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TCP previous segment not captured and Dup ACK at regular intervals - what is going on?

asked 2019-01-06 18:28:11 +0000

User001100 gravatar image

My Wi-Fi bandwidth regularly drops from 144Mbs to 48Mbps (Windows 10, 802.11n). At the same time, the download speed temporarily goes to zero.

It does not seem to be hardware related - I have tested the USB dongle ( I use on another devices, and it works fine. Reinstalling Windows is also useless.

However, in Wireshark, I found that whenever the bandwidth drops there are "TCP previous segment not captured" and "TCP dup ACK" messages. What is going on?

Connection speed Normal bandwidth Dropped bandwidth Wireshark results when the bandwidth drops

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We would need to see a capture of your wireless connectivity taken by another system with a wireless adapter in monitor mode.

I doubt someone would be able to help you here based on the information you provided.

You may want to move to MS forum

net_tech gravatar imagenet_tech ( 2019-01-06 23:20:35 +0000 )edit

Time span on Wireshark screenshot you've provided is 587.435 - 587.289 = 0.146 seconds. It doesn't correlate to throughput drops on the first screenshot. Please capture and share a trace containing larger time period.

Packet_vlad gravatar imagePacket_vlad ( 2019-01-09 07:35:53 +0000 )edit


Do you have found the reason and a solution ? U I hava the same issue for 6 months. I have changed my AP Hp to a Cisco and finally a Xiaomi and I have issue yet.

Thanks !


Croute gravatar imageCroute ( 2019-11-24 14:46:07 +0000 )edit

Very late answer, but it was a combination of problems: dying Wi-Fi antenna + poor signal. Resetting / auto-reconfiguring the channels of the router + buying a new antenna mostly solved the problems. A clean Windows might have helped too.

User001100 gravatar imageUser001100 ( 2022-06-16 21:30:28 +0000 )edit

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answered 2019-01-15 21:20:23 +0000

SYN-bit gravatar image

Why the WiFi adapter continuously changes its speed is something I can not answer, but the "TCP previous segment not captured" and "TCP dup ACK" messages can be explained.

In your screenshot (please supply a network capture file next time, it makes analyzing so much easier), frame 36036 mentions that a segment of the TCP stream is missing. This is probably caused by the switch to a lower bandwidth in the middle of sending a packet. All the following data packets are triggering a duplicate ACK as that is the way TCP tells the other side there is still some data missing.

Then in frame 36080 the segment that was lost is being retransmitted which stops the duplicate ack's as there are no more gaps in the data.

Nothing wrong in the pcap file, except of course that packets are being dropped due to the switching in WiFi speeds.

To be able to analyze the reason of the WiFi speed changes, you might want to capure in monitor mode on another system to see the WiFi management frames. This will tell you signal strengths, amount of retransmissions at the WiFi layer, etc.

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Asked: 2019-01-06 18:28:11 +0000

Seen: 1,386 times

Last updated: Nov 24 '19