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.TL
The KISS TNC:
A simple Host-to-TNC communications protocol
.AU
Mike Chepponis, K3MC
.AU
Phil Karn, KA9Q
.AB
The KISS\**
.FS
"Keep It Simple, Stupid"
.FE
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.
.AE
.NH
Introduction
.PP
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.
.PP
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.
.PP
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 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.
.PP
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.
.PP
It should be stressed that the KISS TNC was intended only as a stopgap.
Ideally, host computers would have HDLC interfaces 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.
.NH
Asynchronous Frame Format
.PP
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.
.PP
The special characters are:
.PP
.TS
center box tab(/);
l l l .
Abbreviation/Description/Hex value
=
FEND/Frame End/C0
FESC/Frame Escape/DB
TFEND/Transposed Frame End/DC
TFESC/Transposed Frame Escape/DD
.TE
.PP
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.
.NH
Transparency
.PP
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,
Transposed Frame Escape).
.PP
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.
.PP
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.
.PP
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.
.NH
Control of the KISS TNC
.PP
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.
.PP
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):
.PP
.TS
box expand tab(/);
l l l .
Command/Function/Comments
=
0/Data frame/T{
The rest of the frame is data to be sent on the HDLC channel.
T}
_
1/TXDELAY/T{
The next byte is the transmitter keyup
delay in 10 ms units.
The default start-up value is 50 (i.e., 500 ms).
T}
_
2/P/T{
The next byte is the persistence parameter, p, scaled to the
range 0 - 255 with the following formula:
.sp
$ P = p * 256 - 1 $
.sp
The default value is P = 63 (i.e., p = 0.25).
T}
_
3/SlotTime/T{
The next byte is the slot interval in 10 ms units.
The default is 10 (i.e., 100ms).
T}
_
4/TXtail/T{
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.
T}
_
5/FullDuplex/T{
The next byte is 0 for half duplex, nonzero for full
duplex. The default is 0 (i.e., half duplex).
T}
_
6/SetHardware/T{
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.
T}
_
FF/Return/T{
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.
T}
.TE
.PP
The following types are defined in frames to the host:
.TS
box expand tab(/);
l l l .
Type/Function/Comments
=
0/Data frame/T{
Rest of frame is data from the HDLC channel
T}
.TE
.PP
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.
.NH
Buffer and Packet Size Limits
.PP
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 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
available RAM.
.PP
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 kbps design.
.PP
.NH
Persistence
.PP
The P and SlotTime parameters are used to implement true p-persistent CSMA.
This works as follows:
.PP
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.\**
.FS
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).
.FE
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.
.PP
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 [1] through [13] for details.
.PP
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.
.PP
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 \fIchannel access\fR protocol that can yield dramatic
performance improvements regardless of the higher level protocol in use;
we urge it be added to \fIevery\fR TNC, whether or not it supports KISS.
.NH
Implementation History
.PP
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 implementations exist:
.PP
.TS
box expand tab (/);
l l l .
TNC type/Author/Comments
=
TAPR TNC-2 & clones/Mike Chepponis, K3MC/T{
First implementation, most widely
used. Exists in both downloadable
and dedicated ROM versions.
T}
_
TAPR TNC-1 & clones/Marc Kaufman, WB6ECE/T{
Both download and dedicated ROM
versions.
T}
_
VADCG TNC & Ashby TNC/Mike Bruski, AJ9X/Dedicated ROM.
_
AEA PK-232 & PK-87/Steve Stuart, N6IA/T{
Integrated into standard AEA
firmware as of 21 January 1987.
The special commands "KISS ON"
and "KISS OFF" (!) control entry
into KISS mode.
T}
_
Kantronics/Mike Huslig/T{
Integrated into standard Kantronics
firmware as of July 1987.
T}
.TE
.PP
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"\** command.  The download version is handy for
.FS
For "Howie", of course.
.FE
occasional KISS operation, while the dedicated version is much more
convenient for full-time or demo KISS operation.
.PP
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.
.NH
Credits
.PP
The combined "Howie + downloader" ROM for the TNC-2 was contributed by
WA7MXZ.
This document was carefully typeset by Bob Hoffman, N3CVL.
.PP
.NH
Bibliography
.PP
.IP 1.
Tanenbaum, Andrew S., "Computer Networks"   pp. 288-292.
Prentice-Hall  1981.
.IP 2.
Tobagi, F. A.: "Random Access Techniques for Data Transmission over
Packet Switched Radio Networks," Ph.D. thesis, Computer Science
Department, UCLA, 1974.
.IP 3.
Kleinrock, L., and Tobagi, F.: "Random Access Techniques for Data
Transmission over Packet-Switched Radio Channels," Proc. NCC,
pp. 187-201, 1975.
.IP 4.
Tobagi, F. A., Gerla, M., Peebles, R.W., and Manning, E.G.: "Modeling
and Measurement Techniques in Packet Communications Networks," Proc.
IEEE, vol. 66, pp. 1423-1447, Nov. 1978.
.IP 5.
Lam, S. S.: "Packet Switching in a Multiaccess Broadcast Channel",
Ph.D. thesis, Computer Science Department, UCLA, 1974.
.IP 6.
Lam, S. S., and Kleinrock, L.: "Packet Switching in a Multiaccess
Broadcast Channel:  Dynamic Control Procedures," IEEE Trans. Commun.,
vol COM-23, pp. 891-904, Sept. 1975.
.IP 7.
Lam, S. S.: "A Carrier Sense Multiple Access Protocol for Local
Networks,"  Comput. Networks, vol 4, pp. 21-32, Feb. 1980
.IP 8.
Tobagi, F. A.: "Multiaccess Protocols in Packet Communications
Systems," IEEE Trans. Commun., vol COM-28, pp. 468-488, April 1980c.
.IP 9.
Bertsekas, D., and Gallager, R.: "Data Networks", pp. 274-282
Prentice-Hall 1987.
.IP 10.
Kahn, R. E., Gronemeyer, S. A., Burchfiel, J., and Kungelman, R. C.
"Advances in Packet Radio Technology," Proc. IEEE.  pp. 1468-1496.  1978.
.IP 11.
Takagi, H.: "Analysis of Polling Systems," Cambridge, MA
MIT Press 1986.
.IP 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.
.IP 13.
Rivest, R. L.: "Network Control by Bayessian Broadcast," Report
MIT/LCS/TM-285.  Cambridge, MA.  MIT, Laboratory for Computer Science.
1985.
.IP 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.
.IP 15.
Karn, P., "Why Do We Even Need TNCs Anyway", Gateway, vol. 3 no. 2, September
5, 1986.
