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The KISS TNC: A simple Host-to-TNC communications
protocol
Mike Chepponis, K3MC
Phil Karn, KA9Q
ABSTRACT
The KISS[1] 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.
1. Introduction
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 com-
plete 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 full- or half-duplex
radio channel, and a special asynchronous, full duplex frame
"Keep It Simple, Stupid"
July 14, 1990
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 flexibil-
ity 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 was intended only
as a stopgap. Ideally, host computers would have HDLC inter-
faces of their own, making separate TNCs unnecessary. [15]
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 inter-
preted as delimiting an empty frame.
3. Transparency
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
July 14, 1990
with the two character sequence FESC TFESC (Frame Escape, Tran-
sposed Frame Escape).
As characters arrive at the receiver, they are appended to
a 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 particu-
lar, 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 unneces-
sary (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 neces-
sary to allow the host to control a few TNC parameters, namely
the transmitter keyup delay, the transmitter persistence vari-
ables 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 indi-
cates 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 defini-
tion Port 0. In multi-port TNCs, the upper 4 bits of the type
indicator byte can specify one of up to sixteen ports. The fol-
lowing commands are defined in frames to the TNC (the "Command"
field is in hexadecimal):
July 14, 1990
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., 500 ms).
2 P The next byte is the persistence
parameter, p, scaled to the range
0 - 255 with the following
formula:
P = p * 256 - 1
The default value is P = 63
(i.e., p = 0.25).
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 function 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 pro-
vision 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
a higher level protocol (typically TCP, but AX.25 in the con-
nected 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 simul-
taneously). In practice, however, many (if not most) user con-
nections 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
July 14, 1990
retransmitting the packet at a later time, just as it does when-
ever 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 buffer-
ing as possible, limited only by available RAM.
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 chan-
nels. They are essential for good performance when operating on
high speed modems such as the new WA4DSY 56 kbps design.
6. Persistence
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.[2] 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 con-
tinuing). Note that p = 1 means "transmit as soon as the
channel clears"; in this case the p-persistence algorithm degen-
erates 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
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).
July 14, 1990
remains stable under heavy load. See references [1] through
[13] for details.
We believe that optimum p and SlotTime values could be
computed automatically. This could be done by noting the chan-
nel 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 supports KISS.
7. Implementation History
The original idea for a simplified host/TNC protocol is due
to Brian Lloyd, WB6RQN. Phil Karn, KA9Q, organized the specifi-
cation and submitted an initial version on 6 August 1986. As of
this writing, the following KISS TNC implementations exist:
TNC type Author Comments
TAPR TNC-2 & clones Mike Chepponis, K3MC First implementation,
most widely used.
Exists in both
downloadable and
dedicated ROM.
TAPR TNC-1 & clones Marc Kaufman, WB6ECE Both download and
dedicated ROM.
VADCG TNC Mike Bruski, AJ9X Dedicated ROM.
AEA PK-232 & PK-87 Steve Stuart, N6IA Integrated into
standard AEA firmware
as of 21 Jan 1987.
The special commands "KISS ON" and "KISS OFF" (!) control entry
into KISS mode.
Kantronics Mike Huslig Integrated into
standard Kantronics
firmware as of July
1987.
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. Since the TNC-1 and TNC-2 KISS versions were written
by different authors than the original AX.25 firmware, and
because the original source code for those TNCs was not made
available, running KISS on these TNCs requires 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"[3] command. The
July 14, 1990
download version is handy for occasional KISS operation, while
the dedicated version is much more convenient for full-time or
demo KISS operation.
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.
8. Credits
The combined "Howie + downloader" ROM for the TNC-2 was
contributed by WA7MXZ. This document was carefully typeset by
Bob Hoffman, N3CVL.
9. Bibliography
1. Tanenbaum, Andrew S., "Computer Networks" pp. 288-292.
Prentice-Hall 1981.
2. Tobagi, F. A.: "Random Access Techniques for Data Transmis-
sion over Packet Switched Radio Networks," Ph.D. thesis,
Computer Science Department, UCLA, 1974.
3. Kleinrock, L., and Tobagi, F.: "Random Access Techniques
for Data Transmission over Packet-Switched Radio Channels,"
Proc. NCC, pp. 187-201, 1975.
4. Tobagi, F. A., Gerla, M., Peebles, R.W., and Manning, E.G.:
"Modeling and Measurement Techniques in Packet Communica-
tions Networks," Proc. IEEE, vol. 66, pp. 1423-1447, Nov.
1978.
5. Lam, S. S.: "Packet Switching in a Multiaccess Broadcast
Channel", Ph.D. thesis, Computer Science Department, UCLA,
1974.
6. Lam, S. S., and Kleinrock, L.: "Packet Switching in a Mul-
tiaccess Broadcast Channel: Dynamic Control Procedures,"
IEEE Trans. Commun., vol COM-23, pp. 891-904, Sept. 1975.
7. Lam, S. S.: "A Carrier Sense Multiple Access Protocol for
Local Networks," Comput. Networks, vol 4, pp. 21-32, Feb.
1980
8. Tobagi, F. A.: "Multiaccess Protocols in Packet Communica-
tions Systems," IEEE Trans. Commun., vol COM-28, pp. 468-
488, April 1980c.
July 14, 1990
9. Bertsekas, D., and Gallager, R.: "Data Networks", pp. 274-
282 Prentice-Hall 1987.
10. Kahn, R. E., Gronemeyer, S. A., Burchfiel, J., and Kungel-
man, R. C. "Advances in Packet Radio Technology," Proc.
IEEE. pp. 1468-1496. 1978.
11. Takagi, H.: "Analysis of Polling Systems," Cambridge, MA
MIT Press 1986.
12. Tobagi, F. A., and Kleinrock, L. "Packet Switching in Radio
Channels: Part II - The Hidden Terminal Problem in CSMA and
Busy-Tone Solution," IEEE Trans. Commun. COM-23 pp. 1417-
1433. 1975.
13. Rivest, R. L.: "Network Control by Bayessian Broadcast,"
Report MIT/LCS/TM-285. Cambridge, MA. MIT, Laboratory for
Computer Science. 1985.
14. Karn, P. and Lloyd, B.: "Link Level Protocols Revisited,"
ARRL Amateur Radio Fifth Computer Networking Conference,
pp. 5.25-5.37, Orlando, 9 March 1986.
15. Karn, P., "Why Do We Even Need TNCs Anyway", Gateway, vol.
3 no. 2, September 5, 1986.
July 14, 1990
