[API Proposal]: [HTTP/3] Support multiple HTTP/3 connections

[ManickaP]

Background and motivation

Similarly to HTTP/2, we want to introduce support for multiple HTTP/3 connections.

Both HTTP/2 and HTTP/3 have stream limits. As RFC Bidirectional Streams suggests, the minimum is 100, which is the current Kestrel default. As #35088 states, this becomes a bottleneck for service-to-service communication, e.g. gRPC.

Implementation depends on #101534.

Original issue: #51775

For comparison with HTTP/2 see the original issue #35088

API Proposal

namespace System.Net.Http;

public sealed partial class SocketsHttpHandler : System.Net.Http.HttpMessageHandler
{
    public bool EnableMultipleHttp3Connections { get { throw null; } set { } }
}

API Usage

var client = new HttpClient(new SocketsHttpHandler()
{
    EnableMultipleHttp3Connections = true
})
{
    DefaultRequestVersion = HttpVersion.Version30,
    DefaultVersionPolicy = HttpVersionPolicy.RequestVersionExact
};

Alternative Designs

N/A

Risks

None. It's an opt-in, default will be false, keeping the original behavior in place.

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[JamesNK]

A customer has run into this problem - grpc/grpc-dotnet#2404

They have more than 100 concurrent requests and this limitation is preventing new requests from being sent.

[JamesNK]

Is EnableMultipleHttp3Connections the right name? What about EnableMultipleMultiplexedConnections that works for both HTTP/2 and HTTP/3.

I imagine whoever wants multiple connections for HTTP/2 would also want multiple for HTTP/3. A property that sets both would be good.

[ManickaP]

We cannot rename the existing EnableMultipleHttp2Connections so the naming suggested here follows the same pattern. With your suggestion we'd have:

• EnableMultipleHttp2Connections for HTTP/2
• EnableMultipleMultiplexedConnections for both HTTP/2 and HTTP/3

I will mention you suggestion during API review as an alternative to get the feel for it.