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perm filename TAC[UP,DOC] blob sn#687997 filedate 1982-11-09 generic text, type C, neo UTF8

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C REC PAGE DESCRIPTION
C00001 00001
C00014 00002 BBN Report No. 4780
C00015 00003 1. Introduction
C00018 00004 The TAC runs on either a Honeywell 316 computer or a BBN
C00020 00005 2. Using the TAC
C00023 00006 if need be, can instruct the TAC to execute these options to meet
C00025 00007
C00028 00008 Some TAC ports are preset to a known rate. The individual
C00030 00009 Once the desired rate is set, type:
C00032 00010 For C/30 TACs, terminals using ports on the TAC must be set
C00035 00011 <Control-Q>
C00037 00012 2.1.3 Device Rate Command
C00039 00013 output, all the above rates apply (including 4800 and 9600 baud).
C00041 00014 the TAC will send the second "@" as data to the host. If the
C00043 00015 Commands can consist of several words. For exampεe:
C00045 00016 return><Linefeed> or <Carriage-return><Null>. The default is
C00048 00017 If the user desires complete transparency on input and/or
C00050 00018 3. TCP/IP and NCP Protocols
C00052 00019 terminal user tells the TAC to stop connection attempts.
C00055 00020 a connection. This may occur if the host
C00058 00021 3.4 Closing a TCP Connection
C00060 00022 3.5 TCP Abnormal Conditions
C00062 00023 3.5.2 TAC Rings Bells When Input is Typed
C00065 00024 about once a minute until the host accepts the
C00069 00025 The TAC has restarted. The connection has
C00071 00026 To open a NCP connection, the user types:
C00074 00027 TAC's IMP down
C00076 00028 3.8.2 NCP Abnormal Conditions
C00079 00029 <sitename> TAC <version #>:<port #>
C00080 00030 4. TAC Commands
C00082 00031 Some commands include one or more parameters following the
C00084 00032 The TAC supports the following rates: input and output - 75,
C00086 00033
C00089 00034 Certain terminals, especially the Teletype Model 37, require
C00092 00035 output.)
C00094 00036 4.5 Transmission Characteristics
C00097 00037 These modes will not transmit until the right character is
C00099 00038
C00102 00039 The TAC sets the port protocol to TCP.
C00105 00040 network the TAC is on. "net#:" is not given
C00108 00041 The other type of Open command available can only be used in
C00111 00042 For non-ARPANET type networks, the parameters
C00114 00043 4.10 Reset Command
C00117 00044 - Sets transmission mode to TRANSMIT EVERY 0.
C00120 00045 Note, also, that a request for these options does not guarantee
C00122 00046 @BINARY INPUT START (@B I S)
C00125 00047 4.11.2 Echo modes
C00128 00048 commands. The TAC does not support the Telnet Remote
C00131 00049 a process, which generates output, to run to
C00136 00050 back to, but not including, the last <Carriage-
C00140 00051 internal buffer transmission capability or a micro-computer)
C00143 00052 NOTE: when the TAC sends the XOFF to the port, it expects
C00146 00053 Certain TAC options will cause a deadlock if set while a
C00149 00054 or a receive-only port, it is useful if the user of another port
C00151 00055 4.15 Commands Requiring Authorization
C00153 00056 an access attempt. To permit such an attempt, the port has to
C00156 00057 a 32-bit socket number. For TCP, the TAC assigns TCP port
C00159 00058 This command simultaneously initiates "@S T H" and
C00162 00059 @HOST 1/6
C00164 00060 @PROTOCOL BOTH (@P B)
C00165 00061 5. Special Operational Issues
C00168 00062 To contact the NOC immediately, telephone (617)661-0100. If
C00170 00063 The specific parameter options that can be tailored for a
C00173 00064 a custom herald message to all of its users. When a user closes
C00176 00065 descriptive.
C00177 00066 6. Appendix A - Command Summary
C00180 00067 @DEVICE CODE EXTRA-PADDING (@D C E)
C00183 00068 * @HOST # (@H #)
C00187 00069 receiving port.
C00190 00070 @SEND ERASE LINE (@S E L)
C00194 00071 7. Appendix B - TAC Messages to the Terminal User
C00197 00072 The remote host has reset the connection to the
C00201 00073 TAC's IMP going down in <mins> mins for <hrs> hrs <mins> mins.
C00203 00074 8. Appendix C - Connection of Terminals to the TAC
C00205 00075
C00206 00076
C00208 00077 The EIA RS-232 modem connector can be used with a modem or a
C00210 00078
C00211 00079 The EIA RS-232-C terminal connector is designed for
C00212 00080
C00213 00081 Current loop connectors are for asynchronous 20ma current
C00214 00082 9. Appendix D - Device Rate Manipulation
C00217 00083 When setting a device rate for a port the user types:
C00219 00084
C00221 00085 10. Appendix E - Internet Addressing: Non-ARPANET-type Networks
C00224 00086 For class C networks: the first THREE bytes are the network
C00226 00087
C00227 00088 Table of Contents
C00234 00089 4.15 Commands Requiring Authorization ................. 53
C00237 ENDMK
C⊗;
BBN Report No. 4780

TAC Users' Guide

September 1982

Bolt Beranek and Newman Inc.
10 Moulton St.
Cambridge, MA 02138

UPDATE HISTORY

Originally written September 1981 by Robert Hinden.
Completely rewritten September 1982 by Robin S. Clifford.

1. Introduction

The Terminal Access Controller (TAC) system provides the

hardware and software necessary to allow a user at a terminal to

connect to hosts on packet-switching networks. The TAC gives the

user a transparent connection to a remote host, as if he were

directly connected to that host.

The TAC User's Guide describes how to use the TAC, details

the available commands and features, and discusses general

information regarding the TAC. This Guide may be used as a

simple reference, or can be read through.

The TAC supports two host-to-host protocols: the Department

of Defense standard Transmission Control Protocol/Internet

Protocol (TCP/IP), and the ARPANET Network Control Protocol

(NCP). In addition, the TAC supports the Initial Connection

Protocol (ICP) for NCP, old and new Telnet protocols, and the

1822 host-to-IMP protocol. A user can connect to a remote host

using either TCP/IP or NCP protocols. (It should be noted that

the DoD intends to stop supporting the NCP protocol as of 1

January 1983.)

-1-

The TAC runs on either a Honeywell 316 computer or a BBN

C/30 system. The H-316 has 32K of memory and a 63-port Multi-

Line Controller (MLC). The C/30 has 64K of memory and one or two

32-port asynchronous multiplexors. (NOTE: port 0 cannot be used.

The C/30 allows up to 31 or 63 ports, depending on whether it has

one or two multiplexors.) The TAC is a host connected to an IMP

but separate from it (unlike the older TIP which runs a logically

separate program within the IMP hardware). Connection of a TAC to

a host is via an 1822 hardware interface. The TAC can be either

a local or distant host, but cannot, as yet, run as a VDH or HDH

host. There can be more than one TAC on an IMP.

For the user, the TAC is very similar to the TIP in its

operation. It supports most of the TIP's commands and features,

as well as providing additional features. For information

regarding the differences between the TIP and the TAC, see

Section 5.3.

-2-

2. Using the TAC

The TAC is designed to allow the terminal user to

communicate with a remote host as if directly connected to that

host. Once the connection is established, the user will

normally ignore the TAC and use the remote host.

Prior to actually connecting to the TAC, the terminal user

will be concerned with terminal hardware, power requirements,

modem equipment (for dial-in access), etc. Although these

concerns are not directly related to the TAC, the user should be

sure that the equipment chosen is compatible with the TAC. (See

Appendix C - "Connection of Terminals to the TAC" - for the

equipment requirements of the TAC. See Appendix D - "Device Rate

Manipulation" - for limitations of device rates on the TAC.)

Once the correct equipment has been selected, the user can

then go about connecting to the TAC, connecting to the remote

host, and, when done, closing the connection.

The TAC maintains a set of default parameters. It is advised

that the user accept these default parameters as they have proven

to meet the needs of most TAC users. There are, however, a

number of optional parameters available in the TAC. The user,

-3-

if need be, can instruct the TAC to execute these options to meet

specific needs. (See Section 4 - "TAC Commands" - for a complete

description of all commands and options available to the user.)

The rest of this section describes different uses of the

TAC, how to connect to the TAC and the remote host, hunting to

the proper terminal speed, etc. It is intended for general use.

Section 4 - "TAC Commands" - contains detailed descriptions of

specific uses of the TAC. If the user has problems with the

TAC, contact the individual site liaison or the NOC (See Section

5.1 - "Dealing with the NOC") for assistance.

2.1 Connecting to the TAC

Once the equipment requirements have been met, the user is

ready to connect to the TAC. To connect, the terminal has to be

wired directly to the TAC or connected via a dial-up modem. The

connection to the TAC is called a "port" and will be referred to

as such throughout this guide.

When the connection is made, the user types:

-4-

<Break>

<Control-Q> (Hold the <Ctrl> key down and press the
"Q" key at the same time. NOTE: the
C/30 TAC cannot hunt to 9600 baud using
<Control-Q>. Another character can be
used instead, such as <Carriage-return>.)

The TAC prints the "herald" message:

or, the TAC will print a garbled message or nothing.

If the TAC prints the herald message, the port is set to the

correct rate for the terminal. The user can proceed to open a

connection to a remote host. If a different rate is desired,

follow the procedure described later in this section.

If the TAC prints a garbled message or nothing, the terminal

and the port are set at different speeds. As most TAC ports are

set to hunting ("hunting" means the TAC port will recognize the

terminal's speed from the first character typed and set itself to

that rate), the user should try to hunt to the correct rate.

