What is the purpose of a SIP Proxy Server?

The purpose of a SIP Proxy Server is to route requests and responses for SIP transactions. In the Session Initiation Protocol (SIP) architecture, the proxy server acts as an intermediary between users, facilitating the signaling required to establish and modify multimedia sessions such as VoIP calls. It analyzes SIP messages and ensures they reach the correct destination by determining the next hop in the network. This capability is central to the operation of SIP, as it handles not just the initial setup but also assists with registration, authentication, and call management.

In contrast, the other options represent functions that are not directly associated with the central role of a SIP Proxy. Initiating sessions directly is typically the responsibility of the SIP endpoints themselves, not the proxy. Managing bandwidth allocation may be handled by Quality of Service (QoS) mechanisms or other network components, but is not a function of the SIP Proxy. Encrypting communication data is more closely related to security protocols like TLS (Transport Layer Security), which operates at a different layer than SIP signaling handled by the proxy.