Timestamp option

• For LFNs, measuring RTTs once per window, and disabling it during retransmissions (Karn’s algo) is not good. Window sizes in old n/ws = 8 segments.

• Timestamp option => can place a timestamp value in any segment => can calculate RTT for each received ACK.

• Receiver simply echoes timestamp value in ack

• If acks are delayed, the timestamp value returned corresponds to the earliest segment being acked. Correct because “RTT” includes delay in acking too.

• Segments lost/retransmitted => RTT overestimated

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