Designing buffer management policies at an IP/ATM gateway

The buffer management policy used at an IP/ATM gateway can significantly impact end-to-end performance for IP traffic (particularly apparent when IP traffic is transported using the ATM ABR service class). The ABR service is designed to achieve near-loss free operation in the core of the ATM network by adaptively shaping traffic at the edge of the ATM network. This can cause buffer build-ups to occur at the IP/ATM gateway due to transient overloads. Since the buffering available at a IP/ATM gateway is small relative to the worst case buffer build up, the buffer management policy has a significant impact in determining the loss rate seen by individual connections. Moreover, for connections that use TCP as the transport protocol, throughput is directly proportional to the number of buffers available to a connection. Therefore, for this class of connections, the buffer management policy also affects the average throughput seen by a connection. Our results show that it is desirable for a buffer management policy at an IP/ATM gateway to incorporate first a mechanism to ensure that an individual connection receives a minimal buffer allocation that is proportional to the product of its bottleneck rate and round-trip delay, and second a mechanism to avoid synchronized losses among TCP connections passing through the same IP/ATM gateway. Rate proportional buffering enables a gateway to provide isolation between connections that have different (downstream) bottleneck links. Avoiding loss synchronization allows more effective use of the shared buffers. This improves the overall utilization of network links. We also show that using buffer management policies, developed in the context of IP routers, and ATM switches, at an IP/ATM gateway can often lead to under-utilization of link capacity and unfairness