WebSocket and Session Initiation Protocol (SIP) are two communication protocols used for real-time, two-way communication in applications such as chat apps, collaboration software, multiplayer games, VoIP, and video conferencing. WebSocket enables low-latency communication with full duplex capabilities, while SIP is a signaling protocol designed specifically for interactive communication sessions like voice and video calls over the internet. Both protocols reuse HTTP's infrastructure to send information from the server to clients in real-time, but SIP is better suited for voice and video applications due to its support for multimedia sessions on multiple devices. The choice between WebSocket and SIP depends on the app's requirements, with WebSocket ideal for instant messaging, collaboration, and real-time updates, while SIP is better for VoIP calling and supporting signaling and control of voice, video, and text applications.