[ih] Why FTP uses two ports?

[John Day]

The primary reason for using two ports was so 
that FTP commands didn't get stuck behind the 
data transfer.  Things could be a lot slower in 
those days and a fair amount could be buffered. 
If one started a transfer and then wanted to 
abort it, the command would get there and be 
acted on rather than wait behind the data 
transfer which might have queued quite a lot.

Secondarily, it enabled 3rd party transfers. 
Third, on the TIPs, you were the FTP user 
process.  You were typing the commands and 
getting the replies.  (This is why FTP commands 
are text and why FTP uses Telnet and why FTP 
replies have a number (for a program to act on) 
and an arbitrary string of text for the human to 
see.)  The TIP could hardwire sockets to a 
printer or card reader so that the file would go 
to the printer and the user could still type 
commands.

BTW, I know that many textbooks and well-known 
professors describe Telnet as a remote login 
protocol.  It isn't.  It is a terminal device 
driver protocol.  This is why this class of 
protocols use to be called virtual terminal 
protocols.  Remote log in is one application 
built using Telnet.  Telnet was also used in FTP, 
RJE, CCNRJE, and SMTP.

Take care,
John

At 0:33 -0400 2012/06/21, Louis A. Mamakos wrote:
>On Jun 21, 2012, at 12:07 AM, Eduardo A. Suárez wrote:
>
>>  Hello,
>>
>>  I want to know if anyone remembers the origins 
>>of FTP and the reason why FTP uses two ports.
>
>You can perform 3rd party file transfers, with 
>one client orchestrating the transfer between two
>different servers.
>
>>
>>  Thanks, Eduardo.-
>>
>>  --
>>  Eduardo A. Suarez
>>  Facultad de Ciencias Astronómicas y Geofísicas - UNLP
>>  FCAG: (0221)-4236593 int. 172/Cel: (0221)-15-4557542/Casa: (0221)-4526589
>>
>>  ----------------------------------------------------------------
>>  This message was sent using IMP, the Internet Messaging Program.
>
>Ha!  Back in the day, IMP was a very different thing..
>
>Louis Mamakos