KERMIT PROTOCOL MANUAL Sixth Edition Frank da Cruz

KERMIT PROTOCOL MANUAL Sixth Edition Frank da Cruz
KERMIT PROTOCOL MANUAL
Sixth Edition
Frank da Cruz
Columbia University Center for Computing Activities
New York, New York 10027
June 1986
Copyright (C) 1981,1986
Trustees of Columbia University in the City of New York
Permission is granted to any individual or institution to copy or
use this document and the programs described in it, except for
explicitly commercial purposes.
Table of Contents
Page i
Table of Contents
1. Introduction
4
1.1. Background
4
1.2. Overview
4
2. Definitions
6
2.1. General Terminology
6
2.2. Numbers
6
2.3. Character Set
7
2.4. Conversion Functions
7
2.5. Protocol Jargon
8
3. System Requirements
9
4. Printable Text versus Binary Data
11
4.1. Printable Text Files
11
4.2. Binary Files
11
5. File Transfer
13
5.1. Conditioning the Terminal
14
5.2. Timeouts, NAKs, and Retries
14
5.3. Errors
15
5.4. Heuristics
15
5.5. File Names
16
5.6. Robustness
17
5.7. Flow Control
17
5.8. Basic Kermit Protocol State Table
18
6. Packet Format
20
6.1. Fields
20
6.2. Terminator
21
6.3. Other Interpacket Data
21
6.4. Encoding, Prefixing, Block Check
22
7. Initial Connection
23
8. Optional Features
27
8.1. 8th-Bit and Repeat Count Prefixing
27
8.2. Server Operation
28
8.2.1. Server Commands
29
8.2.2. Timing
30
8.2.3. The R Command
31
8.2.4. The K Command
31
8.2.5. Short and Long Replies
31
8.2.6. Additional Server Commands
32
8.2.7. Host Commands
34
8.2.8. Exchanging Parameters Before Server Commands
34
Table of Contents
Page ii
8.3. Alternate Block Check Types
35
8.4. Interrupting a File Transfer
37
8.5. Transmitting File Attributes
37
8.6. Advanced Kermit Protocol State Table
45
9. Performance Extensions
49
9.1. Long Packets
49
9.2. Sliding Windows
52
9.2.1. Overall Sequence of Events
52
9.2.2. Questions and Answers about Sliding Windows
60
9.2.3. More Q-and-A About Windows
63
10. Kermit Commands
66
10.1. Basic Commands
66
10.2. Program Management Commands
66
10.3. Terminal Emulation Commands
67
10.4. Special User-Mode Commands
67
10.5. Commands Whose Object Should Be Specified
67
10.6. The SET Command
69
10.7. Macros, the DEFINE Command
71
11. Terminal Emulation
72
12. Writing a Kermit Program
74
12.1. Program Organization
74
12.2. Programming Language
76
12.3. Documentation
76
12.4. Bootstrapping
77
I. Packet Format and Types
78
II. List of Features
80
III. The ASCII Character Set
82
Index
i
PREFACE TO THE SIXTH EDITION
The sixth edition (June 1986) of the Kermit Protocol Manual is being
issued for
two major reasons: to correct minor errors in the fifth edition, and to
include
new sections on two major protocol extensions: long packets and
sliding windows. No attempt has been made to reorganize, rewrite, or otherwise
improve
the protocol manual.
The Kermit protocol has been presented in an
entirely
different -- hopefully more thorough, organized, coherent, and useful
(if not
more formal) -- manner in the book, "Kermit, A File Transfer
Protocol," by
Frank da Cruz, Digital Press, Bedford MA (1986), ISBN 0-932376-88-6, DEC
order
number EY-6705E-DP. If you have the book, you won't need this protocol
manual.
On the other hand, if you don't have the book, this manual should still
contain
all the necessary information. The Kermit Protocol Manual will continue
to be
freely distributed in perpetuity.
The bare-bones C-language Kermit program that appeared as an appendix in
previous editions has been removed. It was not a particularly good example
of how
to write a Kermit program, and made the manual unnecessarily thick. For
sample
Kermit programs, see the source code for any of the hundreds of
Kermit implementations, or follow the program fragments in the book.
PREFACE TO THE FIFTH EDITION
The fifth edition (March 1984) attempts to clarify some fine points
that had
been left ambiguous in the 4th edition, particularly with respect to
when and
how prefix encoding is done, and when it is not, and about switching
between
block check types.
A mechanism is suggested (in the Attributes
section) for
file archiving, and several attributes have been rearranged and some
others
added (this should do no harm, since no one to date has attempted to
implement
the attributes packet). A more complete protocol state table is
provided, a
few minor additions are made to the collection of packet types.
PREFACE TO THE FOURTH EDITION
The fourth edition (November 1983) of the Kermit Protocol Manual
incorporates
some new ideas that grew from our experience in attempting to implement
some of
the features described in earlier editions, particularly user/server
functions.
These include a mechanism to allow batch transfers to be interrupted
gracefully
for either the current file or the entire batch of files; a "capability
mask";
a protocol extension for passing file attributes. In addition, numbers
are now
written in decimal notation rather than octal, which was confusing
to many
readers. Also, several incompatible changes were made in minor areas
where no
attempts at an implementation had yet been made; these include:
- The format and interpretation of the operands to the server
commands.
- Usurpation of the reserved fields 10-11 of the Send-Init packet,
and
addition of new reserved fields.
Most of the remaining material has been rewritten and reorganized, and
much new
material added, including a section on the recommended vocabulary for
documentation and commands.
The previous edition of the Protocol Manual attempted to define "protocol
ver-
Page 2
sion 3"; this edition
is an
unorganized, disorderly,
imposed
on Kermit implementors
their implementations. Rather,
basic
functionality of Kermit,
abandons that concept.
Since Kermit development
distributed enterprise, no requirement can be
to include a certain set of capabilities in
in
this
edition
we
attempt
to
define
the
and then describe various optional functions.
The key principle is that any implementation of Kermit should work
with any
other, no matter how advanced the one or how primitive the other. The
capabily
mask and other Send-Init fields attempt to promote this principle.
A FEW WORDS...
Before deciding to write a new version of Kermit, please bear in mind
that the
philosophy of Kermit has always been that is not, and never should
become, a
commercial product, sold for profit. Its goal is to promote
communication and
sharing, and Kermit itself should be freely shared, and not sold.
Media and
reproduction costs may be recouped if desired, but profit should not be
the motive. Vendors of commercial software, however, may request permission
to include Kermit with, or in, their programs provided certain conditions
are met,
including that credit for the protocol be given to Columbia and that the
price
of the product not be raised substantially beyond media and reproduction
costs
for inclusion of Kermit. Contact the Kermit group at Columbia if you
have any
questions about this. Prospective Kermit implementors should check with
us in
any case, to be sure that someone else has not already done, or started
to do,
the same thing you propose to do.
