problems completing large file transfers | 5.9.x

Using 5.9.3 or 5.9.4 (simply haven't upgraded everything) -- recently, noticed a fairly frequent problem uploading large-ish (500 Mb -> 1.5 Gb) files to one of my box.com folders. Is use FTP as the protocol, wil TLS/SSL ecplicit encryption, pointed at port 21. This has worked fine in past, and still works without problems *most of the time*. But when it fails (which is frequent enough to prompt me posting ths), I usually get a popup window with something like the following (this is a cut-and-paste of the error message I got on last failure). I've flipped looging on for future, but thought I'd start with this.

Using TLSv1, cipher TLSv1/SSLv3: AES128-SHA, 2048 bit RSA, AES128-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA1

Copying files to remote side failed.
/abc/bigfile.zip: Error on output file.

Doesn't appear that there is some sort of mismatch (which might happen if - say - the ssl/ssh libs on the server side are 'wrong', in some fashion, which has in fact happened before with box.com), but perhaps I'm missing some obvious diagnostic.

I'll post debug level 2 logs next time the error happens.

IIRC, ftp uses two connections. One, a command/control channel. Two, a channel opened for each data transfer. The control channel might be timing out due to inactivity while idle - waiting for the data transfer to complete.

Firewall/NAT problem? Perhaps TCP Keepalive needs to be enabled at your end (not box.com) or the keepalive interval made shorter.

workaround (if timeout not fixable): split the files into smaller pieces.

cooch17 wrote:

I'll post debug level 2 logs next time the error happens.

Also, check server-side logs. It may explain the strange "Error on output file" error.