Media agnostic (Separation of signaling and media)
Ability to support multiple media within the same session
Text-based and extensible (Heterogeneity is the norm)
User Agents (UA): Endpoint devices that originate or terminate the SIP signaling. They can be clients (UAC) that initiates a request, or a server (UAS) that terminates a request (most endpoints do both)
Proxies route SIP requests to their destination. They sit in the signaling path between endpoints, and may add parameters to a request or reject a request, but they do not initiate requests
Registrars handle REGISTER requests from endpoints to bind a user to the device. This information is then stored in a location service database for the proxy’s domain.
Back-to-Back User Agent(B2BUA): Similar to a proxy, except that it terminates the SIP dialog on both sides. Commonly used to provide services because it can manipulate the signaling between endpoints