Appeared in IEEE/ACM Transactions on Newworking, Dec 1997. This is a much-extended and revised version of a paper that appeared at ACMSIGCOMM, 1996 A Comparison of Mechanisms for Improving TCP Performance over Wireless links Hari Balakrishnan, Venkata N. Padmanabhan, Srinivasan Seshan and Randy h. Katz lari, padmanab, ss, randy )@cs. berkeley. edu Computer Science Division, Department of EECS, University of california at Berkeley Abstract the estimated round-trip delay and the mean linear deviation from it. The sender identifies the loss of a packet either by Reliable transport protocols such as TCP are tuned to per- the arrival of several duplicate cumulative acknowled form well in traditional networks where packet losses occur ments or the absence of an acknowledgment for the packet mostly because of congestion. However, networks with within a timeout interval equal to the sum of the smoothed wireless and other lossy links also suffer from significant round-trip delay and four times its mean deviation. TCP losses due to bit errors and handoffs. TCP responds to all reacts to packet losses by dropping its transmission(conges- losses by invoking congestion control and avoidance algo- tion) window size before retransmitting packets, initiating rithms, resulting in degraded end-to-end performance in congestion control or avoidance mechanisms(e.g, slow wireless and lossy systems. In this paper, we compare sev- start[13) and backing off its retransmission timer(Karn's eral schemes designed to improve the performance of TCP Algorithm[16]). These measures result in a reduction in the in such networks. We classify these schemes into three load on the intermediate links, thereby controlling the con- broad categories: end-to-end protocols, where loss recovery gestion in the network is performed by the sender; link-layer protocols, that pro- vide local reliability; and split-connection protocols, that Unfortunately, when packets are lost in networks for rea- break the end-to-end connection into two parts at the base sons other than congestion, these measures result in an station. We present the results of several experiments per- unnecessary reduction in end-to-end throughput and hence formed in both lan and Wan environments. using sub-optimal performance. Communication over wireless throughput and goodput as the metrics for comparison links is often characterized by spor gh bit-error rates and intermittent connectivity due to handoffs. TCP perfor Our results show that a reliable link-layer protocol that is mance in such networks suffers from significant throughput TCP-aware provides very good performance. Furthermore, gh it is possible to achieve good performance without splitting the end-to-end connection at the base station We also dem- Recently, several schemes have been proposed to the allevi- onstrate that selective acknowledgments and explicit loss ate the effects of non-congestion-related losses on TCP per- notifications result in significant performance improve- loss links 3, 7, 28]. These schemes choose from a variety of mechanisms, such as local retransmissions, split-TCP con- nections, and forward error correction, to improve end-to- L. ntroduction end throughput. However, it is unclear to what extent each The increasing popularity of wireless networks indicates of the mechanisms contributes to the improvement in per- that wireless links will play an important role in future inter formance. In this paper, we examine and compare the effec networks. Reliable transport protocols such as TCP [24. 261 tiveness of these schemes and their variants, and have been tuned for traditional networks comprising wired experimentally analyze the individual mechanisms and the links and stationary hosts. These protocols assume conges- degree of performance improvement due to each tion in the network to be the primary cause for packet losses There are two different approaches to improving TCP per and unusual delays. TCP performs well over such networks formance in such lossy systems. The first approach hides by adapting to end-to-end delays and congestion losses. The any non-congestion-related losses from the TCP sender and TCP sender uses the cumulative acknowledgments it therefore requires no changes to existing sender implemen- receives to determine which packets have reached the tations. The intuition behind this approach is that since the receiver,and provides reliability by retransmitting lost problem is local, it should be solved locally, and that the It maintains a ort layer need not be aware of the ch the individual links. Protocols that adopt this appr attempt to make the lossy link appear as a higher quality 1.WebpageUrlhttp://daedalus.cs.berkeley.edu. link with a reduced effective bandwidth. As a result most of rinivasan Seshan is now at IBM TJ. Watson Research Center Hawthorne,NY(srini@watson.ibm.com) the losses seen by the TCP sender are caused by congestionA Comparison of Mechanisms for Improving TCP Performance over Wireless Links Hari Balakrishnan, Venkata