Troubleshoot protocol issues

Last updated Apr 21, 2026 This guide covers common HTTP/2 and HTTP/3 issues, including origin incompatibility, multiplexing errors, and browser errors, with steps to diagnose and resolve them.

H2 to Origin - Origin incompatibility

  • The origin's max_concurrent_streams is negotiated during the handshake process.
  • If a GOAWAY(0) is received, it is likely due to a server restart or another reason causing the server to refuse new streams.
  • For more information, refer to RFC 9113 - SETTINGS_MAX_CONCURRENT_STREAMS ↗︎.

H2 Multiplexing - Origin incompatibility/issues

  • Multiplexing issues can arise due to incorrect server configurations.
  • Use netlogs ↗︎ to identify SETTINGS_MAX_CONCURRENT_STREAMS violations or unexpected GOAWAY frames.
  • For more information, refer to Stream Concurrency Issues ↗︎.

Generic browser errors

Common browser errors include:

  • ERR_HTTP2_PROTOCOL_ERROR
  • ERR_HTTP3_PROTOCOL_ERROR
  • ERR_QUIC_PROTOCOL_ERROR

These errors do not necessarily indicate a protocol-level issue. Follow these steps:

  1. Attempt reproduction using HTTP/1.1.
  2. If the issue persists in HTTP/1.1, address the underlying error before testing HTTP/2 or HTTP/3.
  3. If the issue does not persist, analyze netlogs for HTTP/2 or HTTP/3-specific issues.

For more information, refer to Chromium URL Request Header ↗︎.

Chrome stalls or fails only on HTTP/3

If the issue reproduces only in Chrome over HTTP/3 and disappears when HTTP/3 is disabled, the problem may be related to a browser-side QUIC handling issue rather than your origin server. This is a known Chrome issue (crbug.com/41161335 ↗︎) — Cloudflare's QUIC implementation is not the cause.

Symptoms can include:

  • Large downloads stall unexpectedly.
  • Pages with many concurrent requests hang for one to three minutes and then fail.
  • Chrome reports ERR_QUIC_PROTOCOL_ERROR or ERR_HTTP3_PROTOCOL_ERROR after the connection stops making progress.
  • Issue does not reproduce in Firefox or Safari.
  • Issue resolves after disabling QUIC in chrome://flags.

How to isolate the issue

  1. Temporarily disable HTTP/3 for the zone.
  2. Test the same request again over HTTP/2.
  3. If the issue disappears over HTTP/2, capture a NetLog for Chrome and compare the behavior.

Test immediately: In Chrome, go to chrome://flags, search for "QUIC", set it to Disabled, then relaunch Chrome.

Resolution

If the issue is limited to specific hostnames, you can apply a more targeted workaround: create a Response Header Modification Transform Rule to remove the Alt-Svc header for the affected hostname.

  1. In the Cloudflare dashboard, go to the Rules Overview page.
  2. Select Create rule > Response Header Transform Rule.
  3. Set the matching expression to your hostname: (http.host eq "example.com").
  4. Under Modify response header, select Remove and enter Alt-Svc as the header name.

This forces Chrome to use HTTP/2 for that hostname without disabling HTTP/3 globally. However, proxied hostnames can also advertise HTTP/3 through generated HTTPS records, so disabling HTTP/3 for the zone is the most reliable way to force HTTP/2 while you troubleshoot.

After changing Alt-Svc, remember that browsers may cache the advertised alternative service for up to 24 hours.

Was this helpful?

YesNo

On this page

Troubleshoot protocol issues - 图1Docs

{"@context":"https://schema.org","@type":"TechArticle","@id":"https://developers.cloudflare.com/speed/optimization/protocol/troubleshooting/protocol-troubleshooting/#page","headline":"Troubleshoot protocol issues","description":"Resolve common HTTP/2 and HTTP/3 connection issues.","url":"https://developers.cloudflare.com/speed/optimization/protocol/troubleshooting/protocol-troubleshooting/","inLanguage":"en","image":"https://developers.cloudflare.com/speed/optimization/protocol/troubleshooting/protocol-troubleshooting/og.png?v=e2ad8e7dd09c8337","dateModified":"2026-04-21","publisher":{"@type":"Organization","name":"Cloudflare","description":"One platform for your apps, agents, and workforce. Build, secure, and scale without managing infrastructure","url":"https://www.cloudflare.com/","sameAs":["https://github.com/cloudflare","https://www.linkedin.com/company/cloudflare","https://x.com/cloudflare"],"logo":{"@type":"ImageObject","url":"https://developers.cloudflare.com/logo.svg"},"address":{"@type":"PostalAddress","streetAddress":"101 Townsend St","addressLocality":"San Francisco","addressRegion":"CA","postalCode":"94107","addressCountry":"US"},"contactPoint":[{"@type":"ContactPoint","contactType":"Customer Support","url":"https://support.cloudflare.com/","availableLanguage":["English"]},{"@type":"ContactPoint","contactType":"Sales","url":"https://www.cloudflare.com/contact/","availableLanguage":["English"]}]},"isPartOf":{"@type":"WebSite","@id":"https://developers.cloudflare.com/#website","name":"Cloudflare Docs","url":"https://developers.cloudflare.com/"},"keywords":["Debugging"]}

description: View personalized speed optimization recommendations for your site. title: Recommendations image: https://developers.cloudflare.com/speed/optimization/recommendations/og.png?v=b6939aff23d935e7

Skip to content

Documentation Index
Fetch the complete documentation index at: https://developers.cloudflare.com/speed/llms.txt
Use this file to discover all available pages before exploring further.