Remote User Telnet service
A way for hosts with limited resources to provide remote terminal access via a well-known port.
Postel, J., "Remote User Telnet service", RFC 818, DOI 10.17487/RFC0818, November 1982, <https://www.rfc-editor.org/info/rfc818>.
@misc{rfc818,
series = {Request for Comments},
number = 818,
howpublished = {RFC 818},
publisher = {RFC Editor},
doi = {10.17487/RFC0818},
url = {https://www.rfc-editor.org/info/rfc818},
author = {J. Postel},
title = {{Remote User Telnet service}},
pagetotal = 2,
year = 1982,
month = nov,
} RFC text
Network Working Group J. Postel
Request for Comments: 818 ISI
November 1982
The Remote User Telnet Service
This RFC is the specification of an application protocol. Any host that
implements this application level service must follow this protocol.
This RFC was suggested by Mike Mulligan some months ago when he was at
BBN.
In the ARPANET Host-to-Host Network Control Protocol (NCP) and in the
Internet Transmission Control Protocol (TCP) well known sockets or ports
are used to identify services. The general notion is that there are a
few types of services that are distinct and useful enough to use the NCP
or TCP demultiplexing mechanism directly.
The most common of these is the Server Telnet which generally speaking
defines the network terminal access procedure for a system executive.
That is, making a connection to the server Telnet port actually puts the
caller in contact with the system executive, for example, the TOPS20
EXEC or the Unix Shell.
On some small hosts there may be very limited functionality and no
executive. In such cases it may be useful to designate specific well
known ports for specific applications.
This memo specifies that the specific service of User Telnet may be
accessed (on hosts that choose to provide it) by opening a connection to
port 107 (153 octal). The Telnet Protocol is to be used on the
connection from the originating user to the server.
EXAMPLE: REMOTE TELNET SERVICE ON THE BBN TC68K
The TC68K is a Terminal Concentrator based on the Motorola MC68000
microprocessor. It is used at Bolt Beranek & Newman to provide access
by terminals to the FiberNet, a local area network.
The custom hardware provides one network connection, sixteen RS232
terminal connections, and a programmable timer.
The software is based on the Micro-Operating System (MOS) using the IP,
ICMP, TCP, and Telnet protocols. A user TC-Telnet application provides
an interface to allow the user to use the network to connect to a host,
Remote User Telnet Service
providing a network virtual terminal. A server Telnet also exists on
the TC68K to serve as a front end for devices that have no awareness of
the net. This is used for remote printer/plotters and computers with no
network software.
The TC68Ks at BBN are distributed about several buildings. To provide
an operational tool to test remote TC68Ks, the TC68K software was
configured to put a user Telnet back to back with a server Telnet. An
operator can open a connection to a remote TC68K and appear to be a
terminal local to that unit. This verifies that the network path
between the two units is operational and provides the operator with
access to statistics that are kept as part of the standard user
TC-Telnet application.
Operator's Local Remote Remote
Terminal <=TTY=> user <=FiberNet=> server <=PTY=> user
TC-Telnet Telnet TC-Telnet
This solution was attractive as the only extra piece of software
necessary for this was the "Pseudo Teletype" (PTY) device driver for
MOS. This "device" appears as a terminal to its application, but what
it is really doing is providing a character stream between two
processes.
AI ELI5
On this page
§1The Remote User Telnet Service
RFC 818 specifies a service that allows a user to access limited hosts over a network using the Telnet protocol. This RFC was suggested by Mike Mulligan at BBN and addresses the case where a host has very limited functionality and no system executive—the command shell that a user would normally interact with.
The service is accessed by making a connection to port 107 (octal 153). Once connected, the standard Telnet protocol is used for the interaction, allowing a user to send commands and receive responses as if they were at the host's terminal.
The RFC says: if you want to talk to a tiny computer far away, connect to port 107, and you can type just like you are sitting next to it.
§2Why This Service Was Needed
In the early ARPANET and Internet, hosts used well-known ports and sockets to offer distinct services. The most common of these was the Server Telnet port, which connected users to a system executive like TOPS-20 EXEC or the Unix Shell.
However, some small hosts had very limited functionality and no executive at all. For these systems, designating specific well-known ports for specific applications made sense. This RFC standardizes such a port for what is called the Remote User Telnet service.
Big computers have a special place you can call to talk to them. Little computers without such a place needed one too, so this RFC created a standard spot.
§3Example: The BBN TC68K Terminal Concentrator
The RFC provides a detailed example: the TC68K, a terminal concentrator based on the Motorola MC68000 at Bolt Beranek and Newman. It had one network connection, sixteen RS232 terminal connections, and ran the Micro-Operating System (MOS) with IP, ICMP, TCP, and Telnet protocols.
The TC68K software included a user Telnet application for network access and a server Telnet for connecting non-network-aware devices. To test remote units, BBN connected a user Telnet back-to-back with a server Telnet using a Pseudo Teletype (PTY) device driver, allowing operators to appear as a local terminal to a remote TC68K and verify the network path was working.
At BBN, they made a small box to route terminals to a network. To test if a far-away box worked, they sent Telnet through the network as if it were a local terminal.