WebSockets support in ASP.NET Core | Microsoft Docs
"This article explains how to get started with WebSockets in ASP.NET Core. WebSocket (RFC 6455) is a protocol that enables two-way persistent communication channels over TCP connections. It's used in apps that benefit from fast, real-time communication, such as chat, dashboard, and game apps."
Docs/aspnetcore/fundamentals/websockets/sample at master · aspnet/Docs · GitHub
Docs/Startup.cs at master · aspnet/Docs · GitHub
NuGet Gallery | Microsoft.AspNetCore.WebSockets 2.0.2
With Classic ASP.NET & IIS 8 (Win 2012 or Win 8 and newer OS required)
The simple WebSocket application in ASP.NET in C# for Visual Studio 2013
Hands On Lab: Real-Time Web Applications with SignalR | Microsoft Docs
Writing a WebSocket server in C# - Web APIs | MDN
"WebSockets are an advanced technology that makes it possible to open an interactive communication session between the user's browser and a server. With this API, you can send messages to a server and receive event-driven responses without having to poll the server for a reply."
Alternative to WebSockets are "SSE", Server-Sent events ("server push" only, not bi-directional)
Server-sent events - Web APIs | MDN
EventSource - Web APIs | MDN