Study On Congestion Control Based On Network Measurement In Wired/Wireless Networks

Rapid development of wireless networks (including cellular network, satellite network, mobile ad hoc network and even sensor network) and mobile computing becomes a new character of the Internet. People want to access the network services at any time, at any place and with any method. Though network bandwidth increases under optical rule, yet it never met bandwidth requirement of people. Especially, in wired/wireless hybrid neworks, because the properity of high bit error rate, mobile handoff, unstable link state and high bandwidth delay product, the tradictional TCP-based congestion control mechanisms suffer great performance degradation. Considering the factors of wired/wireless hybrid networks, this paper proposes enhanced congestion mechanisms based network measurement to improve the performance of hybrid networks. The main work and contributions are presented in the following aspects:1.To accurately gain network state, end system based queue length of bottleneck link and end-to-end available bandwidth measurement methods are proposed.Accurate measurement of network parameters and detecting right network states play important roles in congestion control, this paper addresses measurement methods of several key network parameters. An RTT-based realtime queue length of the bottleneck link measurement method is introduced, and a TCP-based end-to-end realtime available bandwidth measurement method is designed. The two parameters objectively reflect the real state of network, which supply good evidence for design of new congestion control algorithm in wireless network. The measurement methods can be easily embedded into source end algorithm of TCP without any modification of other node in the network and conveniently widely deployed in the Internet.2. To distinguish various situation of data transmiting in wireless network, measurement based end-to-end congestion control mechanism is proposed.This mechanism consists of a phase-divided smooth slow start algorithm, which increases sending rate with small amplitude at connection start and at transition phase to congeston avoidance and weakens harm to network performance caused by bursty traffic. It also employs network measurement technology, obtaining the network parameters, such as round trip time (RTT), network bandwidth and queue length of the bottleneck link, avoids mistake behavior of terminal nodes for theblindness of network state, modifies AIMD strategy of TCP according to network states and packet loss reasons, and improves network performance. This algorithm is implemented in NS2 and many simulation experiments are run with various scenarios, and simulation results show that TCP Yuelu decreases network oscillation and improves network performance, and good fairness and friendliness are achieved.3.To improve AQM mechanism performance^ dynamic adaptive fair RED algorithm is proposedTCP based congestion control mechanisms are partial to short RTT connections, after investigation limitation of RED, a fair bandwidth allocation mechanism called CRED is proposed. It randomly drops the packets of a flow that occupies buffer exceeding the fair allocation value. CRED also grants higher priority to flows with QoS requirement, and adjusts buffer allocation according to queue length adaptively. It improves the fairness of bandwidth allocation, maintains high throughput and assures QoS of service. In order to get over the limitation of statistic parameter setting, a dynamic adaptive RED algorithm is introduced. It employs a high-order function to mark or discard fragments with lower probability near low threshold and higher probability near high threshold respectively. Its parameter, Pmax, is adaptively adjusted according to average and instantaneous queue length, and it also achieves the smoothness from part to complete packet marking or discarding.4.To keep friendly to TCP flows and maintain QoS of streaming media, A measurement-based streaming media rate control mechanism is introducedMultimedia services are widely applied in wireless mobile network, a dynamic and adaptive rate control mechanism for streaming media is proposed to maintain friendly to TCP flows. In this method, the receiver distinguishes packet drops between congestion and wireless link error to get the accurate situation of networks, and combines the occupation of the receiver buffer, which reflects how long the temporarily stored streaming media will play. And tradeoff between TCP-friendly and QoS of streaming media services is achieved through multi-level rate adjustment and dynamic adaptively changing requirement of QoS, and high network utility is also held. Results of mass simulations show that this method is friendly to TCP, stable and of good performance.