Need help on how to send large UDP data size

[Archer_03]

# Network Environment

Device A (connected wirelessly) <-----> Wireless Access Point <-----> L3 Switch <-----> Device B (connected by wire)

• The L3 switch and Wireless Access Point are connected by wired.
• The Wireless Access Point is PoE.
• The model number of the Wireless Access Point is: C9130AXI-Q
• The model number of the L3 Switch is: C9200-24PXG-E (w/ Essential License)
• Device A: Linux Ubuntu OS PC
• Device B: Windows 10 OS PC

# Issue

• When sending a UDP packet that has a data size of more than 36000 bytes from Device A to Device B, Device B will not be able to receive it.
• But when sending UDP packets from Device B to Device A, even if the data size exceeds 36000 bytes, Device A can receive it without problems.

# Additional information

• When sending UDP 50K packet from Linux Device, I have seen it being fragmented when I was analyzing the packets that are received on the Windows Device.
• However, only 36K out of 50K has arrived at Windows Device side.
• I assume that my wireless access point has been fragmenting the data packet base on its MTU setting. (around 1500)

# Theory
My theory is that either my wireless access point or L3 switch is not sending the data after 36K to the Windows device.
Which I am assuming that there maybe a setting on both my wireless access point or L3 switch that is stopping my data to be sent.

# Questions
Please kindly tell/show me how to change the settings of the access point or L3 switch so that I can send UDP packets that has a size of more than 36000 bytes from Device A to Device B.

[Georg Pauwen]

Hello,

what is/are device(s) A and B ?

[Joseph W. Doherty]

Without knowing specific details on all the devices involved, including their configurations, cannot say why what you described is happening.

That noted, I suspect, if your sending 36K sized packets, likely your media's MTU doesn't support that size, and if not, to be transmitted, packet would need to be fragmented.  Further, also assuming much if not all the media is Ethernet, then the question arises whether some of the media is using non-standard "jumbo" Ethernet, and if some is not, or it doesn't support the jumbo size being used in one direction.

BTW, also assuming media does not support a MTU of 36K, although with proper fragmentation, .you should be able to send a packet that large (also BTW, sometimes on ping tests from Cisco devices, I've sent 64K sized packets, successfully), fragmentation, alone, can create its own set of issues and so is best avoid, if possible (which is why, almost always, sending devices don't generate packets larger than the sending device's MTU).

[Archer_03]

Thank you for your kind reply @Joseph W. Doherty, @Georg Pauwen .
Below is the additional information about the issue and the devices.

# Devices
- Device A: Linux Ubuntu OS PC
- Device B: Windows 10 OS PC

# Additional information
- When sending UDP 50K packet from Linux Device, I have seen it being fragmented by analyzing the packets that are received on the Windows Device.
- However, only 36K out of 50K has arrived at Windows Device side.
- I assume that my wireless access point has been fragmenting the data packet base on its MTU setting. (around 1500)

# Theory
My theory is that either my wireless access point or L3 switch is not sending the data after 36K to the Windows device.
Which I am assuming that there maybe a setting on both my wireless access point or L3 switch that is stopping my data to be sent.

[Joseph W. Doherty]

BTW, packet fragmentation is only done across L3 boundaries.  Generally, wireless APs are L2 only (although they may host/connect to multiple L2 domains).

[NetAlex2349]

Is the interface which is connected to the Wireless AP on the Multi-Layer Switch configured as (Full duplex)? I think this is an issue with the switch configuration more specificly the duplex settings. It sounds like your switch is only sending the information one way and this may be why your not recieving but can still send from Device B. If its not the Duplex Configs then try connecting the L3 Switch and the AP using a different cable like a cat7 or plug your existuing cable into a gigabit interface on the layer 3 switch. The medium through which you are transmitting may not be suitable for what you are trying to do. This may also be the cause of this issue. The issue might also be that you might need to ajust your MTU size. If it is not big enough the packets will be fragmented and therefor somewhat corrupted once they are reassembled as a datagram in the network stack after being recived. soloution: On the Windows computer open the command prompt and do a few simple ping tests. >ping (website name) -f -l 1500 >ping (website name) -f -l 1400 etc.. etc... S tart with a packet size of 1500 bytes, the MTU magic number is somewhere around there. If you get a message that the packet had to be fragmented to be transmitted then reduce your ping size in small increments and find your mtu + 28 bytes for ip and icmp headers This will find your networks mtu size and you can use this to change it in the cmd. Don't forget to add 28 to account for the IP header and ICMP. >netsh interface ipv4 set "interface name" mtu= store=persistant change the mtu size to one that suits your network. In short its likely an MTU Missmatch

[Archer_03]

Thank you for your reply @NetAlex2349

- Our cable is already cat7
- In addition, we have already tried ping test too.

But the issue still exist.
Always and only 36K bytes out of 50K byte of data can be received on Linux Device OS during our ping test.
But when sending 50K bytes data from Linux Device to Windows Device, the Windows Device will received the 50K bytes without problem.

[Georg Pauwen]

Hello,

not sure if I missed something in this developing thread, but for the sake of testing, can you try this setup between two Windows machines (one on each side) and two Linux machines ?

[Archer_03]

Thank you for the kind reply @Georg Pauwen

There were some questions from the other users about my problem,
so I have added more details about the issue in my original post.
(you can refer to it for more information)

Regarding your comment, we have already tested it between 2 Linux machines.
But the issue still exist.

I.E. Only 36K bytes out of 50K byte of data can be received on Linux machine that is directly connected to my L3 switch.

Even though I tested using 2 Linux machine, in the end, the setup must always be like below.
In which I must find a solution for it.

Windows Machine (connected wirelessly) <-----> Wireless Access Point <-----> L3 Switch <-----> Linux Machine (connected by wire)