user agent (UA)
An entity that carries out specific functions on behalf of its owner or user. Such functions usually entail requesting some service or responding to a request for service. When requesting service, the User Agent acts as a User Agent Client (UAC), and when responding to a request for service, it acts like a User Agent Server (UAS). For example, a SIP User Agent is a logical entity that can act as both a UAC and a UAS. As a UAC, it creates a new SIP request on behalf of the user. This request is sent to the UAS, which generates a response to the SIP request. The UAC and UAS maintain the client/server roles for the duration of the transaction.