Kermit is distributed from Columbia University on magnetic tape and
certain
other media, and over various networks. Write to the following
address for
further information, including complete ordering instructions:
Kermit Distribution
Columbia University Center for Computing Activities
7th Floor, Watson Laboratory
612 West 115th Street
New York, NY 10025
ACKNOWLEDGEMENTS
Bill Catchings and I designed the basic Kermit protocol at Columbia
University
in 1981. For ideas, we looked at some of the ANSI models (X3.57,
X3.66), the
ISO OSI model, some real-world "asynchronous protocols" (including the
Stanford
Dialnet and TTYFTP projects, the University of Utah Small FTP project),
as well
as at file transfer on full-blown networks like DECnet and ARPAnet.
Bill wrote the first two programs to implement the protocol, one
for the
DEC-20, one for a CP/M-80 microcomputer, and in the process worked out
most of
the details and heuristics required for basic file transfer. Meanwhile,
Daphne
Tzoar and Vace Kundakci, also of Columbia, worked out the additional
details
necessary for IBM mainframe communication, while writing IBM VM/CMS and
PC-DOS
versions.
Much credit should also go to Bernie Eiben of Digital Equipment
Corporation for
promoting widespread use of Kermit and for adding many insights into
how it
should operate, to Nick Bush and Bob McQueen of Stevens Institute of
Technol-
Page 3
ogy, for many contributions to the "advanced" parts of the protocol,
and for
several major Kermit implementations, and to Leslie Spira and her group
at The
Source Telecomputing for adding full-duplex sliding window capability
to the
Kermit protocol.
Thanks to the many people all over the world who have contributed new
Kermit
implementations, who have helped with Kermit distribution through
various user
groups, and who have contributed to the quality of the protocol and its
many
implementations by reporting or fixing problems, criticizing the
design, or
suggesting new features. In particular, thanks to Ted Toal of Nevada
City, CA,
for a detailed list of corrections to the fifth edition of this manual.
The Kermit protocol was named after Kermit the Frog, star of the
television
series THE MUPPET SHOW. The name is used by permission of Henson
Associates,
Inc., New York City.
DISCLAIMER
No warranty of the software nor of the accuracy of the documentation
surrounding it is expressed or implied, and neither the authors nor Columbia
University
acknowledge any liability resulting from program or documentation errors.
Introduction
Page 4
1. Introduction
This manual describes the Kermit protocol. It is assumed that you
understand
the purpose and operation of the Kermit file transfer facility,
described in
the Kermit Users Guide, and basic terminology of data communications
and computer programming.
1.1. Background
The Kermit file transfer protocol is intended for use in an environment
where
there may be a diverse mixture of computers -- micros, personal
computers,
workstations, laboratory computers, timesharing systems -- from a
variety of
manufacturers.
All these systems need have in common is the ability
to communicate in ASCII over ordinary serial telecommunication lines.
Kermit was originally designed at Columbia University to meet the need
for file
transfer between our DECSYSTEM-20 and IBM 370-series mainframes and
various
microcomputers. It turned out that the diverse characteristics of these
three
kinds of systems resulted in a design that was general enough to fit
almost any
system. The IBM mainframe, in particular, strains most common
assumptions
about how computers communicate.
1.2. Overview
The Kermit protocol is specifically designed for character-oriented
transmission over serial telecommunication lines. The design allows for the
restrictions and peculiarities of the medium and the requirements of diverse
operating
environments -- buffering, duplex, parity, character set, file
organization,
etc.
The protocol is carried out by Kermit programs on each end of the
serial
connection sending "packets" back and forth; the sender sends file names,
file
contents, and control information;
(positively or
negatively) each packet.
the receiver acknowledges
The packets have a layered design, more or less in keeping with the
ANSI and
ISO philosophies, with the outermost fields used by the data link
layer to
verify data integrity, the next by the session layer to verify
continuity, and
the data itself at the application level.
Connections between systems are established by the ordinary user. In a
typical
case, the user runs Kermit on a microcomputer, enters terminal emulation,
connects to a remote host computer (perhaps by dialing up), logs in, runs
Kermit
on the remote host, and then issues commands to that Kermit to start a
file
transfer, "escapes" back to the micro, and issues commands to that
Kermit to
start its side of the file transfer. Files may be transferred singly
or in
groups.
Basic Kermit provides only file transfer, and that is provided for
sequential
files only, though the protocol attempts to allow for various types of
sequential files.
Microcomputer implementations of Kermit are also
expected to
provide terminal emulation, to facilitate the initial connection.
More advanced implementations simplify the "user interface" somewhat by
allowing the Kermit on the remote host to run as a "server", which can
transfer
files in either direction upon command from the local "user" Kermit. The
serv-
Introduction
Page 5
er can also provide additional functionality, such as file
management, messages, mail, and so forth. Other optional features also exist,
including a
variety of block check types, a mechanism for passing 8-bit data
through a
7-bit communication link, a way to compressing a repeated sequence of
characters, and so forth.
As local area networks become more popular, inexpensive, and
standardized, the
demand for Kermit and similar protocols may dwindle, but will never
wither away
entirely.
Unlike hardwired networks, Kermit gives the ordinary user the
power
to establish reliable error-free connections between any two computers;
this
may always be necessary for one-shot or long-haul connections.
Definitions
Page 6
2. Definitions
2.1. General Terminology
TTY: This is the term commonly used for a device which is connected to
a computer over an EIA RS-232 serial telecommunication line. This device is
most
commonly an ASCII terminal, but it may be a microcomputer or even a
large
multi-user computer emulating an ASCII terminal.
Most computers
provide
hardware (RS-232 connectors and UARTs) and software (device drivers) to
support
TTY connections; this is what makes TTY-oriented file transfer protocols
like
Kermit possible on almost any system at little or no cost.
LOCAL: When two machines are connected, the LOCAL machine is the one
which you
interact with directly, and which is in control of the terminal.
The
"local
Kermit" is the one that runs on the local machine. A local Kermit
always communicates over an external device (the micro's communication port, an
assigned
TTY line, etc).
REMOTE: The REMOTE machine is the one on the far side of the connection,
which
you must interact with "through" the local machine. The "remote Kermit"
runs
on the remote machine.
A remote Kermit usually communicates over
its own
"console", "controlling terminal", or "standard i/o" device.
HOST: Another word for "computer", usually meaning a computer that can
provide
a home for multiple users or applications. This term should be avoided
in Kermit lore, unless preceded immediately by LOCAL or REMOTE, to denote which
host
is meant.
SERVER: An implementation of remote Kermit that can accept commands in
packet
form from a local Kermit program, instead of directly from the user.
