《计算机网络和 与因特网》课件第五章可靠传输服务.pptVIP

Computer Networks and Internets 《计算机网络与因特网》课件 林坤辉 PART III Internetworking Chapter 5 TCP: Reliable Transport Service T C P：可靠传输服务 5.2 The Need for Reliable Transoprt可靠传输的必要性 The application write data to the device, but does not need to verity that data arrives intact. Instead, the application relies on the underlying computer system to ensure reliable transfer; the system guarantees that data will not be lost, duplicated, or delivered out of order. 5.3 The Transmission Control Protocol传输控制协议 TCP is the transport level protocol. TCP provides reliable transport service. Most internet applications are built to use TCP. TCP provides a completely reliable(no data duplication or loss), connection-oriented, full-duplex stream transport service. 5.4 The Service TCP Provides To ApplicationsTCP为应用提供的服务 Connection orientation Point-To-point communication Complete reliability Full duplex communication Stream interface Reliable connection startup Graceful connection shutdown 5.6 Achieving Reliability实现可靠性 The major problems are: Unreliable delivery by the underlying communication system. Computer reboot. 5.7 Packet Loss and Retransmission包丢失与重发 By implementing a retransmission scheme. IP数据报文丢失的处理 目标端 TCP程序对于每一个收到的IP数据报文，向源端发送一个响应报文作为确认 源端 TCP程序重传丢失的IP报文 重传条件：一定时间内没有收到响应报文 重传时间：根据一定算法确定的超时等待时间 重传时间的确定 确定的重传时间 始终按照时间T作为重传时间 2的指数级确定方式 按照2的指数作为重传时间 根据一段时间内发送-响应周期的数值分布，调整重传时间为2的n次方 按照一定统计算法进行迭代调整 根据一段时间内的发送-响应周期的分布，调整重传时间为一段时间内的统计结果Ts和前一次重传时间Ti的加权值：T(i+1)= aTi+bTs 2的指数级确定方式举例 已知算法： 出现超时，则重传时间加倍 如果最近３个数据包发送-响应时延小于Ti/2(不足3个按照实际个数计算)，则T(i+1) = Ti/2 已知条件 一次只发送一个数据报文，得到响应再发送下个 初始重传时间：T0= 16ms 响应时间序列：12ms, 10ms, 18ms, 33ms, 6ms, 8ms, 10ms 重传时间结果 16ms(T0), 16ms, 16ms, 32ms, 64ms, 64ms, 64ms, 32ms 5.8 Adaptive Retransmission自适应重发 TCP monitors current delay on each connection, and adapts the retransmission timer to accommodate changing conditions. TCP estimates round-trip delay. TCP records the time at whi