[ih] Why FTP uses two ports?

[John Day]

They were unidirectional.  But I don't remember that had anything to 
do with it.  It did mean you only had to open a data connection in 
one direction.  The only thing the limitation on sockets required was 
the use of ICP, the Initial Connection Protocol.

But it really was so that you could send commands during a transfer 
and they wouldn't get queued up behind a lot of the file transfer, 
AND that it had to be usable by the TIPs, which did not have a file 
system.  And as Dave said, the data connection could be binary.  It 
could take awhile to drain a pipe in those days.

There were some FTPs done early that only did one full duplex 
connection.  I think the UK protocol was like that.  But I don't 
remember how they handled the abort situation.

John


At 10:20 -0400 2012/06/21, Noel Chiappa wrote:
>I have this bit vaguely set that NCP sockets were unidirectional, whereas TCP
>ports are bidirectional. Was that a factor at all (if my memory of how things
>worked is even true :-)?
>
>        Noel