WebSocket per-message compression in ASP.NET Core 6

Back in 2017, I've written about WebSocket per-message compression in ASP.NET Core. Back then the built-in support was deep in backlog with no foreseeable delivery, so I've created my own hackey implementation. Luckily it's 2021 and the built-in support is about to come with ASP.NET Core 6. I was very eager to archive my repository so I've decided to quickly take a look at what changes are needed to benefit from this new capability.

Enabling and Configuring Compression Extension Negotiation

In order to allow compression extension negotiation between client and server, you need to use AcceptWebSocketAsync overload which takes WebSocketAcceptContext as parameter and set DangerousEnableCompression to true.

Let's say you have a WebSocket implementation which supports subprotocol negotiation.

WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync(textSubProtocol?.SubProtocol);

The version with compression negotiation enabled would look like below.

WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync(new WebSocketAcceptContext
{
    SubProtocol = textSubProtocol?.SubProtocol,
    DangerousEnableCompression = true
});

You might be wondering why such a "scary" property name. It's not because things may fail. If the client doesn't support compressions (or doesn't support compression with specific parameters), the negotiated connection will have compression disabled. It's about security. Similarly to HTTPS, encrypted WebS