Problems About Resource Assignment And Congestion Control In The Internet

Generally we consider resource allocation and congestion control issues in the Internet assuming that it is a connectionless, packet-switched network with best effort service; and TCP end-to-end congestion control mechanism makes the Internet converge to steady state with effective and fair resource allocation assuming that packet loss probability is not a function of flow rates. However, in no all scenarios of network do assumptions above come into existence. On the one hand, there indeed exist some network situations where packet loss probability is a function of flow rates and it is difficult for TCP to converge to steady state. On the other hand, some new network technologies, for optical burst switching (OBS) as an example, use different ways to allocate network resources from those used in packet-switched network like Internet.In this dissertation several problems involved the two hands are studied. For the first hand TCP congestion control performance in two network situations where packet loss probability is a function of flow rates is studied. For the second hand the channel scheduling problem in OBS network is discussed. These problems are quite different from those existing, thus the traditional resource assignment and congestion control mechanism, in which link bandwidth and buffer space in routers are viewed as the target resources, will be challenged. In this dissertation the problems above are looked into, and relevant technology solutions are carried out. The contributions are three-fold as following.The first contribution is that TCP performance in the input-queued switch is studied for the first time. It is found that the TCP congestion control performance in the input-queued switch is different from traditional one, and that it is impossible to achieve efficiency and fairness among TCP flows at the same time only by queue management. The reasons why drop tail and drop front queue management mechanisms do not work are analyzed. The queue management and switching arbitrating must be in conjunction, and a scheme, which combines heuristic fair arbitrating (AFS) and queue management mechanism of early drop front randomly (rEDF), is proposed. In proposed scheme, switch arbitration strategy of hFS unevenly allows input ports to transfer packets to output ports while packets at head of any other input ports involved in conflicts have to be dropped by the policy of rEDF with a probability. Simulation results prove that the proposed scheme can achieve better tradeoff between throughput and fairness. The second contribution discusses TCP congestion control performance in OBS network. It is found that significant throughput unfairness exists among TCP flows that share the OBS network through edge nodes. The cause is that the interaction of burst dropping and TCP transmitting rates back-off mechanism. In other words, in OBS network flows with different rates undergo different burst dropping probability, i.e., a TCP flow with a lower rate will undergo more burst dropping than one with a higher rate, and burst dropping always makes TCP enter slow start due to retransmission timeout, which worsen throughput difference between flows with different rates. The reasons why TCP does retransmission timeouts are analyzed. Burst dropping models are proposed to explain TCP throughput unfairness. It is observed that offset time will be a good choice to control TCP fairness, and then an adaptive offset time scheduling (AOS) algorithm is proposed. AOS assigns bursts offset time value adaptive to flow rate, i.e., makes a burst with shorter offset time for the recent flow rate increasing. The simulation results show that the fairness can be significantly improved by proposed AOS scheme.For the third contribution, the optimization problem about channel scheduling in OBS network is studied. It is found that for the channel utilization the offline OBS-GS algorithm is even worse than online LAUC algorithm. An optimal scheduling scheme is proposed to maximum channel utilization. The problem of optimum channel scheduling is mapped to a problem of colouring graph, which is formulated as an integer linear programming problem solvable in polynomial time by making use of interval graph good characteristics. Discrimination to smaller burst and the alleviation with adjusted weight are discussed. Analysis results present the minimal low bound and maximal bound of channel utilization of proposed schemes.