Which component is responsible for maintaining the SIP session state?

The User Agent is responsible for maintaining the SIP session state because it actively participates in the establishment, modification, and termination of sessions. In SIP architecture, a User Agent can be either a User Agent Client (UAC) or a User Agent Server (UAS). The UAC initiates the requests to start the session, while the UAS responds to those requests and can also initiate its own requests.

Throughout the SIP session, the User Agent keeps track of the current state of that session. This includes information such as the session's existence, who is participating, and any changes that may occur during the lifetime of the session. As SIP is designed to be stateless in nature at the server level, it relies heavily on the User Agent to maintain session information for effective communication.

While other components like Proxy Servers and Transaction Servers play significant roles in routing SIP messages and handling SIP transactions, they do not maintain the session state in the same way User Agents do. Media Servers, on the other hand, are primarily focused on handling the actual media streams but do not manage SIP signaling or session state. Thus, the User Agent uniquely fulfills the requirement for maintaining the SIP session state.