SampleRTT measured time from segment transmission until ACK receipt ignore

Samplertt measured time from segment transmission

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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
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Transport Layer 3-62 100 150 200 250 300 350 1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 EstimatedRTT = (1- α )*EstimatedRTT + α *SampleRTT exponential weighted moving average influence of past sample decreases exponentially fast typical value: α = 0.125 TCP round trip time, timeout RTT (milliseconds) RTT: gaia.cs.umass.edu to fantasia.eurecom.fr sampleRTT EstimatedRTT time (seconds)
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Transport Layer 3-63 timeout interval: EstimatedRTT plus safety margin large variation in EstimatedRTT -> larger safety margin estimate SampleRTT deviation from EstimatedRTT: DevRTT = (1- β )*DevRTT + β *|SampleRTT-EstimatedRTT| TCP round trip time, timeout (typically, β = 0.25) TimeoutInterval = EstimatedRTT + 4*DevRTT estimated RTT safety margin
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Transport Layer 3-64 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
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Transport Layer 3-65 TCP reliable data transfer TCP creates rdt service on top of IP s unreliable service pipelined segments cumulative acks single retransmission timer retransmissions triggered by: timeout events duplicate acks let s initially consider simplified TCP sender: ignore duplicate acks ignore flow control, congestion control
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Transport Layer 3-66 TCP sender events: data rcvd from app: create segment with seq # seq # is byte-stream number of first data byte in segment start timer if not already running think of timer as for oldest unacked segment expiration interval: TimeOutInterval timeout: retransmit segment that caused timeout restart timer ack rcvd: if ack acknowledges previously unacked segments update what is known to be ACKed start timer if there are still unacked segments
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Transport Layer 3-67 TCP sender (simplified) wait for event NextSeqNum = InitialSeqNum SendBase = InitialSeqNum Λ create segment, seq. #: NextSeqNum pass segment to IP (i.e., send ) NextSeqNum = NextSeqNum + length(data) if (timer currently not running) start timer data received from application above retransmit not-yet-acked segment with smallest seq. # start timer timeout if (y > SendBase) { SendBase = y /* SendBase–1: last cumulatively ACKed byte */ if (there are currently not-yet-acked segments) start timer else stop timer } ACK received, with ACK field value y
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Transport Layer 3-68 TCP: retransmission scenarios lost ACK scenario Host B Host A Seq=92, 8 bytes of data ACK=100 Seq=92, 8 bytes of data X timeout ACK=100 premature timeout Host B Host A Seq=92, 8 bytes of data ACK=100 Seq=92, 8 bytes of data timeout ACK=120 Seq=100, 20 bytes of data ACK=120 SendBase=100 SendBase=120 SendBase=120 SendBase=92
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Transport Layer 3-69 TCP: retransmission scenarios X cumulative ACK Host B Host A Seq=92, 8 bytes of data ACK=100 Seq=120, 15 bytes of data timeout Seq=100, 20 bytes of data ACK=120
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