Abstract: Mechanisms able to provide the AF PHB in DiffServ networks constitute a significant research topic. Most of the current implementations of AF use RED or similar queue management techniques. Analyzed in this parper are some shortcomings inherent in RED and its improved algorithms in terms of fairness and packet drop precedence, and then proposed is a novel queue management algorithm named TCRED （three color random early drop） using a same suite of RED parameter to control green, yellow and red packets. TCRED calculates the average queue length of these colored packets respectively, accounting for the effect of both packet arrival and packet departure, and computes packet drop probability based on calculated average queue length. The algorithm implements relative drop precedence among different colored packets, assures the customers of their booked bandwidth, and realizes graceful upgrade and degrade of packet drop precedence; it thus well meets the requirements of AF PHB.