TCP latency modeling 1 TCP latency modeling Q

TCP latency modeling 1

TCP latency modeling Q: How long does it take to Notation, assumptions: receive an object from a r Assume one link between Web server after sending client and server of rate R a request? r Assume: fixed congestion r TCP connection establishment window, W segments r data transfer delay r S: MSS (bits) r O: object size (bits) r no retransmissions m no loss, no corruption 2

TCP latency modeling Optimal Setting: Time = O/R Two cases to consider: r WS/R > RTT + S/R: m ACK for first segment in window returns before window’s worth of data sent r WS/R < RTT + S/R: m wait for ACK after sending window’s worth of data sent 3

TCP latency Modeling Case 1: latency = 2 RTT + O/R K: = O/WS Case 2: latency = 2 RTT + O/R + (K-1)[S/R + RTT - WS/R] 4

TCP Latency Modeling: Slow Start r Now suppose window grows according to slow start. r Will show that the latency of one object of size O is: where P is the number of times TCP stalls at server: - where Q is the number of times the server would stall if the object were of infinite size. - and K is the number of windows that cover the object. 5

TCP Latency Modeling: Slow Start (cont. ) Example: O/S = 15 segments K = 4 windows Q=2 P = min{K-1, Q} = 2 Server stalls P=2 times. 6

TCP Latency Modeling: Slow Start (cont. ) 7

Flow Control 8

TCP Flow Control flow control sender won’t overrun receiver’s buffers by transmitting too much, too fast Rcv. Buffer = size or TCP Receive Buffer Rcv. Window = amount of spare room in Buffer receiver: explicitly informs sender of (dynamically changing) amount of free buffer space m Rcv. Window field in TCP segment sender: keeps the amount of transmitted, un. ACKed data less than most recently received Rcv. Window receiver buffering 9

TCP: setting timeouts 10

TCP Round Trip Time and Timeout Q: how to set TCP timeout value? r longer than RTT note: RTT will vary r too short: premature timeout m unnecessary retransmissions r too long: slow reaction to segment loss m Q: how to estimate RTT? r Sample. RTT: measured time from segment transmission until ACK receipt m ignore retransmissions, cumulatively ACKed segments r Sample. RTT will vary, want estimated RTT “smoother” m use several recent measurements, not just current Sample. RTT 11

TCP Round Trip Time and Timeout Estimated. RTT = (1 -x)*Estimated. RTT + x*Sample. RTT r Exponential weighted moving average r influence of given sample decreases exponentially fast r typical value of x: 0. 1 Setting the timeout r Estimted. RTT plus “safety margin” r large variation in Estimated. RTT -> larger safety margin Timeout = Estimated. RTT + 4*Deviation = (1 -x)*Deviation + x*|Sample. RTT-Estimated. RTT| 12