I study the Behavior TCP and did a small experiment about TCP.
In the experiment, [10.0.0.1] host send a TCP flow to [10.0.0.2] host.
I found that a re-transmission occur. To find the cause, I used Wireshark.
The captured packets is the below. the two capture's reference time is different due to the different start time of Wireshark.
From the captured, I found that the [10.0.0.2] host send an ACK slowly than usual.
It makes [10.0.0.1] host send an re-transmission.
At first, I think it would be due to CPU utilization but it wouldn't when seeing it by htop command.
Is there any reason for [10.0.0.2] send a ACK slowly?
Could you give me a little hint for trouble shooting?
Thank you for reading.
<10.0.0.2 side>
1735203 51.249349738 10.0.0.1 10.0.0.2 TCP 1514 35250 → 50021
[ACK] Seq=16124342 Ack=1 Win=58 Len=1448 TSval=902298 TSecr=902296
1735204 51.249349999 10.0.0.1 10.0.0.2 TCP 1514 35250 → 50021
[ACK] Seq=16125790 Ack=1 Win=58 Len=1448 TSval=902298 TSecr=902296
1735205 51.249350340 10.0.0.1 10.0.0.2 TCP 866 35250 → 50021
[PSH, ACK] Seq=16127238 Ack=1 Win=58 Len=800 TSval=902298 TSecr=902296
1735207 51.258283589 10.0.0.1 10.0.0.2 TCP 866
[TCP Retransmission] 35250 → 50021 [PSH, ACK] Seq=16127238
Ack=1 Win=58 Len=800 TSval=902301 TSecr=902298
1735208 51.258288919 10.0.0.2 10.0.0.1 TCP 66 50021 → 35250
[ACK] Seq=1 Ack=16128038 Win=32766 Len=0 TSval=902301 TSecr=902298
1735209 51.258296563 10.0.0.2 10.0.0.1 TCP 78
[TCP Dup ACK 1735208#1] 50021 → 35250 [ACK] Seq=1 Ack=16128038
Win=32766 Len=0 TSval=902301 TSecr=902301
1736003 51.263706308 10.0.0.1 10.0.0.2 TCP 1514 35250 → 50021
[ACK] Seq=16128038 Ack=1 Win=58 Len=1448 TSval=902302 TSecr=902301
1736004 51.263724813 10.0.0.2 10.0.0.1 TCP 66 50021 → 35250
[ACK] Seq=1 Ack=16129486 Win=32766 Len=0 TSval=902302 TSecr=902302
<10.0.0.1 side>
1735214 58.247766104 10.0.0.1 10.0.0.2 TCP 1514 35250 → 50021
[ACK] Seq=16125790 Ack=1 Win=58 Len=1448 TSval=902298 TSecr=902296
1735215 58.247766625 10.0.0.1 10.0.0.2 TCP 866 35250 → 50021
[PSH, ACK] Seq=16127238 Ack=1 Win=58 Len=800 TSval=902298 TSecr=902296
1735216 58.247807101 10.0.0.2 10.0.0.1 TCP 66 50021 → 35250
[ACK] Seq=1 Ack=16092486 Win=32766 Len=0 TSval=902298 TSecr=902298
1735219 58.257898525 10.0.0.1 10.0.0.2 TCP 866
[TCP Retransmission] 35250 → 50021 [PSH, ACK] Seq=16127238
Ack=1 Win=58 Len=800 TSval=902301 TSecr=902298
1735220 58.257914605 10.0.0.2 10.0.0.1 TCP 66 50021 → 35250
[ACK] Seq=1 Ack=16128038 Win=32766 Len=0 TSval=902301 TSecr=902298
1735221 58.257921378 10.0.0.2 10.0.0.1 TCP 78
[TCP Dup ACK 1735220#1] 50021 → 35250 [ACK] Seq=1
Ack=16128038 Win=32766 Len=0 TSval=902301 TSecr=902301
1735982 58.263316595 10.0.0.1 10.0.0.2 TCP 1514 35250 → 50021
[ACK] Seq=16128038 Ack=1 Win=58 Len=1448 TSval=902302 TSecr=902301
1735983 58.263317587 10.0.0.1 10.0.0.2 TCP 1514 35250 → 50021
[ACK] Seq=16129486 Ack=1 Win=58 Len=1448 TSval=902302 TSecr=902301
