计算机网络基椽—第八章(传输层)(全)

第八章 传输层 8.1 传输层概述 2. 传输层的功能 3. 传输层协议与数据链路层协议的区别 8.2 因特网的传输协议--- TCP和UDP TCP消息格式 TCP消息头部字段 The TCP Service Model (a) Four 512-byte segments sent as separate IP datagrams. (b) The 2048 bytes of data delivered to the application in a single READ CALL. Compare with file read&write 建立TCP连接：三次握手 连接释放：文雅释放 TCP连接管理的有限状态自动机 TCP的窗口管理 TCP窗口管理的问题 TCP Transmission Policy efficiency TCP/IP overhead=20 bytes, TCP head + 20 bytes IP head When used in TELNET connection, 1 data bytes carried with 40 bytes overhead (41+40+40+41) Improvement algorithm Delay acknowledgement and window update reduce the load placed by the receiver Nagle’s algorithm send one segment unless either of the following condition exist: acknowledgement to last segment has been received buffered data fills half the window or a maximum segment TCP Transmission Policysilly window syndrome Clark’s solution The receiver should not send a window update until it can handle the MSS or its buffer is half empty, whichever is smaller TCP Transmission Policy A receiver avoids advertising a small window, A sender uses an adaptive scheme to delay transmission so it can clump data into large segments TCP Congestion Control The reason for packet timeout Noise on a transmission line Packet discard at a congested router TCP congestion control algorithm Assumption: all timeouts are caused by congestion Two flow factor receiver capacity: receiver window network capacity: congestion window TCP Congestion Controlnetwork capacity and receiver capacity (a) A fast network feeding a low capacity receiver. (b) A slow network feeding a high-capacity receiver. TCP的流量控制 TCP Congestion ControlSlow Start Algorithm An example of the Internet congestion algorithm. SS Examples Consider the effect of using slow start on a line with a 10-msec round-trip time and no congestion. The receive window is 24 KB and the maximum segment size is 2 KB. How long does it take before the first full window can be sent?