Use HTTP/3 with the ASP.NET Core Kestrel web server

HTTP/3 is an approved standard and the third major version of HTTP. This article discusses the requirements for HTTP/3. HTTP/3 is fully supported in ASP.NET Core 7.0 and later.

Important

Apps configured to take advantage of HTTP/3 should be designed to also support HTTP/1.1 and HTTP/2.

HTTP/3 requirements

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 fall back to other HTTP protocols.

Windows

• Windows 11 Build 22000 or later OR Windows Server 2022.
• TLS 1.3 or later connection.

Linux

• libmsquic package installed.

libmsquic is published via Microsoft's official Linux package repository at packages.microsoft.com. To install this package:

1. Add the packages.microsoft.com repository. See Linux Software Repository for Microsoft Products for instructions.
2. Install the libmsquic package using the distro's package manager. For example, apt install libmsquic=1.9* on Ubuntu.

Note: .NET 6 is only compatible with the 1.9.x versions of libmsquic. Libmsquic 2.x is not compatible due to breaking changes. Libmsquic receives updates to 1.9.x when needed to incorporate security fixes.

macOS

HTTP/3 isn't currently supported on macOS and may be available in a future release.

Getting started

HTTP/3 is not enabled by default. Add configuration to Program.cs to enable HTTP/3.

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel((context, options) =>
{
    options.ListenAnyIP(5001, listenOptions =>
    {
        listenOptions.Protocols = HttpProtocols.Http1AndHttp2AndHttp3;
        listenOptions.UseHttps();
    });
});

The preceding code configures port 5001 to:

• Use HTTP/3 alongside HTTP/1.1 and HTTP/2 by specifying HttpProtocols.Http1AndHttp2AndHttp3.
• Enable HTTPS with UseHttps. HTTP/3 requires HTTPS.

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 Configure endpoints for the ASP.NET Core Kestrel web server.

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.

Localhost testing

• Browsers don't allow self-signed certificates on HTTP/3, such as the Kestrel development certificate.

• 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:

  • Set HttpRequestMessage.Version to 3.0, or
  • Set HttpRequestMessage.VersionPolicy to HttpVersionPolicy.RequestVersionOrHigher.

HTTP/3 benefits

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.

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 8. It may be available in a future release.