USER: In addition to its usual use to denote the person using a
system or
program, "user" will also be used refer to the local Kermit program,
when the
remote Kermit is a server.
2.2. Numbers
All numbers in the following text are expressed in decimal (base
notation
unless otherwise specified.
10)
Numbers are also referred to in terms of their bit positions in a
computer
word. Since Kermit may be implemented on computers with various word
sizes, we
start numbering the bits from the "right" -- bit 0 is the least
significant.
Bits 0-5 are the 6 least significant bits; if they were all set to
one, the
value would be 63.
A special quirk in terminology, however, refers to the high order
bit of a
character as it is transmitted on the communication line, as the "8th
bit".
More properly, it is bit 7, since we start counting from 0. References
to the
"8th bit" generally are with regard to that bit which ASCII transmission
sets
aside for use as a parity bit. Kermit concerns itself with whether
this bit
can be usurped for the transmission of data, and if not, it may
resort to
"8th-bit prefixing".
Definitions
Page 7
2.3. Character Set
All characters are in ASCII (American national Standard Code for
Information
Interchange) representation, ANSI standard X3.4-1968. All
implementations of
Kermit transmit and receive characters only in ASCII. The ASCII
character set
is listed in Appendix III.
ASCII character mnemonics:
NUL
SOH
SP
CR
LF
CRLF
DEL
Null, idle, ASCII character 0.
Start-of-header, ASCII character 1 (Control-A).
Space, blank, ASCII 32.
Carriage return, ASCII 13 (Control-M).
Linefeed, ASCII 10 (Control-J).
A carriage-return linefeed sequence.
Delete, rubout, ASCII 127.
A control character is considered to be any byte whose low order 7 bits
are in
the range 0 through 31, or equal to 127. In this document, control
characters
are written in several ways:
Control-A
This denotes ASCII character 1, commonly referred to as
"Control-A".
Control-B is ASCII character 2, and so forth.
CTRL-A This is a common abbreviation for "Control-A". A control
character is
generally typed at a computer terminal by holding down the key
marked
CTRL and pressing the corresponding alphabetic character, in this
case
"A".
^A
"Uparrow" notation for CTRL-A.
control
characters in this fashion.
Many computer systems "echo"
A printable ASCII character is considered to be any character in the
range 32
(SP) through 126 (tilde).
2.4. Conversion Functions
Several conversion functions are useful in the description of the
protocol and
in the program example. The machine that Kermit runs on need operate
only on
integer data; these are functions that operate upon the numeric value of
single
ASCII characters.
tochar(x) = x+32
Transforms the integer x, which is assumed to lie in the range 0
to 94,
into a printable ASCII character; 0 becomes SP, 1 becomes "!", 3
becomes
"#", etc.
unchar(x) = x-32
Transforms the character x, which is assumed to be in the
range
(SP through tilde), into an integer in the range 0 to 94.
printable
ctl(x) = x XOR 64
Maps between control characters and their printable
representations,
preserving the high-order bit. If x is a control character, then
Definitions
Page 8
x = ctl(ctl(x))
that is, the same function is used to controllify and
uncontrollify. The
argument is assumed to be a true control character (0 to 31, or
127), or
the result of applying CTL to a true control character (i.e. 63
to 95).
The transformation is a mnemonic one -- ^A becomes A and vice versa.
2.5. Protocol Jargon
A Packet is a clearly delimited string of characters, comprised of
"control
fields" nested around data; the control fields allow a Kermit program to
determine whether the data has been transmitted correctly and completely. A
packet
is the unit of transmission in the Kermit protocol.
ACK stands for "Acknowledge". An ACK is a packet that is sent to
acknowledge
receipt of another packet. Not to be confused with the ASCII character
ACK.
NAK stands for "Negative Acknowledge". A NAK is a packet sent to say
that a
corrupted or incomplete packet was received, the wrong packet was
received, or
an expected packet was not received. Not to be confused with the ASCII
character NAK.
A timeout is an event that can occur if expected data does not arrive
within a
specified amount of time. The program generating the input request can
set a
"timer interrupt" to break it out of a nonresponsive read, so that
recovery
procedures may be activated.
System Requirements
Page 9
3. System Requirements
The Kermit protocol requires that:
- The host can send and receive characters using 7- or 8-bit ASCII
encoding over an EIA RS-232 physical connection, either hardwired
or
dialup.
- All printable ASCII characters are acceptable as input to the
host
1
and will not be transformed in any way . Similarly, any
intervening
network or communications equipment ("smart modems", TELENET,
terminal concentrators, port selectors, etc) must not transform or
swallow any printable ASCII characters.
- A single ASCII control character can pass from one system to
the
other without transformation.
This character is used for
packet
synchronization. The character is normally Control-A (SOH, ASCII
1),
but can be redefined.
- If a host requires a line terminator for terminal input, that
terminator must be a single ASCII control character, such as CR or
LF,
distinct from the packet synchronization character.
- When using a job's controlling terminal for file transfer, the
system
must allow the Kermit program to set the terminal to no echo,
infinite width (no "wraparound" or CRLF insertion by the
operating
system), and no "formatting" of incoming or outgoing characters
(for
instance, raising lowercase letters to uppercase, transforming
control characters to printable sequences, etc). In short, the
terminal
must be put in "binary" or "raw" mode, and, hopefully, restored
afterwards to normal operation.
- The host's terminal input processor should be capable of receiving
a
single burst of 40 to 100 characters at normal transmission
speeds.
This is the typical size of packet.
Note that most of these requirements rule out the use of Kermit
through IBM
3270 / ASCII protocol converters, except those (like the Series/1 or
7171 running the Yale ASCII package) that can be put in "transparant mode."
Kermit does not require:
- That the connection run at any particular baud rate.
- That the system can do XON/XOFF or any other kind of flow
control.
System- or hardware-level flow control can help, but it's not
necessary. See section 5.7.
_______________
1
If they are translated to another character set, like EBCDIC, the
Kermit
program must be able to reconstruct the packet as it appeared
on the
communication line, before transformation.
System Requirements
Page 10
- That the system is capable of full duplex operation.
Any mixture
of
half and full duplex systems is supported.
- That the system can transmit or receive 8-bit bytes. Kermit
will
take advantage of 8-bit connections to send binary files; if an 8bit
connection is not possible, then binary files may be sent using
an
optional prefix encoding.
Printable Text versus Binary Data
Page 11
4. Printable Text versus Binary Data
For transmission between unlike systems, files must be assigned
either of
two catagories: printable text or binary.
to
A printable text file is one that can make sense on an unlike system -- a
document, program source, textual data, etc. A binary file is one that
will not
(and probably can not) make sense on an unlike system -- an executable
program,
numbers stored in internal format, etc. On systems with 8-bit bytes,
printable
2
ASCII files will have the high order bit of each byte set to zero (since
ASCII
is a 7-bit code) whereas binary files will use the high order bit of each
byte
for data, in which case its value can vary from byte to byte.
