Timer Backoff/Karn’s Algorithm

• Timer backoff: If timeout, RTO = 2*RTO {exponential backoff}
• Retransmission ambiguity problem:
  • During retransmission, it is unclear whether an ack refers to a packet or its retransmission. Problem for RTT estimation
  • Karn/Partridge: don’t update RTT estimators during retransmission.
    • Restart RTO only after an ack received for a segment that is not retransmitted

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