Session Initiation Protocol (SIP) is a communication protocol used to initiate, maintain, and terminate real-time sessions that involve video, voice, and messaging applications. It is widely used in Voice over Internet Protocol (VoIP) services, allowing users to make phone calls over the internet. SIP helps establish connections between different devices and networks, enabling seamless communication. SIP operates by sending messages between endpoints, such as phones or computers, to set up and control the session. It can handle various multimedia formats and is essential for applications like video conferencing and instant messaging. Its flexibility and scalability make it a popular choice for modern communication systems.