Many computers have no way to distinguish a printable file from a
binary file
-- especially one originating from an unlike system -- so the user may
have to
give an explicit command to Kermit to tell it whether to perform these
conversions.
4.1. Printable Text Files
A primary goal of Kermit is for printable text files to be useful on the
target
system after transfer. This requires a standard representation for text
during
transmission. Kermit's standard is simple: 7-bit ASCII characters,
with
"logical records" (lines) delimited by CRLFs. It is the responsibility
of systems that do not store printable files in this fashion to perform the
necessary
conversions upon input and output. For instance, IBM mainframes might
strip
trailing blanks on output and add them back on input; UNIX would prepend
a CR
to its normal record terminator, LF, upon output and discard it upon
input. In
addition, IBM mainframes must do EBCDIC/ASCII translation for text files.
No other conversions (e.g. tab expansion) are performed upon text files.
This
representation is chosen because it corresponds to the way text
files are
stored on most microcomputers and on many other systems. In many common
cases,
no transformations are necessary at all.
4.2. Binary Files
Binary files are transmitted as though they were a sequence of
characters. The
difference from printable files is that the status of the "8th bit"
must be
preserved. When binary files are transmitted to an unlike system, the
main objective is that they can be brought back to the original system (or one
like
it) intact; no special conversions should be done during transmission,
except
to make the data fit the transmission medium.
For binary files, eight bit character transmission is permissible as
long as
the two Kermit programs involved can control the value of the parity
bit, and
_______________
2
There are some exceptions, such as systems that store text files
in socalled "negative ASCII", or text files produced by word processors that
use the
high order bit to indicate underline or boldface attributes.
Printable Text versus Binary Data
Page 12
no intervening communications equipment will change its value. In that
case,
the 8th bit of a transmitted character will match that of the
original data
byte, after any control-prefixing has been done. When one or both sides
cannot
control the parity bit, a special prefix character may be
inserted, as
described below.
Systems that do not store binary data in 8-bit bytes, or whose word size
is not
a multiple of 8, may make special provisions for "image mode" transfer
of binary files. This may be done within the basic protocol by having the two
sides
implicitly agree upon a scheme for packing the data into 7- or 8-bit
ASCII
characters, or else the more flexible (but optional) file attributes
feature
may be used.
The former method is used on PDP-10 36-bit word
machines, in
which text is stored five 7-bit bytes per word; the value of the "odd
bit" is
sent as the parity bit of every 5th word.
File Transfer
Page 13
5. File Transfer
The file transfer protocol takes place over a transaction. A transaction
is an
exchange of packets beginning with a Send-Init (S) packet, and ending
with a
3
Break Transmission (B) or Error (E) packet , and may include the
transfer of
one or more files, all in the same direction. In order to minimize the
unforseen, Kermit packets do not contain any control characters except one
specially
designated to mark the beginning of a packet. Except for the packet
marker,
only printable characters are transmitted.
The following sequence
characterizes basic Kermit operation; the sender is the machine that is
sending
files; the receiver is the machine receiving the files.
1. The sender transmits a Send-Initiate (S) packet to specify
its
parameters (packet length, timeout, etc; these are explained
below).
2. The receiver sends an ACK (Y) packet, with its own parameters in
the
data field.
3. The sender transmits a File-Header (F) packet, which contains
the
file's name in the data field. The receiver ACKs the F packet,
with
no data in the data field of the ACK (optionally, it may contain
the
name under which the receiver will store the file).
4. The sender sends the contents of the file, in Data (D) packets.
Any
data not in the printable range is prefixed and replaced by a
printable equivalent. Each D packet is acknowledged before the next
one
is sent.
5. When all the file data has been sent, the sender
OfFile (Z) packet. The receiver ACKs it.
sends
an
End-
6. If there is another file to send, the process is repeated
beginning
at step 3.
7. When no more files remain to be sent, the sender transmits an
EndOf-Transmission (B) packet.
The receiver ACKs it. This ends
the
transaction, and closes the logical connection (the physical
connection remains open).
Each packet has a sequence number, starting with 0 for the Send Init.
The acknowledgment (ACK or NAK) for a packet has the same packet number as the
packet
being acknowledged. Once an acknowledgment is successfully received the
packet
number is increased by one, modulo 64.
If the sender is remote, it waits for a certain amount of time
(somewhere in
the 5-30 second range) before transmitting the Send-Init, to give the
user time
to escape back to the local Kermit and tell it to receive files.
Each transaction starts fresh, as if no previous transaction had
place.
taken
_______________
3
A transaction should also be considered terminated when one side
or the
other has stopped without sending an Error packet.
File Transfer
Page 14
For example, the sequence number is set back to zero, and parameters are
reset
to their default or user-selected values.
5.1. Conditioning the Terminal
Kermit is most commonly run with the user sitting at a microcomputer,
connected
through a communications port to a remote timesharing system. The
remote Kermit is using its job's own "controlling terminal" for file transfer.
While the
microcomputer's port is an ordinary device, a timesharing job's
controlling
terminal is a special one, and often performs many services that would
interfere with normal operation of Kermit. Such services include echoing
(on full
duplex systems), wrapping lines by inserting carriage return linefeed
sequences
at the terminal width, pausing at the end of a screen or page full of
text,
displaying system messages, alphabetic case conversion, control
character intepretation, and so forth.
Mainframe Kermit programs should be
prepared to
disable as many of these services as possible before packet
communication
begins, and to restore them to their original condition at the end of a
transaction. Disabling these services is usually known as "putting the
terminal in
binary mode."
Kermit's use of printable control character equivalents, variable
packet
lengths, redefinable markers and prefixes, and allowance for any
characters at
all to appear between packets with no adverse effects provide a great
deal of
adaptability for those systems that do not allow certain (or any) of
these features to be disabled.
5.2. Timeouts, NAKs, and Retries
If a Kermit program is capable of setting a timer interrupt, or setting
a time
limit on an input request, it should do so whenever attempting to read a
packet
from the communication line, whether sending or receiving files. Having
read a
packet, it should turn off the timer.
If the sender times out waiting for an acknowledgement, it should send
the same
packet again, repeating the process a certain number of times up to a
retry
limit, or until an acknowledgement is received.
If the receiver
times out
waiting for a packet, it can send either a NAK packet for the expected
packet
or another ACK for the last packet it got. The latter is preferred.
