HttpClient does not retry when HTTP_1_1_REQUIRED returned in GOAWAY

[drmathias]

Description

Given a request is sent using HTTP/2 and a server responds with GOAWAY error code HTTP_1_1_REQUIRED, the HttpClient appears to retry using HTTP/2 rather than attempting to downgrade to HTTP/1.1

RFC9113 indicates that HTTP_1_1_REQUIRED can be returned per endpoint.

The endpoint requires that HTTP/1.1 be used instead of HTTP/2.

An example is a reverse proxy that accepts HTTP/2 connections and is configured to forward requests to multiple servers. One server only supports HTTP/1.1, so a policy is configured on the proxy to return HTTP_1_1_REQUIRED back to the client for HTTP/2 requests sent to those specific endpoints.

Reproduction Steps

var handler = new SocketsHttpHandler
{
    ConnectCallback = async (context, cancellationToken) =>
    {
        var socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
        await socket.ConnectAsync(IPAddress.Loopback, port, cancellationToken);
        return new NetworkStream(socket, ownsSocket: true);
    },
    SslOptions = new SslClientAuthenticationOptions
    {
        RemoteCertificateValidationCallback = (_, _, _, _) => true
    }
};

using var httpClient = new HttpClient(handler)
{
    BaseAddress = new Uri("https://example.com"),
    DefaultRequestVersion = HttpVersion.Version20,
    DefaultVersionPolicy = HttpVersionPolicy.RequestVersionOrLower
};

await httpClient.GetAsync("/");

Expected behavior

HTTPClient retries the request using HTTP/1.1 and caches the HTTP version.

Actual behavior

Retries using HTTP/2 observed when debugging until the following exception gets thrown.

Exception thrown: 'System.Net.Http.HttpRequestException' in System.Private.CoreLib.dll: 'An error occurred while sending the request.'
Inner exceptions found, see $exception in variables window for more details.
Innermost exception System.ObjectDisposedException : Cannot access a disposed object.
Object name: 'System.Net.Security.SslStream'.
at System.ThrowHelper.ThrowObjectDisposedException(Object instance)
at System.Net.Security.SslStream.g__ThrowExceptional|131_0(ExceptionDispatchInfo e)
at System.Net.Security.SslStream.WriteAsync(ReadOnlyMemory`1 buffer, CancellationToken cancellationToken)
at System.Net.Http.Http2Connection.d__54.MoveNext()

Regression?

No

Known Workarounds

No response

Configuration

.NET 10

Other information

This was discussed on this issue.
#63715

However I think the conclusion not to retry with HTTP/1.1 should be revisited. It should not be assumed that the server is misconfigured if it is returning a specific error code.

[dotnet-policy-service]

Tagging subscribers to this area: @karelz, @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[MihaZupan]

.NET 11 added similar automatic retry support for H2 => H1 downgrades, specific to receiving a session auth response - #123827.

We should consider expanding that to HTTP_1_1_REQUIRED.