generates messages that result in loss recovery. Although option allows us to identify what percentage of the end-to- this figure only shows data packets being lost, our experi- end performance degradation is associated with TCP ments have wireless errors in both directions incorrect invocation of congestion control algorithms when it does a fast retransmission of a packet lost on the wireless 3.1 End-To-End Schemes hop. The E2E-ELN-RXMT protocol is an enhancement of the previous one, where the sender retransmits the packet on Internet, the current de facto standard for TCP implementa. receiving the first duplicate acknowledgement with the ELN tions is TCP Reno [26]. We call this the E2E protocol, and ment in the case of TCP Reno), in addition to not shrinking use it as the standard basis for performance comparison its window size in response to wireless losses The EZ E-NEWRENO protocol improves the pertormance In practice, it might be difficult to identify which packets are lost due to errors on a lossy link. However, in our expe remaining in fast recovery mode if the first new acknowl iments we assume sufficient knowledge at the receiver ments are indicative of multiple packet losses within the gies for the ELn mechanism in Section f2 tion.We edgment received after a fast retransmission is"partial, 1.e, about wireless losses to generate ELN information. We is less than the value of the last byte transmitted when the describe some possible implementation pol nd strate fast retransmission was done. Such partial acknowledge original window of data. Remaining in fast recovery mode 3.2 Link-Layer Schemes enables the connection to recover from losses at the rate of one segment per round trip time, rather than stall until a Unlike TCP for the transport layer, there is no de facto stan- coarse timeout as TCP-Reno often would [9, 12 dard for link-layer protocols. Existing link-layer protocol choose from techniques such as Stop-and-Wait, Go-Back-N The E2E-SMART and E2E-IETF-SACK protocols add Selective Repeat and Forward Error Correction to provide SMART-based and IETF selective acknowledgments reliability. Our base link-layer algorithm, called LL, uses respectively to the standard TCP Reno stack. This allows cumulative acknowledgments to determine lost packets that the sender to handle multiple losses within a window of out- are retransmitted locally from the base station to the mobile standing data more efficiently. However, the sender still assumes that losses are a result of congestion and invokes leverages off TCP acknowledgments instead of generating he end-to-end performance degradation is associated with taining a smoothed round-trip time estimate, with a mini standard TCP's handling of error detection and retransmis- mum timeout granularity of 200 ms to limit the overhead of sion. We used the SMArT-based scheme [17] only for the processing timer events. This still allows the LL scheme to LAN experiments. This scheme is well-suited to situations retransmit packets several times before a typical TCP Reno transmitter would time out. LL is equivalent to the snoop one-hop wireless systems such as ours. Unlike the scheme agent that does not suppress any duplicate acknowledg- proposed in [17], we do not use any special techniques to ments, and does not attempt in-order delivery of packets detect the loss of a retransmission the sender retransmits a across the link(unlike protocols proposed in [15],[22) packet when it receives a SMART acknowledgment only if While the use of TCP acknowledgments by our LL protocol the same packet was not retransmitted within the last round- renders it atypical of traditional ARQ protocols, we believe trip time. If no further SMART acknowledgments arrive, the that it still preserves the key feature of such protocols: the sender falls back to the coarse timeout mechanism to ability to retransmit packets locally, independently of and ecover from the loss. We used the IETF selective acknowl- on a much faster time scale than TCP. Therefore, we expect edgement scheme both for the Lan and the wan experi- the qualitative aspects of our results to be applicable to gen- ments. Our implementation is based on the RFC and takes eral link-layer protocols appropriate congestion control actions upon receiving SACK information [41 We also investigated a more sophisticated link-layer proto- col (Ll-SMART) that uses selective retransmissions to The E2E-ELN protocol adds an Explicit Loss Notification improve performance. The LL-SMART protocol performs (ELN) option to TCP acknowledgments. When a packet is this by applying a SMART-based acknowledgment scheme dropped on the wireless link, future cumulative acknowl- at the link layer. Like the LL protocol, LL-SMART uses edgments corresponding to the lost packet are marked to TCP acknowledgments instead of generating its own and identify that a non-congestion related loss has occurred. limits its minimum timeout to 200 ms. LL-SMART is Upon receiving this information with duplicate acknowl- equivalent to the snoop agent performing retransmissions edgments, the sender may perform retransmissions without based on selective acknowledgements but not suppressing invoking the associated congestion-control procedures. This duplicate acknowledgments at the base stationgenerates messages that result in loss recovery. Although this figure only shows data packets being lost, our experi￾ments have wireless errors in both directions. 