Final answer:
Random Early Detection (RED) is the congestion-avoidance mechanism among the Quality of Service (QoS) tools listed. RED prevents congestion by predicting it and dropping packets early to reduce the rate of traffic flow, hence avoiding queue overflows.
Step-by-step explanation:
From the list of Quality of Service (QoS) tools mentioned, the congestion-avoidance mechanism is Random Early Detection (RED). Congestion avoidance mechanisms are employed in network environments to prevent network congestion before it becomes problematic. RED works by monitoring the average queue size and dropping packets early when it anticipates that congestion is about to occur, hence encouraging transmissions to slow down ahead of time. This proactive measure helps to prevent queues from becoming full, thus avoiding the more abrupt packet loss that would have occurred had the queue become full. Other mechanisms listed, such as Traffic shaping and Weighted Fair Queuing (WFQ), are used to manage bandwidth and ensure fair transmission opportunities to different kinds of traffic but do not focus specifically on avoiding congestion like RED does. Differentiated Services Code Point (DSCP) is a method for classifying and managing network traffic and setting priorities but is not in itself a congestion-avoidance technique.