The packet starvation effect in CSMA/CD LANs and a solution

Abstract

In this paper we explore the packet starvation effect (PSE) that occurs in Ethernet controllers due to the unfairness of the CSMA/CD algorithm. The PSE causes some packets to experience latencies up to 100 times the average or to completely starve out due to 16 collisions. The PSE causes some packets to experience high delays at realistic offered loads as low as 40% and causes complete starvation of some packets at offered loads as low as 60%. The PSE makes CSMA/CD LANs unsuitable for real-time traffic except at offered loads much less than 100%. It is the limiting factor in the usable bandwidth of the bus. As an alternative to CSMA/CD, we present the fair dual distributed queue (FDDQ) algorithm. Under high load, FDDQ uses a single reservation mini-dot per packet and a tree-based collision resolution algorithm (CRA) to maintain two distributed queues of waiting senders. This provides two priority FCFS access to the network. FDDQ provides utilizations and average latencies very similar to those of CSMA/CD but is fair even at extremely high offered loads. The protocol is stable for a constant number of senders, is simple enough to be practical, should be implementable in firmware, and completely eliminates the PSE.<>