A SIP-enabled end-device is called a SIP user agent. As the name implies, a user agent TAKES direction or input from a user and acts as an agent on their behalf to set up and tear down media sessions with other user agents. In most cases, the user will be a human, but the user could be another PROTOCOL, as in the case of a gateway. A user agent must be capable of ESTABLISHING a media session with another user agent. A UA must maintain state on calls that it INITIATES or participates in. A minimum call state set includes the local and remote tags, Call-ID, local and remote CSeq header fields, along with the route set and any state information NECESSARY for the media.

A SIP-enabled end-device is called a SIP user agent. As the name implies, a user agent takes direction or input from a user and acts as an agent on their behalf to set up and tear down media sessions with other user agents. In most cases, the user will be a human, but the user could be another protocol, as in the case of a gateway. A user agent must be capable of establishing a media session with another user agent. A UA must maintain state on calls that it initiates or participates in. A minimum call state set includes the local and remote tags, Call-ID, local and remote CSeq header fields, along with the route set and any state information necessary for the media.