Why am I getting TCP Previous Segment not captured?

asked 2018-08-07 16:14:17 +0000

martinjplajp
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I'm working with a new embedded device and having performance issues.

I'm suspecting the devices tcp/ip stack, but need to understand the capture better.

I'm hoping someone here can give me some insight...

I see the "TCP Previous segment not captured" and after that, a retransmission occurs, causing a 1 to 2 second delay. The device needs to stream and I'm trying to understand what's going on.

Any help / insight would be greatly appreciated.

J.

https://www.cloudshark.org/captures/0...

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answered 2018-08-08 11:02:14 +0000

Packet_vlad

1055 ●5 ●75 ●32 http://packettrain.net

updated 2018-08-08 11:05:15 +0000

We see "TCP Previous segment not captured" message when Wireshark observes a packet that has TCP SEQ number bigger than we expected in this TCP stream.

For example look at packet No. 134.

It has SEQ of 11701, but we expected SEQ of 10241 (previous packet in this stream had SEQ 10165, and we add segment lenght of 76, so we wanted to see 10165+76). So exactly 1460 Bytes are missing (one-MSS TCP packet).

In your case the packet was actually lost somewhere because we observe subsequent Dup ACKs and Retransmission (receiver and sender reacting to packet loss).

To have an ability to dig deeper we need to know some additional details about network environment you're capturing in and where is the capture point located. And also please capture TCP 3-way handshake if possible (beginning of the connection).

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Thank you for your response.

The setup for this device is a dedicated 100 mb ethernet direct link to the device from the PC via a usb-ethernet adapter, so no traffic from any other device should be present. Capture point is on the PC communicating with the device. (192.168.2.2)

Here is a capture from the power on of the device, thru a transfer that exhibits the issue.

https://www.cloudshark.org/captures/2...

Thanks again :)

martinjplajp ( 2018-08-08 12:31:15 +0000 )edit

The link to Pcap is corrupted this time..

Packet_vlad ( 2018-08-09 09:58:16 +0000 )edit

Sorry about that. Try this one. https://www.cloudshark.org/captures/2...

martinjplajp ( 2018-08-09 12:25:29 +0000 )edit

So your device is 192.168.2.105? The one periodically sending small bursts of data?

There's one confusing part in the trace - pair of packets 271+272, 316+317 etc. We have the same source IP and dest IP, BUT mac addresses are rotating. How is that?

Packet_vlad ( 2018-08-10 13:04:40 +0000 )edit

I see what you are saying. Does that have anything to do with the 2nd packet being a retransmission? Is there an easy wait to get the mac address of my PC ethernet adapter to verify? The embedded device is relatively new and may have issues in the Stack, so nothing is off the table here.

martinjplajp ( 2018-08-10 21:08:09 +0000 )edit

In Windows you can run ipconfig /all in command line, spot your USB-NIC and it's MAC address in the output.

Packet_vlad ( 2018-08-15 06:40:57 +0000 )edit

Host Windows Side: Physical Address. . . . . . . . . : 00-24-9B-30-2F-89 Embedded Device: 12:34:56:78:9a:bc

martinjplajp ( 2018-08-15 12:33:58 +0000 )edit

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answered 2018-08-16 13:33:13 +0000

martinjplajp
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Was related to incorrect stack configuration on the embedded device.

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Hello,

Could you please elaborate what was the lwip stack configuration issue?

Facing something similar.

Thanks

Vivan5 ( 2022-07-02 14:28:18 +0000 )edit

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