Our experimental testbed consists of IBM ThinkPad laptops ng BSD/OS) and Pentium-based personal computers running BSD/OS 2.1 from bsdl. The machines are interconnected a10 TCP Rece Mbps ethernet and 915 MHz at&T WaveLANs [271,a 2 Mbps WaveLAN Y ning Silos) shared-medium wireless Lan with a raw signalling band (lossy link Pentium-based Pc width of 2 Mbps. The network topology for our experiments is shown in Figure 2. The peak throughput for TCP bulk transfers is 1. 5 Mbps in the local area testbed and 1 Mbps in the wide area testbed in the absence of congestion tional 16 Internet hops between the souree and base sta- or wireless losses. These testbed topologies represent typi Figure 2. Experimental topology. There were an addi- cal scenarios of wireless links and mobile hosts such as cel- tion during the WAN experiments lular wireless networks. In addition, our experiments focus on data transfer to the mobile host. which is the common We added TCP awareness to both the LL and LL-SMArT case for mobile applications (e.g, Web accesses) protocols, resulting in the LL-TCP-AWARE and lL- SMART-TCP-AWARE schemes. The LL-TCP-AWARE In order to measure the performance of the protocols under protocol is identical to the snoop protocol, while the LL- controlled conditions, we generate errors on the lossy link SMART-TCP-AWARE protocol uses SMART-based tech- using an exponentially distributed bit-error model. The niques for further optimization using selective repeat. LL- receiving entity on the lossy link generates an exponential SMART-TCP-AWARE iS the best link-layer protocol in our distribution for each bit-error rate and changes the tcp experiments- it performs local retransmissions based on checksum of the packet if the error generator determines selective acknowledgments and shields the sender from that the packet should be dropped. Losses are generated in duplicate acknowledgments caused by wireless losses both directions of the wireless channel. so tcp acknowl edgments are dropped too. The TCP data packet size in 3.3 Split-Connection Schemes experiments is 1400 bytes. We first measure and analyze the Like i-tcp our sPlit scheme uses an intermediate host to performance of the various protocols at an average error rate of one every 64 KBytes(this corresponds to a bit-error rate divide a TCP connection into two separate TCP connec- of about 1.9x10-6) Note that since the exponential distribu ions. The implementation avoids data copying in the inter tion has a standard deviation equal to its mean, there are mediate host by passing the pointers to the same buffer several occasions when multiple packets are lost in close between the two TCP connections. A variant of the SPLIT succession. We then report the results of some burst error approach we investigated, SPLIT-SMART, uses a SMART- situations, where between two and six packets are dropped based selective acknowledgment scheme on the wireless connection to perform selective retransmissions. There is formance of many of these protocols across a range of erro little chance of reordering of packets over the wireless con- rates from one every 16 KB to one every 256 KB. The nection since the intermediate host is only one hop away choice of the exponentially distributed error model is moti from the final destination vated by our desire to understand the precise dynamics of each protocol in response to a wireless loss, and is not an 4. Experimental Results attempt to empirically model a wireless channel. While the actual performance numbers will be a function of the exact In this section, we describe the experiments we performed error model, the relative performance is dependent on how and the results we obtained, including detailed explanations the protocol behaves after one or more losses in a single for observed performance. We start by describing the exper- TCP window. Thus, we expect our overall conclusions to be imental testbed and methodology. We then describe the per- pplicable under other patterns of wireless loss as well formance of the various link-layer, end-to-end and split- Finally, we believe that though wireless errors are generated artificially in our experiments, the use of a real testbed still valuable in that it introduces realistic effects such as 4.1 Experimental Methodolog wireless bandwidth limitation. media access contention We performed several experiments to determine the protocol processing delays, etc, which are hard to model mance and efficiency of each of the protocols. The realistically in a simulation cols were implemented as a set of modifications to the In our experiments, we attempt to ensure that losses are only OS TCP/IP(Reno)network stack. To ensure a fair basis for due to wireless errors(and not congestion). This allows us comparison, none of the protocols implementations intro- to focus on the effectiveness of the mechanisms in handling duce any additional data copying at intermediate points such losses. The WAN experiments are performed across 16 from sender to receiverWe added TCP awareness to both the LL and LL-SMART protocols, resulting in the LL-TCP-AWARE and LL￾SMART-TCP-AWARE schemes. The LL-TCP-AWARE protocol is identical to the snoop protocol, while the LL￾SMART-TCP-AWARE protocol uses SMART-based tech￾niques for further optimization using selective repeat. LL￾SMART-TCP-AWARE is the best link-layer protocol in our experiments — it performs local retransmissions based on selective acknowledgments and shields the sender from duplicate acknowledgments caused by wireless losses. 