RSX11M/DECNET VIRTUAL TERMINAL SUPPORT 
	  
		     Greg Thompson (Informatics/PMI) 
	  
				4/16/78
	  
		Version 1.0 for RSX11M V3.0 and DECNET V1.2

  
	  
	   This  document  describes the virtual terminal support available on
	RSX11M  systems with DECNET which allows a terminal on one DECNET host
	to  logically be connected as a terminal on another DECNET host.  Once
	a  connection  is  established  the initiating terminal appears on the
	remote  host  just  like  any other remote terminal and is required to
	log-in  to  the remote system using HELLO as usual.  The connection is
	as  fully  transparent  as  possible  including the support of running
	RMDEMO or EDT over the virtual terminal connection. 
	  
	   The  support consists of two tasks and one loadable driver entitled
	CON,  TELNET,  and  RTDRV.   It  requires the DECNET software and many
	functions  of  the  full  user-oriented  terminal  driver.   The RTDRV
	requires  loadable  device driver support but can be easily changed to
	be  a resident driver if desired.  The software, as provided, supports
	up  to  eight  virtual  terminals on each host, although the number of
	terminals  supported  is  simply  an  assembly parameter in TELNET and
	RTDRV  and  may  be modified if desired.  Only RTDRV need be built for
	the  particular host and the software does not require a new sysgen as
	long as the required functions are already available. 
	  
	   The  CONNECT task is the user side of the virtual terminal software
	and  allows  a user to connect to the virtual terminal server software
	on  a  remote host.  Even though the CON task is a privileged task (so
	write-break-throughs  will  work), it is based on APR0 and need not be
	rebuilt  for  each system.  The connection is initiated by issuing the
	command  "CON  <name  of host>" where <name of host> is a valid DECNET
	host  name for the local host.  When the connection is established the
	virtual   terminal  driver  on  the  remote  host  returns  a  message
	indicating to which  virtual terminal the user is connected.  The  CON
	task  has  two  local commands which allow the user to disconnect from
	the  remote  host  and  to temporarily suspend a connection inorder to
	issue commands to the local host after which he (or she) may reconnect
	using  the  RESUME  command.  Local commands are preceeded by a number
	sign  "#"  as the first character of a line.  If it is desired to send
	to  the remote host a line with the first character a number sign, two
	number  signs  should  be  used  instead.   The local commands;  #END,
	#CLOSE,  #QUIT  or #EXIT close the connection while #SUSPEND or #PAUSE
	suspend  the  connection.   If  a  user  forgets  to  issue BYE before
	disconnecting, a BYE is automatically issued for him. 
	  
	   Even though DECNET does not support route through, issuing a CON on
	the  remote  host  inorder  to  connect  to  a  host  local  to  it is
	explicitly  allowed.  Of course the user must still have an account on
	the  remote  hosts  inorder  to  log-in and each remote host must have
	allowed virtual terminal connections by installing the TELNET task and
	loading  the  virtual  terminal  driver  (RT:).   Some caution must be
	observed  when using CON from the operators console since CON attaches
	the terminal (with unsolicited input AST) and any messages intended to
	the  terminal  are held up until the connection is ended or suspended.
	This  is particularly a problem on our systems since our HELLO and BYE
	tasks report the logging in and out of users to the operators console.
	  
	   TELNET  (from  ARPANET's  TELecommunications NETwork protocol) is a
	non-privileged  server  task  that  interfaces  between DECNET and the
	virtual  terminal  driver.   Since it is a non-privileged task it also
	does  not need to be rebuilt for each host.  It contains 24 buffers of
	142  bytes  each  (both  assembly  parameters)  which  are dynamically
	allocated  to connections as necessary inorder to keep the exchange of
	messages between CONNECT and RTDRV as rapid as possible.
	  

  
	  
	   RTDRV  is  the  loadable  device  driver  for  the virtual terminal
	support.   It  implements  additional  terminal type devices under the
	device  name  "RTx:".  where x ranges from 0 to one less the number of
	virtual  terminals  supported.   It  accept all of the functions codes
	supported  by the TT: driver along with IO.CON, IO.XMT, IO.RCV, IO.DIS
	and  IO.CTL  which  the TELNET task uses to communicate to the driver.
	To  disable  virtual  terminal connections to a node simply unload the
	RT: driver on that node.  When no user is connected to a given virtual
	terminal  number  the  DISABLED  bit  for  that terminal is set and is
	cleared as soon as someone is connected to that virtual terminal. 
	  
	   The   sizes   of   the  software  modules  (with  current  assembly
	parameters) are 2.5K words for the CONNECT task, 2.9K words for TELNET
	and  .9K  words for the virtual terminal driver.  The virtual terminal
	software  was  written under contract to NASA/Ames Research Center and
	is therefore in the public domain.
 	 
	   The only currently known bug which will be fixed in the ungrade to
	DECNET V2 is the fact that when a task attaches RTx: the unsolicited
	input is not tossed away and this can cause problems when used with
	EDT if you type to fast.