What is a Session Initiation Protocol or SIP?
SIP is widely used for controlling - establishing, modifying, and terminating - communication sessions like voice and video calls over the internet. Other applications include multimedia sessions like video conferencing, instant messaging, file transfer and online games. SIP itself does not provide services, but rather a conduit for services. It is simply a component of a complete multimedia architecture.
SIP provides five functions: user location, user availability, user capability, session setup, and session management. Like a public switched phone network, it sets up/establishes, ties together and tears down multimedia communications.
SIP utilizes two end points from installed user agents (UA) on communication devices like an iPhone, mobile phone, wireless device or computer. Direct communication occurs when the devices recognize the other's IP address. Usually, SIP servers allow for an infrastructure that can route and authorize communication.