The duplicate ACKs should not cause any retransmits (unless the sender
is badly broken), because they contain a high enough ACK number.
The problem is really that a TCP sender cannot recover from a too short RTT
estimate; if the RTT is longer and it doesn't get any acks it'll assume
packet loss and never has a chance to find out about the longer RTT, because
that only works with new ACKs.
The standard initial RTT is 3 seconds; but your RTT is 5.2s.
What you can do is to change the initial RTT on the sender side. On Linux
it can be done with the "irtt" option of route or the equivalent one of
iproute2. Most other OS have a similar way to configure the IRTT.
-Andi
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