《计算机网络——自顶向下方法及Internet特色》幻灯片Lecture33.ppt

Chapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control 3.5 TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 full duplex data: bi-directional data flow in same connection MSS: maximum segment size (基于链路层的MTU) connection-oriented: Three way handshaking (exchange of control msgs) init’s sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver point-to-point (end system): one sender, one receiver reliable, in-order byte steam: no “message boundaries” pipelined: TCP congestion and flow control set window size send receive buffers 3.5.1 TCP segment structure 1. TCP seq. #’s and ACKs Seq. #’s: byte stream “number” of first byte in segment’s data ACKs: seq # of next byte expected from other side cumulative ACK Q: how receiver handles out-of-order segments A: TCP spec doesn’t say, - up to implementor Two ways 2. TCP Round Trip Time and Timeout Q: how to set TCP timeout value? longer than RTT but RTT varies too short: premature timeout unnecessary retransmissions too long: slow reaction to segment loss Q: how to estimate RTT? SampleRTT: measured time from segment transmission until ACK receipt ignore retransmissions SampleRTT will vary, want estimated RTT “smoother” average several recent measurements, not just current SampleRTT 2. TCP Round Trip Time and Timeout Example RTT estimation: 2. TCP Round Trip Time and Timeout Setting the timeout EstimtedRTT plus “safety margin” large variation in EstimatedRTT - larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT: 3.5.2 TCP reliable data transfer TCP creates rdt service on top of IP’s unreliable service Pipelined segments Cumulative acks TCP uses single retransmission timer Retrans