If a packet from the sender is garbled or lost in transmission (the
latter is
detected by a timeout, the former by a bad checksum), the receiver sends
a NAK
for the garbled or missing packet. If an ACK or a NAK from the
receiver is
garbled or lost, the sender ignores it; in that case, one side or the
other
will time out and retransmit.
A retry count is maintained, and there is a retry threshold,
normally set
around 5.
Whenever a packet is resent -- because of a timeout, or
because it
was NAK'd -- the counter is incremented. When it reaches the
threshold, the
transaction is terminated and the counter reset.
If neither side is capable of timing out, a facility
intervention
must be available on the local Kermit. Typically, this
sampling
the keyboard (console) periodically; if input, such as
then the
same action is taken as if a timeout had occurred. The
keeps a
for manual
will work
by
a CR, appears,
local
Kermit
File Transfer
Page 15
running display of the packet number or byte count on the screen to
allow the
user to detect when traffic has stopped. At this point, manual
intervention
should break the deadlock.
Shared systems which can become sluggish when heavily used should adjust
their
own timeout intervals on a per-packet basis, based on the system load, so
that
file transfers won't fail simply because the system was too slow.
Normally, only one side should be doing timeouts, preferably the side
with the
greatest knowledge of the "environment" -- system load, baud rate,
and so
forth, so as to optimally adjust the timeout interval for each packet.
If both
sides are timing out, their intervals should differ sufficiently to
minimize
collisions.
5.3. Errors
During file transfer, the sender may encounter an i/o error on the disk,
or the
receiver may attempt to write to a full or write-protected device.
Any
condition that will prevent successful transmission of the file is called a
"fatal
error". Fatal errors should be detected, and the transfer shut down
gracefully, with the pertinent information provided to the user. Error
packets
provide a mechanism to do this.
If a fatal error takes place on either the sending or receiving side, the
side
which encountered the error should send an Error (E) packet. The E
packet contains a brief textual error message in the data field.
Both the
sender and
receiver should be prepared to receive an Error packet at any time
during the
transaction. Both the sender and receiver of the Error packet should
halt, or
go back into into user command mode (a server should return to server
command
wait). The side that is local should print the error message on the
screen.
There is no provision for sending nonfatal error messages, warnings, or
information messages during a transaction. It would be possible to add such
a feature, but this would require both sides agree to use it through setting
of a
bit in the capability mask, since older Kermits that did not know about
such a
feature would encounter an unexpected packet type and would enter the
fatal error state.
In any case, the utility of such a feature is questionable,
since
there is no guarantee that the user will be present to see such messages
at the
time they are sent; even if they are saved up for later perusal in a
"message
box", their significance may be long past by the time the user reads
them. See
the section on Robustness, below.
5.4. Heuristics
During any transaction, several heuristics are useful:
1. A NAK for the current packet is equivalent to an ACK for the
previous packet (modulo 64). This handles the common situation
in
which a packet is successfully received, and then ACK'd, but the
ACK
is lost. The ACKing side then times out waiting for the next
packet
and NAKs it.
The side that receives a NAK for packet n+1
while
waiting for an ACK for packet n simply sends packet n+1.
2. If packet n arrives more than once, simply ACK it
it.
and
discard
File Transfer
Page 16
This
can
happen when the first ACK was lost.
Resending the ACK
is
necessary and sufficient -- don't write the packet out to
the
file
again!
3. When opening a connection, discard the contents of the line's
input
buffer before reading or sending the first packet.
This is
especially important if the other side is in receive mode (or acting
as
a server), in which case it may have been sending out periodic
NAKs
for your expected SEND-INIT or command packet. If you don't
do
this, you may find that there are sufficient NAKs to prevent
the
transfer -- you send a Send-Init, read the response, which is an
old
NAK, so you send another Send-Init, read the next old NAK, and
so
forth, up to the retransmission limit, and give up before getting
to
the ACKs that are waiting in line behind all the old NAKs.
If
the
number of NAKs is below the cutoff, then each packet may be
transmitted multiply.
4. Similarly, before sending a packet, you should clear the input
buffer (after looking for any required handshake character). Failure
to
clear the buffer could result in propogation of the repetition of
a
packet caused by stacked-up NAKs.
5. If an ACK arrives for a packet that has already been ACK'd,
simply
ignore the redundant ACK and wait for the next ACK, which should
be
on its way.
5.5. File Names
The syntax
avoid
for
file
names
can vary widely from system to system.
To
problems, it is suggested that filenames be represented in the File
Header (F)
packet in a "normal form", by default (that is, there should be an
option to
override such conversions).
1. Delete all pathnames and attributes from the file
specification.
The file header packet should not contain directory or device
names;
if it does, it may cause the recipient to try to store the file
in
an inaccessible or nonexistent area, or it may result in a
very
strange filename.
2. After stripping any pathname, convert the remainder of the
file
specification to the form "name.type", with no restriction on
length
(except that it fit in the data field of the F packet), and:
a. Include no more than one dot.
b. Not begin or end with a dot.
c. The name and type fields contain digits and uppercase
letters.
Special characters like "$", "_", "-", "&", and so forth should be
disallowed,
since they're sure to cause problems on one system or another.
The recipient, of course, cannot depend upon the sender to follow this
convention, and should still take precautions. However, since most file
systems embody the notion of a file name and a file type, this convention will
allow
these items to be expressed in a way that an unlike system can
understand. The
particular notation is chosen simply because it is the most common.
File Transfer
Page 17
The recipient must worry about the length of the name and type fields
of the
file name. If either is too long, they must be truncated.
If the
result
(whether truncated or not) is the same as the name of a file that
already exists in the same area, the recipient should have the ability to take some
special action to avoid writing over the original file.
Kermit implementations that convert file specifications to normal
form by
default should have an option to override this feature.
This would be
most
useful when transferring files between like systems, perhaps used in
conjunction with "image mode" file transfer. This could allow, for instance,
one UNIX
system to send an entire directory tree to another UNIX system.
5.6. Robustness
A major feature of the Kermit protocol is the ability to transfer
multiple
files. Whether a particular Kermit program can actually send multiple
files
depends on the capabilities of the program and the host operating
system (any
Kermit program can receive multiple files).
If a Kermit program can send multiple files, it should make every
attempt to
send the entire group specified. If it fails to send a particular
file, it
should not terminate the entire batch, but should go on the the next
one, and
proceed until an attempt has been made to send each file in the group.
Operating in this robust manner, however, gives rise to a problem:
the user
must be notified of a failure to send any particular file.
Unfortunately, it
is not sufficient to print a message to the screen since the user may
not be
physically present. A better solution would be to have the sender
optionally
keep a log of the transaction, giving the name of each file for which
an at-
tempt was made, and stating whether the attempt was successful, and if
not, the
reason.
Additional aids to robustness are described in the Optional
Features
section, below.
