1 Transport Layer Transport Layer RTT Estimation RTT Estimation - PDF document

Transport Layer Transport Layer Outline Mobile network  Transport-layer  Connection-oriented Global ISP services transport: TCP Transport Layer  Multiplexing and  segment structure Home network demultiplexing  reliable data transfer Regional ISP  flow control  Connectionless  connection management Yanmin Zhu transport: UDP  Principles of congestion Institutional network  Principles of reliable Department of Computer control data transfer Science and Engineering  TCP congestion control CSE Department CSE Department 1 2 Transport Layer Transport Layer TCP segment structure TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 32 bits  point-to-point:  full duplex data: URG: urgent data counting source port # dest port # (generally not used) by bytes  one sender, one receiver  bi-directional data flow sequence number of data ACK: ACK # in same connection  reliable, in-order byte (not segments!) acknowledgement number valid  MSS: maximum segment steam: head not U A P R S Receive window PSH: push data now F size len used  no “message boundaries” # bytes (generally not used) checksum Urg data pnter  connection-oriented: rcvr willing  pipelined: to accept  handshaking (exchange RST, SYN, FIN: Options (variable length)  TCP congestion and flow connection estab of control msgs) init’s control set window size (setup, teardown sender, receiver state commands)  send & receive buffers before data exchange application data  flow controlled: Internet (variable length) checksum application application  sender will not writes data reads data (as in UDP) socket socket overwhelm receiver door door TCP TCP send buffer receive buffer segment CSE Department CSE Department 3 4 Transport Layer Transport Layer TCP Round Trip Time and Timeout TCP seq. #’s and ACKs simple telnet scenario Seq. #’s:  byte stream Host A Host B S D Q: how to set TCP timeout “number” of first User value? byte in segment’s types data  longer than RTT host ACKs ‘C’ ACKs:  but RTT varies receipt of RTT1  seq # of next byte ‘C’, echoes expected from  too short : premature timeout other side back ‘C’  unnecessary  cumulative ACK host ACKs retransmissions RTT2 receipt Q: how receiver handles  too long : slow reaction to out-of-order segments of echoed segment loss  A: TCP spec doesn’t ‘C’ say, - up to time implementor CSE Department CSE Department 5 6 1

Transport Layer Transport Layer RTT Estimation RTT Estimation – Basic Idea  SampleRTT : measured time from segment EstimatedRTT transmission until ACK receipt = (1-  )*EstimatedRTT +  *SampleRTT  ignore retransmissions  SampleRTT will vary, want estimated RTT  Exponential weighted moving average “smoother”  influence of past sample decreases  average several recent measurements, not just exponentially fast current SampleRTT  typical value:  = 0.125 CSE Department CSE Department 7 8 Transport Layer Transport Layer Setting the timeout Example RTT estimation: RTT: gaia.cs.umass.edu to fantasia.eurecom.fr  EstimtedRTT plus “ safety margin ” 350  large variation in EstimatedRTT -> larger safety margin  first estimate of how much SampleRTT deviates from EstimatedRTT: 300 250 DevRTT = (1-  )*DevRTT + RTT (milliseconds)  *|SampleRTT-EstimatedRTT| 200 (typically,  = 0.25) 150 Then set timeout interval: 100 TimeoutInterval = EstimatedRTT + 4*DevRTT 1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 time (seconnds) SampleRTT Estimated RTT CSE Department CSE Department 9 10 TCP Transport Layer Transport Layer Outline TCP reliable data transfer  Retransmissions are  TCP creates rdt  Transport-layer  Connection-oriented triggered by: service on top of IP’s services transport: TCP unreliable service  timeout events  Multiplexing and  segment structure  duplicate acks  Pipelined segments demultiplexing  reliable data transfer  flow control  Cumulative acks  Connectionless  connection management  Initially consider transport: UDP  TCP uses single  Principles of congestion simplified TCP sender retransmission timer  Principles of reliable control  ignore duplicate acks data transfer  TCP congestion control  ignore flow control, congestion control CSE Department CSE Department 11 12 2

Transport Layer Transport Layer TCP sender events: Illustration data rcvd from app: timeout:  Create segment with  retransmit segment seq # that caused timeout Segment Segment  seq # is byte-stream  restart timer number of first data Ack rcvd: byte in segment  If acknowledges  start timer if not previously unacked already running (think segments of timer as for oldest  update what is known to Seq # unacked segment) be acked  expiration interval:  start timer if there are outstanding segments SendBase NextSeqNumber TimeOutInterval CSE Department CSE Department 13 14 Transport Layer Transport Layer 3 Events NextSeqNum = InitialSeqNum TCP （ 3 ） ACK SendBase = InitialSeqNum loop (forever) { sender switch(event) （ 1 ） Called above ACK(y) (simplified) event: data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) start timer pass segment to IP Comment: NextSeqNum = NextSeqNum + length(data) • SendBase-1: last cumulatively event: timer timeout retransmit not-yet-acknowledged segment with ack’ed byte smallest sequence number Example: start timer • SendBase-1 = 71; y= 73, so the rcvr event: ACK received, with ACK field value of y Seq # if (y > SendBase) { wants 73+ ; SendBase = y y > SendBase, so if (there are currently not-yet-acknowledged segments) that new data is start timer SendBase NextSeqNumber } acked } /* end of loop forever */ （ 2 ） Timeout CSE Department CSE Department 15 16 Transport Layer Transport Layer TCP: retransmission scenarios TCP: retransmission scenarios Host A Host B Host A Host B Seq=92 timeout timeout X loss Sendbase = 100 Seq=92 timeout SendBase = 120 SendBase SendBase = 100 = 120 time time premature timeout lost ACK scenario CSE Department CSE Department 17 18 3

Transport Layer Transport Layer TCP retransmission scenarios (more) Fast Retransmit Idea: duplicate ACKs Host A Host B Time-out period often relatively long: long delay before resending lost packet timeout  If sender receives 3 X  Detect lost segments loss ACKs for the same via duplicate ACKs. data, it supposes that  Sender often sends SendBase segment after ACKed many segments back-to- = 120 back data was lost:  If segment is lost,  fast retransmit: resend there will likely be many segment before timer time duplicate ACKs. expires Cumulative ACK scenario CSE Department CSE Department 19 20 Transport Layer Transport Layer Fast retransmit algorithm: Host A Host B event: ACK received, with ACK field value of y X if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } timeout else { increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) { resend segment with sequence number y } a duplicate ACK for fast retransmit time already ACKed segment Resending a segment after triple duplicate ACK CSE Department CSE Department 21 22 Transport Layer Transport Layer Outline TCP Flow Control flow control sender won’t overflow  Transport-layer  Connection-oriented  receive side of TCP receiver’s buffer by services transport: TCP connection has a transmitting too much, receive buffer:  Multiplexing and  segment structure too fast demultiplexing  reliable data transfer  flow control  Connectionless  speed-matching  connection management transport: UDP service: matching the  Principles of congestion send rate to the  Principles of reliable control data transfer receiving app’s drain  TCP congestion control rate  Potential problem: app process may be slow at reading from buffer CSE Department CSE Department 23 24 4