L
Lasse Reichstein Nielsen
[FTP data transfer]
Actually, passive mode data transfer has been in the FTP protocol
almost since the beginning (RFC 542 from 1973). It is not new, and
not caused by firewalls.
It is true that the growth of private computers, possibly behind
firewalls, or just NAT routers, and using browsers as FTP clients
have cause passive FTP connections to become much more prevalent.
Having separate data and control lines, along with both passive and
active data transfer, allow you to use two control lines to initiate a
direct transfer between two FTP servers, controlled by a third computer.
Pretty nifty, actually.
/L
Because this nowadays typically causes firewall problems at
the client, a new "passive mode" FTP has been defined, in which,
instead of initiating the data session, the server requests the
client, via the command session, to initiate it, "active mode" being a
retronym for the original design. Passive mode must be requested
explicitly, since not all software supports it.
Actually, passive mode data transfer has been in the FTP protocol
almost since the beginning (RFC 542 from 1973). It is not new, and
not caused by firewalls.
It is true that the growth of private computers, possibly behind
firewalls, or just NAT routers, and using browsers as FTP clients
have cause passive FTP connections to become much more prevalent.
I don't know why they didn't just use a single session with a side
channel implemented in the data format in the first place, unless it
was simply to squeeze out the last drop of bandwidth.
Having separate data and control lines, along with both passive and
active data transfer, allow you to use two control lines to initiate a
direct transfer between two FTP servers, controlled by a third computer.
Pretty nifty, actually.
/L