西安交通大学计算机网络与通讯课件第三章 Transport Layer_B.pptVIP

TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 full duplex data: bi-directional data flow in same connection MSS: maximum segment size connection-oriented: handshaking (exchange of control msgs) init’s sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver point-to-point: one sender, one receiver reliable, in-order byte steam: no “message boundaries” pipelined: TCP congestion and flow control set window size send receive buffers TCP segment structure 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 TCP: reliable data transfer TCP: reliable data transfer TCP ACK generation [RFC 1122, RFC 2581] TCP: retransmission scenarios TCP Flow Control receiver: explicitly informs sender of (dynamically changing) amount of free buffer space RcvWindow field in TCP segment sender: keeps the amount of transmitted, unACKed data less than most recently received RcvWindow TCP Round Trip Time and Timeout Q: how to set TCP timeout value? longer than RTT note: RTT will vary 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, cumulatively ACKed segments SampleRTT will vary, want estimated RTT “smoother” use several recent measurements, not just current SampleRTT TCP Round Trip Time and Timeout Setting the timeout EstimtedRTT plus “safety margin” large variation in EstimatedRTT - larger safety margin TCP Connection Management Recall: TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow) client: connection initiator Socket clientSocket = new Socket(hostname,port number); server: contacted by