Implementation notes:
The System.Net.WebSockets.WebSocket.SendAsync API is poorly suited to compression/extension. Compression is a per-message flag set on the first frame using the RSV1 bit. SendAsync sends frames and only takes the buffer, message type, an EndOfMessage bool, and a cancellationToken. There's no simple way to indicate if the data is or should be compressed / transformed. Possible solutions:

Enable compression for all messages, don't provide a per message option
Add a new overload of SendAsync with a compression / RSV1 flag. (Requires changing the core WebSocket abstraction)
Add a property to WebSocket bool CompressOutput that can be toggled between messages (Requires changing the core WebSocket abstraction)
Overload WebSocketMessageType with CompressedBinary and CompressedText. As an enum we may be able to temporarily pass data through here by casting non-standard values. The enum values already do not map directly to the WebSocket frame type values.

ReceiveAsync is easier, it could:

Decompress the content internally
Add a flag to WebSocketReceiveResult indicating if the content was compressed.
Use the same WebSocketMessageType overloads from above indicating if the content is compressed (but only if it does not decompress it internally, don't want to confuse consumers).

The compression extension could not be correctly implemented as wrapper over the current WebSocket API due to the framing requirement. Compression either needs to be built in or the API needs to expose additional frame fields (e.g. RSV1).

I'll consider the extension negotiation separately as the handshake APIs differ in System.Net, System.Web, and Microsoft.AspNetCore.

We also need to verify that the Sec-WebSocket-Extensions header(s) flow well end-to-end. Intermediaries like ANCM have had problems with other optional headers like Sec-WebSocket-Protocol. For that matter IIS and WinHttp would have to implement this extension as well, their APIs don't allow ANCM to proxy opaque websocket frames.

Implement WebSocket compression #19

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