3.3 Split-Connection Schemes Like I-TCP, our SPLIT scheme uses an intermediate host to divide a TCP connection into two separate TCP connec￾tions. The implementation avoids data copying in the inter￾mediate host by passing the pointers to the same buffer between the two TCP connections. A variant of the SPLIT approach we investigated, SPLIT-SMART, uses a SMART￾based selective acknowledgment scheme on the wireless connection to perform selective retransmissions. There is little chance of reordering of packets over the wireless con￾nection since the intermediate host is only one hop away from the final destination. 4. Experimental Results In this section, we describe the experiments we performed and the results we obtained, including detailed explanations for observed performance. We start by describing the exper￾imental testbed and methodology. We then describe the per￾formance of the various link-layer, end-to-end and split￾connection schemes. 4.1 Experimental Methodology We performed several experiments to determine the perfor￾mance and efficiency of each of the protocols. The proto￾cols were implemented as a set of modifications to the BSD/ OS TCP/IP (Reno) network stack. To ensure a fair basis for comparison, none of the protocols implementations intro￾duce any additional data copying at intermediate points from sender to receiver. Our experimental testbed consists of IBM ThinkPad laptops and Pentium-based personal computers running BSD/OS 2.1 from BSDI. The machines are interconnected using a 10 Mbps Ethernet and 915 MHz AT&T WaveLANs [27], a shared-medium wireless LAN with a raw signalling band￾width of 2 Mbps. The network topology for our experiments is shown in Figure 2. The peak throughput for TCP bulk transfers is 1.5 Mbps in the local area testbed and 1.35 Mbps in the wide area testbed in the absence of congestion or wireless losses. These testbed topologies represent typi￾cal scenarios of wireless links and mobile hosts, such as cel￾lular wireless networks. In addition, our experiments focus on data transfer to the mobile host, which is the common case for mobile applications (e.g., Web accesses). In order to measure the performance of the protocols under controlled conditions, we generate errors on the lossy link using an exponentially distributed bit-error model. The receiving entity on the lossy link generates an exponential distribution for each bit-error rate and changes the TCP checksum of the packet if the error generator determines that the packet should be dropped. Losses are generated in both directions of the wireless channel, so TCP acknowl￾edgments are dropped too. The TCP data packet size in our experiments is 1400 bytes. We first measure and analyze the performance of the various protocols at an average error rate of one every 64 KBytes (this corresponds to a bit-error rate of about 1.9x10-6 ). Note that since the exponential distribu￾tion has a standard deviation equal to its mean, there are several occasions when multiple packets are lost in close succession. We then report the results of some burst error situations, where between two and six packets are dropped in every burst (Section 4.5). Finally, we investigate the per￾formance of many of these protocols across a range of error rates from one every 16 KB to one every 256 KB. The choice of the exponentially distributed error model is moti￾vated by our desire to understand the precise dynamics of each protocol in response to a wireless loss, and is not an attempt to empirically model a wireless channel. While the actual performance numbers will be a function of the exact error model, the relative performance is dependent on how the protocol behaves after one or more losses in a single TCP window. Thus, we expect our overall conclusions to be applicable under other patterns of wireless loss as well. Finally, we believe that though wireless errors are generated artificially in our experiments, the use of a real testbed is still valuable in that it introduces realistic effects such as wireless bandwidth limitation, media access contention, protocol processing delays, etc., which are hard to model realistically in a simulation. In our experiments, we attempt to ensure that losses are only due to wireless errors (and not congestion). This allows us to focus on the effectiveness of the mechanisms in handling such losses. The WAN experiments are performed across 16 TCP Source 10 Mbps Ethernet TCP Receiver 2 Mbps WaveLAN (lossy link) Pentium-based PC running BSD/OS) Base Station (Pentium PC running BSD/OS) (Pentium laptop running BSD/OS) Figure 2. Experimental topology. There were an addi￾tional 16 Internet hops between the source and base sta￾tion during the WAN experiments