The KISS TNC: A simple Host-to-TNC communications protocol
Mike Chepponis, K3MC, Phil Karn, KA9Q
The KISS TNC provides direct computer to TNC communication using a simple
protocol described here. Many TNCs now implement it, including the TAPR TNC-1
and TNC-2 (and their clones), the venerable VADCG TNC, the AEA PK-232/PK-87
and all TNCs in the Kantronics line. KISS has quickly become the protocol of
choice for TCP/IP operation and multi-connect BBS software.
Standard TNC software was written with human users in mind; unfortunately,
commands and responses well suited for human use are ill-adapted for host
computer use, and vice versa. This is especially true for multi-user servers
such as bulletin boards which must multiplex data from several network
connections across a single host/TNC link. In addition, experimentation with
new link level protocols is greatly hampered because there may very well be no
way at all to generate or receive frames in the desired format without
reprogramming the TNC.
The KISS TNC solves these problems by eliminating as much as possible from the
TNC software, giving the attached host complete control over and access to the
contents of the HDLC frames transmitted and received over the air. This is
central to the KISS philosophy: the host software should have control over
all TNC functions at the lowest possible level.
The AX.25 protocol is removed entirely from the TNC, as are all command
interpreters and the like. The TNC simply converts between synchronous HDLC,
spoken on the fullor half-duplex radio channel, and a special asynchronous,
full duplex frame format spoken on the host/TNC link. Every frame received on
the HDLC link is passed intact to the host once it has been translated to the
asynchronous format; likewise, asynchronous frames from the host are
transmitted on the radio channel once they have been converted to HDLC format.
Of course, this means that the bulk of AX.25 (or another protocol) must now be
implemented on the host system. This is acceptable, however, considering the
greatly increased flexibility and reduced overall complexity that comes from
allowing the protocol to reside on the same machine with the applications to
which it is closely coupled.
It should be stressed that the KISS TNC is intended only as stopgap. Ideally,
host computers would have HDLC interfaces of their own, making separate TNCs
unnecessary.  Unfortunately, HDLC interfaces are rare, although they are
starting to appear for the IBM PC. The KISS TNC therefore becomes the ``next
best thing'' to a real HDLC interface, since the host computer only needs an
ordinary asynchronous interface.
2. Asynchronous Frame Format
The ``asynchronous packet protocol'' spoken between the host and TNC is very
simple, since its only function is to delimit frames. Each frame is both
preceded and followed by a special FEND (Frame End) character, analogous to an
HDLC flag. No CRC or checksum is provided. In addition, no RS-232C
handshaking signals are employed.
The special characters are:
Abbreviation Description Hex value
FEND Frame End C0
FESC Frame Escape DB
TFEND Transposed Frame End DC
TFESC Transposed Frame Escape DD
The reason for both preceding and ending frames with FENDs is to improve
performance when there is noise on the asynch line. The FEND at the beginning
of a frame serves to ``flush out'' any accumulated garbage into a separate
frame (which will be discarded by the upper layer protocol) instead of
sticking it on the front of an otherwise good frame. As with back-to-back
flags in HDLC, two FEND characters in a row should not be interpreted as
delimiting an empty frame.
Frames are sent in 8-bit binary; the asynchronous link is set to 8 data bits,
1 stop bit, and no parity. If a FEND ever appears in the data, it is
translated into the two byte sequence FESC TFEND (Frame Escape, Transposed
Frame End). Likewise, if the FESC character ever appears in the user data, it
is replaced with the two character sequence FESC TFESC (Frame Escape, Tran-
sposed Frame Escape).
As characters arrive at the receiver, they are appended to buffer containing
the current frame. Receiving a FEND marks the end of the current frame.
Receipt of a FESC puts the receiver into ``escaped mode'', causing the
receiver to translate a following TFESC or TFEND back to FESC or FEND,
respectively, before adding it to the receive buffer and leaving escaped mode.
Receipt of any character other than TFESC or TFEND while in escaped mode is an
error; no action is taken and frame assembly continues. A TFEND or TESC
received while not in escaped mode is treated as an ordinary data character.
This procedure may seem somewhat complicated, but it is easy to implement and
recovers quickly from errors. In particular, the FEND character is never sent
over the channel except as an actual end-of-frame indication. This ensures
that any intact frame (properly delimited by FEND characters) will always be
received properly regardless of the starting state of the receiver or
corruption of the preceding frame.