This procedure is described below. If hunting is unsuccessful,

contact the individual site liaison or the NOC for assistance.

-5-

Some TAC ports are preset to a known rate. The individual

site liaison is responsible for such assignments and should be

contacted if there are questions (see Section 5.2 - "Site

Tailoring and Default Values"). If the port is set to a

specific rate, change the terminal's speed to the correct rate.

If there are still problems, contact the NOC.

The user may want to change the port speed, even for a

preset rate port. This can be done by using the Device Rate

command (see Section 4.2 - "Device Rate"). It should be noted

that ports with preset rates may be protected from change. This

ensures that the port will be predictably set for all users who

access it.

2.1.1 Hunting

If the port the terminal is connected to is set for hunting,

the user should set the terminal to the desired input and output

rates. Both input and output rates must initially be the same.

(For split rates and output greater than 2400 baud, see Section

2.1.2 - "Split Rate Hunting".)

-6-

Once the desired rate is set, type:

<Break>

<Control-Q>

The TAC prints:

Typing <Break> puts the port (if it is a hunting port) into

hunting mode. If there are problems, <Break> returns the port

to this hunting mode (except when there is an open connection to

a remote host. If there is such a connection, <Break> will

cause the TAC to send a Telnet "break" to the remote host).

<Control-Q> (ASCII DC1, octal 021) is the character the TAC uses

to deduce the terminal rate.

The TAC can hunt to any standard rate from 110 to 9600 baud.

The <Control-Q> will set the input and output rates to the output

rate deduced from the terminal. If this is sufficient, the TAC

will type the herald message. The user can then continue to

open a connection to a remote host, or set up any special

requirements for the TAC.

-7-

For C/30 TACs, terminals using ports on the TAC must be set

to the same input/output rate. Legal rates are: 75, 110, 134.5,

150, 300, 600, 1200, 1800, 2400, 4800 and 9600 baud. A terminal

may hunt to any of these rates. See Section 2.1 - "Connecting

to the TAC" - for information on hunting to 9600 baud.

For H-316 TACs, there is a hardware limitation that

prohibits input rates to be greater than 2400 baud. The H-316

TAC, therefore, has the capability to have different input and

output rates. If the user wants to have the output rate greater

than 2400 baud, there are two methods of achieving this. (Both

methods require switching the terminal's rates. Consult the

manufacturer's documentation for the procedure.) The first

method is called "split rate" hunting (see Section 2.1.2). The

second method is via the Device Rate command (see Section 2.1.3).

2.1.2 Split Rate Hunting (H-316 only)

To set a port to a split rate, with output greater than 2400

baud, set the terminal speeds for input and output to the desired

output rate. Then, type:

<Break>

-8-

<Control-Q>

The TAC will print:

Set Input Rate, Then Type ↑Q

Set the input rate on the terminal to a rate of 2400 baud or

less, leaving the output rate at its initial setting. Then,

type:

<Control-Q>

The TAC will print:

The TAC is ready for the user to connect to a remote host,

or to set up specific options in the TAC. (Note: the above

method, in effect, hunts twice. First, for the output rate,

then, for the input rate. <Break> is only typed at the very

beginning of the process. If it is typed a second time, the

port will return to its initial hunting state.)

-9-

2.1.3 Device Rate Command

To change the device rate of a port via the Device Rate

command, set the terminal rate to some input/output value (2400

baud or less for input for H-316 TACs). Then, type:

<Break>

<Control-Q>

The TAC prints its herald message:

The user can then use the Device Rate command to change the

input/output rate of the port. The command syntax is:

@DEVICE RATE # (or, @D R #)

# is the parameter that specifies the rate. See Section

4.2 - "Device Rate" - in the Command section for interpretation

of #.

Once the device rate has been changed for the port, set the

terminal rate to the correct setting.

The Device Rate command allows the TAC port to be set to the

following rates: for input, 75, 110, 134.5, 150, 300, 600, 1200,

1800, 2400 baud (plus 4800 and 9600 baud for the C30 TAC); for

-10-

output, all the above rates apply (including 4800 and 9600 baud).

2.2 Giving a TAC Command

When the user wishes to communicate with the TAC (e.g., to

open a connection to a remote host, set TAC parameters, reset a

port, etc.), TAC commands must be given. These commands need to

be flagged in some way that the TAC can recognize. This flag is

called the intercept character. The default for the intercept

character is the "@" symbol. All commands to the TAC must be

preceded by the intercept character for the TAC to accept them.

Once the command is typed to the TAC, it is executed with a

<Carriage-return> if the user is satisfied with the command. If

the user is not satisfied with the command, <rubout> will abort

it.

If the user wants to include the intercept character (as

text) in the data stream to the remote host, typing the intercept

character twice will cause the second character to be sent to the

host. For example, if the user types:

-11-

the TAC will send the second "@" as data to the host. If the

port is in remote echo mode (see Section 4.11.2 - "Echo Modes"),

the user will see "@@@" from the TAC. The first two "@" are

echoed by the TAC, the last "@" is echoed from the remote host.

Generally, commands to the TAC can be given at any time.

They need not start on a new line. The TAC does not distinguish

between upper or lower case letters.

Between the intercept character and the <Carriage-return>,

there should be one or more words for the command, followed,

perhaps, by one or more parameters. The TAC only recognizes the

first letter of each word in a command. Therefore, commands can

be abbreviated. For example, if opening a connection to a

remote host, the full command would be:

@OPEN 10:2/49 <Carriage-return> (TCP only)

@OPEN 2/49 <Carriage-return> (TCP or NCP)

The TAC will also recognize:

@O 10:2/49 <Carriage-return> (TCP only)

@O 2/49 <Carriage-return> (TCP or NCP)

-12-

Commands can consist of several words. For example:

@DEVICE CODE ASCII <Carriage-return>

This command can be abbreviated to:

@D C A <Carriage-return>

The spaces are required. @DCA <Carriage-return> would be

interpreted by the TAC as @D, and is not valid.

2.3 Sending and Receiving Data: TAC Parameters

Once a connection to a remote host is open, sending data to

that host is straightforward. The TAC permits the user to

communicate with the host as if directly connected. The TAC

saves 7-bit ASCII characters in an input buffer and transmits

them at appropriate intervals in messages to the host. Several

commands exist in the TAC to control the frequency of

transmission of characters (see Section 4.5 - "Transmission

Characteristics").

The TAC does not perform code translation. The single

exception (in accordance with Telnet protocol) is that a

<Carriage-return> is automatically translated to <Carriage-

-13-

return><Linefeed> or <Carriage-return><Null>. The default is

<Carriage-return><Linefeed>. Some TAC ports may be preset to

<Carriage-return><Null>. For further discussion, see Section

4.4 - "Linefeed Insertion".

As mentioned earlier, the TAC interprets the "@" symbol to

be the command intercept character. If the "@" is a frequently

used character, the user may want to change the intercept

character to something else. This is described in Section 4.7 -

"Intercept Character Changes".

Telnet protocol defines several standard representations for

control functions, such as "break" and "interrupt process".

These are intended as a convenience to the user of several

different host systems. The user is thus not required to learn

the conventions of each host. The TAC provides several commands

to insert these standard function codes in the data stream.

These commands are described in Section 4.12 - "Telnet Standard

Control Functions".

Receiving data from the remote host is also straightforward.

The TAC receives characters and sends them to the terminal

without translation.

-14-

If the user desires complete transparency on input and/or

output (all eight bits, no intercept character, parity or fill

characters), the TAC allows 8-bit binary mode - if the host

agrees. Section 4.11 - "Connection Options" - describes binary

mode more fully.

As stated earlier, the user is advised to accept the default

parameters of the TAC. The novice user will be able to use the

TAC with a minimum of experience. However, if the experienced

user has more complex needs, the TAC has the versatility to

satisfy them.

-15-

3. TCP/IP and NCP Protocols

3.1 TCP and IP Protocols

The Transmission Control Protocol (TCP) and the Internet

Protocol (IP) are highly reliable protocols for host-to-host

communication within a packet-switched computer network and

between such networks.

The TCP/IP protocols have been declared, by the Department

of Defense, as the basis for a DoD standard host-to-host

communications protocol. Support for the ARPANET NCP

(Network Control Protocol) will be withdrawn sometime in the

future.

3.2 TCP Operation

A TCP connection consists of opening a connection to a

remote host, sending and receiving data, and closing the

connection. However, it should be kept in mind that TCP

will attempt to keep a connection to a host active even if that

host does not acknowledge data from a TAC or is down. TCP will

try to send data to a host about once a minute or until the

-16-

terminal user tells the TAC to stop connection attempts.

3.3 Opening a TCP Connection

There are two ways to open a connection to a host via TCP.

The first uses a "net#:host#/IMP#" combination. The second

method utilizes a four-field internet address and is only

permitted in TCP. It is described in Section 4.9 - "Connection

Control".

The first method is used as follows:

User types: @OPEN net#:host#/IMP# <Carriage-return>

TAC prints: "TCP Trying..."

The TAC will print one of the following:

Open
The remote host agrees to connect. The user may
then continue.

Destination host dead
Remote host is not communicating with the network.

Host down until <day> at <hour>:<minutes> <timezone>.
Remote host is not communicating with the
network. The day and time when the host was
most recently scheduled to come up is indicated.

Refused
The remote host rejected the attempt to establish

-17-

a connection. This may occur if the host
does not support TCP and/or Telnet. The TAC
port will now be idle.

