client/server transfer

In a client/server transfer, there are only two entities involved in the transfer, the client entity and the server entity. We use the term entity here rather than process because in the implementation provided in GT4, the server entity may actually run as two or more separate processes.

The client will either move data from or to his local host. The client will decide whether or not he wishes to connect to the server to establish the data channel or the server should connect to him (MODE E dictates who must connect).

If the client wishes to connect to the server, he will send the PASV (passive) command. The server will start listening on an ephemeral (random, non-privileged) port and will return the IP and port as a response to the command. The client will then connect to that IP/Port.

If the client wishes to have the server connect to him, the client would start listening on an ephemeral port, and would then send the PORT command which includes the IP/Port as part of the command to the server and the server would initiate the TCP connect. Note that this decision has an impact on traversing firewalls. For instance, the client's host may be behind a firewall and the server may not be able to connect.

Finally, now that the data channel is established, the client will send either the RETR "filename" command to transfer a file from the server to the client (GET), or the STOR "filename" command to transfer a file from the client to the server (PUT).