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Modbus TCP [Retransmission], [RST]

asked 2023-05-08 10:12:53 +0000

Ramu gravatar image


I'm new to wireshark and packet capturing and was wondering if someone here could help me understand what i'm looking at how to understand the captured file. Captured file is attached

We have a setup where communication from our software(Client) is disrupted randomly to a PLC over modbus TCP connection. The PLC and Client are on different locations, communicated over internet(cloud) as sketched on the drawing, I have also inserted a trace route if its of any help.

The fault i'm trying to understand start from timeline 09:53:29 as seen from screenshot below.


image:alt text


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answered 2023-05-08 11:00:22 +0000

hugo.vanderkooij gravatar image

The packet capture hints at packet loss. However as you only see the packets at one end of what seems like a multihop connection there is no telling where things went wrong.

Reset with capturing op as many steps including the client and the host and compare them. So if anythnig stcks out as seeing packet going out on client but never arrive at server.

The more steps in between you can measure the better the insight into the cause will be.

But if the network in between is not in your span of control you may end up haveing a tough conversation about network quality issues.

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Hi Hugo, thanks for the answer. I was also leaning towards packet loss because of the tcp retransmission. And as you said the network in between is not in my control, but how can I narrow down as much as possible to where this problem lies. I think I will be to do a packet capture on the other end also, but thats it. Is it a way to find out if this is related to the PLC?

Ramu gravatar imageRamu ( 2023-05-08 14:25:07 +0000 )edit

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Asked: 2023-05-08 10:12:53 +0000

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Last updated: May 08