Retransmitting
This indicates that TCP is having to
retransmit many times in attempting to
open a connection to a host. The host may
not support TCP and is ignoring the connection
attempt.

Destination Unreachable
There is no path from the TAC to the remote host
through the communication networks.

Can't
The user already has an active connection to
another host. It is necessary to close or
reset the connection before another attempt can be
made.

Open error
An error occurred while the OPEN attempt was in
progress. This probably indicates a host
error and should not happen often. If it is
recurrent, contact the NOC (see section 5.1)

TAC's IMP down
The IMP that connects the TAC to the network
is down.

A "connection" consists of a data path in both directions,

between the TAC and a remote host. When the "OPEN" occurs,

the connection is complete. A port may only have one open

connection at a time.

-18-

3.4 Closing a TCP Connection

Before a host connection is closed, the user must first log

out of the remote host. Follow whatever procedures the

host requires. Then, to close the connection between the TAC

and the host, type:

@CLOSE <Carriage-return>

The TAC will print "Closed" when the connection to the host is

terminated. The host has agreed to end the connection in

both directions. The "Closed" message should appear in a

second or two. However, sometimes the host does not respond

to the @CLOSE command. If this happens, the user should reset

the connection by typing:

@RESET <Carriage-return>

Once a connection is closed or reset, the user can then open a

new connection.

-19-

3.5 TCP Abnormal Conditions

Once a connection is open, the TAC (and network) will

normally provide an invisible link to a remote host.

However, problems will sometimes occur, such as slow response,

pauses, or broken connections. Some of the most common

problems are described below.

3.5.1 Slow Reponse

Slow response occurs for a number of reasons but is

usually transient. It can be caused by heavy load on some

part of the network, the host, or the TAC. Or, a line or

node down on the network can cause heavy load on alternate

routes, thus causing slow response. If the problem does

not clear within a short period of time and is not host

related, contact the NOC (see Section 5.1 - "Dealing with the

NOC").

-20-

3.5.2 TAC Rings Bells When Input is Typed

If the TAC's buffers are full when a character is typed, it

will discard the character and ring the terminal's "bell" by

sending <Bell> (ASCII BEL, octal 007). (<Bell> will also be

sent when Telnet negotiations fail.) The buffers will be full

only if congestion exists on the network or in the host,

preventing the TAC from sending buffered characters. If the

problem persists for several minutes, the user can try to flush

the buffers using the @FLUSH and/or @SEND SYNC commands.

(See Section 4.6 - "Clear Input Buffer" - and Section 4.12 -

"Telnet Standard Control Funtions".)

3.5.3 TAC Error Messages

In the event of problems with the connection between a

terminal and a remote host, the TAC attempts to keep the user

informed. The TAC will give the following error messages

if a problem should arise:

Retransmitting
TCP has to retransmit many times to get the TAC
data accepted by the remote host. The
message will appear after TCP has retransmitted
the data five times. It will then appear

-21-

about once a minute until the host accepts the
data or the TAC port is reset.

Host closing connection
The remote host has closed its connection to
the TAC. The TAC will close its connection to
the host. The TAC port will then be idle.

Destination host dead
The remote host is not communicating with the
network.

Destination unreachable
No path exists from the TAC to the remote host
through the communications networks.

Host down until <day> at <hour>:<minutes> <timezone>.
The remote host is not communicating with the
network. The day and time that the host was
most recently scheduled up is indicated.

Host reset connection
The remote host reset the connection to the
TAC. The TAC port is now idle.

TAC going down in <mins> mins for <hrs> hrs <mins> mins.
The TAC is going down in the time indicated.

TAC going down NOW
The TAC will go down immediately.

TAC's IMP going down in <mins> mins for <hrs> hrs <mins> mins
The TAC's IMP is going down in the time
indicated. Although the TAC will still respond,
it will be isolated from the network.

TAC's IMP going down NOW
The TAC's IMP will go down immediately.

TAC's IMP down
The TAC's IMP is down. The TAC is isolated
from the network.

-22-

The TAC has restarted. The connection has
been reset.

3.6 NCP Protocol

The Network Control Protocol (NCP) is a host-to-host

communication protocol used on the ARPANET. It is limited to

use on the ARPANET and will be replaced by TCP in the near

future (scheduled date - 1 January 1983).

3.7 NCP Operation

NCP operation is very similar to TCP. However, it is not

as reliable or versatile as TCP and will close a connection to a

remote host when problems are encountered.

3.8 Opening an NCP Connection

Opening a connection to a remote host using NCP is the same

as with the first method for TCP (see Section 3.3 - "Opening a

TCP Connection"). It cannot be used except on ARPANET

hosts supporting NCP.

-23-

To open a NCP connection, the user types:

@OPEN host#/IMP#

The TAC prints "NCP trying...". The user will get one of

the following responses:

Open
The remote host agrees to connect. The user
may continue.

Destination host dead
The remote host is not communicating with
the network.

Host down until <day> at <hour>:<minutes> <timezone>.
The remote host is not communicating with
the network. The day and time when the host
was most recently scheduled to come up is
indicated.

Refused
The remote host has rejected the connection
attempt.

Destination unreachable
There is no path from the TAC to the remote host
through the communications network.

Can't
The TAC already has a connection to a remote
host. The user must close or reset the
connection before another attempt can be made.

Open error
An error occurred while the OPEN attempt was in
progress. This probably indicates a host
error and should not happen often. If it is
recurrent, contact the NOC (see section 5.1).

-24-

TAC's IMP down
The IMP that connects the TAC to network is down.

In any case except the OPEN, the TAC port will be idle

after receiving the above responses.

3.8.1 Closing an NCP Connection

To close an NCP connection, type:

@CLOSE <Carriage-return>

The TAC will print "Closed". The TAC port is now idle.

To reset an NCP connection, type:

@RESET <Carriage-return>

The TAC will print:

Once a connection is closed or reset, the user can open a new

connection.

-25-

3.8.2 NCP Abnormal Conditions

Problems with an NCP connection to a remote host are

similar to those using TCP. Error messages for NCP are listed

below. For other types of abnormal conditions, see Section

3.5 - "TCP Abnormal Conditions".

Closed
The remote host has closed the connection.

Destination host dead
The remote host is not up to the network.

Host reset connection
The remote host has reset the connection to
the TAC.

Host down until <day> at <hour>:<minutes> <timezone>.
The remote host is not communicating with
the network. The day and time that the host is
scheduled to come up is indicated.

TAC going down in <mins> mins for <hrs> hrs <mins> mins
The TAC is going down in the time indicated.

TAC going down NOW
The TAC will go down immediately.

TAC's IMP going down in <mins> for <hrs>hrs <mins> mins
The TAC's IMP is going down in the time
indicated. Although the TAC will still respond,
it will be isolated from the network.

TAC's IMP going down NOW
The TAC's IMP will go down immediately.

TAC's IMP down
The TAC's IMP is down. The TAC is
isolated from the network.

-26-

<sitename> TAC <version #>:<port #>
The TAC has restarted. The connection has
been reset.

-27-

4. TAC Commands

Some of the TAC commands available to the user were covered

in Section 2 - "Using the TAC". These and other commands will

now be explained in more detail. This section is for reference

and should be used to clarify the options available on the TAC.

The commands are local to the TAC and should not be confused with

host-level commands.

4.1 TAC Command Language

In order to communicate with a TAC, it is necessary to flag

a command with a character the TAC can identify. This is

called the intercept character. The "@" symbol is the default

intercept character, but can be changed to another character by

using the "intercept" command (see Section 4.7 - "Intercept

Character Changes").

The command itself is made up of words separated by spaces.

Commands can be abbreviated, since the first character of each

word is the only one recognized by the TAC. Upper or lower

case letters can be used.

-28-

Some commands include one or more parameters following the

command text. These parameters are in decimal, with the

exception of the NCP socket number, which is in octal.

Once typed, the command is executed with a <Carriage-

return>. A command can be aborted by using <Rubout> (ASCII

DEL, octal 177) or by typing an illegal command. The TAC will

print "Bad" or "Can't". Most commands can be given at any time

and can begin anywhere on a line.

If the intercept command is doubled (i.e., typing "@@" or

two of the selected intercept characters) it is treated as a

single text character. It will be transmitted to the host as

normal text.

4.2 Device Rate

Although the TAC can hunt to a variety of baud rates, it is

sometimes useful to set a port to a specific speed. For

example, many lineprinters are receive-only devices and are

unable to transmit their baud rate to the TAC to set the port

speed.

-29-

The TAC supports the following rates: input and output - 75,

110, 134.5, 150, 300, 600, 1200, 1800, 2400, 4800 and 9600.

(Note: the Honeywell 316 TAC can only run up to 2400 baud for

input, but can run split speeds. The C/30 TAC must have input

and output the same.) The TAC also supports externally clocked

rates (terminal provides the clock). Rates may be pre-set in

an individual site's parameter file (see Section 5.2 - "Site

Tailoring and Default Values"). A port may have its rate

specified as permanent, in which case it cannot be changed via

user command.

To set the device rate for a port, type:

@DEVICE RATE # (@D R #)
# is a decimal number representing a 10-bit field.
It sets the input rate, output rate and character
size. Since most ASCII terminals use 8-bit
characters, the following list gives examples of
available rates with 8-bit characters.

-30-

Decimal Parameter I/O Rate

113 75 baud
178 110
243 134.5
308 150
373 300
438 600
503 1200
568 1800
633 2400
698 4800 (C/30 TAC only)
763 9600 (C/30 TAC only)
754 110 input/9600 output
(H-316 TAC only)
757 300 input/9600 output
(H-316 TAC only)
761 2400 input/9600 output
(H-316 TAC only)
1023 Externally clocked

(Note: See Appendix D for more information regarding rates and

character size.)

