cp-QuicForcedOrigins

Header Value Format

The header value must contain one or more origins using the following format:

hostname:port

Multiple origins can be specified as a comma-separated list.

Example

Cp-QuicForcedOrigins: www.cloudflare.com:443,ot.www.cloudflare.com:443

The following example shows the header configuration within the test.

Configuring the Cp-QuicForcedOrigins header

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Verification

After configuring the header, Chrome establishes connections to the specified origins using HTTP/3 immediately.

This means:

  • The initial document request uses HTTP/3
  • All subsequent requests to the configured origins also use HTTP/3

HTTP/3 enforced for all requests
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Summary

The Cp-QuicForcedOrigins header allows Catchpoint synthetic tests to bypass the normal HTTP/3 discovery process and establish HTTP/3 connections from the beginning of a test run.

This ensures consistent protocol behavior and is particularly useful when validating:

  • HTTP/3 connectivity
  • HTTP/3 performance
  • Protocol-specific application behavior