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IETF RFC 158
TELNET Protocol
Last modified on Thursday, May 22nd, 2008
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Network Working Group T. O'Sullivan
Request for Comments: 158 Raytheon
NIC: 6768 19 May 1971
TELNET Protocol
This request for comment is being circulated by the TELNET committee
to solicit comments, evaluation, and requests for modification of the
proposed protocol presented here. Unless comments are received by
the writer within two weeks of the date of this RFC, they will not be
considered in the preparation of official TELNET PROTOCOL document.
The proposed document is the result of the work of the committee. It
represents a TELNET protocol felt to be adequate for initial
implementation.
Readers are referenced to the following previous releases of
information:
1. Conventions for Using an IBM 2741 Terminal or a User Console
for Access to Network Server HOSTS
Joel Winett, RFC 110 (NIC #5809)
2. Response to RFD 110
Wayne Hathaway, RFC 135 (NIC #6712)
3. Level III Server Protocol for the Lincoln Laboratory 360/67
HOST
Joel Winett, RFC 109 (NIC #5808)
4. First Cut at a Proposed TELNET Protocol
J. Melvin, D. Watson, RFC 97 (NIC #5740)
5. ASCII Format for Network Interchange
V. Cerf, RFC 20 (NIC #4722)
6. Discussion of TELNET PROTOCOL
Tom O' Sullivan, RFC 139 (NIC 6717)
(Although relevant to the obsoleted RFC 137 (NIC 6714) many of
the examples still hold. A replacement discussion document RFC
159 (NIC 6769) will be forthcoming in the near future).
T. O'Sullivan PAGE 1 
RFC 158 TELNET PROTOCOL 19 May 1971
TELNET PROTOCOL
A Proposed Document
T. O'Sullivan for the TELNET Committee
Will Crowther BBN
Bob Long SDC
John Melvin SRI-ARC
Bob Metcalfe Harvard
Ed Meyer MAC
Tom O'Sullivan (Chairman) Raytheon
Joel Winett MIT-LL
T. O'Sullivan PAGE 2
RFC 158 TELNET PROTOCOL 19 May 1971
Telnet Protocol
TELNET is a third-level protocol, the function of which is to make a
terminal (or process) at a using site appear to the system or a
process at a serving site as logically equivalent to a terminal
"directly" connected to the serving site. In performing this
function, the protocol attempts to minimize the amount of information
each HOST must keep about the characteristics of other HOSTS.
Definitions
Protocol Levels (see Figure 1)
Level 1
HOST-IMP protocol specified by BBN in NIC 5735, Specifications
for the Interconnection of a HOST and an IMP (BBN Report 1822)
Level 2
HOST-HOST protocol performed by NCPs as described in Document
Number 1 (NIC 5413) and subsequent amendments, see RFC 107 (NIC
#5806)
One view of the NCP's function is that it takes information
from the net and routes it to receiving processes via
mechanisms internal to each HOST; conversely, processes use
the NCP, via internal system calls, to have information
routed to other processes in the net (via the other
processes' NCPs).
Level 3 (see Figure 2)
Level 3 is, by definition, the place to which and from which
the NCP communicates internally in its own host.
This level may be equivalent to the user process level in
some systems, but this may not be the case in all systems.
In using sites, the TELNET process operates at this level.
In serving sites, the TELNET server operates at this level.
Initial Connection Protocol (ICP)
An agreed-upon sequence of level 3 exchanges between two processes
which is, in general, used to synchronize the connection dialogue
between the processes, e.g., RFC 80 (NIC #5608) #1, as revised by
subsequent information.
T. O'Sullivan PAGE 3
RFC 158 TELNET PROTOCOL 19 May 1971
Serving Site
The HOST into which the TELNET process is directing the user's
keyboard input and from which the TELNET process is receiving
control information and data effecting the user's terminal. At
the serving site, a TELNET server is executing.
Using Site
The HOST in which the TELNET process is executing.
Sending Site
The HOST transmitting data, could be either using site or serving
site.
Receiving Site
Converse of sending site.
User
The person or process "driving" the TELNET process.
In providing services the TELNET protocol will use established
network conventions, specifically the Network Control Program, and
Initial Connection Protocol referenced in the above definitions,
using a byte size of 8 bits on the permanent connection.
The TELNET protocol provides for a Network Virtual Terminal (NVT)
through which users may transmit and receive data over connections
between the using site and the serving site.
The code of the NVT will be full 7 bit ASCII. The seven-bit code
will be transmitted in eight-bit bytes, the high order bit set to
zero.
It will be the responsibility of the using site to provide its users
with a means of producing all 128 ASCII codes, as well as a selected
set of special TELNET control signals (see Figure 3).
The ASCII character ESC will be employed by the user as an escape
signal indicating that the next character(s) has special meaning.
The meaning assigned to escape code will be serving site defined and
therefore may not be consistant across the network.