This asynchronous framing protocol is identical to ``SLIP'' (Serial Line IP),
a popular method for sending ARPA IP datagrams across asynchronous links. It
could also form the basis of an asynchronous amateur packet radio link
protocol that avoids the complexity of HDLC on slow speed channels.
4. Control of the KISS TNC
Each asynchronous data frame sent to the TNC is converted back into ``pure''
form and queued for transmission as a separate HDLC frame. Although removing
the human interface and the AX.25 protocol from the TNC makes most existing
TNC commands unnecessary (i.e., they become host functions), the TNC is still
responsible for keying the transmitter's PTT line and deferring to other
activity on the radio channel. It is therefore necessary to allow the host to
control a few TNC parameters, namely the transmitter keyup delay, the
transmitter persistence variables and any special hardware that a particular
TNC may have.
To distinguish between command and data frames on the host/TNC link, the first
byte of each asynchronous frame between host and TNC is a ``type'' indicator.
This type indicator byte is broken into two 4-bit nibbles so that the
low-order nibble indicates the command number (given in the table below) and
the high-order nibble indicates the port number for that particular command.
In systems with only one HDLC port, it is by definition Port 0. In multi-port
TNCs, the upper 4 bits of the type indicator byte can specify one of up to
sixteen ports. The following commands are defined in frames to the TNC (the
``Command'' field is in hexadecimal):
Command Function Comments
0 Data frame The rest of the frame is data to be sent
on the HDLC channel.
1 TXDELAY The next byte is the transmitter keyup
delay in 10 ms units. The default
start-up value is 50 (i.e.,500ms).
2 P The next byte is the persistence parame-
ter, p, scaled to the range 0 - 255 with
the following formula:
P = p * 256 - 1
The default value is P = 63 (i.e., p =
3 SlotTime The next byte is the slot interval in 10
ms units. The default is 10 (i.e., 100ms).
4 TXtail The next byte is the time to hold up the
TX after the FCS has been sent, in 10 ms
units. This command is obsolete, and is
included here only for compatibility with
some existing implementations.
5 FullDuplex The next byte is 0 for half duplex, nonzero
for full duplex. The default is 0
(i.e., half duplex).
6 SetHardware Specific for each TNC. In the TNC-1, this
command sets the modem speed. Other
implementations may use this command
for other hardware-specific functions.
FF Return Exit KISS and return control to a higher-
level program. This is useful only
when KISS is incorporated into the TNC
along with other applications.
The following types are defined in frames to the host:
Type Function Comments
0 Data frame Rest of frame is data from the HDLC channel
No other types are defined; in particular, there is no provision for
acknowledging data or command frames sent to the TNC. KISS implementations
must ignore any unsupported command types. All KISS implementations must
implement commands 0,1,2,3 and 5; the others are optional.
5. Buffer and Packet Size Limits
One of the things that makes the KISS TNC simple is the deliberate lack of
TNC/host flow control. The host computers run higher level protocol
(typically TCP, but AX.25 in the connected mode also qualifies) that handles
flow control on an end-to-end basis. Ideally, the TNC would always have more
buffer memory than the sum of all the flow control windows of all of the
logical connections using it at that moment. This would allow for the worst
case (i.e., all users sending simultaneously). In practice, however, many (if
not most) user connections are idle for long periods of time, so buffer memory
may be safely ``overbooked''. When the occasional ``bump'' occurs, the TNC
must drop the packet gracefully, i.e., ignore it without crashing or losing
packets already queued. The higher level protocol is expected to recover by
``backing off'' and retransmitting the packet at a later time, just as it does
whenever a packet is lost in the network for any other reason. As long as
this occurs infrequently, the performance degradation is slight; therefore the
TNC should provide as much packet buffering as possible, limited only by
Individual packets at least 1024 bytes long should be allowed. As with buffer
queues, it is recommended that no artificial limits be placed on packet size.
For example, the K3MC code running on a TNC-2 with 32K of RAM can send and
receive 30K byte packets, although this is admittedly rather extreme. Large
packets reduce protocol overhead on good channels. They are essential for
good performance when operating on high speed modems such as the new WA4DSY 56
The P and SlotTime parameters are used to implement true p-persistent CSMA.
