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Abstract-TCP-Friendly Rate Control (TFRC) is a congestion control mechanism that can compete fairly with TCP while maintaining a much smoother transmission rate. The smoothness is achieved at the cost of responsiveness, which makes it slow to settle down when handed-over to a different network. In this paper we will study how to improve the TFRC performance when the available bandwidth is changed during a handover. An enhancement is proposed that can eliminate most of the heavy packet loss during a bandwidth-decreased handover. The definition and conditions of a smooth TFRC handover is also provided. The improvement over the original TFRC is proved with simulation results.

Abstract-In this paper we propose a vertical handover mechanism for TCP to deal with the delay and bandwidth change during the handover. The proposed mechanism relies on the interaction between the sender and the receiver during the handover and requires no a prior information of the new path. The adaptation is triggered by a handover notification at the receiver and finishes in about two round-trip time. Furthermore, it solves the problem of packet reordering and spurious retransmission timeout as well which are also common during vertical handovers. Simulation results show that our mechanism improves TCP performance in various vertical handover scenarios.Index Terms-TCP, Handover, Congestion Control, Mobility.I. INTRODUCTION It is well-known that TCP does not perform well during a handover. A first reason is the possible packet loss during the outage time when the mobile node changes its network connection. Since TCP regards all packet loss as indication of congestion, it will back-off its sending rate unnecessarily. This problem can be readily solved in the network layer with lossless handover techniques such as buffer-and-forward and make-before-break. Unfortunately, this is not the end of problems. When the handover takes place between different type of networks, or in other words, when TCP experiences a vertical handover, the abrupt change in network delay and bandwidth will still affect its performance.When the new path has a higher bandwidth or a lower delay, packets following the new path may overtake those in the old one and arrive at the receiver earlier. Consecutive out-oforder packets can generate enough duplicate ACKs to trigger false fast retransmission while no packet is really lost. On the other hand, when the new path has a lower bandwidth or a higher delay, packets following the new path may not be acknowledged in time, which causes premature RTOs at the TCP sender. All these wrongly triggered congestion responses not only cause unnecessary packet retransmission but also greatly decrease the TCP throughput regardless of the real network conditions. Furthermore, since TCP does not control the sending rate directly but only the number of in-flight packets using a sliding-window based mechanism, it will try to keep the same amount of packets in the network also after a handover. If the Bandwidth-Delay Product (BDP) is changed during a handover -which is very likely the case for a vertical one -TCP will either overrun or under-utilize the capacity of the new path. Because the Additive-Increase MultiplicativeDecrease (AIMD) scheme used in TCP for window adjusting

Abstract-TCP-Friendly Rate Control(TFRC) is an equationbased congestion control mechanism that competes fairly with TCP but has a much lower throughput variation, which makes it a better choice for streaming over the Internet. It is known that in a mobile network environment, after a handover TFRC can overshoot or underutilize the new link if the conditions there differ from those of the old link. There are different factors that affect the TFRC performance during a handover. In this paper we focus on the impact of the change in the round trip time, an aspect that is largely overlooked in the literature compared with bandwidth disparity. We find that even if the same bandwidth is assured on the new link, changes in the link latency will still cause TFRC performance degradation after the handover. A fast link adaptation mechanism is thus proposed to address the problem. Simulations show that this mechanism helps TFRC to adjust quickly to the new link without consecutive packet loss or longtime bandwidth underutilization.

the Random Early Detection (RED) is a widely used active queue management (AQM) algorithm for congestion avoidance. It monitors the average queue length to detect incipient congestion and notifies the connections of congestion to adjust their sending rate. During a handover, all the active connections of the mobile node will be diverted to the new network and increase the traffic there immediately. Because RED uses an exponential weighted moving average (EWMA) to calculate the queue length, it will be too slow to track this rapid change and fail to react correctly. In this paper we propose a dual RED algorithm to accommodate the handed-over connections quickly while keeping the link utilization high. The improvement is verified with simulations.