A fast adaptation mechanism for TCP vertical handover

Abstract

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

TCP Mobility Enhancement for Vertical Handovers

“…To overcome these drawbacks of SS, other probing techniques are proposed. For example in [16] the Quick-Start technique from [17] is used to probe the BDP; [18] and [19] use PathChirp algorithm to find the available bandwidth continuously; in a previous work [20] we also proposed to use packet-pair from [21] to probe the bandwidth and multiply it with the RTT to get the BDP, or probe only within the new network [22]. However, QuickStart needs the support of intermediate routers which is not very practical for the moment, and packet-pair probing needs time to converge.…”

Which Transport Protocol for Hybrid Terrestrial and Satellite Systems?

“…Some are general (as TCP options), while others are designed to change a particular TCP behavior or tends to adapt TCP functioning in the conditions of a given media technology (e.g. TCP Sack, Quick-Start), and some are completely specific to the vertical handover purpose (Freeze TCP [2], Fast Adaptation Mechanism [3]).…”