3.1 End-To-End Schemes Although a wide variety of TCP versions are used on the Internet, the current de facto standard for TCP implementa￾tions is TCP Reno [26]. We call this the E2E protocol, and use it as the standard basis for performance comparison. The E2E-NEWRENO protocol improves the performance of TCP-Reno after multiple packet losses in a window by remaining in fast recovery mode if the first new acknowl￾edgment received after a fast retransmission is “partial”, i.e, is less than the value of the last byte transmitted when the fast retransmission was done. Such partial acknowledge￾ments are indicative of multiple packet losses within the original window of data. Remaining in fast recovery mode enables the connection to recover from losses at the rate of one segment per round trip time, rather than stall until a coarse timeout as TCP-Reno often would [9, 12]. The E2E-SMART and E2E-IETF-SACK protocols add SMART-based and IETF selective acknowledgments respectively to the standard TCP Reno stack. This allows the sender to handle multiple losses within a window of out￾standing data more efficiently. However, the sender still assumes that losses are a result of congestion and invokes congestion control procedures, shrinking its congestion window size. This allows us to identify what percentage of the end-to-end performance degradation is associated with standard TCP’s handling of error detection and retransmis￾sion. We used the SMART-based scheme [17] only for the LAN experiments. This scheme is well-suited to situations where there is little reordering of packets, which is true for one-hop wireless systems such as ours. Unlike the scheme proposed in [17], we do not use any special techniques to detect the loss of a retransmission. The sender retransmits a packet when it receives a SMART acknowledgment only if the same packet was not retransmitted within the last round￾trip time. If no further SMART acknowledgments arrive, the sender falls back to the coarse timeout mechanism to recover from the loss. We used the IETF selective acknowl￾edgement scheme both for the LAN and the WAN experi￾ments. Our implementation is based on the RFC and takes appropriate congestion control actions upon receiving SACK information [4]. The E2E-ELN protocol adds an Explicit Loss Notification (ELN) option to TCP acknowledgments. When a packet is dropped on the wireless link, future cumulative acknowl￾edgments corresponding to the lost packet are marked to identify that a non-congestion related loss has occurred. Upon receiving this information with duplicate acknowl￾edgments, the sender may perform retransmissions without invoking the associated congestion-control procedures. This option allows us to identify what percentage of the end-to￾end performance degradation is associated with TCP’s incorrect invocation of congestion control algorithms when it does a fast retransmission of a packet lost on the wireless hop. The E2E-ELN-RXMT protocol is an enhancement of the previous one, where the sender retransmits the packet on receiving the first duplicate acknowledgement with the ELN option set (as opposed to the third duplicate acknowledge￾ment in the case of TCP Reno), in addition to not shrinking its window size in response to wireless losses. In practice, it might be difficult to identify which packets are lost due to errors on a lossy link. However, in our exper￾iments we assume sufficient knowledge at the receiver about wireless losses to generate ELN information. We describe some possible implementation policies and strate￾gies for the ELN mechanism in Section 5.2. 3.2 Link-Layer Schemes Unlike TCP for the transport layer, there is no de facto stan￾dard for link-layer protocols. Existing link-layer protocols choose from techniques such as Stop-and-Wait, Go-Back-N, Selective Repeat and Forward Error Correction to provide reliability. Our base link-layer algorithm, called LL, uses cumulative acknowledgments to determine lost packets that are retransmitted locally from the base station to the mobile host. To minimize overhead, our implementation of LL leverages off TCP acknowledgments instead of generating its own. Timeout-based retransmissions are done by main￾taining a smoothed round-trip time estimate, with a mini￾mum timeout granularity of 200 ms to limit the overhead of processing timer events. This still allows the LL scheme to retransmit packets several times before a typical TCP Reno transmitter would time out. LL is equivalent to the snoop agent that does not suppress any duplicate acknowledg￾ments, and does not attempt in-order delivery of packets across the link (unlike protocols proposed in [15], [22]). While the use of TCP acknowledgments by our LL protocol renders it atypical of traditional ARQ protocols, we believe that it still preserves the key feature of such protocols: the ability to retransmit packets locally, independently of and on a much faster time scale than TCP. Therefore, we expect the qualitative aspects of our results to be applicable to gen￾eral link-layer protocols. We also investigated a more sophisticated link-layer proto￾col (LL-SMART) that uses selective retransmissions to improve performance. The LL-SMART protocol performs this by applying a SMART-based acknowledgment scheme at the link layer. Like the LL protocol, LL-SMART uses TCP acknowledgments instead of generating its own and limits its minimum timeout to 200 ms. LL-SMART is equivalent to the snoop agent performing retransmissions based on selective acknowledgements but not suppressing duplicate acknowledgments at the base station