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I am working on an FTP client for kicks and I am trying to understand the workflow of data connections.

As I understand, the initial (command) connection is permanent until you quit. However, I am unsure of the data connection - is it re-initiated per-command? So you call PORT ... or PASV, get a second connection, do a LIST, get the results, connection closes, start over?

Also, do you need to call PASV (or PORT ...) again after each connection closes? It seems that when I try to test some things out using a passive connection, I cannot re-connect to the same port after the first command has returned the results and closed the data connection. I can keep calling PASV -> Data Connect -> Run Command -> Get Results -> Data Connection closed -> PASV, but it seems like it's not how it's meant to run?

Also, if someone has a good material on FTP that is more terse than the RFC I really appreciate it.

Martin Prikryl
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Christian P.
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1 Answers1

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You have to open a new connection every time. It's only the closing of the connection, how you (or the server) can tell that the transfer completed (at least in the common "stream mode").

You cannot even reuse the local/remote port number combination, as when a TCP connection is closed, it enters TIME_WAIT mode and the port number combination cannot be used for some time. So for two immediately consecutive transfers you have to use a different port number combination anyway.

Refer to RFC 959, section 3.3. Data management:

Reuse of the Data Connection: When using the stream mode of data transfer the end of the file must be indicated by closing the connection. This causes a problem if multiple files are to be transfered in the session, due to need for TCP to hold the connection record for a time out period to guarantee the reliable communication. Thus the connection can not be reopened at once.

There are two solutions to this problem. The first is to negotiate a non-default port. The second is to use another transfer mode.

A comment on transfer modes. The stream transfer mode is inherently unreliable, since one can not determine if the connection closed prematurely or not. The other transfer modes (Block, Compressed) do not close the connection to indicate the end of file. They have enough FTP encoding that the data connection can be parsed to determine the end of the file. Thus using these modes one can leave the data connection open for multiple file transfers.

See also:

Martin Prikryl
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  • Alright, that makes perfect sense (and matches the behaviour I have seen). Does this mean you could actually mix and match PASV and PORT between transfers? Not that you would want to. – Christian P. Jul 23 '15 at 13:17
  • Yes you can. But it does not make sense. You pick a [mode that best suits your network configuration](http://winscp.net/eng/docs/ftp_modes). I.e. you typically use `PASV` nowadays, as the client is typically behind a firewall or NAT or both. – Martin Prikryl Jul 23 '15 at 13:18