N. Padmanabhan, Srinivasan Seshan and Randy H. Katz1 {hari,padmanab,ss,randy}@cs.berkeley.edu Computer Science Division, Department of EECS, University of California at Berkeley Abstract Reliable transport protocols such as TCP are tuned to per￾form well in traditional networks where packet losses occur mostly because of congestion. However, networks with wireless and other lossy links also suffer from significant losses due to bit errors and handoffs. TCP responds to all losses by invoking congestion control and avoidance algo￾rithms, resulting in degraded end-to-end performance in wireless and lossy systems. In this paper, we compare sev￾eral schemes designed to improve the performance of TCP in such networks. We classify these schemes into three broad categories: end-to-end protocols, where loss recovery is performed by the sender; link-layer protocols, that pro￾vide local reliability; and split-connection protocols, that break the end-to-end connection into two parts at the base station. We present the results of several experiments per￾formed in both LAN and WAN environments, using throughput and goodput as the metrics for comparison. Our results show that a reliable link-layer protocol that is TCP-aware provides very good performance. Furthermore, it is possible to achieve good performance without splitting the end-to-end connection at the base station. We also dem￾onstrate that selective acknowledgments and explicit loss notifications result in significant performance improve￾ments. 1. Introduction The increasing popularity of wireless networks indicates that wireless links will play an important role in future inter￾networks. Reliable transport protocols such as TCP [24, 26] have been tuned for traditional networks comprising wired links and stationary hosts. These protocols assume conges￾tion in the network to be the primary cause for packet losses and unusual delays. TCP performs well over such networks by adapting to end-to-end delays and congestion losses. The TCP sender uses the cumulative acknowledgments it receives to determine which packets have reached the receiver, and provides reliability by retransmitting lost packets. For this purpose, it maintains a running average of 1. Web page URL http://daedalus.cs.berkeley.edu. Srinivasan Seshan is now at IBM T.J. Watson Research Center, Hawthorne, NY (srini@watson.ibm.com). the estimated round-trip delay and the mean linear deviation from it. The sender identifies the loss of a packet either by the arrival of several duplicate cumulative acknowledg￾ments or the absence of an acknowledgment for the packet within a timeout interval equal to the sum of the smoothed round-trip delay and four times its mean deviation. TCP reacts to packet losses by dropping its transmission (conges￾tion) window size before retransmitting packets, initiating congestion control or avoidance mechanisms (e.g., slow start [13]) and backing off its retransmission timer (Karn’s Algorithm [16]). These measures result in a reduction in the load on the intermediate links, thereby controlling the con￾gestion in the network. Unfortunately, when packets are lost in networks for rea￾sons other than congestion, these measures result in an unnecessary reduction in end-to-end throughput and hence, sub-optimal performance. Communication over wireless links is often characterized by sporadic high bit-error rates, and intermittent connectivity due to handoffs. TCP perfor￾mance in such networks suffers from significant throughput degradation and very high interactive delays [8]. Recently, several schemes have been proposed to the allevi￾ate the effects of non-congestion-related losses on TCP per￾formance over networks that have wireless or similar high￾loss links [3, 7, 28]. These schemes choose from a variety of mechanisms, such as local retransmissions, split-TCP con￾nections, and forward error correction, to improve end-to￾end throughput. However, it is unclear to what extent each of the mechanisms contributes to the improvement in per￾formance. In this paper, we examine and compare the effec￾tiveness of these schemes and their variants, and experimentally analyze the individual mechanisms and the degree of performance improvement due to each. There are two different approaches to improving TCP per￾formance in such lossy systems. The first approach hides any non-congestion-related losses from the TCP sender and therefore requires no changes to existing sender implemen￾tations. The intuition behind this approach is that since the problem is local, it should be solved locally, and that the transport layer need not be aware of the characteristics of the individual links. Protocols that adopt this approach attempt to make the lossy link appear as a higher quality link with a reduced effective bandwidth. As a result, most of the losses seen by the TCP sender are caused by congestion. Appeared in IEEE/ACM Transactions on Networking, Dec.1997. This is a much-extended and revised version of a paper that appeared at ACM SIGCOMM, 1996