What type of entities can send and receive SIP messages?

The correct answer is that User Agents, which include both clients and servers, can send and receive SIP messages. In the Session Initiation Protocol (SIP) architecture, User Agents play a crucial role in establishing, modifying, and terminating multimedia sessions. A User Agent Client (UAC) is responsible for initiating requests and sending messages, while a User Agent Server (UAS) responds to those requests. This dual functionality of clients and servers under the User Agent umbrella allows for comprehensive communication in SIP-based networks.

The other options do not capture the full scope of SIP communication. Servers alone do not encompass the whole system, as clients are also essential participants in the SIP protocol. Likewise, stating that only clients can send and receive SIP messages fails to recognize the importance of servers in this process. Network switches, while critical components of data transmission and routing, do not engage directly in generating or responding to SIP messages, as they operate at a different layer of the OSI model and are responsible for forwarding packets rather than handling SIP signaling.