T. O'Sullivan PAGE 4
RFC 158 TELNET PROTOCOL 19 May 1971
It will be the responsibility of the serving site to specify for
users how the NVT code will be used to represent the codes normally
generated by a local terminal. The serving sites specification of
this representation is expected, where reasonable, to map on a one-
for-one basis for ASCII graphics and controls that are provided
through local terminals. The serving site will also specify how the
escape conventions will be interpreted by the system.
The end of a line will be represented in the NVT as carriage return
(X'0D') followed by line feed. (X'0A')
The protocol assumes that initially the serving site will not provide
any echo to the using site.
Each TELNET control signal for which code must be sent over the
connection will be represented in the NVT by an eight-bit code, with
the high order bit set to one. Following are the special codes
established to date. (U) indicates that in most implementations the
user would be expected to have the ability to signal the TELNET
process from his terminal to initiate the code.
Code X'A0'
Source: Both Sites (U)
Meaning: A DATA TYPE[1] signal indicating that code will be
transmitted by NVT, i.e., using the seven-bit ASCII
conventions.
Code X'80'
Source: Using Site (U)
Meaning: Order using site NCP to send an INS and insert X'80' in
data stream.
Code X'81'
Source: Using Site (U)
Meaning: Break or Attention, and reverse break.
Code X'82'
Source: Both Sites
Meaning: No op
Code X'83'
Source: Both Sites
Meaning: Don't Echo
T. O'Sullivan PAGE 5
RFC 158 TELNET PROTOCOL 19 May 1971
Code X'84'
Source: Both Sites
Meaning: You Echo
Code X'85'
Source: Serving Site
Meaning: Hide your input [2]
Some special TELNET control signals are required to permit the user
on some systems to send control information to the using site TELNET
process[3]. These do not require a corresponding control code for
transmission. The local TELNET control signals are:
1. Transmit all data to this point.
2. Suppress transmission of end of line, send all other data.
Data is to be forwarded to the NCP for transmission as convenient,
but at least at the end of line, end of line suppression, and
transmit signals. If the normal line length of the sending site is
greater than the allocation given by the receiving site, the sending
sites NCP, TELNET process, or TELNET server must be prepared to send
line segments in convenient lengths until the full line has been
sent.
A minimum implementation for TELNET for both using site and serving
site follows:
Using Site
1.) Provide User (human or process) with ability to cause all
128 ASCII codes to be transmitted in the required 8 bit
field to the serving site.
2.) Ignore (and strip) all TELNET control characters received
from the serving site.
3.) Provide echo or local print capability to local user
terminals.
4.) Provide for CR-LF end of line convention.
5.) Implement local TELNET controls (See discussion above of
local TELNET control signals) for transmit or suppress
end of line.
T. O'Sullivan PAGE 6
RFC 158 TELNET PROTOCOL 19 May 1971
Serving Site
1.) Provide (and announce) one for one mapping between ASCII
and Serving Site character and control set (or if Serving
Site set greater than 128, a sub set.)
2.) Ignore (and strip) all TELNET control characters received
from the Using Site.
3.) Assume Using Site will provide local terminal echo or
print capability.
4.) Provide for CR-LF end of line convention.
This document will be revised as necessary to provide conventions for
data types in addition to the NVT ASCII type.
|<------- 32 ------->|<-8->|<-8->|<-- 16 -->|<-8->|<---
+--------------------+-----+-----+----------+-----+------------------
| leader | x |size | count | x | TEXT
+--------------------+-----+-----+----------+-----+------------------
|<---- level 1 ---->|
message leader
|<------------------ level 2 ------------------>|
message preamble
level 3
|<- message text..-->
Figure 1. Network Message on Link 2-31
Indicating Portions of Interest to Various Levels
T. O'Sullivan PAGE 7
RFC 158 TELNET PROTOCOL 19 May 1971
USING HOST Serving HOST
-----------------------+ +----------------------
| |
\ | | /
Sub- \ -----------------| +-+ +-+ |-----------------/
Sys- \ | |I| |I| | NCP /
tems+---> <--->|M|---NETWORK--|M|<---> ^ /
| \ NCP | |P| |P| | +-----|-----/
| \ | +-+ +-+ | | v /
| \ | | | TELNET / USER
TELNET )___________| |--|Protocol( PROCESS
| ) | | |Server <--->Sub
| / | | | ^^ \Systems
| / TTY | | +----||-----\ETC
User +---> HANDLER <---> Local | TTY vv \
Pro- / | Terminals | Handles \
cesses/-----------------| |-----------------\
/ | | \
| |
-----------------------+ +----------------------
<---> TELNET path path
Figure 2. Current and Candidate Future TELNET Paths
T. O'Sullivan PAGE 8
RFC 158 TELNET PROTOCOL 19 May 1971
+---------------------------+----+----+----+----+----+----+----+----+
|\ b8 -> | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| \ b7 -> | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 |
| \ b6 -> | 0 | 0 | 1 | 1 | 0 | 0 | 1 | 1 |
|B \ b5 -> | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 |
| I +---+---+---+---+------+----+----+----+----+----+----+----+----+
+ T | b | b | b | b |\COL->| | | | | | | | |
