A User Agent Client (UAC) is a term commonly used in the context of Session Initiation Protocol (SIP) communication, which is a protocol used for establishing, maintaining, and terminating real-time communication sessions over the internet. SIP is widely used for services like voice and video calls, instant messaging, and presence information.

In SIP architecture, a User Agent Client is a network element that initiates SIP requests and interacts with other components to establish and manage communication sessions. The UAC plays a central role in initiating calls, sending SIP messages, and handling responses from User Agent Servers (UASs) within the SIP network.

Here’s how a User Agent Client (UAC) is typically involved in SIP communication:

1. Initiating a Session: A User Agent Client (UAC) sends a SIP request, such as an INVITE, to initiate a communication session. The request specifies the recipient’s address and other relevant information.
2. Handling Responses: The UAC waits for responses from the network. These responses come from User Agent Servers (UASs) and provide information about the status of the requested action, such as call acceptance or redirection.
3. Call Establishment: Once the UAC receives a positive response, it continues the call establishment process. This might involve negotiating media parameters, codecs, and other session details.
4. Managing the Session: Throughout the communication session, the UAC continues to exchange SIP messages with UASs to manage the session. This includes handling call changes, updating session information, and responding to session-related events.
5. Termination: When the communication session is concluded, the UAC sends a SIP request to terminate the session, such as a BYE request for a voice call.

User Agent Clients are essential for initiating and managing communication sessions in SIP-based networks. They play a key role in the establishment of real-time communication services, helping users make voice calls, participate in video conferences, exchange instant messages, and more. UACs and User Agent Servers (UASs) together create a robust SIP ecosystem that enables various forms of real-time communication over the internet.