Request For Comments - RFC818
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Network Working Group J. Postel
Request for Comments: 818 ISI
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
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,
Postel [Page 1]
RFC 818 November 1982
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
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
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
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