5.7. Flow Control
On full duplex connections, XON/XOFF flow control can generally be used
in conjunction with Kermit file transfer with no ill effects. This is because
XOFFs
are sent in the opposite direction of packet flow, so they will not
interfere
with the packets themselves. XON/XOFF, therefore, need not be
implemented by
the Kermit program, but can done by the host system.
If the host
system
provides this capability, it should be used -- if both sides can
respond
XON/XOFF signals, then buffer overruns and the resulting costly
packet
retransmissions can be avoided.
Beware, however, of the following situation: remote Kermit is sending
periodic
NAKs, local system is buffering them on the operating system level
(because the
user has not started the local end of the file transfer yet); local line
buffer
becomes full, local systems sends XOFF, remote starts buffering them up
on its
end, user finally starts file transfer on local end, clears buffer,
local
operating system sends XON, and then all the remotely buffered NAKs
show up,
causing the packet echoing problem described above, despite the buffer
clearing.
Flow control via modem signals can also be used when available.
File Transfer
Page 18
Note that flow control should not be confused with "handshake"
"line
turnaround" techniques that are used on simplex or half-duplex
communication
lines. In fact, the two techniques are mutually exclusive.
or
5.8. Basic Kermit Protocol State Table
The Kermit protocol can be described as a set of states and
transitions, and
rules for what to do when changing from one state to another. State
changes
occur based on the type of packets that are sent or received, or errors
that
may occur. Packets always go back and forth; the sender of a file always
sends
data packets of some kind (init, header, data) and the receiver always
returns
ACK or NAK packets.
Upon entering a given state, a certain kind
sent or
is expected to arrive -- this is shown on top
state.
As a result of the action, various responses
in the
EVENT column. For each event, an appropriate
protocol
enters a NEW STATE.
of packet is either being
of the description of that
may occur; these are shown
ACTION is taken, and the
The following table specifies basic Kermit operation. Timeouts and
error conditions have been omitted from the following table for simplicity, but
the action is as described above. Server operation and some of the advanced
features
are also omitted. A full-blown state table is given subsequently.
File Transfer
Page 19
STATE
EVENT
ACTION
NEW STATE
-- SEND STATES -Send Send-Init Packet:
S
Get NAK,bad ACK (None)
Get good ACK
Set remote's params, open file
(Other)
(None)
S
SF
A
Send File-Header Packet
SF
Get NAK,bad ACK (None)
Get good ACK
Get bufferful of file data
(Other)
(None)
SF
SD
A
Send File-Data Packet
SD
Get NAK,bad ACK
Get good ACK
(End of file)
(Other)
(None)
Get bufferful of file data
(None)
(None)
SD
SD
SZ
A
Send EOF Packet
SZ
Get NAK,bad ACK
Get good ACK
(No more files)
(Other)
(None)
Get next file to send
(None)
(None)
SZ
SF
SB
A
Send Break (EOT) Packet
SB
Get NAK,bad ACK (None)
Get good ACK
(None)
(Other)
(None)
SB
C
A
-- RECEIVE STATES -Wait for Send-Init Packet
R
Get Send-Init
ACK w/local params
(Other)
(None)
RF
A
Wait for File-Header Packet
RF
Get Send-Init
ACK w/local params
(previous ACK was lost)
Get Send-EOF
ACK (prev ACK lost)
Get Break
ACK
Get File-Header Open file, ACK
(Other)
(None)
RF
RF
C
RD
A
Wait for File-Data Packet
RD
Get previous
packet(D,F)
ACK it again
Get EOF
ACK it, close the file
Get good data
Write to file, ACK
RD
RF
RD
(Other)
(None)
A
-- STATES COMMON TO SENDING AND RECEIVING -C
A
(Send Complete)
("Abort")
start
start
Packet Format
Page 20
6. Packet Format
6.1. Fields
The Kermit protocol is built around exchange of packets of the
format:
following
+------+-------------+-------------+------+------------+-------+
| MARK | tochar(LEN) | tochar(SEQ) | TYPE |
DATA
| CHECK |
+------+-------------+-------------+------+------------+-------+
where all fields consist of ASCII characters.
The fields are:
MARK
The synchronization character that marks the beginning of the
packet.
This should normally be CTRL-A, but may be redefined.
LEN
this
The number of ASCII characters
field,
in
within
the
packet
other words the packet length minus two.
that
follow
Since this
number
is transformed to a single character via the tochar() function,
packet
character counts of 0 to 94 (decimal) are permitted, and 96
(decimal)
is the maximum total packet length. The length does not include
endof-line or padding characters, which are outside the packet
and are
strictly for the benefit of the operating system or
communications
equipment, but it does include the block check characters.
SEQ
The packet sequence number, modulo 64, ranging from 0 to 63.
Sequence
numbers "wrap around" to 0 after each group of 64 packets.
TYPE
types
The packet type, a single ASCII character.
are required:
D
Y
N
S
B
F
Z
Data packet
Acknowledge (ACK)
Negative acknowledge (NAK)
Send initiate (exchange parameters)
Break transmission (EOT)
File header
End of file (EOF)
The following packet
E
Q
T
Error
Reserved for internal use
Reserved for internal use
The NAK packet is used only to indicate that the expected
packet was
not received correctly, never to supply other kinds of
information,
such as refusal to perform a requested service. The NAK packet
always
has an empty data field. The T "packet" is used internally by
many
Kermit programs to indicate that a timeout occurred.
DATA
given
The "contents" of the packet, if any contents are required in the
type of packet, interpreted according to
the
packet
type.
Control
characters (bytes whose low order 7 bits are in the ASCII control
range
0-31, or 127) are preceded by a special prefix character,
normally "#",
and "uncontrollified" via ctl(). A prefixed sequence may not be
broken
across packets. Logical records in printable files are delimited
with
CRLFs, suitably prefixed (e.g. "#M#J"). Logical records need
not cor-
Packet Format
Page 21
respond to packets.
Any prefix characters are included in
the
count.
Optional encoding for 8-bit data and repeated characters is
described
later. The data fields of all packets are subject to prefix
encoding,
except the S, I, and A packets and their acknowledgements,
which must
not be encoded.
CHECK
A block check on the characters in the packet between, but not
including, the mark and the block check itself. The check for each
packet is
computed by both hosts, and must agree if a packet is to be
accepted.
A single-character arithmetic checksum is the normal and required
block
check. Only six bits of the arithmetic sum are included.
In
order
that all the bits of each data character contribute to this
quantity,
bits 6 and 7 of the final value are added to the quantity
formed by
bits 0-5.
Thus if s is the arithmetic sum of the ASCII
characters,
then
check = tochar((s + ((s AND 192)/64)) AND 63)
capable
This is the default block check, and all Kermits must be
of
performing it. Other optional block check types are described
later.
