Timeout Estimation

• SampleRTT from segment transition until ACK receipt
• Average several recent measurements
• Estimated = $(1-\alpha)e + \alpha s$ where $\alpha$ is typically $\dfrac{1}{8}\approx 10\%$
• Set timeout as EstimatedRTT + 4*std devRTT
• DevRTT = $(1-\beta)d + \beta*|s-e|$ where $\beta$ is typically $\dfrac{1}{4}$
• On RTO expiry
  • Retransmit
  • Double value
  • Start timer
  • Once new RTT, update using formula

3-way Handshake

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Flow Control

• TCP receiver advertises free buffer rwnd
• Sender limits amount of unACKed data
• Guarantees buffer won’t overflow

Congestion Control

• Assumption: all lost TCP segments are due to internet congestion
• Detecting loss
  • Timeout event
  • Fast retransmit
• Additive increase
• Multiplicative decrease (50%)
• Golden rule (AIMD)
• Motivated by TCP fairness—approaches fairness line