ANALYSIS OF DISCARDING POLICIES IN HIGH-SPEED NETWORKS

Networked applications generate messages that are segmented into small er, fixed or variable size packets, before they are sent through the n etwork, In high-speed networks, acknowledging individual packets is im practical; so when congestion builds up, and packets have to be droppe d, entire messages are lost. For a message to be useful, all packets c omprising it must arrive successfully at the destination. The problem is therefore which packets to discard so that as many complete message s are,delivered, and so that congestion is alleviated or avoided altog ether. In this paper, selective discarding policies, as a means for co ngestion avoidance, are studied and compared to nondiscarding policies . The partial message discard policy discards packets of tails of corr upted messages. An improvement to this policy is the early message dis card that drops entire messages and not just message tails. A common p erformance measure of network elements is the effective throughput whi ch measures the utilization of the network links but which ignores the application altogether. We adopt a new performance measure-goodput-wh ich reflects the utilization of the network from the application's poi nt of view and thus better describes network behavior. We develop and analyze a model for systems which employ discarding policies, The anal ysis shows a remarkable performance improvement when any message-based discarding policy is applied, and that the early message discard poli cy performs better than the others, especially under high load. We com pute the optimal parameter setting for maximum goodput at different in put loads, and investigate the performance sensitivity to these parame ters.