Throughput Enhancement of TCP over Wireless Links

Abstract

The congestion control mechanisms of Transmission Control Protocol (TCP) are very effective in providing best effort service in wired networks, where packet losses are mainly due to congestion in the network. In wireless mobile networks, more often than not, loss of packets is because of corruption of data on the wireless link. The TCP sender responds to these losses as if they are due to congestion, by reducing its congestion window, thereby reducing the rate of flow of packets. The reduction in congestion window is a necessity when network is experiencing congestion to avoid congestion collapse but it is not required if packet losses occur due to corruption of data on the wireless link. This unnecessary reduction in congestion window for corruption losses is the main reason for poor throughput of data transfer in wireless networks. The reduction in congestion window for corruption losses can be avoided if TCP can successfully differentiate between packet losses due to congestion and corruption.

We suggest enhancements to TCP that, if implemented, will help the TCP receiver in separately identifying corruption losses and congestion losses. The enhancements are suggested over and above standard TCP NewReno and we call this new scheme as "NewRenoEln (NewReno with Explicit Loss Notification)". We suggest that the TCP sender attach a separate checksum for the TCP header with the packet. Since the length of the TCP header is much smaller as compared to the length of the TCP packet, there is a large probability that the TCP receiver will receive the header portion of the TCP packet without error even if the data portion of the packet is corrupted. Once the header information is found to be correct for a corrupted packet, the receiver can generate reliable Explicit 5oss Notification (ELN) for the sender.

We derive an expression for the probability of a receiver generating successful Explicit Loss Notification, assuming a generic link layer protocol that is used for data transfer over wireless link. With this analysis, we show that there is large probability that receiver will generate successful ELN for various channel conditions We also suggest modifications to the sender behavior on receiving successful Explicit Loss Notification from the receiver. With these modifications, the TCP sender will recover from corruption losses without any reduction in congestion window.

There is also a need to develop a unified analytical approach for the evaluation of TCP performance. We develop an analytical approach for the performance evaluation of NewRenoEln scheme. We compare the throughput results obtained by analytical calculations with results obtained by simulation and find them to be very close to each other.

We also compare the performance of the proposed scheme NewRenoEln and the standard NewReno TCP via simulation as well as analytical approach, and find considerable improvement in throughput over wireless links.