.NET Core 3 HTTP/2 Support

The System.Net.Http.HttpClient type supports the HTTP/2 protocol. If HTTP/2 is enabled, the HTTP protocol version is negotiated via TLS/ALPN, and HTTP/2 is used if the server elects to use it.

The default protocol remains HTTP/1.1, but HTTP/2 can be enabled in the following two different ways.

You can set the HTTP request message to use HTTP/2, as shown below.

var client = new HttpClient() { BaseAddress = new Uri("https://localhost:5001") };

// HTTP/1.1 request
using (var response = await client.GetAsync("/"))
    Console.WriteLine(response.Content);

// HTTP/2 request
using (var request = new HttpRequestMessage(HttpMethod.Get, "/") { Version = new Version(2, 0) })
using (var response = await client.SendAsync(request))
    Console.WriteLine(response.Content);

You can also change HttpClient to use HTTP/2 by default as shown below.

var client = new HttpClient()
{
    BaseAddress = new Uri("https://localhost:5001"),
    DefaultRequestVersion = new Version(2, 0)
};

// HTTP/2 is default
using (var response = await client.GetAsync("/"))
    Console.WriteLine(response.Content);

Use ASP.NET Core with HTTP/2

HTTP/2 is supported with ASP.NET Core in the following IIS deployment scenarios.

• Windows Server 2016 or later / Windows 10 or later
• IIS 10 or later
• TLS 1.2 or later connection

When hosting out-of-process: Public-facing edge server connections use HTTP/2, but the reverse proxy connection to the Kestrel server uses HTTP/1.1.

• For an in-process deployment, when an HTTP/2 connection is established, HttpRequest.Protocol reports HTTP/2.
• For an out-of-process deployment, when an HTTP/2 connection is established, HttpRequest.Protocol reports HTTP/1.1.

Advanced HTTP/2 features to support gRPC

Additional HTTP/2 features in IIS support gRPC, including support for response trailers and sending, reset frames.

Requirements to run gRPC on IIS:

• In-process hosting.
• Windows 10, OS Build 20300.1000 or later. May require the use of Windows Insider Builds.
• TLS 1.2 or later connection

Trailers

HTTP Trailers are similar to HTTP Headers, except they are sent after the response body is sent. For IIS and HTTP.SYS, only HTTP/2 response trailers are supported.

if (httpContext.Response.SupportsTrailers())
{
    httpContext.Response.DeclareTrailer("trailername");	

    // Write body
    httpContext.Response.WriteAsync("Hello world");

    httpContext.Response.AppendTrailer("trailername", "TrailerValue");
}

• SupportsTrailers ensures that trailers are supported for the response.
• DeclareTrailer adds the given trailer name to the Trailer response header. Declaring a response's trailers is optional but recommended. If DeclareTrailer is called, it must be before the response headers are sent.
• AppendTrailer appends the trailer.

Many times when you are developing an application, you want to use an unencrypted connection.

• If you know the target endpoint will be using HTTP/2, you can turn on unencrypted connections for HTTP/2.
• You can turn it on by setting the DOTNET_SYSTEM_NET_HTTP_SOCKETSHTTPHANDLER_HTTP2UNENCRYPTEDSUPPORT environment variable to 1 or by enabling it in the app context:

AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2UnencryptedSupport", true);