My question is why a TCP flow make a re-transmission when a network has enough link bandwidth.
For finding a cause, I used a wireshark. I got the below captured at a host side (10.0.0.1)
25434 50.968633785 10.0.0.1 10.0.0.2 TCP 33370 42848 → 50004 [ACK] Seq=14063654 Ack=1 Win=29696 Len=33304 TSval=290016 TSecr=290011
25435 50.968637512 10.0.0.1 10.0.0.2 TCP 31274 42848 → 50004 [PSH, ACK] Seq=14096958 Ack=1 Win=29696 Len=31208 TSval=290016 TSecr=290011
25492 50.977183364 10.0.0.1 10.0.0.2 TCP 866 [TCP Retransmission] 42848 → 50004 [PSH, ACK] Seq=14127366 Ack=1 Win=29696 Len=800 TSval=290019 TSecr=290011
25513 50.979382486 10.0.0.2 10.0.0.1 TCP 66 50004 → 42848 [ACK] Seq=1 Ack=14096958 Win=16776192 Len=0 TSval=290018 TSecr=290016
25514 50.979383087 10.0.0.2 10.0.0.1 TCP 66 50004 → 42848 [ACK] Seq=1 Ack=14128166 Win=16776192 Len=0 TSval=290018 TSecr=290016
25543 50.981453868 10.0.0.2 10.0.0.1 TCP 78 [TCP Dup ACK 25514#1] 50004 → 42848 [ACK] Seq=1 Ack=14128166 Win=16776192 Len=0 TSval=290019 TSecr=290019 SLE=14127366 SRE=14128166
In summary,
(1) [.1 -> .2 ] SEQ = 14063654, ACK = 1, LEN = 33304
(2) [.1 -> .2 ] SEQ = 14096958, ACK = 1, LEN = 31208
(3) [.1 -> .2 ][Retr] SEQ = 14127366, ACK = 1, LEN = 800
(4) [.2 -> .1 ] SEQ = 1, ACK = 14096958, LEN = 0
(5) [.2 -> .1 ] SEQ = 1, ACK = 14128166, LEN = 0
(6) [.2 -> .1 ] SEQ = 1, ACK = 14128166, LEN = 0, SLE =14127366, SRE=14128166
I totally do not know why the re-transmission occur at (3)
I guess the SACK (6) occur due to (3).
As you see, (4), (5) is the ack due to (1), (2) respectively.
I think the re-transmission (3) is not needed.
Could you give me a little hint about the reason of the re-transmission?
Thank you so much and sorry for re-uploading.
