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TCP-Eifel. TCP-Eifel [13] is a technique that is specially designed to detect and handle spurious retransmissions. Detection of Spurious Retransmissions. TCP-Eifel uses the TCP timestamp option to solve the ambiguity of retransmissions. TCP-Eifel includes in every transmitted packet the transmission time and this timestamp is echoed back by the receiver. In addition, the sender records the time of the rst retransmission. When an ACK acknowledging a recently retransmitted packet arrives, the sender can compare the timestamp on the ACK with the recorded time to determine whether the retransmission is spurious or not. If the timestamp on the ACK is smaller than the recorded time (which means that the ACK is generated due to a packet that was transmitted prior to the particular retransmission), the retransmission is likely to be spurious. If it is spurious, the sender will restore the cwnd and ssthresh values before the retransmission. Response after Detecting Spurious Retransmission. If only one retransmission of the oldest unacknowledged segment was performed, the stored ssthresh and cwnd values will be restored. If more than one retransmission was performed, ssthresh is halved. If exactly two such retransmissions were performed, cwnd is set to the ssthresh (which had been halved previously). If more than two retransmissions were performed, cwnd is set to one. Therefore, the more retransmissions, the more conservative the sender gets. TCP-Eifel is robust to a sudden increase in the packet delivery time. It identi es spurious retransmissions and avoids unnecessary go-back-N retransmissions for packets that are not lost but are just delayed. However, TCP-Eifel requires the use of the TCP timestamp option or some modi cations to the TCP header to enable the detection of spurious retransmissions. Moreover, extra memory and processing is needed to store the timestamp of each transmitted but not yet acknowledged packets. Forward RTO-Recovery (F-RTO). F-RTO [22] can be considered as an improvement to Eifel algorithm. It is used to avoid further, unnecessary retransmissions when a spurious retransmission has been detected by the scheme. Detecting Spurious Retransmissions after a Retransmission Timeout (RTO) Event. When the rst ACK after a retransmission due to a RTO arrives, F-RTO does not immediately continue with further retransmissions but instead checks if the ACK advances the congestion window to determine whether to perform further retransmissions or to continue sending new data. There are two cases: 1. If the rst ACK after a RTO-triggered retransmission advances the window, F-RTO transmits two new segments instead of continuing retransmissions. If the next ACK also advances the window, the RTO is likely to be spurious, because this second ACK must be triggered by an originally transmitted segment.
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VARIOUS TCP SCHEMES
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2. If either one of the two ACKs after a RTO is a duplicate ACK, the sender continues retransmissions in a manner similar to that of the operations of conventional TCP. F-RTO facilitates the detection of spurious timeouts that are caused by a sudden increase in the packet delay. It does not require any modi cation to the TCP header, nor does it require the use of the TCP timestamp option. However, F-RTO does not explicitly handle spurious retransmissions that are caused by spurious fast retransmits [13]. The spurious retransmissions that are caused by packet reordering cannot be handled by F-RTO. Besides, the determination of spurious retransmissions of F-RTO requires a default of two new segments to be sent after receiving the rst ACK after a retransmission. However, it may not be possible sometimes due to the window restriction. If no new segments can be sent, the TCP sender has no choice but to follow the conventional TCP RTO recovery by entering the slow start phase. 9.4.5 Exploiting the Buffering Capability In conventional TCP, a route failure can cause TCP to perform poorly. In fact, the poor performance of TCP, to a signi cant extent, is due to the inef ciency of TCP to preserve the work that was already done. For instance, when a route failure occurs, many of the transmitted but not yet acknowledged packets are dropped at the intermediate nodes (or delayed due to the broken route) and need to be retransmitted. The representative schemes that make use of the buffering capability to avoid unnecessary retransmissions to enhance TCP performance include Split-TCP [23] and TCP-BUS [24]. Split-TCP. Split-TCP [23] is a scheme that separates the functionalities of TCP congestion control and reliability. It makes use of proxies set up along the connection to improve TCP performance and fairness. TCP with Proxies. For a TCP connection, certain nodes along the route become proxies for the connection. The proxies become responsible for ensuring that packets will arrive at a subsequent proxy or at the destination. One way to see this is that a TCP connection is split into several small TCP connections. Each of these small connections has its own local acknowledgment mechanism that can control its own rate and ensure reliable arrival of packets at the next proxy. With Split-TCP, the buffering of packets at the proxies allows any dropped packets to be recovered from the most recent proxy instead of the need to retransmit it all the way back from the source. Furthermore, by dividing a long connection into several short segments, Split-TCP enables better pipelining of data transmissions and alleviates the capture effect (due to the IEEE802.11 MAC) due to other coexisting short TCP connections. However, an extra buffer is required at each proxy to store all the outstanding packets that have not been acknowledged. Furthermore, an additional state such as
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