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Extremely high rate of Dup ACK and Retransmission - WiFi interference?

asked 2018-03-28 21:49:10 +0000

riverton gravatar image

updated 2018-03-28 21:51:18 +0000

I’m required to set up WiFi devices that make HTTP GET requests of a local server, but I’m not allowed to make changes to the network. I would like to suggest to the owner that their network has problems, which are preventing the WiFi devices from communicating with the server. I suspect there is a high level of WiFi interference - a quick scan shows that every AP in their building is on 2.4Ghz channel 1.

Is the capture shown in the image below consistent with WiFi interference? This is a single client, which would normally make that HTTP GET request 1 - 3 times a second. On this network it is sometimes the case that the response takes up to a minute, with dozens of Dup ACK and Retransmission packets in between. Wired clients have better (though not perfect) connectivity and do not exhibit this extremely high rate of Dup ACK and Retransmission.

link: Capture Window Image

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answered 2018-03-29 00:10:01 +0000

Bob Jones gravatar image

All APs on the same channel can cause CCI (co channel interference) and is not usually a recommended layout (unless you have a Meru system - they do something like this, or so I have heard).

The single biggest issue with wireless, in my opinion, is packet loss. Your results are consistent with this, and likely latency as well. However, latency often comes from packet loss; loss leads to retransmissions at 802.11 (retry) then further at transport layer (TCP retransmission). That looks like a wired trace of the problem and is informative (could be unencrypted or open wireless as well, but you did not show enough to decide). A wireless trace could show things like retry percentages, maybe channel utilization if available in the beacon, beacon loss rates, etc., all which can strengthen your argument.

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Thank you Bob. I was on-site again over the weekend. I took a Ubiquiti system of ours with me. I asked the client to disable the network our devices were using (not their other networks). I then set up a single AP in the center of the venue and placed it on a clear channel, after which traffic improved dramatically. I made extensive Wireshark captures before and after, then reported my findings to the client. It also turns out they have a number of APs that are in the wrong place, which contributes to the issues. Luckily they have agreed with my estimation of the problem and will take steps to remedy.

Your answer helped make up my mind on how to proceed, it is much appreciated.

riverton gravatar imageriverton ( 2018-04-04 23:28:54 +0000 )edit

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Asked: 2018-03-28 21:49:10 +0000

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Last updated: Mar 29 '18