Noop

Certain FTP servers require NOOP commands to be sent periodically to keep the connection alive. FluentFTP supports sending a NOOP command at the specified interval.

Settings

• NoopInterval - Time to wait (in milliseconds) between sending NOOP commands to keep the control socket alive during long file transfers. Setting this interval too low will have a negative impact on performance. Setting this interval to 0 disables NOOP commands altogether. Decrease this setting if you are getting timeouts during file transfers. Read the PR notes for suggested values for this property. Default: 15000 (15 seconds).

What settings are required to enable NOOP support?

Currently if you need to reliably use NOOPs, you need to either turn off encryption for best results or try .NET Framework, which will sometimes glitch with v40+.

Also, your experience will vary depending on the server type and its configuration.

What is the current status of NOOP support?

Tested against server: Proftpd.

See the full issue for more info.

Framework	NOOP works?	SSL Buffering	NOOP Responses from Server
NET 4.7	Yes	Any	200,200,200,200,200,200,200,200,200,200,200,200,226
NET 4.7	Yes	Turned off	200,200,200,200,200,200,200,200,200,200,200,200,226
.NET 6+	Yes	Any	226,200,200,200,200,200,200,200,200,200,200,200,200
.NET 6+	No	Turned off	200 and then failure because the server is not sending all NOOP responses right away.

Some additional information from the web

You might want to read:

Essay on NOOP handling of different servers

Essay on Keep Alive Techniques in general (proftpd) (here, read the section on FTP Keep Alive in detail).

A synopsis would be:

Being able to solve problems where your control connection is being dropped because of a too long idle state really depends on many factors. There are a lot of places in between the client and server where this can happen, it can happen on the server too.

The servers configuration might discourage keep alives, depending on its configuration.

The way a server reacts to keep alive tests (such as NOOP) will differ not only by its configuration - different servers are programmed differently.

Last but not least, we see different behaviour on the client side, depending on encryption, ssl protocol and the underlying .NET frameworks.