The block check is based on the ASCII values of all the
characters in
the packet, including control fields and prefix characters.
Non-ASCII
systems must translate to ASCII before performing the block
check calculation.
6.2. Terminator
Any line terminator that is required by the system may be appended
to the
packet; this is carriage return (ASCII 15) by default. Line
terminators are
not considered part of the packet, and are not included in the
checksum.
Terminators are not necessary to the protocol, and are
to it,
as are any characters that may appear between packets. If a
cannot do
single character input from a TTY line, then a terminator will
required when
sending to that host. The terminator can be specified in the
connection exchange.
count or
invisible
host
be
initial
Some Kermit implementations also use the terminator for another
reason -speed. Some systems are not fast enough to take in a packet and
decode it
character by character at high baud rates; by blindly reading and
storing all
characters between the MARK and the EOL, they are able to absorb the
incoming
characters at full speed and then process them at their own rate.
6.3. Other Interpacket Data
The space between packets may be used for any desired purpose.
Handshaking
characters may be necessary on certain connections, others may require
screen
control or other sequences to keep the packets flowing.
Packet Format
Page 22
6.4. Encoding, Prefixing, Block Check
MARK, LEN, SEQ, TYPE, and CHECK
always
literal single-character fields,
extended by
one or two additional check
encoded by
tochar() or taken literally, but
never contain 8-bit data.
are control fields.
Control fields are
except that the CHECK field may be
characters.
Each control field is
never prefixed.
The control fields
The DATA field contains a string of data characters in which any
control
characters are encoded printably and preceded with the control prefix.
The
decision to prefix a character in this way depends upon whether its low
order 7
bits are in the ASCII control range, i.e. 0-31 or 127. Prefix characters
that
appear in the data must themselves be prefixed by the control prefix,
but unlike control characters, these retain their literal value in the packet.
The
character to be prefixed is considered a prefix character if its loworder 7
bits corresponds to an active prefix character, such as # (ASCII
35),
regardless of the setting of its high-order bit.
During decoding, any character that follows the control prefix, but is
not in
the control range, is taken literally. Thus, it does no harm to
prefix a
printable character, even if that character does not happen to be an
active
prefix.
The treatment of the high order ("8th") bit of a data byte is as follows:
- If the communication channel allows 8 data bits per character,
then
the original value of the 8th bit is retained in the prefixed
character. For instance, a data byte corresponding to a Control-A with
the
8th bit set would be send as a control prefix, normally "#",
without
the 8th bit set, followed by ctl(^A) with the 8th bit set. In
binary
notation, this would be
00100011 11000001
In this
calculations.
case,
the 8th bit is figured into all block check
- If the communication channel or one of the hosts requires
parity
on
each character, and both sides are capable of 8th-bit prefixing,
then
the 8th bit will be used for parity, and must not be included in
the
block
check.
8th
bit prefixing is an option feature described
in
greater detail in Section 8, below.
- If parity is being used but 8th-bit prefixing is not being done,
then
the value of the 8th bit of each data byte will be lost and
binary
files will not be transmitted correctly. Again, the 8th bit does
not
figure into the block check.
The data fields of all packets are subject to prefix encoding, except S,
I, and
A packets, and the ACKs to those packets (see below).
Initial Connection
Page 23
7. Initial Connection
Initial connection occurs when the user has started up a Kermit program
on both
ends of the physical connection. One Kermit has been directed (in one
way or
another) to send a file, and the other to receive it.
The receiving Kermit waits for a "Send-Init" packet from the sending
Kermit.
It doesn't matter whether the sending Kermit is started before or
after the
receiving Kermit (if before, the Send-Init packet should be
retransmitted
periodically until the receiving Kermit acknowledges it). The data
field of
the Send-Init packet is optional; trailing fields can be omitted (or
left
blank, i.e. contain a space) to accept or specify default values.
The Send-Init packet contains a string of configuration information in
its data
field. The receiver sends an ACK for the Send-Init, whose data field
contains
its own configuration parameters. The data field of the Send-Init and
the ACK
to the Send-Init are literal, that is, there is no prefix encoding.
This is
because the two parties will not know how to do prefix encoding until
after the
configuration data is exchanged.
It is important to note that newly invented fields are added at
right, so
that old Kermit programs that do not have code to handle the
fields will
act as if they were not there. For this reason, the default
for any
field, indicated by blank, should result in the behavior that
before
the new field was defined or added.
the
new
value
occurred
1
2
3
4
5
6
7
8
9
10...
+------+------+------+------+------+------+------+------+------+------| MAXL | TIME | NPAD | PADC | EOL | QCTL | QBIN | CHKT | REPT | CAPAS
+------+------+------+------+------+------+------+------+------+------The fields are as follows (the first and second person "I" and "you"
used
are
to distinguish the two sides). Fields are encoded printably using the
tochar()
function unless indicated otherwise.
1. MAXL
to 94
The maximum length packet I want
(decimal).
to
receive,
a
number
up
(This really means the biggest value I want to
see in a
LEN field.)
You respond with the maximum you want me to send.
This
allows systems to adjust to each other's buffer sizes, or to
the condition of the transmission medium.
2. TIME
while
The number of seconds after which I want you to
waiting
for a packet from me.
time
me
out
You respond with the amount of
time I
should wait for packets from you.
to
This allows the two sides
accommodate
to different line speeds or other factors that could
cause
timing problems.
Only one side needs to time out.
If
both
sides
time out, then the timeout intervals should not be close
together.
3. NPAD
incoming
The
number
of
padding
characters
packet; you respond in kind.
I want to precede each
Padding may be necessary
when
sending
to
a half duplex system that requires some time to change the
direction of transmission, although in practice
this
situation
is
more
commonly handled by a "handshake" mechanism.
4. PADC
The control character I need for padding, if any,
transformed by
ctl() (not tochar()) to make it printable.
You respond
kind.
in
Initial Connection
Page 24
Normally NUL (ASCII 0), some systems use DEL (ASCII 127).
This
field
is to be ignored if the value NPAD is zero.
5. EOL
You
The character I need to terminate an incoming packet, if
any.
respond in kind.
Most systems that require a line
terminator for
terminal input accept carriage return for this purpose (note,
because
there is no way to specify that no EOL should be sent, it
would have
been better to use ctl() for this field rather than
tochar(), but
it's too late now).
6. QCTL
control
(verbatim)
The printable ASCII character I will use to quote
characters, normally and by default "#".
the
You respond
with
one
you will use.
The following fields relate to
Kermit
protocol, described in section 8.
