A modification of TCP flow control for improving end-to-end TCP performance over networks with wireless links

End-to-end Transmission Control Protocol (TCP) performance is one of the more important issues in wireless Internet services. This paper proposes the improvement of end-to-end TCP performance via a TCP-aware link layer protocol called Adaptive TCP (A-TCP). The key idea behind the protocol is that an A-TCP agent, which is located in each base station, makes a mobile host look as if it has a wired link with the base station. This concept is referred to in this paper as the virtual host model. In order to implement this model, the A-TCP agent performs three functions: local retransmission, sender freezing and A-TCP flow control. A-TCP flow control is an original proposal and is also the principal factor for improving end-to-end TCP performance in a wireless Internet environment. In A-TCP flow control, the A-TCP agent marks the window field of each acknowledgment segment with a retransmission buffer size. Therefore, the TCP congestion controls, which happen in a TCP sender, are not caused by wireless link overflow. Performance evaluations were conducted via computer simulations and mathematical analyses. The results of the evaluations show not only that the A-TCP improves end-to-end TCP performance by at least 20%, which is higher compared to other TCP approaches, but also that the A-TCP can provide near-optimal performance in wireless bottleneck conditions. In addition, this paper proposes an A-TCP agent architecture, which consists of virtual host objects and several other functional blocks, and describes a new hand-off policy for the virtual host model, called transport-level soft hand-off.