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principles of network and system administration phần 8

Các câu hỏi rõ ràng là liệu các chi phí như vậy là hợp lý bởi tính chính xác của Biểu thuế kết quả, độ chính xác này được phản ánh vào việc cung cấp ổn định giá cả tốt hơn và công bằng. Kết quả thực nghiệm cho thấy một sự cải thiện đáng kể đạt được thời gian, | 246 CHARGING FLEXIBLE CONTRACTS that in all circumstances the system maintains stability and achieves a good utilization of its links. We describe two mechanisms for implementing appropriate Pj s. The first of these called RED is a proposal for preventing packet losses in the Internet. The basic idea is that the routers should monitor the average queue size of outgoing links and when this size exceeds some threshold they should randomly place ECN marks on outgoing packets as congestion indications doing so with a probability that increases linearly in the average queue size. These ECN marks eventually reach the sender. Originally RED was intended to be used in combination with TCP the idea being that TCP should react to a congestion mark as if a packet loss had occurred. The second mechanism for implementing appropriate Pj s is the virtual queue approach in which an algorithm runs an on-line simulation of a virtual queue of a proportionally smaller size i.e. in which the buffer size and service rate are multiplied by some factor 6 1. It feeds the queue with the same traffic or with a fraction 0 of the traffic randomly chosen depending on the variant of the implementation . The algorithm waits until the virtual queue overflows and then marks all subsequently arriving packets until it empties. The idea is that the virtual queue will overflow before the actual queue does and so most packets that cause overflow in the actual queue will be marked in the virtual queue. Also virtual queues produce larger rates of congestion signals. Using this approach bursty flows receive more marks. Both of the above algorithms have many parameters and tuning them appropriately takes experiment study and skill gained by experience. There are many subtleties. For instance since in RED the burstiness of the marking process affects the burstiness of the traffic that results from the flow control one may be tempted to reduce such burstiness by averaging the queue length process. The danger