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Can someone explain the dup ack at packet no. 13? Thank you in advance Capture dup ack

asked 23 Sep '14, 12:52

becco981's gravatar image

accept rate: 0%

edited 29 Mar '15, 19:07

Guy%20Harris's gravatar image

Guy Harris ♦♦

Packet #06 client sends 445 bytes
Packet #07 server acks 445 bytes (immediate ack, application hasn't read the data yet)
Packet #08 client sends 9 bytes (Nagle disabled at the client)
Packet #09 is the server sending 506 bytes
Packet #10 is the server sending a window_update (the appl now has read 445 bytes from #6)
Packet #11 is the client acking packet #9 (ack# 507) (immediate ack
Packet #12 is the server acking 9 bytes from packet 8, application hasn't read the data yet) Packet #13 is the second ack for packet # 9

So wireshark is right in telling you that frame 13 is a duplicate ack.
Why the (Linux) client sends an immediate ack to an empty ack from the server I don't know. I think the scenario is caused by both TCP stacks sending immediate (not delaying) acknowledgements.

I would say the trace is showing how disabling Nagle and delayed_ACK might cause some unwanted packets to flow over the wire.

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answered 26 Sep '14, 08:50

mrEEde's gravatar image

accept rate: 20%

edited 28 Sep '14, 22:02

Hi mrEEde,do you think that seq no. of packet #10 is right(seq 1),i think it should be 507 and then in packet#12 seq no. comes back to normal 507.

(27 Sep '14, 07:02) kishan pandey

Well spotted, Kishan!

The seq# of packet 10 indicates that it was created earlier by TCP (between packet 7 and 8). All those packets are traced in a very short timeframe of less than 100 µs so when the capture point cuts them the outbound packets are out_of_order already.

(28 Sep '14, 21:49) mrEEde
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question asked: 23 Sep '14, 12:52

question was seen: 1,854 times

last updated: 29 Mar '15, 19:07

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