iperf test shows large number of TCP: duplicate ACK / retransmission & out-of-order

I carried out a network throughput test using iperf and captured packets at both ends. I see a almost 10% packets highlighted in tcp.analysis.flag with following characteristic:

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“TCP duplicate ack” followed by “TCP fast restransmission / TCP restransmission” & “TCP out-of-order” occurs every second with rare occurrence of TCP ack for unseen segment.

Here is a printscreen using filter on the receiver side (filter: tcp.analysis.flags):

https://i.stack.imgur.com/bNSbD.png

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Here is the capture from sender side:

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https://i.stack.imgur.com/spbqg.png

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I have used iperf to send data in TCP mode for a period of 500 seconds using IPv4 only.

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Only capture filter was "host" followed by IP (sender or receiver). I also observed that major errors (duplicate ack/re-transmission) occur every 3.5 seconds. While few set of errors occur every second. Concluding through high number of duplicate ACKs, I feel there is definite packet drop? Am I correct or am I missing anything.

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Here is statistics>tcptrace from Wireshark:

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sender (client) side (client to server view):

https://i.stack.imgur.com/2iYiF.png

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Thank you in advance.