4.3 Padding and Parity

Some terminals and lineprinters need extra time after

<Carriage-return> <Linefeed> for the print head to return to

margin. Others, CRTs for example, need no padding. The TAC

provides several options for padding output to a terminal.

-31-

Certain terminals, especially the Teletype Model 37, require

even parity. The TAC supports even parity or no parity. The

default on the TAC is no parity.

The default for the TAC is to insert no padding and no

parity on output. (The exception is: when the TAC hunts to 300

baud, it automatically sets extra padding.) The options

available are to insert padding on output to slow carriage-return

terminals or lineprinters and to set even parity. The desired

option can be pre-set in the site parameter file (see Section 5.2

- "Site Tailoring and Default Values"). It should be noted

that binary output mode suppresses any padding and parity.

To select the desired padding or parity, type:

@DEVICE CODE ASCII (@D C A)
The TAC will not insert padding on output to the
terminal. Any prior padding is disabled.

@DEVICE CODE EXTRA-PADDING (@D C E)
The TAC pads the output with <Nulls> after a
<Carriage-return> is sent by the terminal. The
number of <Nulls> sent is related to the
terminal's output rate. The faster the rate,
the more <Nulls> inserted. This option is
useful with terminals that have a slow carriage-
return, such as Texas Instrument or Execuport
terminals.

@DEVICE CODE OTHER-PADDING (@D C O)
The TAC pads output to an ODEC lineprinter.
(This brand printer requires special timing for

-32-

output.)

@DEVICE CODE 37 (@D C 37)
The TAC sets the parity of the port to even
parity.

To change any of the padding or parity settings, the user

must send an @D C A first. Also, padding and parity cannot be

set when flow control is enabled.

4.4 Linefeed Insertion

When a terminal sends a <Carriage-return>, Telnet protocol

requires the TAC to send a <Linefeed> or a <Null> character.

The default for the TAC is to insert <Linefeed>. Linefeed

insertion is suppressed in binary input mode. To set the

character insertion, type:

@INSERT LINEFEED (@I L)
TAC inserts <Linefeed> after the terminal sends
<Carriage-return>.

@CLEAR INSERT LINEFEED (@C I L)
TAC inserts <Null> after the terminal sends
<Carriage-return>.

-33-

4.5 Transmission Characteristics

Once a connection to a remote host is established, the TAC

normally sends every character to the host as soon as it is

typed. However, the user can specify when the TAC should send

data. The following commands represent the options available.

They are maximum values, since the TAC may send data sooner than

specified if its input buffer fills up.

To change the transmission rate, type:

@TRANSMIT EVERY # (@T E #)
TAC attempts to send every # characters. # is a
decimal number. The initial TAC mode is TRANSMIT
EVERY 1.

@TRANSMIT ON LINEFEED (@T O L)
TAC transmits to the remote host whenever a
<Linefeed> is received from the terminal.

@TRANSMIT ON MESSAGE-END (@T O M)
TAC transmits to the remote host whenever a
<Control-S> is received from the terminal.

@TRANSMIT NOW (@T N)
TAC immediately sends all characters stored in
input buffers to the remote host. This does not
change other transmission modes previously set.
The TAC may always be forced to transmit the
contents of its buffers by using the @T N command.

@TRANSMIT EVERY 0 (@T E 0)
Used to reset the TRANSMIT ON MESSAGE-END and the
TRANSMIT ON LINEFEED modes.

-34-

These modes will not transmit until the right character is

received, or the buffer is full, or an @T N is sent.

4.6 Clear Input Buffer

@FLUSH (@F)
TAC clears any unsent characters in the input
buffers.

4.7 Intercept Character Changes

Although the "@" is the default intercept character, it is

sometimes useful to change it to an alternate character.

(NOTE: the intercept character may be specified as permanent in

the site file and thus be unchangeable via user command. See

Section 5.2 - "Site Tailoring and Default Values".

To change the intercept character, type:

@INTERCEPT # (@I #)
# is the decimal number representing the ASCII
character desired.

Some of the common intercept characters are listed below

along with their decimal value:

-35-

Decimal value ASCII character

004 EOT <Control-D>
016 DLE <Control-P>
025 EM <Control-Y>
033 !
035 #
037 %
039 '
064 @
092 \
094 ↑
095 ← (Underscore)
124 |
126 } (Tilde)

@INTERCEPT ESC (@I E)
Sets intercept character to "@". "@" is the
default.

@INTERCEPT NONE (@I N)
No intercept character is recognized. The port
is in a pseudo "7-bit binary mode". Once this
mode is enabled, a reset must be given from
another port on a TAC to disable this mode on the
TAC port.

4.8 Protocol Selection

The protocols available to the user are TCP or NCP.

Unless specified in the site parameter file, the default is TCP.

To select the desired protocol, type:

@PROTOCOL TCP (@P T)

-36-

The TAC sets the port protocol to TCP.

@PROTOCOL NCP (@P N)
The TAC sets the port protocol to NCP.

The protocol may be changed only when no open connection to

a remote host exists.

4.9 Connection Control: Open and Close

The Open and Close commands permit the user to establish or

terminate a connection to a remote host. A list of the

messages the TAC will print can be found in Appendix B - "TAC

Messages to the Terminal User".

There are two types of Open commands. The first type can

be used in TCP or NCP, as follows:

@OPEN net#:host#/imp#;port# (@O net#:host#/imp#;port#)*

The TAC will open a connection to the specified
remote host.

The "net#:" field is the number of the network to
which the remote host is connected. It can only
be used in TCP mode. The number can be from 0
to 255 and is decimal. The default is the
←←←←←←←←←←←←←←←
* ";port#" is a TCP-specific term and should not be confused with
the term "port" which refers to the TAC port to which the user's
device is attached.

-37-

network the TAC is on. "net#:" is not given
when in NCP mode.

The "host#/" field is the number of the remote
host on the IMP. The number can be from 0 to
255 and is decimal. If not specified, the
default is zero. In TCP mode, the "host#/"
field is mapped in the upper 8-bits of the "local
address" portion of the internet address.

The "imp#" field is the IMP number on which the
remote host is connected. There is no default
and it must be specified. The number can be from
0 to 65,535. It is decimal. In TCP mode, the
"imp#" field is mapped in the lower 16-bits of the
internet "local address".

The ";port#" field is the number of the port or
socket the TAC will connect to. This number is
decimal in TCP mode and octal in NCP mode. In
either mode it is optional. The default value is
the new Telnet socket (23 decimal, 27 octal).
In TCP mode, it is the 16-bit port that the TAC
will connect to. In NCP mode, it is the 32-bit
socket that ICP will use as a logger socket.

The following are all valid examples of connecting to the

Telnet socket (23) on host 0 of IMP 5 on network 10:

@O 10:0/5;23 (TCP only)

@O 0/5;23

@O 5;23

@O /5

@O 10:/5

-38-

The other type of Open command available can only be used in

TCP mode. It is formed as follows:

@OPEN a.b.c.d;port# (@O a.b.c.d;port#)

For ARPANET-type networks, the formula is defined
as:

"a." is the network number. For example,
the ARPANET number is 10. The default is
the network the TAC is on.

"b." is the host number on the IMP. The
default is zero.

"c." is the logical host number. The
default is zero.

"d" is the IMP number the remote host is
connected to. There is no default. The
IMP number must be given.

";port#" is the TCP port to which the
connection will be made. It is optional.
It defaults to Telnet (23). It is a decimal
number.

(NOTE: if the default is desired as a
parameter, it is sufficient to just enter the
period after a parameter. For example,
@O 10...5 is a legal address.) **

←←←←←←←←←←←←←←←
** Other valid examples of the Open commands using TCP are:

@O ...5
@O ..5
@O .5
@O 5

All open a connection to host 0 on IMP 5.

-39-

For non-ARPANET type networks, the parameters
"a.b.c.d" specify a 32-bit Internet address.
See Appendix E for a complete breakdown of
the Internet address.

If a user wishes to communicate between TAC ports, the TCP

";port#" number or the NCP socket number has to be calculated for

the remote port.

To define the ";port#" for TCP, the formula is:

(decimal)port # X 256 + 23 = TCP ;port#

The NCP socket number is calculated by putting double the

octal port number in the left 16-bits and a 2 or 3 (transmit or

receive) in the right 16-bits. See Section 4.14.4 - "Low Level

NCP Protocol Commands" - for an example of the NCP socket number.

To terminate a connection, use the Close command. It

applies to both TCP and NCP connections.

To close a connection to a remote host, type:

@CLOSE (@C)
The TAC connection to the remote host is
terminated. The TAC port is now idle.

-40-

4.10 Reset Command

The Reset command will restore the TAC port to its initial

state. It applies to both TCP and NCP modes. It is especially

useful, however, when the port is in TCP mode. If a remote

host goes down or becomes unreachable, TCP will keep

retransmitting to that host about once a minute. The CLOSE

command will not be acknowledged. In this event, the Reset

will close the connection completely. The port will then be

ready for any other connection attempts.

To reset a TAC port, type:

@RESET (@R)
The TAC closes any existing connections.
Terminal parameters are returned to their preset
values. The TAC will print the herald. This
command does return the port to a hunting state.

The Reset command will clear certain options that have been

previously set on a port. Reset will do the following:

- Simultaneously do a "give back" on the user's
port, the port the user is controlling and will
force any other port to relinquish control. (See
Section 4.14.3 - "Controlling Another Port".)

