What is a User Agent (UA) in SIP?

A User Agent (UA) in SIP refers to a software application that acts on behalf of a user to send and receive SIP messages. This encompasses any device or application that can send SIP requests, initiate calls, and manage session parameters. User Agents interact directly with the SIP protocol to establish, modify, or terminate communication sessions.

In the realm of SIP, a User Agent can be classified into two types: User Agent Client (UAC), which initiates requests, and User Agent Server (UAS), which responds to those requests. This dual functionality allows User Agents to fully facilitate communication between parties by orchestrating the call setup and management process.

The other options do not accurately describe the role of a User Agent. For instance, a type of SIP server that routes messages pertains more to proxy servers or registrar servers, which handle signal routing and registration of endpoints in a SIP network. A device used for recording SIP calls is a specific application or function that, while potentially incorporating UA elements, does not define what a User Agent is. Lastly, a protocol for managing network traffic does not pertain specifically to SIP or its User Agents, as SIP is itself a signaling protocol rather than a traffic management protocol. Thus, the definition that focuses on the function of