\ S| 4 | 3 | 2 | 1 | \ | | | | | | | | |
\ | | | | | |\ | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
\ | | | | | | | | | v \ | | | | | | | | |
\ | v | v | v | v |ROW \ | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 0 | 0 | 0 | 0 |NUL |DLE | SP | 0 | @ | P | \ | p |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 0 | 0 | 1 | 1 |SOH |DC1 | ! | 1 | A | Q | a | q |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 0 | 1 | 0 | 2 |STX |DC2 | " | 2 | B | R | b | r |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 0 | 1 | 1 | 3 |ETX |DC3 | # | 3 | C | S | c | s |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 1 | 0 | 0 | 4 |EOT |DC4 | $ | 4 | D | T | d | t |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 1 | 0 | 1 | 5 |ENO |NAC | % | 5 | E | U | e | u |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 1 | 1 | 0 | 6 |ACK |SYN | & | 6 | F | V | f | v |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 1 | 1 | 1 | 7 |BEL |ETB | ' | 7 | G | W | g | w |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 0 | 0 | 0 | 8 | BS |CAN | ( | 8 | H | X | h | x |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 0 | 0 | 1 | 9 | HT | EM | ) | 9 | I | Y | i | y |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 0 | 1 | 0 | 10 | LF |SUB | * | : | J | Z | j | z |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 0 | 1 | 1 | 11 | VT |ESC | + | ; | K | [ | k | { |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 1 | 0 | 0 | 12 | FF | FS | , | < | L | \ | l | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 1 | 0 | 1 | 13 | CR | GS | - | = | M | ] | m | } |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 1 | 1 | 0 | 14 | S0 | RS | . | > | N | ^ | n | ~ |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 1 | 1 | 1 | 15 | S1 | US | / | ? | O | _ | o |DEL |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
Code Structure 8 7 6 5 4 3 2 1
--- --- --- --- --- --- --- --- --- ---
T. O'Sullivan PAGE 9
RFC 158 TELNET PROTOCOL 19 May 1971
+---------------------------+----+----+----+----+----+----+----+----+
|\ b8 -> | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| \ b7 -> | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 |
| \ b6 -> | 0 | 0 | 1 | 1 | 0 | 0 | 1 | 1 |
|B \ b5 -> | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 |
| I +---+---+---+---+------+----+----+----+----+----+----+----+----+
+ T | b | b | b | b |\COL->| | | | | | | | |
\ S| 4 | 3 | 2 | 1 | \ | | | | | | | | |
\ | | | | | |\ | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
\ | | | | | | | | | v \ | | | | | | | | |
\ | v | v | v | v |ROW \ | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 0 | 0 | 0 | 0 |'80'| |'A0'| | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 0 | 0 | 1 | 1 |'81'| | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 0 | 1 | 0 | 2 |'82'| | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 0 | 1 | 1 | 3 |'83'| | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 1 | 0 | 0 | 4 |'84'| | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 1 | 0 | 1 | 5 |'85'| | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 1 | 1 | 0 | 6 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 0 | 1 | 1 | 1 | 7 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 0 | 0 | 0 | 8 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 0 | 0 | 1 | 9 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 0 | 1 | 0 | 10 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 0 | 1 | 1 | 11 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 1 | 0 | 0 | 12 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 1 | 0 | 1 | 13 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 1 | 1 | 0 | 14 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
| 1 | 1 | 1 | 1 | 15 | | | | | | | | |
+---+---+---+---+------+----+----+----+----+----+----+----+----+
'XX' = HEX designation for codes assigned to TELNET Control Signals.
Figure 3. Official Network Virtual Terminal Code
T. O'Sullivan PAGE 10
RFC 158 TELNET PROTOCOL 19 May 1971
Endnotes
[1] A one-byte DATA TYPE signal is sent as the first byte of data
over a connection. A default is employed if the first byte over a
connection has the high order bit set to zero, and it is assumed that
the seven-bit ASCII NVT convention will be employed. Most
implementations and applications may expect the DATA TYPES to be
symmetrical at any point in time. (i.e. both using a serving site
using the same DATA TYPE.) Other data types for which codes are
currently assigned are:
X'A1' Transparency
X'A2' EBCDIC
X'A3' Special String to TELNET (I'll use your code)
X'A4' End Special String to TELNET (I'll use my code)
[2] i.e. suppress printing of password.
[3] In some cases, for prolonged [periods of special treatment, local
implementation may dictate permitting the user to set a "mode" to
prevail until explicitly discarded.
[This RFC was put into machine readable form for entry]
[into the online RFC archives by Lorrie Shiota 2/02]
T. O'Sullivan PAGE 11
TELNET Protocol
RFC TOTAL SIZE: 21025 bytes
PUBLICATION DATE: Thursday, May 22nd, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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