- Returns echo mode to LOCAL ECHO, with REMOTE ECHO
desired (when connected to a remote host).

- Sets port to allow 8-bit binary mode when
requested.

-41-

- Sets transmission mode to TRANSMIT EVERY 0.

- Returns "wild" mode to not wild.

- Inserts <Linefeed> after <Carriage-return>.

Reset does not change the Device Rate setting for a port.

That rate will remain the same as it was before the Reset command

was given. Reset does not alter flow control, padding or

parity.

4.11 Connection Options: Binary and Echo Modes

Telnet protocol allows the user (TAC) and server hosts to

negotiate transmission characteristics and define a default set

of parameters. It then permits each side of the connection to

negotiate for non-standard options. Hosts are not required to

accept non-standard options and can refuse to accept them without

understanding what they have just refused. This permits

sophisticated hosts to define complex options without burdening

simple hosts.

The TAC provides two options for the user. These options

are Binary (input and/or output) Mode and Remote Echo Mode.

The TAC to host connection must exist to use these options.

-42-

Note, also, that a request for these options does not guarantee

acceptance.

4.11.1 Binary Input and Output Modes

Normally, the TAC sends and receives data to and from a

remote host in 7-bit characters. The Binary Mode option allows

all eight bits to be sent or received over a connection to the

remote host. Usually, the TAC user will not need the Binary

Mode option.

When binary mode is requested, the TAC sets an internal flag

for the mode the user desires. It will then attempt to get the

remote host to agree to implement the mode. It is important to

note that a connection to a remote host must exist for the binary

options to function. The TAC will passively accept a binary

mode request from a remote host. (NOTE: If the user sets @B I E

or @B O E (see below) on a TAC port, the port will reject any

requests for binary mode from a remote host or a remote terminal

on a TAC.)

The commands for binary mode(s) are:

-43-

@BINARY INPUT START (@B I S)
Enables eight-bit binary input mode.

@BINARY INPUT END (@B I E)
Disables eight-bit binary input mode.

@BINARY OUTPUT START (@B O S)
Enables eight-bit binary output mode.

@BINARY OUTPUT END (@B O E)
Disables eight-bit binary output mode.

When Binary Mode(s) are in effect, the TAC passes all eight

bits of data to and/or from the remote host. In Binary Output

Mode, there is no padding or parity added to the output. In

Binary Input Mode, <Linefeed> and <Null> are not inserted after

<Carriage-return>. NOTE: the intercept character and local TAC

commands cannot be given with binary input mode enabled. The

remote host or a remote terminal on a TAC must terminate binary

input mode on the port.

Although binary input and output are independent in the TAC,

some hosts only negotiate both together. Since binary mode can

only be executed on an open connection, closing the connection

returns the TAC port to non-binary mode and allows local command

execution.

-44-

4.11.2 Echo modes

Echoing is the process of outputting characters typed as

input by the user back to the terminal. This process can occur

at the terminal (ECHO HALFDUPLEX), in the TAC (ECHO LOCAL), or in

the remote host (ECHO REMOTE).

The commands to set echo mode are:

@ECHO HALFDUPLEX (@E H)
The TAC assumes the terminal is physically
providing its own echo of characters. The TAC
will not echo input characters from the terminal
and will attempt to negotiate local echo with the
remote host. The TAC will send to the terminal
any characters it inserts in the data stream
(<Linefeed> after <Carriage-return>, for example).
This mode is used for physical halfduplex
terminals or special-purpose terminals connected
to the TAC to eliminate almost all echoing.

@ECHO LOCAL (@E L)
The TAC echoes all characters received from the
terminal.

@ECHO REMOTE (@E R)
If a connection to a remote host is open, the TAC
requests the host to echo. The TAC will still
echo TAC-level commands.

The default for the TAC, when not actively connected to a

host, is ECHO LOCAL. When a connection is active, the default

changes to ECHO REMOTE. Remote echo requires that a TAC-to-host

connection exist. Most users will not need to use the echo

-45-

commands. The TAC does not support the Telnet Remote

Controlled Transmission and Echoing (RCTE) option.

4.12 Telnet Standard Control Functions

The function of the Telnet protocol is to provide a standard

interface for terminal devices and terminal-oriented processes

throughout the network. The Telnet protocol defines a standard

representation for certain functions, described below. These

representations have standard, but not required, meaning.

(NOTE: a system that does not provide such functions to local

users need not provide them to network users. The standard

representation for a function may be treated as a no-operation by

the host.) A system that provides the function to the local users

is obliged to provide the same function to the network user

transmitting the standard representation for the function. The

user may still use the server host's method of invoking the

function. The TAC commands simply send the appropriate Telnet

function codes to the host if the connection is open (or

opening).

@SEND ABORT OUTPUT (@S A O)
(AO) - Many systems provide a function that allows

-46-

a process, which generates output, to run to
completion without sending the output to the
user's terminal. This function also clears any
output already produced but not yet output to the
user's terminal. AO is the standard
representation for implementing this function.
The server host should allow the current process
to run, or appear to run, to completion. It
will not send its output to the user terminal.
A SYNC is also sent to the user.

@SEND ARE YOU THERE (@S A Y T)
(AYT) - Many systems provide a function that gives
the user some visible (i.e., printable) or
audible evidence that the host is up and running.
The user can implement this function if the host
system is unexpectedly silent for a long period of
time due to heavy load, unanticipated long
computations, etc. AYT is the standard
representation for this function. When the
function is implemented, the server host should
send back to the NVT (network virtual terminal)
some visible or audible evidence that the AYT was
received.

@SEND BREAK (@S B)
(BRK) - The interpretation of the BREAK function
is entirely up to the host. Many server hosts
ignore it. The function is provided because it is
a signal outside the ASCII set and is given local
meaning in many host systems. It indicates that
the Break or Attention key was hit.

@SEND ERASE CHARACTER (@S E C)
(EC) - The server host may provide a function to
delete the last preceding character from the
stream of data from the user. The function is
used to edit keyboard input when typing mistakes
are made. EC is the standard representation for
this function.

@SEND ERASE LINE (@S E L)
(EL) - The server host may provide a function to
delete all the characters from the data stream

-47-

back to, but not including, the last <Carriage-
return><Linefeed> sequence sent over the Telnet
connection. This function is used to edit user
keyboard input. EL is the standard
representation for this function.

@SEND INTERRUPT PROCESS (@S I P)
(IP) - The server host may provide a function to
suspend, interrupt, abort, or terminate the
operation of a user process. This function is
useful if a user believes the process is in an
unending loop or if an unwanted process is
inadvertently activated. IP is the standard
representation for this function. The TAC
automatically follows IP with a SYNC character.
This function is not yet supported in TCP mode.

@SEND SYNC (@S S)
The NVT has a key labelled "SYNC". Real
terminals do not have such a key. The function
is unique to network use. The "SYNC" key is a cue
to the server host that there is an important
message buffered in an inaccessible place. The
TAC and the server host go to the trouble to get
the SYNC indication over a different channel to
bypass the normal buffering conventions. Normal
use of this function would be @S B followed by
@S S.

4.13 Flow Control Options

Certain devices, such as lineprinters, need a means of

preventing their internal buffers from over-flowing when data is

sent from the TAC at a rapid rate. Likewise, the TAC may need

a means of keeping its buffers from over-flowing when data is

sent at too rapid a rate from a device (such as a terminal with

-48-

internal buffer transmission capability or a micro-computer)

connected to the TAC. These requirements are accomplished by

the use of input and output flow control.

The TAC provides XON-XOFF flow control for the input

(terminal to TAC) and output (TAC to terminal) directions. The

XOFF flow control character is <Control-S> (ASCII DC3, octal

023). The XON flow control character is <Control-Q> (ASCII

DC1, octal 021).

With Output Flow Control enabled, the TAC will stop sending

output to a port when it receives an XOFF from the terminal.

The TAC will start sending data when it receives an XON from the

terminal. This is normally used for lineprinters and terminals

which provide slow scrolling of output.

With Input Flow Control enabled, the TAC will accept data

from the port until the TAC's input buffer is approximately 85%

full. The TAC will then send an XOFF to the port to signal the

port to stop sending data. After the TAC's input buffer

empties to about 70%, the TAC will send an XON to the port to

signal the terminal to resume sending data.

-49-

NOTE: when the TAC sends the XOFF to the port, it expects

the port to stop sending input within a few characters. If the

terminal continues sending, the TAC's input buffer may overflow,

causing characters to be lost. To prevent lost characters

during high speed transmission through the TAC, the user should

have the remote host NOT echo the data back to the user. This

can be accomplished via the remote host, or, if that is not

possible, by setting the TAC port to ECHO HALF-DUPLEX during the

high speed transmission of data. This will prevent characters

from being lost due to delays caused by the remote host's echoing

of characters sent.

The commands for Input Flow Control are:

@FLOW-CONTROL INPUT START (@F I S)
Enables flow control from the terminal to the TAC.

@FLOW-CONTROL INPUT END (@F I E)
Disables flow control from the terminal to the
TAC.

The commands for Output Flow Control are:

@FLOW-CONTROL OUTPUT START (@F O S)
Enables flow control from the TAC to the terminal.

@FLOW-CONTROL OUTPUT END (@F O E)
Disables flow control from the TAC to the
terminal.

