You have been hired to redesign a network due to issues with congestion. How will a router function if the QoS mechanism of congestion avoidance is integrated into the existing network design?

FIFO (First In First Out) is a basic queuing strategy with no intelligence about traffic priority or queue depth; it does not perform any congestion avoidance and simply drops all packets when the queue is full (tail-drop).

WRED (Weighted Random Early Detection) is the QoS congestion avoidance mechanism that monitors queue depth and probabilistically drops packets - with higher drop probability for lower-priority traffic - before the queue reaches its maximum, preventing tail-drop and TCP synchronization issues.

Rate limiting (traffic policing) enforces a defined bandwidth contract by dropping or remarking traffic that exceeds a configured rate; this is a traffic conditioning mechanism, not a congestion avoidance mechanism.

NBAR (Network-Based Application Recognition) is a deep packet inspection classification tool used to identify application traffic, not a mechanism that manages or avoids congestion.

PBR (Policy-Based Routing) directs packets to specific next hops based on defined criteria; it is a routing mechanism and does not mark packets for QoS treatment in the context of congestion avoidance.