Add WebSockets conceptual doc

Author: Katya Sokolova

docs/fundamentals/networking/websockets.md

@@ -0,0 +1,44 @@
+# WebSockets support in .NET
+
+The WebSocket Protocol enables two-way communication between a client to a remote host that has opted-in to communications from that code. The <xref:System.Net.WebSockets.Client.ClientWebSocket?displayProperty=fullName> exposes the ability to establish WebSocket connection via an opening handshake: it is created and sent by `ConnectAsync` method.
+
+## HTTP/2 vs HTTP/1.1 WebSockets
+
+WebSockets over HTTP/1.1 uses a single TCP connection, therefore it is managed by connection-wide headers (see [RFC 6455](https://www.rfc-editor.org/rfc/rfc6455)). Here is an example of how to establish WebSocket over HTTP/1.1:
+
+```c#
+ClientWebSocket ws = new();
+ws.ConnectAsync(uri, cancellationToken);
+```
+
+A different approach must be taken with HTTP/2 due to its multiplexing nature: here, WebSockets are established per stream (see [RFC 8441](https://www.rfc-editor.org/rfc/rfc8441)). With HTTP/2 it is possible to use one connection for multiple web socket streams together with ordinary HTTP streams and extends HTTP/2's more efficient use of the network to WebSockets. There is a special overload of `ConnectAsync` which accepts `HttpMessageInvoker` to allow reusing existing pooled connections:
+
+```c#
+var handler = new SocketsHttpHandler();
+ClientWebSocket ws = new();
+ws.ConnectAsync(uri, new HttpMessageInvoker(handler), cancellationToken);
+```
+
+## How to set up HTTP version and policy
+
+By default, `ClientWebSocket` uses HTTP/1.1 to send an opening handshake and allows downgrade. It can be changed before calling `ConnectAsync`:
+
+```c#
+ClientWebSocket ws = new();
+
+ws.Options.HttpVersion = HttpVersion.Version20;
+ws.Options.HttpVersionPolicy = HttpVersionPolicy.RequestVersionOrHigher;
+
+ws.ConnectAsync(uri, new HttpMessageInvoker(handler), cancellationToken);
+```
+
+## Incompatible options
+
+`ClientWebSocket` has properties <xref:System.Net.WebSockets.Client.ClientWebSocketOptions?displayProperty=fullName> that the user can set up before the connection is established. However, when `HttpMessageInvoker` is provided, it also has these properties. To avoid ambiguity, in that case, properties should be set on `HttpMessageInvoker`, and `ClientWebSocketOptions` should have default values. Otherwise, if `ClientWebSocketOptions` are changed, overload of `ConnectAsync` will throw an <xref:System.ArgumentException>.
+
+## Compression
+
+WebSocket supports per-message deflate as defined in [RFC 7692](https://tools.ietf.org/html/rfc7692#section-7). It is controlled by <xref:System.Net.WebSockets.Client.ClientWebSocketOptions.DangerousDeflateOptions?displayProperty=fullName>. When present, the options are sent to the server during the handshake phase. If the server supports per-message-deflate and the options are accepted, the <see cref="WebSocket"/> instance will be created with compression enabled by default for all messages.
+
+> [!IMPORTANT]
+> Before using it, please be aware that enabling compression makes the application subject to CRIME/BREACH type of attacks. It is strongly advised to turn off compression when sending data containing secrets by specifying `DisableCompression` flag for such messages.