TCP Round Trip Time and Timeout

Q: how to set TCP timeout value?

• longer than RTT

  • note: RTT will vary

• too short: premature timeout

  • unnecessary retransmissions

• too long: slow reaction to segment loss

Q: how to estimate RTT?

• SampleRTT: measured time from segment transmission until ACK receipt

  • ignore retransmissions, cumulatively ACKed segments

• SampleRTT will vary, want estimated RTT “smoother”

  • use several recent measurements, not just current SampleRTT

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