[HTTP/3](https://quicwg.org/base-drafts/draft-ietf-quic-http.html) is the third and upcoming major version of HTTP. This article discusses requirements for HTTP/3 and how to configure Kestrel to use it.
> HTTP/3 is available in .NET 6 as a *preview feature*. The HTTP/3 specification isn't finalized and behavioral or performance issues may exist in HTTP/3 with .NET 6.
> For more information on preview feature support, see [the preview features supported section](https://github.com/dotnet/designs/blob/main/accepted/2021/preview-features/preview-features.md#are-preview-features-supported).
> Apps configured to take advantage of HTTP/3 should be designed to also support HTTP/1.1 and HTTP/2. If issues are identified in HTTP/3, we recommended disabling HTTP/3 until the issues are resolved in a future release of ASP.NET Core. Significant issues are reported at the [Announcements GitHub repository](https://github.com/aspnet/Announcements/issues).
HTTP/3 has different requirements depending on the operating system. If the platform that Kestrel is running on doesn't have all the requirements for HTTP/3 then it's disabled, and Kestrel will fallback to other HTTP protocols.
1. Add the `packages.microsoft.com` repository. See [Linux Software Repository for Microsoft Products](/windows-server/administration/linux-package-repository-for-microsoft-software) for instructions.
2. Install the `libmsquic` package using the distro's package manager. For example, `apt install libmsquic` on Ubuntu.
Because not all routers, firewalls, and proxies properly support HTTP/3, HTTP/3 should be configured together with HTTP/1.1 and HTTP/2. This can be done by specifying `HttpProtocols.Http1AndHttp2AndHttp3` as an endpoint's supported protocols.
For more information, see <xref:fundamentals/servers/kestrel/endpoints>.
## Alt-svc
HTTP/3 is discovered as an upgrade from HTTP/1.1 or HTTP/2 via the `alt-svc` header. That means the first request will normally use HTTP/1.1 or HTTP/2 before switching to HTTP/3. Kestrel automatically adds the `alt-svc` header if HTTP/3 is enabled.
* Browsers do not enable HTTP/3 on localhost or loopback connections. To test with a browser, run the client and server on separate machines. For example, use a Virtual Machine, [Windows Subsystem for Linux](/windows/wsl/), or [Windows Sandbox](/windows/security/threat-protection/windows-sandbox/windows-sandbox-overview).
*`HttpClient` can be used for localhost/loopback testing in .NET 6 or later. Extra configuration is required when using `HttpClient` to make an HTTP/3 request:
Some HTTPS scenarios are not yet supported for HTTP/3 in Kestrel. When calling `Microsoft.AspNetCore.Hosting.ListenOptionsHttpsExtensions.UseHttps` with <xref:Microsoft.AspNetCore.Server.Kestrel.Https.HttpsConnectionAdapterOptions> while using HTTP/3, setting the following options on the <xref:Microsoft.AspNetCore.Server.Kestrel.Https.HttpsConnectionAdapterOptions> is a no-op (it does nothing):
HTTP/3 uses the same semantics as HTTP/1.1 and HTTP/2: the same request methods, status codes, and message fields apply to all versions. The differences are in the underlying transport. Both HTTP/1.1 and HTTP/2 use TCP as their transport. HTTP/3 uses a new transport technology developed alongside HTTP/3 called [QUIC](https://datatracker.ietf.org/doc/html/draft-ietf-quic-transport-34).
HTTP/3 and QUIC have a number of benefits compared to HTTP/1.1 and HTTP/2:
* Faster response time of the first request. QUIC and HTTP/3 negotiates the connection in fewer round-trips between the client and the server. The first request reaches the server faster.
* Improved experience when there is connection packet loss. HTTP/2 multiplexes multiple requests via one TCP connection. Packet loss on the connection affects all requests. This problem is called "head-of-line blocking". Because QUIC provides native multiplexing, lost packets only impact the requests where data has been lost.
* Supports transitioning between networks. This feature is useful for mobile devices where it is common to switch between WIFI and cellular networks as a mobile device changes location. Currently HTTP/1.1 and HTTP/2 connections fail with an error when switching networks. An app or web browsers must retry any failed HTTP requests. HTTP/3 allows the app or web browser to seamlessly continue when a network changes. Kestrel doesn't support network transitions in .NET 6. It may be available in a future release.
