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Network Congestion Control Managing Internet Traffic phần 4

Không đóng trình duyệt đến khi xuất hiện nút TẢI XUỐNG Tải xuống

Trong các mạng ngày nay chủ yếu là uncongested, mục tiêu vẫn giữ nguyên, nhưng trong khi nó xuất hiện rằng phương pháp kiểm soát tắc nghẽn hiện có amply xử lý với các liên kết quá tải trên mạng Internet qua Các chức năng đánh dấu của các màu ĐỎ trong chế độ 'nhẹ nhàng' • ECN, | 3.3. TCP RTO CALCULATION 67 Taking less implicit feedback into account than there is available may generally be a bad idea the more an end system can learn about the network in between the better. Van Jacobson explained this in a much more precise way in RFC 1323 Jacobson et al. 1992 by pointing out that RTT estimation is actually a signal processing problem. The frequency of the observed signal is the rate at which packets are sent if samples of this signal are taken only once per RTT the signal is sampled at a much lower frequency. This violates Nyquist s criteria and may therefore cause errors in the form of aliasing. This problem is solved in RFC 1323 by the introduction of the Timestamps option which allows a sender to take samples based on almost each and every ACK that comes in. Using the Timestamps option is quite simple. It enables a sender to insert a timestamp in every data segment this timestamp is reflected in the next ACK by the receiver. Upon receiving an ACK that carries a timestamp the sender subtracts the timestamp from the current time which always yields an unambiguous RTT sample. The option is designed to work in both directions at the same time for full-duplex operation and only ACKs for new data are taken into account so as to make it impossible for a transmission pause to artificially prolong an RTT estimate. If a receiver delays ACKs the earliest unacknowledged timestamp that came in must be reflected in the ACK which means that this behaviour influences RTO calculation. This is necessary in order to prevent spurious retransmissions. The Timestamps option has two notable disadvantages first it causes a 12-byte overhead in each data packet and second it is known that it is not supported by TCP IP header compression as specified in RFC 1144 Jacobson 1990 . 3.3.3 Updating RTO calculation The procedure described in RFC 793 does not work well even if all the samples that are taken are always precise. Before we delve into the details here are two