Which protocol is used to monitor active voice call quality?

RTCP works alongside RTP by periodically sending control packets that contain statistics including round-trip delay, jitter, and packet loss percentages. These metrics allow endpoints and monitoring systems to assess and report the quality of an active RTP media stream in real time, making RTCP the correct protocol for active voice call quality monitoring.

RTP (Real-time Transport Protocol) carries the actual encoded voice media payload between endpoints but does not provide quality measurement or reporting functions.

QoS is a broad framework of policies and mechanisms for traffic prioritization, not a protocol that actively monitors or reports on call quality metrics.

STP (Spanning Tree Protocol) operates at Layer 2 to prevent switching loops in Ethernet networks and has no role in voice call quality monitoring.

SDP (Session Description Protocol) is used during signaling to negotiate media parameters such as codec and port before a call is established, not to monitor quality during an active call.