Abstract: The authors propose a robust end-to-end loss differentiation scheme to identify the packet losses because of congestion for transport control protocol (TCP) connections over wired/wireless networks. The authors use the measured round trip time (RTT) values to determine whether the cause of packet loss is because of the congestion over wired path or regular bit errors over wireless paths. The classification should be as accurate as possible to achieve high throughput and maximum fairness for the TCP connections sharing the wired/wireless paths. The accuracies of previous schemes in the literature depends on varying network parameters such as RTT, buffer size, amount of cross traffic, wireless loss rate and congestion loss rate. The proposed scheme is robust in that the accuracy remains rather stable under varying network parameters. The basic idea behind the scheme is to set the threshold for the classification to be a function of the minimum RTT and the current sample RTT, so that it may automatically adapt itself to the current congestion level. When the congestion level of the path is estimated to be low, the threshold for a packet loss to be classified as a congestion loss is increased. This avoids unnecessary halving of the congestion window on packet loss because of the regular bit errors over the wireless path and hence improves the TCP throughput. When the congestion level of the path is estimated to be high, the threshold for a packet loss to be classified as the congestion loss not to miss any congestion loss is decreased and hence improves the TCP fairness. In ns 2 simulations, the proposed scheme correctly classifies the congestion losses under varying network parameters whereas the previous schemes show some dependency on subsets of parameters. Transport control protocol (TCP) congestion control runs under the basic assumption that any packet loss is the indication of congestion. However, the assumption does not hold when the TCP flow path includes wireless part. In such a case the packet loss may not come from congestion but from regular bit errors over wireless path. TCP throughput may be unnecessarily degraded because of the packet loss from the bit errors over wireless part even though there is little congestion. Research on improving performance of the TCP over hybrid wired/wireless paths has focused on differentiating packet losses using information readily available to the TCP: congestion window size, inter-arrival time between ACK packets and changes in a round-trip time (RTT) [1 -4]. However, correct classification based on these metrics has been found to be difficult [2] . The reason is that the nature of losses is weakly correlated with the observable metrics (RTT, congestion window size, inter-arrival of ACKs and Jitter) [5] . In [6] , the timestamp option in TCP is used to measure inter-arrival jitter between data packets. From our simulations, this scheme tends to regard most of the packet loss because of the wireless channel errors. We propose a new end-to-end scheme to precisely infer the nature of packet losses over the wired/wireless networks. (From now on, we call packet losses because of the congestion as congestion losses and the wireless channel errors as wireless losses.) Instead of fixed threshold in estimating the cause of individual packet loss, our scheme employs a moving threshold for relative change of RTT against the minimum RTT. In particular, we use the ratio of a difference between a sample RTT and a mean of RTT over a variance of RTT. We classify a packet loss as the congestion loss if this value exceeds the threshold or classify it as the wireless loss, otherwise. The threshold is defined as a function of minimum and sample RTT, which decreases as congestion level increases. The moving threshold is lowered for the differentiator to be more sensitive to the congestion loss when the network is congested whereas it is increased when the network is unloaded. Our scheme is shown to be more stable in identifying the congestion losses under varying network conditions than from previous schemes [1 -4, 6]. The accuracy of correctly detecting wireless loss on unloaded path is similar to previous schemes as far as the congestion loss rate is kept low. The rest of this paper is organised as follows. We describe related work in Section 2, and in Section 3 we propose a new scheme of differentiating the nature of packet loss and its evaluation is described in Section 4. In Section 5, we conclude.
Lim and Jang REF suggested a robust end-to-end loss differentiation scheme (RELDS) to precisely discriminate between congestion losses and wireless losses.
Robust end-to-end loss differentiation scheme for transport control protocol over wired/wireless networks
IET Communications
IET Communications
Computer Science Mathematics