TCP Timeout and Retransmission

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TCP Timeout and Retransmission
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RTT Measurement

• Err M A
• A ? A g ? Err
• D ? D h ? (Err - D)
• RTO ? A 4D
• M Measured sample, A Running average
• g0.125, h 0.25

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Karns Algorithm

• For retransmitted packets, RTT estimates are not
  updated

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RTT Measurements

• There is at most one ongoing measurement at any
  time

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RTT and clock ticks

• RTTs are measured in ticks, not in seconds
• Tick happen at 500ms intervals

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Measured RTT and RTO Estimate
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RTT Calculations

• RTO of very first packet (SYN)
• Initial values A 0, D 3
• RTO A 2D 0 2 X 3 6 seconds
• Exponential backoff upon timeout
• Example
• Err M A 0.5 2 -1.5
• A A g x Err 2 0.125 x 1.5 1.8125
• D D h (Err - D) 10.25 x (1.5-1) 1.125
• RTO A 4D 1.8125 4 x 1.125 6.3125
• In reality, all calculations are done in integers
  resulting in a value of 6 seconds in this example

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Retransmissions
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Slow Start(cwnd lt ssthresh)

• Increase window by 1 MSS for each new ACK
  received
• It is expected that in each RTT window would
  double
• The start is slow, but growth of window is very
  fast in this phase
• Often a short duration phase
• Most of the lifetime of a connection is in the
  congestion avoidance phase (typically)

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Adjusting TCP parameters

• Initialize Cwnd 1, ssthresh 65535
• TCP sender constrained by Min(cwnd, recvwin)
• Upon congestion (timeout/3 duplicate ACKs)
• ssthresh ? round_MSS(max(2MSS,0.5 x
  min(cwnd,recvwin))
• Upon receiving a new ACK in Congestion Avoidance
  phase (cwnd gt ssthresh)
• cwnd ? cwnd segsize x segsize/cwnd segsize/8
• Upon timeout
• cwnd ? 1 MSS

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Slow Start and Congestion Avoidance ( A Coarse
Visualization)
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Fast Retransmit and Fast Recovery

• Upon 3rd DUPACK
• ssthresh ? round_MSS(max(2MSS,0.5 x
  min(cwnd,recvwin))
• cwnd ? ssthresh 3 MSS
• Retransmit missing segment
• For additional DUPACK
• Increment cwnd by MSS
• Transmit packet if allowed by current window
• When new ack arrives
• set cwnd to ssthresh (the value in the first
  step) 1 MSS

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Duplicate ACKs

• 45 is retransmitted as 63
• Data is buffered till the in-sequence contiguous
  chunk becomes available

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Cwnd and sequence number An Example
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Slow-start and Congestion Avoidance Example
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Duplicate ACK Example
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ICMP Host Unreachable

• TCP does not react to it
• It retries with exponential increase in RTO
• RTO increases to a max of 64 sec

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Repacketization(Example sock sending data to
discard port)