Ref: 10490006
Title: CDC Workstation Does Not Disconnect in PCS/TCP FTP Transaction
date: 3/7/90

Copyright 3Com Corporation, 1991.  All rights reserved.

A CDC workstation may not disconnect correctly from a PCS/TCP FTP
transaction, causing the PC running PCS/TCP to hang.

When an FTP session is established, two connections result.  The first is a
Telnet connection to handle the control functions.  The second is an FTP
connection to handle the file transfer.  As explained in RFC 959 "File
Transfer Protocol" (10/85), section 2.3:  "the protocol requires that the
control connections be open while data transfer is in progress.  It is the
responsibility of the user (PCS/TCP) to request the closing of the control
connections when finished using the FTP service, while it is the server
(CDC workstation, in this example) who takes the action.  The server may
abort data transfer if the control connections are closed without command."

PCS/TCP is a client-only implementation of IP.  It expects the
server or host functions to be handled by whatever it is connecting to.
When the PCS/TCP PC wants to signify the end of data transfer and request
that the control connection be closed, it sends a "quit" signal.  The CDC
workstation replies with a "goodbye" to acknowledge the "quit."  The PC then
expects the CDC workstation to begin a FIN-ACK sequence to close the Telnet
control connection, but the CDC workstation does not send the sequence.  As
a result, the PCS/TCP machine hangs.  Reboot the PC to unhang it.

