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From: jerry@wg3b20.UUCP
Newsgroups: net.unix-wizards
Subject: Re: telnet problem
Message-ID: <618@wg3b20.UUCP>
Date: Wed, 21-May-86 19:20:09 EDT
Article-I.D.: wg3b20.618
Posted: Wed May 21 19:20:09 1986
Date-Received: Sat, 24-May-86 20:45:26 EDT
References: <850@oliveb.UUCP>
Organization: The Wollongong Group., Palo Alto, CA 94303.
Lines: 31

> We have a very serious problem of 'telnet' program in our VAXen 11/{750,785}
> 4.2 BSD Unix.  The symptom seems to me is flow control problem (lost 
> characters, gobble the screen when cat'ing file).  The followings are
> the tests dealt with the 'telnet' program:
> 
> Flow control problem occurs with any of following tests:
> 	1.  Use a VT100, "telnet" from VAX 11/{750,785} 4.2BSD Unix to
> 	    VAX 11/{750,785} 4.2BSD Unix
> 	2.  Use a VT100, "telnet" from VAX 11/(750,785} 4.2BSD Unix to
> 	    VAX 11/750 VMS with Wollongong Group TCP/IP
> 	3.  Use a VT100, "telnet" from VAX 11/{750,785} 4.2BSD Unix to
> 	    VALID workstation based 4.2BSD Unix. And vise versa.
> 

The "problem" has to do with the VT100 sending XON and XOFF as it tries
to throttle output from the terminal driver.  What happens in the above
cases is that the XON is sent across the network where it reaches the
telnet server who then gives it to the terminal driver who then stops
output.  However the beauty of TCP is that it can send a window of
data at a time.  So by the time the server side's terminal server stops
output it is too late, the VT100 has been overriden.  You can either
set the terminal speed down to 4800 baud (or less) or make mods to
the telnet program on the 4.2BSD side to do local XON/XOFF processing.
I did this on Wollongong's VMS telnet and it works fine.  VMS has the
ability to set the terminal in a mode similar to RAW mode but ask to catch
certain characters.  I ask to catch CTRL-S and when I receive
it buffer new characters over the net locally and send a XON across the
net.  When catching CTRL-Q I flush all characters and send a XOFF.

Regards,
Jerry Scott