-50-

Certain TAC options will cause a deadlock if set while a

port has flow control enabled. Therefore, the following

options cannot be set at the same time: Input Flow Control and

Output Flow Control; Input Flow Control and Binary Output Mode;

Output Flow Control and Binary Input Mode; and, Output Flow

Control and Transmit on EOM (which uses <Control-S>). Padding

and parity cannot be set when flow control is enabled. Also,

when Input Flow Control is enabled, the TAC will not output, to

the terminal, the characters <Control-S> or <Control-Q> sent from

the remote host. When Output Flow Control is enabled,

<Control-S> and <Control-Q> cannot be entered into the data

stream from the terminal.

Flow control can be permanently enabled for a port in the

site parameter file (see Section 5.2 - "Site Tailoring and

Default Values"). If it is not enabled in the site file, hanging

up a data set will disable any flow control mode.

4.14 Controlling Another Port

Normally, the terminal user only sends commands to the TAC

port being used. However, it is sometimes necessary to give

commands from one port to another. In the case of a hung port

-51-

or a receive-only port, it is useful if the user of another port

can send commands to it.

To send commands to another port, the command has to be

preceded by the number of that port. The port number must be

in octal. For example, to send a command to port 16 (octal),

type:

@16 DEVICE RATE 633 (@16 D R 633)
The device rate for port 16 would be set to 2400
baud for input and output rates.

Since only one port at a time can control another port, it

is necessary to give up control when finished sending commands to

the other port.

To give up control of another port, type:

@# GIVE BACK (@# G B)
# is the octal number of the port one wishes to
stop controlling.

NOTE: To issue a command requiring authorization, the

receiving port must have that authorization set.

-52-

4.15 Commands Requiring Authorization

Certain ports, due to regulatory requirements, need to have

authorization given to allow certain parameter changes from

another port on a TAC. This authorization has to be approved by

the individual site liaison and executed by the NOC. (See

Section 5 - "Special Operational Issues".)

The following commands require authorization on the port of

the TAC to which such commands are directed.

4.15.1 Open

Authorization is required to set the ";port#" parameter in

the OPEN command. See Section 4.9 - "Connection Control: Open

and Close" - for a complete discussion of the OPEN command.

4.15.2 Wild Mode

Sometimes it is useful for a remote host to be able to

access a passive device connected to a port on the TAC (such as a

lineprinter or tape punch). Normally, the TAC will refuse such

-53-

an access attempt. To permit such an attempt, the port has to

be put into a state called "wild".

"Wild" mode allows a remote host to send to a selected port.

There are several things one must be remember when using a port

set to "wild". The remote host must know the location of the

wild port: its port or socket number and host address. The

request to send to the port must use the correct protocol: if the

port is in NCP mode, it will not accept a TCP request; if it is

in TCP mode it will not accept an NCP request.

"Wild" mode can be set via user command or can be pre-set in

the site parameter file (see Section 5.2 - "Site Tailoring and

Default Values"). It is useful, for example, if the remote

host uses the port as an output device for spooling a listing

queue to a lineprinter.

The commands for enabling and disabling "wild" mode are:

@SET DEVICE WILD (@S D W)
Enables "wild" mode on a port.

@CLEAR DEVICE WILD (@C D W)
Disables "wild" mode on a port.

A TAC port in "wild" mode will accept a connection made to

its port or socket. TCP uses a 16-bit port number. NCP uses

-54-

a 32-bit socket number. For TCP, the TAC assigns TCP port

numbers using the upper 8-bits set to the device port number (1-

63) and the lower 8-bits set to 23. For NCP, the TAC assigns a

socket number with double the octal port number in the high-order

16-bits and 2 (or 3) set in the low-order 16-bits (for the

sending or receiving socket). (See the end of Section 4.9 -

"Connection Control: Open and Close" - to formulate TCP ;port#

and NCP socket #.)

4.15.3 Low Level NCP Protocol Commands

There are several low-level NCP commands that allow manual

NCP host-to-host protocol. These commands can be used to

connect to a socket other than the logger socket, or to connect

to another TAC port. The commands are valid only in NCP mode,

and can only be used when no connection is active. Most users

will not use these commands. (NOTE: when DoD stops support of

NCP, these commands will be invalid.)

The commands available are:

@HOST # (@H #)
# is the network address of the host or port.

-55-

This command simultaneously initiates "@S T H" and
"@R F H"

@INITIAL CONNECTION PROTOCOL (@I C P)
Starts the NCP initial connection protocol.

@PROTOCOL BOTH (@P B)
Simultaneously sets NCP "@P T T" and "@P T R".

@PROTOCOL TO RECEIVE (@P T R)
Initiates receive side of NCP connection protocol.

@PROTOCOL TO TRANSMIT (@P T T)
Initiates transmit side of NCP connection
protocol.

@RECEIVE FROM HOST # (@R F H #)
# is the network address of the host or port.
Establishes host # parameter for manual
initialization of the connection.

@RECEIVE FROM SOCKET # (@R F S #)
# is the socket number in octal. Establishes
socket # parameter for manual initialization of
the connection.

@SEND TO HOST # (@S T H #)
# is the network address of the host or port.
Establishes host # parameter for manual
initialization of the connection.

@SEND TO SOCKET # (@S T S #)
# is the socket number in octal. Establishes
socket # parameter for manual initialization of
the connection.

As mentioned earlier, one use of the low-level protocol is

to open a connection to a socket other than the standard Telnet

logger socket. The following example demonstrates how to open a

connection to socket 13 on host 1 of IMP 6.

-56-

@HOST 1/6
@RECEIVE FROM SOCKET 13
@INITIAL CONNECTION PROTOCOL

A connection would now exist to the selected socket on the

host/IMP.

Another use for the low-level protocol is to open a

connection to another TAC port. The socket number of the

destination TAC port must be known. The socket number is a

32-bit word. The high order 16-bits contain twice the (octal)

port number. The low order 16-bits contain a 2 (or 3) for the

send (or receive) socket. The following commands demonstrate

how to open a connection to port 3 of the TAC on host 2 of IMP

40:

@SEND TO HOST 2/40
@RECEIVE FROM HOST 2/40
@SEND TO SOCK ET 600002
@RECEIVE FROM SOCKET 600003

At this point, the TAC must initiate the connection.

Type:

@PROTOCOL TO TRANSMIT (@P T T)
@PROTOCOL TO RECEIVE (@P T R)

Or, to initiate both protocols at the same time, type:

-57-

@PROTOCOL BOTH (@P B)

The connection to port 3 of TAC host 2 on IMP 40 is

complete.

-58-

5. Special Operational Issues

5.1 Dealing with the Network Operations Center (NOC)

The Network Operations Center is located at, and operated

by, Bolt Beranek and Newman Inc. in Cambridge, Massachusetts.

It is staffed 24 hours a day, seven days a week. Should problems

occur that the user is unable to diagnose and fix locally, the

NOC can assist by: fixing TAC problems and setting ports; setting

up and changing site parameter files; helping to diagnose

problems on the TAC, remote host, phone lines or terminals;

answering questions, explaining situations and referring problems

through the proper channels.

It is helpful to both the user and the NOC if problems with

the TAC are reported immediately. When contacting the NOC, the

user should try to be as specific as possible. Diagnosing and

fixing a problem can be most readily accomplished if the user can

tell the NOC what TAC system they are using, and the port number

on the TAC. Also, informing the NOC of the time of day the

problem occurs can help relate the problem to other network

events.

-59-

To contact the NOC immediately, telephone (617)661-0100. If

the problems or questions are not critical, network mail can be

sent to the NOC. The address is CONTROL@BBN-NOC.

5.2 Site Tailoring and Default Values

Each TAC site has a site-specific file that contains

parameter information unique to each site. This site file is

included whenever the TAC is reloaded by the NOC. The file

contains the name of the site, the port configuration, and other

information needed by the TAC program.

The site-specific file allows individual ports on a TAC to

be tailored for particular devices. This is very helpful when

equipment such as lineprinters, special devices (micro-computers,

smart terminals), etc. are connected to the TAC.

A liaison at each TAC site is responsible for coordinating

and requesting changes to the site file. If users wish to have

specific options enabled for their TAC ports, they should contact

the site liaison. The NOC can provide the name of the liaison

for any site upon request.

-60-

The specific parameter options that can be tailored for a

TAC port are:

- input and/or output rates

- non-hunting (default is hunting)

- parity (default is none)

- EIA or current loop capability (default is EIA)

- permanent rates (default is changeable rate setting)

- TCP or NCP protocol (default is TCP)

- insert <Linefeed> or <Null> after <Carriage-return>
(default is insert <Linefeed>)

- padding (default is no padding)

- flow control enabled (default is flow control disabled)

- echo mode (default is local echo)

- wild mode (default is not wild)

- intercept character (default is "@")

- permanent intercept character (default is changeable
intercept character assignment)

- transmission frequency (default is every character)

- low-level protocol authorization (default is not
authorized)

- old Telnet authorization (default is not authorized)

In addition to the standard herald message, the TAC can send

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a custom herald message to all of its users. When a user closes

a connection or hunts to speed on a port, a short message can be

sent to the user's terminal along with the normal TAC herald

message. This is useful to warn users of a particular TAC of

extended outages or dial-up access changes. If this feature is

desired, contact the NOC for details.

5.3 Differences Between the TIP and the TAC

Although the TAC operates in a similar manner to the older

TIP systems, there are a number of important differences. These

are:

- TCP/IP protocols are supported on the TAC.

- The TAC uses a different hunting scheme. It can hunt
to higher rates. It can hunt on direct connect or
dial-in terminals.

- The TAC does not implement the Telnet Remote Controlled
Transmission and Echoing (RCTE) option.

- The TAC does not support the IBM 2741 terminal or the
magnetic tape option.

- The TAC does not implement the divert output option.

