# HTTP/3

Category: [Technical SEO](https://seo-geo-glossary.com/en/glossary/category/technical-seo/) › [Servers, HTTP & caching](https://seo-geo-glossary.com/en/glossary/category/technical-seo/servers-http-caching/) · Also known as: HTTP3

**HTTP/3** is the third major version of the HTTP protocol, standardized as RFC 9114 in June 2022. It carries the same HTTP semantics as earlier versions, but sends them over QUIC, a transport protocol built on UDP, instead of TCP.

## How does HTTP/3 differ from HTTP/2?

With HTTP/2, many streams share one TCP connection, so a lost packet delays all active streams. QUIC provides multiplexing itself. If a packet is lost, only the streams whose data was in that packet wait. HTTP/3 also replaces the HPACK header compression of HTTP/2 with QPACK, which works with QUIC's out-of-order delivery. QUIC includes TLS encryption, so HTTP/3 always runs encrypted.

## How do clients find HTTP/3?

A client usually starts with HTTP/1.1 or HTTP/2 over TCP. A server announces HTTP/3 with the `Alt-Svc` response header, for example `Alt-Svc: h3=":443"`, and the client can use QUIC for later requests. A DNS record of type HTTPS (RFC 9460) lets clients learn about HTTP/3 before the first connection. If a UDP connection fails, clients fall back to a TCP-based version.

## HTTP/3 and crawlers

Google's overview of its crawlers and fetchers states support for HTTP/1.1 and HTTP/2. HTTP/3 is not mentioned there. Enabling HTTP/3 on a server or CDN therefore changes how browsers connect, and no effect on Google's crawling is documented. Server logs show which protocol each request used.

## Example

```http
HTTP/2 200
alt-svc: h3=":443"; ma=86400
```

The `ma` value is how long, in seconds, the client may remember the alternative service.

## HTTP versions compared

| Version | Transport | Concurrent requests | Header fields | Use by Google's crawlers |
|---|---|---|---|---|
| HTTP/1.1 (RFC 9112) | TCP | no multiplexing layer; pipelining possible, responses come back in request order; often several TCP connections for parallel requests | whitespace-delimited text fields; often repetitive and verbose according to RFC 9113 | supported; Google's default version |
| HTTP/2 (RFC 9113) | TCP | yes, many requests interleaved on one connection; TCP-level packet loss still stalls all streams | binary frames, HPACK compression | supported since November 2020; no ranking boost; opt out with status code 421 |
| HTTP/3 (RFC 9114) | QUIC over UDP | yes, over independent QUIC streams; one blocked stream does not hold up the others | QPACK compression | not stated by Google; the crawler documentation lists only HTTP/1.1 and HTTP/2 |

Sources: [RFC 9112: HTTP/1.1](https://www.rfc-editor.org/rfc/rfc9112.html); [RFC 9113: HTTP/2](https://www.rfc-editor.org/rfc/rfc9113.html); [RFC 9114: HTTP/3](https://www.rfc-editor.org/rfc/rfc9114.html); [RFC 9000: QUIC](https://www.rfc-editor.org/rfc/rfc9000.html); [Google Crawling Infrastructure: Overview of Google crawlers and fetchers (last updated 2026-06-12)](https://developers.google.com/crawling/docs/crawlers-fetchers/overview-google-crawlers); [Google Search Central Blog: Googlebot will soon speak HTTP/2 (2020-09-17)](https://developers.google.com/search/blog/2020/09/googlebot-will-soon-speak-http2). Last checked: October 8, 2026

## Not to be confused with

- [HTTP/2](https://seo-geo-glossary.com/en/glossary/http-2/index.md): runs over TCP and bundles many requests on one connection. A lost packet delays every stream on that connection. Google documents support for HTTP/2 in its crawlers and says they use the version with the best crawling performance.

## Related terms

- [HTTP/2](https://seo-geo-glossary.com/en/glossary/http-2/index.md)
- [HTTP header](https://seo-geo-glossary.com/en/glossary/http-header/index.md)
- [HTTPS](https://seo-geo-glossary.com/en/glossary/https/index.md)
- [HTTP status code](https://seo-geo-glossary.com/en/glossary/http-status-code/index.md)

## Sources

- [RFC 9114: HTTP/3](https://www.rfc-editor.org/rfc/rfc9114)
- [HTTP/3 – Wikipedia](https://en.wikipedia.org/wiki/HTTP/3)
- [Overview of Google crawlers and fetchers – Google Search Central](https://developers.google.com/crawling/docs/crawlers-fetchers/overview-google-crawlers)

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Last updated: October 8, 2026
URL: https://seo-geo-glossary.com/en/glossary/http-3/