Experimental assessment of RED in wired/wireless networks

The Internet is a heterogeneous environment comprising wired and wireless components, and transport protocols with different error recovery strategies. We use simulations to study the behaviour of random early detection (RED) gateways in such heterogeneous environments. We investigate two issues: (i) the performance trade-offs of the dropping policy of RED over Drop-Tail’s, and (ii) the impact of RED’s active queue management on more sophisticated protocols such as TCP-SACK. Some of our results indicate that RED does not deal adequately with heterogeneity. In particular, it may happen that RED applies congestion avoidance techniques even when wireless errors force some senders to back off prior to actual congestion. Our scenarios involve a non-monotonic transmission behaviour of transport protocols within one and the same congestion epoch; RED may escalate further bandwidth under-utilization by applying false congestion avoidance tactics. Furthermore, our results indicate that RED’s dropping policy of ‘proportional’ fairness, which is realized through a Send-more/Drop-more scheme, can also penalize sophisticated protocols like TCP-SACK, which attempt to recover more aggressively from wireless losses. In summary, our results call for further investigation of RED’s efficiency in heterogeneous networks and naturally raise the concern of RED’s compliance with the end-to-end argument. Copyright # 2004 John Wiley & Sons, Ltd.