Lecture 6

252 eects of conges7on 3 as oered load rises conges7on

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Unformatted text preview: ﬁres, retransmit data as lost Retransmit! 224 17 11/4/13 Timeout Problem •  Timeout should be “just right” –  Too long wastes network capacity –  Too short leads to spurious resends –  But what is “just right”? •  Easy to set on a LAN (Link) –  Short, ﬁxed, predictable RTT •  Hard on the Internet (Transport) –  Wide range, variable RTT 225 Example of RTTs BCNà༎SEAà༎BCN Round Trip Time (ms) 1000 900 800 700 600 500 400 300 200 100 0 0 20 40 60 80 100 120 140 160 180 Seconds 200 226 18 11/4/13 Example of RTTs (2) BCNà༎SEAà༎BCN Round Trip Time (ms) 1000 900 Varia7on due to queuing at routers, changes in network paths, etc. 800 700 600 500 400 300 200 Propaga7on (+transmission) delay ≈ 2D 100 0 0 20 40 60 80 100 120 140 160 180 Seconds 200 227 Example of RTTs (3) Round Trip Time (ms) 1000 Timer too high! 900 800 Need to adapt to the network condi7ons 700 600 500 Timer too low! 400 300 200 100 0 0 20 40 60 80 100 120 140 160 180 Seconds 200 228 19 11/4/13 Adap7ve Timeout •  Keep smoothed es7mates of the RTT (1) and variance in RTT (2) –  Update es7mates with a moving average 1.  SRTTN+1 = 0.9*SRTTN + 0.1*RTTN+1 2.  SvarN+1 = 0.9*SvarN + 0.1*|RTTN+1– SRTTN+1| •  Set 7meout to a mul7ple of es7mates –  To es7mate the upper RTT in prac7ce –  TCP TimeoutN = SRTTN + 4*SvarN 229 Example of Adap7ve Timeout 1000 900 RTT (ms) 800 700 600 SRTT 500 400 300 200 Svar 100 0 0 20 40 60 80 100 120 140 160 180 Seconds...
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This document was uploaded on 04/04/2014.

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