摘要
随着视频点播、电视会议、远程教育、网络交互式游戏等实时性业务的兴起，组播技术得到了广泛的应用。这些实时业务一般在IP组播之上，采用UDP协议进行传输。然而，由于UDP协议和IP组播都不提供拥塞控制机制，导致这些业务和TCP业务共存时出现了带宽占用的不公平性，因此组播拥塞控制便成为一项重要的研究课题。
本文就组播拥塞控制中的拥塞快速响应，反馈抑制，协议的公平性三个方面的问题进行了分析和研究。
第一部分针对网络拥塞的快速响应，从路由器队列管理的角度，提出一种新的算法。所提出的算法在平均队列长度超过一个参考值之后，根据数据包到达队列的速度来进行丢包和标记，这样它能及时发现拥塞，通知发送方进行速率调整，而且该算法具有更好的稳定性和对网络状态的自适应性。
第二部分针对组播应用中的反馈爆炸问题，改进路由器的功能，提出一种新的反馈抑制算法。该算法由主动路由器参与拥塞控制，结合了反馈延时法和主动路由器的反馈汇聚方法来解决反馈爆炸问题；由于主动路由器可以缓存数据进行局部恢复，所以该算法可以应用于可靠组播，且它和一些经典的可靠组播算法相比较，具有更高的带宽利用率和更小的传输延时。
第三部分研究协议的公平性，在现有的TCP友好协议TFMCC的基础上，针对传送实时流媒体时出现的链路振荡问题，提出改进算法。改进的算法根据丢包率和TCP友好性来调整传输速度，并且对传输速度的调整步长进行了平滑。该算法能更好的解决链路振荡问题、满足协议的公平性和TCP友好性。
最后，利用NS网络模拟器对前面三部分提出的各种算法进行了仿真，并且验证了其结果的正确性。
With the springing up of real-time services, such as VOD (Video on Demand), video conferencing, distance learning, network interaction game, multicast is considered to one of the widely used technologies. But multicast based on UDP cannot offer congestion control. At the present time, that existing TCP applications, as the main applications in the Internet, offer congestion control have made it important to ensure the fairness when multicast coexists with TCP protocols. Therefore study of congestion control about multicast is becoming an important subject.
The paper solves three problems of multicast congestion control: quickly response to network congestion, feedback suppressed and fairness of protocol.
Firstly, aiming at quickly response to network congestion, the paper analyses some existing active queue management algorithm and brings forward a new algorithm. When average length of queue exceeds a reference value, the algorithm adjusts the rate of packet loss percentage according to the packet arriving rate. Its queue length keeps around the reference value, so it has more stability and self-adaptability to network congestion.
Secondly, taking feedback implosion into account, the paper uses the active network technique for improving router's functions and brings forward a kind of congestion control that is concerned with the router, which implements feedback aggregation and local recover of lost packet. Having been simulated and analyzed, the algorithm has been testified to have better scalability and utilization of bandwidth.
Thirdly, aiming at fairness of protocol, the paper, based on the existing TCP-Friendly Multicast Congestion Control (TFMCC), considers the problem of the dithering of transmission rate when the multimedia stream is transferred, and proposes an improved algorithm. The improved algorithm adjusts its transmission rate according to the probability of packet loss and TCP-friendliness, and smoothes its rate adaptation. The algorithm can preferably alleviate the dithering of transmission rate and ensure intra-protocols fairness and meet TCP-friendliness.
Finally, NS, a widely used network simulation tool, is used to analyses and validates the performance of forenamed algorithms.
引文
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