Re: simultaneous connections


Bill Barns (barns@gateway.mitre.org)
Tue, 18 Oct 88 08:18:35 EDT


Three party FTP doesn't depend on the symmetry of TCP connection
opening. I suppose there is a kind of symmetry there, but it is not at
the TCP connection handling level. Although there are two servers
involved in a three party FTP operation, they are executing different
state machines, so to speak; the client that sets up the three party
FTP implicitly selects this "polarity" by sending the PASV command to
one of the servers and not to the other one. Once this has been done,
there is a specific assignment of duties implicit in the protocol.
(The server which received the PASV command does only a passive open on
the data connection, and the other one does an active open. There is a
similar allocation of responsibility for closing the connection,
subject to the same twist as in two party FTP where the sender of a
file in a file-structure, stream-mode transfer always initiates the close.)

My admittedly superficial reading of the BFTP RFC led me to believe that
BFTP doesn't alter any of this, but rather provides a different sort of
a frontend to the FTP operation.

Bill Barns / MITRE-Washington / barns@gateway.mitre.org



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