Which protocol is used to monitor active voice call quality?

RTCP operates as the companion control protocol to RTP and periodically sends SR (Sender Report) and RR (Receiver Report) packets between call endpoints. These packets carry metrics such as packet loss percentage, interarrival jitter, and round-trip delay, giving both endpoints real-time visibility into call quality. This monitoring function makes RTCP the correct answer for any question about assessing active voice call quality.

RTP transports the actual encoded voice payload between endpoints in real time but does not collect or report quality statistics - that role belongs to its companion protocol RTCP.

QoS is a broad set of network traffic management techniques and policies used to prioritize voice traffic, not a protocol that actively monitors or reports on call quality metrics.

STP (Spanning Tree Protocol) is a Layer 2 protocol that prevents Ethernet bridging loops and has no function related to voice call quality monitoring.

SDP negotiates media session parameters such as codec type, IP addresses, and port numbers during call setup signaling, but it does not monitor or report on the quality of an active call.