FTP Failing and generic error

Hi, FTPing of results to our server generally works. However, there is one location that is regularly used and FTP fails partway through upload with a missing “ack” from the server. The only difference is that at this site the ISP’s router puts various headers in the packets for routing which means that the effective MTU size is reduced from 1500 to 1432. Has anyone had a similar issue and if so what work arounds have they discovered. The PCs/Laptops are running either latest updates of Windows 10 or 11.

As a starter for 10 just try setting the Passive FTP setting usually fixes it for me.


Hi Michael,
From your post you have obviously done to quite deep level tracking find the issue.

Quick question - does the same thing happen if you use an FTP client like Filezilla to upload the file?
Kind regards,

Hi - Could you confirm which version of Sailwave you are using
V2.29.0 onwards uses a completely different FTP under the hood as they say


Hi, Yes it is the latest version and I get the same issue with FTP Explorer and using native Windows FTP via Windows explorer.

I will try passive mode when I am next down at the club (about 72 hrs).

All, Thanks for the couple of pointers. I do now have it working. It was a combination of issues some with the ISP and some on the internal network plus a missing patch!. I have found that WiFi Access point contention can be an issue so hardwiring your results computer could be a neat solution if that could be an issue for any one. Also setting up QoS so that the traffic gets the appropriate bandwidth at least as far as your first router/modem. Finally with Windows machines there is a registry key that can be set that means that the PC will check the max size packet that can traverse the route to the web server. Using this will mean that the routers will not be fragmenting the packets meaning that any issues with fragmented packets are mitigated. The article is here: Isolated Domain (Windows) - Windows security | Microsoft Docs It talks about isolated domains but the key point is the EnablePMTU registry key. Hope this helps others.

Thank you for posting what you have found out. That is some detective work and very well done on tracing issue through.
It is useful information going forward to have in the information pool.

Kind regards,