We have a scheduled task that transfers a large file (~130 MB) between two Windows servers using the Windows command-line FTP program. It's painfully slow (taking about 30 minutes) and occasionally terminates before the transfer has completed. The servers are 2003 (sending) and 2008 (receiving).
If I transfer the file manually, using FileZilla's GUI, it runs considerably faster and is done in a couple of minutes. I can't use this approach though, because we want this to run automatically and FileZilla can't be scripted.
I've also noticed, when I disable the Windows 2003 Firewall, command line FTP is suddenly much faster, and reaches the same speed as FileZilla.
So, it seems I have a workaround in disabling/enabling the firewall before and after the transfer. But after Googling extensively, this doesn't seem to be a common problem, and I get the feeling something is configured wrongly somewhere.
Can anyone shed any light on why disabling the firewall improves the speed so dramatically? And why FileZilla is faster than command-line FTP (when the Firewall is enabled)?
Best Answer
ICMP is an Internet host requirement. Sometimes admins block all ICMP because they "heard somewhere it was a security risk". But ICMP is not optional and TCP relies on it. (At a minimum, 'fragmentation required' must be permitted.)