New flow control schemes of TCP for multimodal mobile hosts

Two control schemes designed specifically to handle changes in the datalink interface for a mobile host are presented. The future mobile environment is expected to involve multimode connectivity to the Internet and dynamic switching of the connection mode depending on network conditions. However, the existing TCP architecture is unable to maintain stable and efficient throughput as a result of such interface changes. The main issues are how to handle the change in host IP address when the datalink interface changes, and how to ensure reliable and continuous TCP flow. Although architectures addressing the first of these issues have already been proposed, the problem of flow control still remains. The first of the proposed schemes, immediate expiration of timeout timer, detects such an interface change, and begins retransmission immediately without waiting for retransmission timeout as in conventional TCP. The second scheme, bandwidth aware slow start threshold, detects the interface changes, and also estimates the new bandwidth so as to set an appropriate slow start threshold when beginning retransmission. Through simulations, the bandwidth aware slow start threshold scheme is demonstrated to provide marked improvements in performance over conventional TCP, particularly in situations where the bandwidth changes significantly and the interface is changed consecutively at short intervals. The immediate expiration of timeout timer scheme is also shown to be effective when the communication interruption associated with interface change is long.