FTPS Keepalives to Control Port for Large File Transfers

It seems WinSCP only sends keepalive commands to the FTPS control port when idle, not when downloading a file...can someone please confirm?

On SmartFTP and FileZilla, it appears they allow sending keepalive commands to the control port when downloading a file via the data port.

Using WinSCP Version 5.1.7 (Build 3446) with Commander interface on Windows 7 Enterprise (OS 6.1.7601 Service Pack 1).

PROBLEM DESCRIPTION

We transfer a 7GB file via FTPS. We get intermittent connectivity errors even though we use passive and have keepalives selected with option "Executing dummy protocol commands" and "Seconds between keepalives" = 30. Session logging shows WinSCP sending keepalives while idle. However, WinSCP does not seem to send keepalive commands to the control port while transferring the file via FTPS (e.g., LIST, NOOP, PWD).

I have attached a log of the activity, for reference.

Thank you.

I regret any confusion; I posted the above request. Thank you.

Yes that's true. Does your server really terminate the session in a middle of a file transfer? Or is it some proxy on the way?

Hi Martin,

Thank you; it seems so, yes--either the FTPS servers or some other piece of routing equipment between the client and the FTPS servers intermittently drops the connection.

We connect to an Axway SecureTransport server, v4.9.1. This server listens on FTPS control port 2121 in an Internet-facing DMZ. For data transfers, the DMZ server directs the client to one or more different backend servers on a non-privileged port (e.g., 5050).

For testing purposes, we have initiated FTPS transfers from various networks using various clients: our work site; an Amazon EC2 server; another work site's equipment; and my personal equipment--all seem to intermittently timeout. The consistent, intermittent failures despite originating network seems (to me) a clue the issue may lie with routing equipment in or near the FTPS server.

FTPS server support staff suggest we try transferring the file using software capable of sending KeepAlives to the FTPS control port during the transfer. They think routing equipment may close the FTPS control port due to lack of activity during a long transfer on the data port, which would timeout the session. I assume the data port, over which the transfer takes place, would not timeout (?).

Thus, we would like to see which FTPS clients may send KeepAlives to the control port during long transfers over the data port (or an option to do so, perhaps). WinSCP also has scripting support and runs on Windows, which would allow us to automate the download.

Thank you again.

Hi Martin,

It appears we now have two logged TRANSFER FAILURES involving FTPS clients sending KeepAlives to the control port during a long transfer on the data port. FTPS clients were FileZilla and SmartFTP...each failed after exactly 30 minutes. Hopefully, it is not the steady stream of KeepAlives to the control port which cause the timeout (?). We had them sent in 15 second intervals.

So, no need to consider moving forward on creating a WinSCP option to send KeepAlives to control port during long transfers unless you determine it has merit.

Thank you.

Thanks for all the information.