Fragmented Packets and packet size

  • 1
  • Problem
  • Updated 2 years ago
  • Not a Problem
  • (Edited)

While trying to fault find an issue with the waterfall and spectrum not displaying on a remote smartlink connection.....

I observed while monitoring the local network traffic on my LAN that the Flex was sending ipv4 packets of 1514 bytes. As my PC has an MTU of 1500 bytes fragmentation and reassembly has to occur.

My router also has a fixed MTU of 1500 bytes on the outbound interface so all packets sent over the internet are being fragmented.

When trying to connect remotely, wireshark shows reassembly errors. "netstat-s" shows a huge number of packets requiring reassembly and zero successfully reassembled.

I understand from conversation with a core network engineer at an ISP that any packet size greater than 1500 bytes is not best practice and can result in issues.

Is this normal to have an MTU on the flex packets greater than 1500 and can it be reduced.

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Andy M5ZAP

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Posted 2 years ago

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Tim - W4TME, Customer Experience Manager

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Packets sent from the radio are not greater than the max for an Ethernet frame (1518 bytes).  Devices will list an MTU of 1500 when it really is 1518 (14 bytes of Ethernet header and 4 bytes of CRC, leaving 1500 bytes of payload).

The VITA-49 data sent for display data is greater than the 1500 byte payload of an Ethernet packet so the TCP/IP stack on the radio is fragmenting the data and the TCP/IP stack on the client is reassembling it.  This is a standard and customary way of handling large data streams over a packet network.

If you are getting the errors you are reporting, the problem is not with the radio, but most likely with your router not allow fragmented packets to pass.  This can be a global setting or implemented as a firewall rule.  Since the maximum sized fragmented packets are not allowed to pass and the ones that are smaller do, Wireshark reports this as reassembly errors.

We have had at least one customer that has this issue and it was resolved when you configured their firewall/router to allow fragmented packets to pass.
(Edited)
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Tim - W4TME, Customer Experience Manager

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Yes.
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Eric - KE5DTO, Official Rep

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Note that the size of the display packets is directly related to the width of the display.  If you're right on the edge of a boundary, you can make the display more narrow to test that boundary.
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Andy M5ZAP

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Thanks Eric I will test that and report back. Any guide as to what width should result in a vita packet size that won't be fragmented ( I assume < 1500)
For my own interest how big can the vita packets be before the necessary fragmentation?
(Edited)
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Tim - W4TME, Customer Experience Manager

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For my own interest how big can the vita packets be before the necessary fragmentation? 

When the data payload is > 1500 bytes
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KB4AAA

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I have spectrum internet as far as I can tell they are the cause of no display of waterfall and spectrum display while using remote smartlink. I have kinda of a workaround to at least display the spectrum so it will be usable without using VPN. If you put Smartsdr in a window and shrink the width down (height dosen't seem to matter)  and open the radio dock on the right side. The spectrum display will start working.
It even works with all 4 slices open

You can even undock a slice and it will work unless you make it to wide

Hopefully this may help others to work remotely until there is some kinda fix for the fragmentation.