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DECnet/E
Virtual Terminal Protocol
(VTP)
Version 1.0
Jan 20,1979
digital equipment corporation - maynard massachusetts
DECnet/E Virtual Terminal Protocal Page 2
1.0 SCOPE
This document describes the functions, characteristics, capabilities, and
operation of the Virtual Terminal Protocol (VTP). It is primarily intended to
assist developers and implementers in understanding how VTP functions within a
system. The document is not intended to address specific implementations.
2.0 FUNCTIONAL DESCRIPTION
The VTP is a user level protocol. Its purpose is to permit users to access
other DECnet/E systems for one DECnet/E as terminal users. Using the programs
that inplement the VTP a remote user can virtualy connect it's terminal to a
remote system. In this connection it (the user not being male or female)
will/may lose some of the features of normal terminal service but will be able
to logon and run most RSTS/E cusp.
2.1 BASIC MESSAGE FLOW
The basic message flow is shown below. The hope here is that Configuration
message will be used to pick one of two modes of operation:
COMPATABLEITY MODE: is where the two operating systems are different. In this
mode the messages with message codes < 127 are valid. If an
implemntation gets a message type that it does not
understand it should send it back in a Unsuported-Message
message. The initial implenetation includes the following
support; echo on/off with local echo and delete. It is
expected that this protocol will grow with time and should
be replaced with an X25 'PAD'. This is a short (2-3 year)
solution to a larger problem.
OTHER MODE: is wher the two operating systems are the same and wish to
implment some more complex protocol. All message types are
valid in this mode and the types from 128 to 255 are used
here.
TERMINAL REMOTE
Request to communicate
Connect Initiate Message
------------------------------_>
Connect Confirm Message
_<------------------------------
Configuration Message
------------------------------_>
Configuration Message
_<------------------------------
DECnet/E Virtual Terminal Protocal Page 3
pick mode for this link
DATA + CONTROL + USPMSG
_<-----------------------------_>
user logged out
DISCONNECT
_<------------------------------
- OR -
user done
DISCONNECT
------------------------------_>
2.2 RULES
The following rules must be followed.
1. All messages must be less than or equal to 137 bytes in length.
3.0 MESSAGE FORMATS
3.1 Notation
The following notation is used to describe the VTP messages:
FIELD (length) : coding = description of field.
where:
FIELD = the name of the field being described.
length = the length of the field, which can be indicated in one of
four ways:
1. A number meaning number of 8-bit bytes (octet).
2. The letters "I-n" means this is an image field, with n being a
number that specifies the maximun length of 8-bit bytes in the image.
The image is preceded by a length of 1-byte count of the length of the
remainder of the field. Image fields are variable in length and may
be null (count=0). All 8-bits of each byte are used as information
bits. The meaning and interpretation of each image field is defined
with that specific field.
coding = the representation type used.
A = 7-bit ASCII
B = binary
DECnet/E Virtual Terminal Protocal Page 4
BM = bit map (in which each bit has a specific meaning)
The following rules apply to the notation:
1. If length and coding are omitted, FIELD represents a number of subfields
specified in description.
2. Any bit or field described as resvered shall be zero unlyess otherwise
specified.
3. All fields are presented to NSP with the least-significant byte first. In an
ASCII field, the left-most character is contained in the low-order byte.
4. All numbers are in decimal unless otherwise specified.
DECnet/E Virtual Terminal Protocal Page 5
3.2 General message Format
All VTP messages have the following form:
+---------+---------+--------+
| | | |
| TYPE | LENGTH | REST |
| (1):B | (2):B | |
+---------+---------+--------+
where:
TYPE (1) : B = This field denotes the type of message this is. The values
from 1 to 127 are for the decnet common (cross system)
protocol and the values from 128 to 255 are for system
specific messages.
LENGTH (2) : B = Denotes the length of the message. The length includes the
full message (including type and length).
REST = Denotes the rest of the message.
DECnet/E Virtual Terminal Protocal Page 6
3.3 Configuration Message
The configuration message is used to pass system configuration information
between both systems involved in a virtual terminal connection The main use of
this message is to determine if the system on the other end is of the same type.
If the other system is different then the session is run in compatability mode
(the message types <128 are valid).
+---------+---------+--------+---------+--------+----------+
| | | | | | |
| CONFIG | LENGTH | OSTYPE | VERSION | MODE | MENU |
| (1):B=1 | (2):B | (1):B | (4):B | (1):BM | (I-6):BM |
+---------+---------+--------+---------+--------+----------+
CONFIG : = The TYPE field with a value of 1.