- Low level protocol commands are only supported for NCP.

- Old format addressing is not supported.

- Some error messages have been changed and made more

-62-

descriptive.

- The TAC is a separate host from the IMP. There can be
more than one TAC on an IMP.

- The TAC commands are exectued by a <Carriage-return>.

- The reset command does not set a port back to hunting
mode.

-63-

6. Appendix A - Command Summary

Following is a list of commands available on the TAC,

followed by a brief description of use for each command. The

commands are presented in capital letters. Lower case words and

"#" are parameters. "*" signifies the command is allowed in NCP

mode only. (NOTE: only the first letter of each word in a

command is recognized by the TAC. It is possible, therefore, to

abbreviate the command to be just the first letter of each word.

The abbreviated format is given in parentheses.)

@BINARY INPUT END (@B I E)
Turn off 8-bit binary input mode.

@BINARY INPUT START (@B I S)
Turn on 8-bit binary input mode.

@BINARY OUTPUT END (@B O E)
Turn off 8-bit binary output mode.

@BINARY OUTPUT START (@B I S)
Turn on 8-bit binary output mode.

@CLEAR DEVICE WILD (@C D W)
Take device out of wild mode.

@CLEAR INSERT LINEFEED (@C I L)
Stop inserting <Linefeed> after <Carriage-return>.
Inserts a <Null> after <Carriage-return>.

@CLOSE (@C)
Close the connection.

@DEVICE CODE ASCII (@D C A)
No padding performed on output.

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@DEVICE CODE EXTRA-PADDING (@D C E)
Insert padding on output for terminals with slow
carriage-return.

@DEVICE CODE OTHER-PADDING (@D C O)
Insert padding in output for lineprinter.

@DEVICE CODE 37 (@D C 37)
The TAC sets the parity of the port to even
parity.

@DEVICE RATE # (@D R #)
Set input and output rates and character size to
#.

@ECHO HALFDUPLEX (@E H)
TAC echoes nothing; terminal generates echo. (The
TAC inserts the <Linefeed> character.)

@ECHO LOCAL (@E L)
TAC generates echo.

@ECHO REMOTE (@E R)
Remote host echoes data; TAC echoes TAC commands.

@FLOW-CONTROL INPUT END (@F I E)
Flow control from terminal to TAC is disabled.

@FLOW-CONTROL INPUT START (@F I S)
Flow control from terminal to TAC is enabled.

@FLOW-CONTROL OUTPUT END (@F O E)
Flow control from TAC to terminal is disabled.

@FLOW-CONTROL OUTPUT START (@F O S)
Flow control from TAC to terminal is enabled.

@FLUSH (@F)
Delete all unsent characters in input buffer.

@# GIVE BACK (@# G B)
Give up control of port #. (# is the octal port
number.)

-65-

* @HOST # (@H #)
Simultaneously sets "@S T H" and "@R F H". # is
the host/imp address.

* @INITIAL CONNECTION PROTOCOL (@I C P)
Starts the NCP initial connection protocol.

@INSERT LINEFEED (@I L)
Insert <Linefeed> after <Carriage-return>.

@INTERCEPT # (@I #)
Set intercept character to #. # is a decimal
number representing an ASCII character.

@INTERCEPT ESCAPE (@I E)
Reset the intercept character back to the default
"@".

@INTERCEPT NONE (@I N)
Enables 7-bit binary mode.

@NEW TELNET (@N T)
Enables new Telnet mode.

@OLD TELNET (@O T)
Enables old Telnet mode. Can only be used with
permission from the NOC.

@OPEN net#:host#/imp#;port# (@O net#:host#/imp#;port#)
Open a connection to a specific host on the
specified network. This may be used for TCP or
NCP connections. ("net#:" is not specified for
NCP.) Authorization required to set the ";port#"
parameter for the receiving port.

@OPEN a.b.c.d;port# (@O a.b.c.d;port#)
Open a connection to a specified internet address.
This is used for TCP connections only.
Authorization required to set the ";port#"
parameter for the receiving port.

* @PROTOCOL BOTH (@P B)
Simultaneously sets NCP "@P T T" and "@P T R"
commands. Authorization required for the

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receiving port.

@PROTOCOL NCP (@P N)
Sets the protocol to NCP.

@PROTOCOL TCP (@P T)
Sets the protocol to TCP.

* @PROTOCOL TO RECEIVE (@P T R)
Initiates receive side of NCP connection protocol.
Authorization required for the receiving port.

* @PROTOCOL TO TRANSMIT (@P T T)
Initiates transmit side of NCP connection
protocol. Authorization required for the
receiving port.

* @RECEIVE FROM HOST # (@R F H)
Establishes host # parameter for manual
initialization of connection protocol. # is the
network address of the host.

* @RECEIVE FROM SOCKET # (@R F S #)
Establishes socket # parameter for manual
initialization of connection protocol. # is the
socket number in octal.

* @RECEIVE FROM WILD (@R F W)
Same as "@R F S <socket #>".

@RESET (@R)
Resets the TAC port. Resets existing connection.

@SEND ABORT OUTPUT (@S A O)
Send the Telnet "AO" command.

@SEND ARE YOU THERE (@S A Y T)
Send the Telnet "AYT" command.

@SEND BREAK (@S B)
Send the Telnet "BRK" command.

@SEND ERASE CHARACTER (@S E C)
Send the Telnet "EC" command.

-67-

@SEND ERASE LINE (@S E L)
Send the Telnet "EL" command.

@SEND INTERRUPT PROCESS (@S I P)
Send the Telnet "IP" command followed by the SYNC
character sequence. (Not yet implemented in TCP.)

@SEND SYNC (@S S)
Send the Telnet "SYNC" character and an "Interrupt
Process" message.

* @SEND TO HOST # (@S T H #)
Establishes the host # parameter for manual
initialization of connection protocol. # is the
network address of the host.

* @SEND TO SOCKET # (@S T S #)
Establishes the socket # parameter for the manual
initialization of connection protocol. # is the
socket number in octal.

@SET DEVICE WILD (@S D W)
Port will accept a connection from any host.
Authorization required for the receiving port.

@TRANSMIT EVERY # (@T E #)
The TAC will attempt to send every # characters to
the remote host. # is a decimal number.

@TRANSMIT NOW (@T N)
The TAC immediately sends all characters stored in
the input buffers to the remote host.

@TRANSMIT ON LINEFEED (@T O L)
The TAC transmits to the remote host whenever a
<Linefeed> is received from the terminal.

@TRANSMIT ON MESSAGE-END (@T O M)
The TAC transmits to the remote host whenever a
<Control-S> is received from the terminal.

@TRANSMIT EVERY 0 (@T E 0)
Resets TRANSMIT ON MESSAGE-END and TRANSMIT ON
LINEFEED modes.

-68-

7. Appendix B - TAC Messages to the Terminal User

Following is a list of messages the TAC may, at times, give

the terminal user.

<sitename> TAC <version#>:<port#>
The TAC has acknowledged the user attempt to
connect to the TAC. <version#> is the software
version number running in the TAC. <port#> is the
octal port number the user is connected to. The
user can now use the TAC to connect to a remote
host.

Bad
The TAC does not recognize the command.

Can't
The TAC could not execute the command.

Closed
The TAC's connection to the remote host is closed.

Destination host dead
The remote host is not communicating with the
network.

Destination unreachable
There is no path from the TAC to the remote host
through the communication networks.

Host closing connection
The remote host has closed its connection to the
TAC. The TAC will close its connection to the
host. The port will then be idle.

Host down until <day> at <hour>:<minutes> <timezone>.
The remote host is not communicating with the
network. The day and time when the host was most
recently scheduled to come up is indicated.

Host reset connection

-69-

The remote host has reset the connection to the
TAC.

NCP Trying...
The TAC is attempting to open a NCP connection.

No
Parameters cannot be set for the specified port.

Not authorized
Low-level protocol authorization is needed.
Contact the NOC.

Num
The TAC expects a number. The command is aborted.

Open
The TAC's connection to the remote host is open.

Open error
An error occurred while the OPEN attempt was in
progress. This probably indicates a host error
and should not happen often. If it is recurrent,
contact the NOC (see Section 5.1).

Refused
The remote host rejected the attempt to establish
a connection. This may occur if the remote host
does not support TCP and/or Telnet. The TAC port
will now be idle.

Retransmitting
This indicates that TCP has to retransmit many
times to open a connection to a remote host, or to
get TAC data accepted by that host. The message
will occur after TCP has retransmitted five times.
It will appear about once a minute until the data
is accepted, or the user resets the connection.

Set Input Rate, Then Type ↑Q
The TAC has hunted to an acceptable output rate,
but too high an input rate. The user must set the
input rate to 2400 baud or less, then type
<Control-Q>. (This applies only to H-316 TACs.)

-70-

TAC's IMP going down in <mins> mins for <hrs> hrs <mins> mins.
The TAC's IMP is going down in the time indicated.
Although the TAC will still respond, it will be
isolated from the network.

TAC's IMP going down NOW
The TAC's IMP is going down immediately.

TAC's IMP down
The TAC's IMP is down. The TAC is isolated from
the network.

TAC going down in <mins> mins for <hrs> hrs <mins> mins.
The TAC is going down in the time indicated.

TAC going down NOW
The TAC is going down immediately.

TCP Trying...
The TAC is attempting to open a TCP connection to
a remote host.

-71-

8. Appendix C - Connection of Terminals to the TAC

The TAC permits a wide variety of terminal types to access

it. The basic requirement for the TAC is that the terminals and

modems using it conform to EIA RS-232 specification. It will

also support terminals that use 20 milliampere current loop to

communicate.