This works as follows:
Whenever the host queues data for transmission, the TNC begins monitoring the
carrier detect signal from the modem. It waits indefinitely for this signal
to go inactive. When the channel clears, the TNC generates a random number
between 0 and 1. If this number is less than or equal to the parameter p,
the TNC keys the transmitter, waits .01 * TXDELAY seconds, and transmits all
queued frames. The TNC then unkeys the transmitter and goes back to the idle
state. If the random number is greater than p, the TNC delays .01 * SlotTime
seconds and repeats the procedure beginning with the sampling of the carrier
detect signal. (If the carrier detect signal has gone active in the meantime,
the TNC again waits for it to clear before continuing). Note that p=1 means
``transmit as soon as the channel clears''; in this case the p-persistence
algorithm degenerates into the 1-persistent CSMA generally used by
conventional AX.25 TNCs.
P-persistence causes the TNC to wait for an exponentially-distributed random
interval after sensing that the channel has gone clear before attempting to
transmit. With proper tuning of the parameters p and SlotTime, several
stations with traffic to send are much less likely to collide with each other
when they all see the channel go clear. One transmits first and the others
see it in time to prevent a collision, and the channel remains stable under
heavy load. See references  through  for details.
We believe that optimum p and SlotTime values could be computed automatically.
This could be done by noting the channel occupancy and the length of the
frames on the channel. We are proceeding with a simulation of the
p-persistence algorithm described here that we hope will allow us to construct
an automatic algorithm for p and SlotTime selection.
We added p-persistence to the KISS TNC because it was a convenient opportunity
to do so. However, it is not inherently associated with KISS nor with new
protocols such as TCP/IP. Rather, persistence is a channel access protocol
that can yield dramatic performance improvements regardless of the higher
level protocol in use; we urge it be added to every TNC, whether or not it
7. Implementation History
The original idea for a simplified host/TNC protocol is due to Brian Lloyd,
WB6RQN. Phil Karn, KA9Q, organized the specification and submitted an initial
version on 6 August 1986. As of this writing, the following KISS TNC
TNC type Author Comments
TAPR TNC-1 Marc Kaufman, Both download and dedicated ROM
& clones WB6ECE versions.
TAPR TNC-2 Mike Chepponis, First implementation, most
& clones K3MC widely used. Exists in both
downloadable and dedicated ROM
VADCG TNC Mike Bruski, Dedicated ROM.
& Ashby TNC AJ9X
AEA PK-232 Steve Stuart, Integrated into standard AEA
PK-87 N6IA firmware as of 21 January 1987.
The special commands ``KISS
ON'' and ``KISS OFF'' (!)
control entry into KISS
Kantronics Mike Huslig Integrated into standard
Kantronics firmware as of
The AEA and Kantronics implementations are noteworthy in that the KISS
functions were written by those vendors and integrated into their standard TNC
firmware. Their TNCs can operate in either KISS or regular AX.25 mode without
ROM changes. The TNC-1 and TNC-2 KISS versions were written by different
authors than the original AX.25 firmware. Because of the specialized
development environment used for the TNC-1 code, and because original source
code for the TNC-2 was not made available, the KISS authors wrote their code
independently of the standard AX.25 firmware. Therefore these TNCs require
the installation of nonstandard ROMs. Two ROMs are available for the TNC-2.
One contains ``dedicated'' KISS TNC code; the TNC operates only in the KISS
mode. The ``download'' version contains standard N2WX firmware with a
bootstrap loader overlay. When the TNC is turned on or reset, it executes the
loader. The loader will accept a memory image in Intel Hex format, or it can
be told to execute the standard N2WX firmware through the ``H'' command.
The download version is handy for occasional KISS operation, while the
dedicated version is much more convenient for full-time or demo KISS
The code for the TNC-1 is also available in both download and dedicated
versions. However, at present the download ROM contains only a bootstrap; the
original ROMs must be put back in to run the original TNC software.
The combined ``Howie + downloader'' ROM for the TNC-2 was contributed by
WA7MXZ. This document was expertly typeset by Bob Hoffman, N3CVL.
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 ``Keep It Simple, Stupid''
 To conform to the literature, here p takes on values between 0 to 1.
However, fractions are difficult to use in a fixed point microprocessor so
the KISS TNC actually works with P values that are rescaled to the range 0
to 255. To avoid confusion, we will use lower-case p to mean the former
(0-1) and upper-case P whenever we mean the latter (0-255).
 For ``Howie'', of course.