Research On TCP/IP Congestion Control Strategies

Internet has experienced an explosively growth since 1990's. Widespread use of computer networks, as well as the appearance of varied network applications has brought forth network congestion as a significant problem. Congestion often results in the decline of Quality of service (Qos) such as transmission delay and throughput, while the network resource utilization like bandwidth and buffers are also affected seriously. Therefore, it is important to solve the congestion problem effectively for improving network performance. How to avoid and control congestion is one of the most active fields in the computer networks.Current TCP/IP networks only provide best effort services and their resources allocation is mainly on end hosts. The end hosts use Transmission Control Protocol (TCP) to implement end-to-end congestion control. It can be said that the success of today's Internet primarily relies on TCP congestion control mechanisms. With the development of Internet and the problems in the implementation of TCP, the current TCP strategies couldn't meet various demands of users any longer. More and more researches indicate that the TCP congestion control mechanisms, while necessary and powerful, are substantially conservative and are not sufficient to provide good service in all circumstances.The traditional technique for managing router queue in Internet usually adopts first in first out (FIFO) scheduling and "Drop tail" queue management algorithms, which have no provision for the detection of incipient congestion when the queue is full. In fact, it will be more effective for detecting and preventing congestion if the routers perform congestion control strategies, so IP congestion control strategies has been discussed widely in recent years. The IETF (Internet Engineering Task Force) has proposed the solution by deploying active queue management in Internet routers.In this paper, Chapter One gives a general introduction to the network congestion control. First the background and the cause of network congestion areintroduced, and then the necessity of implementing congestion control is analyzed. We also discuss the research and development of congestion control strategies, and classify them from different aspects. Finally existing problems in current congestion control are proposed.In chapter two we focus on TCP end-to-end congestion control strategies and the mechanisms implemented in IP routers. First the four kernel algorithms of TCP congestion control are described, and the improved schemes such as New-Reno, SACK and TCP-Vegas are discussed either. Then we introduce "drop tail" algorithm and the principal idea of active queue management. Finally we discuss the related works in IP congestion control, in which RED and ECN algorithm are emphasized.When network stability and congestion nonoccurrence are guaranteed, fairness is an important performance index of the network. Therefore, a new fair window algorithm is proposed in the third part and is realized in IP network in support of ECN mechanism in network layer. The simulation result demonstrates that remarkable fairness among TCP data flow in the multiple bottleneck network can be achieved with the application of this algorithm.Based on control theory, Chapter four provides guidelines for the selection of the control gain for dynamic-RED to stabilize a congested queue at a target and hence to improve network performance. Simulations demonstrate that indeed satisfactory performance can be achieved if the control gain is selected based on the guidelines.On basis of the study of the above chapter, we further explore the control gain selection of another AQM (active queue management) algorithm-PD controller. Though the recently proposed proportional and differential random early detection (PD-RED) scheme has been used in enhancing the performance of queuing networks, one key issue in applications of this scheme as well as of other RED variations is the so-called parameter-tuning problem. The guideline on choosing control gains to meet the stability condition of queue dynamic...