The TAC allows for the use of modems connected via dedicated

or dial-up lines. A variety of manufacturers' modems will work

on the TAC. The requirements for these modems are that they be

similar to Bell 103, Bell 202S and 202T, and Vadic 3400 series

modems.

When selecting modem equipment for use on the TAC, it is

advised that the user contact the NOC for assistance. There are

many modem manufacturers, and selection of a proper modem will

make using the TAC more convenient.

In any case, if there is a question about proper equipment

for TAC use, contact the NOC. They will be able to help the user

in determining what can be best applied to the TAC.

-72-

The tables on the following pages list the pin and signal

allocations for TACs with connectors for modems, EIA terminals

and current loop terminals.

-73-

PIN SIGNAL DESIGNATION

1 Protective Ground

2 Transmitted Data (TD) (from TAC)

3 Received Data (RD) (to TAC)

4 Request to Send (RTS) (from TAC)

6 Data Set Ready (DSR) (to TAC)

7 Signal Ground

8 Data Carrier Detect (DCD) (to TAC)

9 + VDC

10 - VDC

15 Transmitted Clock (TC) (to TAC)

17 Received Clock (RC) (to TAC)

20 Data Terminal Ready (DTR) (from TAC)

Table 1. TAC Signal Allocation for EIA RS-232 Modem Connector

-74-

The EIA RS-232 modem connector can be used with a modem or a

terminal. Table 1 lists the pin assignments for a modem

connected via a cable with no crossings or jumpers - all signals

should run straight through.

If a terminal is to be connected to the port, the cable must

be modified to be a null modem cable. To do this: cross the Data

Set Ready (DSR) pin of the fantail connector to the Data Terminal

Ready (DTR) pin of the terminal connector; cross the DTR pin of

the fantail connector to the DSR pin of the terminal connector;

cross the Request to Send (RTS) pin of the fantail connector to

the Data Carrier Detect (DCD) pin of the terminal connector;

cross the DCD pin of the fantail connector to the RTS pin of the

terminal connector; and, jumpering pin 4 to pin 5 in the terminal

connector. A null modem adapter is available to accomplish the

above.

-75-

PIN SIGNAL ALLOCATION

1 Protective Ground

2 Transmitted Data (TD) (by terminal)

3 Received Data (RD) (by terminal)

4 Request to Send (RTS) (from terminal)

5 Clear to Send (CTS) (from TAC)

6 Data Set Ready (DSR) (from TAC)

7 Signal Ground

8 Data Carrier Detect (DCD) (from TAC)

9 + VDC

10 - VDC

Table 2. TAC Signal Allocation for EIA RS-232 Terminal Connector

-76-

The EIA RS-232-C terminal connector is designed for

terminals with an EIA RS-232 interface. No crossed signals or

jumpers are needed.

-77-

PIN SIGNAL DESIGNATION

1 Spare

2 Transmitter Source (TS)

3 Ground

4 Receiver Return (RR)

5 Spare

6 Spare

7 Transmitter Return (TR)

8 Receiver Source (RS)

9 Spare

Table 3. TAC Signal Allocation for Current Loop Cable Connector

-78-

Current loop connectors are for asynchronous 20ma current

loop terminals. Current source is provided by the system for all

signals. Cable wiring is dependent on the connector pin

assignments at the terminal to be used.

-79-

9. Appendix D - Device Rate Manipulation

In section 4.2 - "Device Rate" - examples were given for

various device rates using 8-bit characters. This appendix will

give the user the breakdown of the device rate parameter to allow

for character size different from 8-bit. It also gives the user

the formula to set a variety of input and output rates.

It should be noted that certain hardware limitations exist

on both the C/30 TAC and the H-316 TAC. The C/30 TAC requires

the terminal connected to a port be set to the same input/output

rate. Legal rates are: 75, 110, 134.5, 150, 300, 600, 1200,

1800, 2400, 4800 and 9600 baud. A terminal may hunt to any of

these rates (with the exception of 9600/9600 baud. Another

character, <Carriage-return> for example, can be used to hunt

instead of <Control-Q>). Also, the C/30 TAC will only recognize

8-bit character size.

For the H-316 TAC, the hardware limitation prohibits input

rates to be greater than 2400 baud. The H-316 TAC, therefore,

has the capability to have different input and output rates.

See Sections 2.1.2 - "Split Rate Hunting" - and 2.1.3 - "Device

Rate Command" - for further discussion.

-80-

When setting a device rate for a port the user types:

@DEVICE RATE # (or, @D R #)

# represents, in decimal, a 10-bit field. The field is

divided as follows:

+-+-+-+-+-+-+-+-+-+-+
| 4 | 2 | 4 |
+-+-+-+-+-+-+-+-+-+-+
| | |
| | +-- Input Rate
| |
| +-- Character Size
|
+-- Output Rate

To determine the device input/output rate and the character

size, use the following table.

-81-

Character Size Input/Output Rates

0 (00) 5-bits 0 hunting

1 (01) 6-bits 1 75 baud

2 (10) 7-bits 2 110

3 (11) 8-bits 3 134.5

4 150

5 300

6 600

7 1200

10 1800

11 2400

12 4800 (output only for H-316)

13 9600 (output only for H-316)

17 synchronous

-82-

10. Appendix E - Internet Addressing: Non-ARPANET-type Networks

The internet addressing scheme makes provision for three

classes of networks: classes A, B and C. The ARPANET is a class

A network. The TAC expects to be connected to a class A network.

The TAC is, however, capable of communicating with host computers

on the other classes of networks. The main difference in

addressing the different classes of networks is in the

interpretation of the four bytes of the internet address.

For class A networks: the first byte is interpreted as the

network number. The following three bytes are interpreted as a

local address. Class A network numbers may range from (decimal)

0 through 127. For example, to connect to host 3 on IMP 49 on

net 10, the address would be 10.3.0.49.

For class B networks: the first TWO bytes are the network

number. The remaining two bytes are the local address. The

first byte of a class B network must be between 128 and 191, and

the second byte may take any value between 0 and 255. An

example of a class B internet address is 128.10.0.2.

-83-

For class C networks: the first THREE bytes are the network

number. There is only one byte of local address. The first

byte of a class C network must have a value between 192 and 223,

and the second and third bytes may take on any value between 0

and 255. An example of a class C internet address is

192.1.2.67.

To access a class B or class C network, or a class A network

which uses the third byte of the internet address in its local

address (unlike the ARPAnet, which expects this byte to be zero),

it is necessary to use the a.b.c.d format for the Open command on

the TAC.

-84-

-85-

Table of Contents

1. Introduction ........................................ 1
2. Using the TAC ....................................... 3
2.1 Connecting to the TAC .............................. 4
2.1.1 Hunting .......................................... 6
2.1.2 Split Rate Hunting (H-316 only) .................. 8
2.1.3 Device Rate Command ............................. 10
2.2 Giving a TAC Command .............................. 11
2.3 Sending and Receiving Data: TAC Parameters ........ 13
3. TCP/IP and NCP Protocols ........................... 16
3.1 TCP and IP Protocols .............................. 16
3.2 TCP Operation ..................................... 16
3.3 Opening a TCP Connection .......................... 17
3.4 Closing a TCP Connection .......................... 19
3.5 TCP Abnormal Conditions ........................... 20
3.5.1 Slow Reponse .................................... 20
3.5.2 TAC Rings Bells When Input is Typed ............. 21
3.5.3 TAC Error Messages .............................. 21
3.6 NCP Protocol ...................................... 23
3.7 NCP Operation ..................................... 23
3.8 Opening an NCP Connection ......................... 23
3.8.1 Closing an NCP Connection ....................... 25
3.8.2 NCP Abnormal Conditions ......................... 26
4. TAC Commands ....................................... 28
4.1 TAC Command Language .............................. 28
4.2 Device Rate ....................................... 29
4.3 Padding and Parity ................................ 31
4.4 Linefeed Insertion ................................ 33
4.5 Transmission Characteristics ...................... 34
4.6 Clear Input Buffer ................................ 35
4.7 Intercept Character Changes ....................... 35
4.8 Protocol Selection ................................ 36
4.9 Connection Control: Open and Close ................ 37
4.10 Reset Command .................................... 41
4.11 Connection Options: Binary and Echo Modes ........ 42
4.11.1 Binary Input and Output Modes .................. 43
4.11.2 Echo modes ..................................... 45
4.12 Telnet Standard Control Functions ................ 46
4.13 Flow Control Options ............................. 48
4.14 Controlling Another Port ......................... 51

-i-

4.15 Commands Requiring Authorization ................. 53
4.15.1 Open ........................................... 53
4.15.2 Wild Mode ...................................... 53
4.15.3 Low Level NCP Protocol Commands ................ 55
5. Special Operational Issues ......................... 59
5.1 Dealing with the Network Operations Center (NOC) .. 59
5.2 Site Tailoring and Default Values ................. 60
5.3 Differences Between the TIP and the TAC ........... 62
6. Appendix A - Command Summary ....................... 64
7. Appendix B - TAC Messages to the Terminal User ..... 69
8. Appendix C - Connection of Terminals to the TAC .... 72
9. Appendix D - Device Rate Manipulation .............. 80

10. Appendix E - Internet Addressing: Non-ARPANET-type
Networks ........................................ 83

TABLES

TAC Signal Allocation for EIA RS-232 Modem Connector ... 74
TAC Signal Allocation for EIA RS-232 Terminal Connector 76
TAC Signal Allocation for Current Loop Cable Connector . 78

-ii-