LENGTH(2) : B = The length of this message.
OSTYPE(1) : B = Operating system type (the sending system). Values in the
range 1-127 are reserved for DIGITAL use; 128-255 are
reserved for user-specified operating systems.
Value OS Type
----- -------
0 Illegal
1 RT-11
2 RSTS/E
3 RSX-11S
4 RSX-11M
5 RSX-11D
6 IAS
7 VAX/VMS
8 TOPS-20
9 TOPS-10
10 OS8
11 RTS-8
VERSION = A Field identifying the protocol and software version
numbers. This field is subdivided as follows:
+--------+--------+--------+---------+---------+
| VERNUM | ECONUM | USRNUM | SOFTVER | USRSOFT |
+--------+--------+--------+---------+---------+
where:
VERNUM(1) : B = VTP version number. This is the same as
DECnet/E Virtual Terminal Protocal Page 7
the first digit of the protocol vesion
number.
ECONUM(1) : B = VTP ECO (Modification) number. This is
the same as the second digit of the
protocol version number.
USRNUM(1) : B = Customer modification level of VTP.
SOFTVER(1) : B = VTP Software version number in binary.
This is the DIGITAL Release number. If
the software is completely user written
this field should be 0.
USRSOFT(1) : B = User software version number in binary.
If the user modifies DIGITAL software,
he should increment this byte to reflect
the modification number. Set to 0 by
DIGITAL.
MODE (1) :BM = This bit map defines the mode for this
sesion.
BIT MEANING (WHEN SET)
--- ------------------
0 DISSCONNECT ON LOGOUT.(SEND TO REMOTE END)
MENU (I-6) : BM = A menue for the rest of the CONFIG
message. There is currently no
additional fields so the menu is null.
All the previous fields are required.
DECnet/E Virtual Terminal Protocal Page 8
3.4 Control Message
The control message is used to inform the other end of the link of a
change in the mode or settings of the terminal.
*** NOTE ***
This message will grow with water and sun light.
+---------+---------+----------+---------+-----------+-------+
| | | | | | |
| CONTROL | LENGTH | MENU | ECHOFLG | DELIMITER | WIDTH |
| (1):B=2 | (2):B | (I-6):BM | (1):BM | (32):BM | (2):B |
+---------+---------+----------+---------+-----------+-------+
where:
CONTROL = The type field with code = 2.
LENGTH (2) : B = The length of the message.
MENU (I-6) : BM = The bit map below specifies which of the indicated
fields, groups of fields, or messages follow. The
field is present if the corresponding bit is set.
BIT Meaning (when set)
--- ------------------
0 ECHO flags
1 DELIMITER BIT MAP
2 WIDTH FIELD
ECHOFLG (1) : BM = The bit map below specifie the action to take if the
corresponding bit is set.
BIT ACTION (when set)
--- -----------------
0 ECHO OFF
1 ECHO ON
The default start up state is for echo at the
terminal end to be on and off at the remote end.
This changes the mode till another command is
received.
DECnet/E Virtual Terminal Protocal Page 9
DELIMITER(32) :BM = This is a bit map where each bit corisponds to an
ASCII character. If the Bit is set then that
character is a delimiter. (EG all bits set = ODT
mode). The standare delimiter set is as follows.
This the setting assumed for this field if the two
ends OSTYPE fields are different.
DELIMITERS DECIMAL
CODE
<CR> 13
<LF> 10
<ESC> 27,?,?
<_^Y> 31
<_^C> 3
<_^T> 24
<_^Z> 26
<_^O>
WIDTH (2) : B = This is the width of the real terminal from the
terminal side. It has a value of 0 from the host
side.
DECnet/E Virtual Terminal Protocal Page 10
3.5 UNSUPORTED MESSAGE MESSAGE
This message is used to tell the other end that it send you something
you dont know what to do with. The procedure from there on is up the
end thatt sent the bad message.
+---------+---------+--------+---------+
| | | | |
| USPMSG | LENGTH | ERRLOC | MESSAGE |
| (1):B=3 | (2):B | (2):B | (I-n):B |
+---------+---------+--------+---------+
DECnet/E Virtual Terminal Protocal Page 11
3.6 DATA MESSAGE
The DATA message is used to send keyboard input to the remote system and
printer output to the local user.
+---------+---------+-----------+
| | | |
| DATA | LENGTH | TERDATA |
| (1):B=4 | (2):B | (I-133):B |
+---------+---------+-----------+
where:
DATA = The type field with code = 4.
LENGTH (2) : B = The length of the message.
TERDATA (I-133) : B The actual data being send across the net.