Real-time web applications need a bidirectional communication channel between the client and the server. This is any persistent connection that allows the server to push additional data to the client when needed. The WebSockets protocol is the modern standard for this kind of communication and is implemented by most browsers.

WebSocket connections are initiated via HTTP, but otherwise do not depend on it. The WebSocket protocol defines a way of sending messages bi-directionally over a TCP connection. TCP is the low-level transport protocol that usually underlies HTTP. WebSockets are still a relatively new technology and not fully supported by all clients and servers. Most modern web browsers today do support WebSockets. However, intermediate servers (proxies, firewalls, and load-balancers) can prevent WebSocket connections from working (either through lack of support or intentionally blocking non-HTTP traffic). In these cases, there are alternative...