TAILIEUCHUNG - artech house a professionals guide to data communication in a tcp ip world 2004 phần 2

và Truyền số liệu trong một thế giới Thư viện của Quốc hội mục-trong-Xuất bản dữ liệu Carne, E. Bryan, 1928 - Một hướng dẫn chuyên nghiệp để truyền dữ liệu trong một thế giới củaBao gồm các thư mục tài liệu tham khảo và chỉ số. | 12 A TCP IP World extreme cases it may command the sender to stop sending until the congestion clears. Changing traffic loads from other senders may affect some of the intermediate nodes. They pass congestion status information along to the receiver. In addition the sender may send special packets to probe conditions along the path. The receiver returns these packets to the sender. On the basis of this information the sender may reduce the transmission unit size so that the intermediate nodes can make buffer capacity available to other circuits. In other situations the intermediate nodes may destroy packets that have been sent in excess of the rate that the network owner has guaranteed to the user. Flow control requires constant monitoring by all the nodes in the network and frequent instructions to the senders to slow down or speed up to accommodate changing conditions. Retransmission Time-Out In TCP all segments containing data must be acknowledged. For each connection TCP maintains a variable whose value is the amount of time within which an ACK is expected for the segment just sent. Called the retransmission time-out RTO if the sender does not receive an ACK by the time RTO expires the segment is retransmitted. To prevent needless repetitions RTO must be greater than the round-trip time RTT for the connection. Since the RTT is likely to vary with traffic conditions it must be monitored continually and the RTO adjusted accordingly. For frames containing data TCP uses an exponential backoff algorithm to determine the RTO of successive retransmissions. Initially when the TCP segment is sent the RTO is set to the value currently known for the connection RTO1 . If the retransmission timer expires without an acknowledgment the segment is resent and the RTO timer is set to 2n RTO1 where n 0 1 2 . . This step is repeated until a maximum number of retransmissions are reached. At that time the connection is abandoned. Segments that contain no data . ACKs are .

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