WebSockets

WebSockets can be used by web applications or web sites to setup a bi-directional (two-way), full duplex communication channel over a single TCP connection.
It features a lightweight protocol, allowing developers to realize realtime use cases. WebSockets do also provide an alternative to heavy use of Ajax, HTTP Long Polling or Comet.

After an initial HTTP based handshake, the TCP connection is kept open, allowing applications to send & receive arbitrary data. Often port 80 or 443 for encrypted WebSocket channels are used.

The WebSocket standard is defined in

The WebSocket API (http://www.w3.org/TR/websockets/): specifies the interface in browsers
The WebSocket Protocol (RFC6455) (https://tools.ietf.org/html/rfc6455): describes the structure of WebSocket frames upon TCP

ZAP is able to:

intercept and show WebSocket messages
set breakpoints on specific types of WebSocket messages
fuzz WebSocket messages (send lots of invalid or unexpected data to a browser or server)
passively scan WebSocket messages and raise alerts with scripts

WebSocket messages are displayed within the WebSockets tab.

The WebSocket add-on adds new scripts and additional endpoints to the ZAP API