7. QBIN
quote
the
use of OPTIONAL features of the
(verbatim) The printable ASCII character
characters
I
want
to
use
to
which have the 8th bit set, for transmitting binary
files
when the parity bit cannot be used for data.
kind
Since
this
of
quoting
increases
both
processor
and transmission overhead,
it is
normally to be avoided.
in
If used, the quote character must be
the
range
ASCII 33-62 ("!" through ">") or 96-126 ("`" through
"~"), but
different from the control-quoting character.
This field
is
interpreted as follows:
Y
I agree to 8-bit quoting if you request it (I don't need
N
&
I will not do 8-bit quoting (I don't know how).
(or any other character in the range 33-62 or 96-126) I
it).
need to
do 8-bit quoting using this character (it will
if
the
be
done
other
Kermit
puts
a Y in this field, or responds with
the same
prefix character, such as &).
The
recommended
8th-bit
quoting
prefix character is "&".
Anything Else : 8-bit quoting will not be done.
Note that this
request, and
the order does
8-bit
communication
whereas a
mainframe that
matter who
sends first,
8th-bit
quoting.
8. CHKT
"1" for
scheme allows either side to initiate the
not matter.
will
For instance, a micro
normally
put
a
"Y"
in
capable
this
uses parity will always put an "&".
this
combination
will
(Verbatim) Check Type, the method for
single-character
checksum
of
field
No
result in election of
detecting
errors.
(the normal and required method),
"2" for
two-character checksum (optional), "3" for three-character
CRC-CCITT
(optional).
If your response agrees, the designated method
will be
used; otherwise the single-character checksum will be used.
9. REPT
The prefix character I will use to indicate a repeated
character.
This can be any printable character in the range ASCII
33-62 or
96-126, but different from the control and 8th-bit prefixes.
SP (32)
denotes no repeat count processing is to be done. Tilde ("~")
is the
recommended and normal repeat prefix. If you don't respond
identically, repeat counts will not be done. Groups of at least
3 or 4
identical characters may be transmitted more efficiently
using a
repeat count, though an individual implementation may wish to
set a
Initial Connection
Page 25
different threshhold.
10-?. CAPAS
A bit mask, in which each bit position corresponds to a
capability of
Kermit, and is set to 1 if that capability is present, or 0 if
it is
not.
Each character contains a 6-bit field
(transformed by
tochar()), whose low order bit is set to 1 if another
capability byte
follows, and to 0 in the last capability byte. The
capabilities
defined so far are:
#1
#2
#3
#4
#5
Reserved
Reserved
Ability to accept "A" packets (file attributes)
Ability to do full duplex sliding window protocol
Ability to transmit and receive extended-length packets
The capability byte as defined so far would then look like:
bit5 bit4 bit3 bit2 bit1 bit0
+----+----+----+----+----+----+
| #1 | #2 | #3 | #4 | #5 | 0 |
+----+----+----+----+----+----+
If all these capabilities were "on", the value of the byte
would
be
76
(octal).
When
capability 6 is added, the capability
mask will
look like this:
bit5 bit4 bit3 bit2 bit1 bit0
+----+----+----+----+----+----+
bit5 bit4 bit3 bit2 bit1 bit0
+----+----+----+----+----+---
| #1 | #2 | #3 | #4 | #5 |
| #6 | -- | -- | -- | -- |
-+
1 |
0
|
+----+----+----+----+----+----+
+----+----+----+----+----+---
-+
CAPAS+1. WINDO
Window size (see section 9.2).
CAPAS+2. MAXLX1
Extended packet length (see section 9.1).
CAPAS+3. MAXLX2
Extended packet length (see section 9.1).
The receiving Kermit responds with an ACK ("Y") packet in the same
format to
indicate its own preferences, options, and parameters. The ACK need not
contain the same number of fields as the the Send-Init. From that point,
the two
Kermit programs are "configured" to communicate with each other
for the
remainder of the transaction. In the case of 8th-bit quoting, one
side must
specify the character to be used, and the other must agree with a "Y"
in the
same field, but the order in which this occurs does not matter.
Similarly for
checksums -- if one side requests 2 character checksums and the other
side
responds with a "1" or with nothing at all, then single-character
checksums
will be done, since not all implementations can be expected to do 2character
checksums or CRCs. And for repeat counts; if the repeat field of the
send-init
and the ACK do not agree, repeat processing will not be done.
All Send-Init fields are optional. The data field may be left totally
empty.
Similarly, intervening fields may be defaulted by setting them to blank.
Ker-
Initial Connection
Page 26
mit implementations should know what to do in these
apply appropriate defaults. The defaults should be:
MAXL:
TIME:
NPAD:
PADC:
EOL:
QCTL:
QBIN:
CHKT:
REPT:
CAPAS:
WINDO:
MAXLX1:
MAXLX2:
cases,
namely
80
5 seconds
0, no padding
0 (NUL)
CR (carriage return)
the character "#"
space, can't do 8-bit quoting
"1", single-character checksum
No repeat count processing
All zeros (no special capabilities)
Blank (zero) - no sliding windows
Blank (zero) - no extended length packets
Blank (zero) - no extended length packets
There are no prolonged negotiations in the initial connection sequence -there
is one Send-Init and one ACK in reply. Everything must be settled in
this exchange.
The very first Send-Init may not get through if the sending Kermit makes
wrong
assumptions about the receiving host. For instance, the receiving host
may require certain parity, some padding, handshaking, or a special end
of line
character in order to read the Send-Init packet. For this reason, there
should
be a way for the user the user to specify whatever may be necessary to
get the
first packet through.
A parity field is not provided in
not be
of use. If the sender requires a
sending it. If the receiver does not
getting the
Send-Init, it will not be able to
the Send-Init packet because it could
certain kind of parity, it will also be
know this in advance, i.e. before
read the Send-Init packet.
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8. Optional Features
The foregoing sections have discussed basic, required operations for any
Kermit
implementation. The following sections discuss optional and advanced
features.
8.1. 8th-Bit and Repeat Count Prefixing
Prefix quoting of control characters is mandatory. In addition,
prefixing may
also be used for 8-bit quantities or repeat counts, when both Kermit
programs
agree to do so. 8th-bit prefixing can allow 8-bit binary data pass
through
7-bit physical links.
Repeat count prefixing can improve the
throughput of
certain kinds of files dramatically; binary files (particularly
executable
programs) and structured text (highly indented or columnar text) tend to
be the
major beneficiaries.
When more than one type of prefixing is in effect, a single data
character can
be preceded by more than one prefix character. Repeat count
processing can
only be requested by the sender, and will only be used by the sender
if the
receiver agrees.
8th-bit prefixing is a special case because its use
is normally not desirable, since it increases both processing and transmission
overhead.
However, since it is the only straightforward mechanism for
binary file
transfer available to those systems that usurp the parity bit, a receiver
must
be able to request the sender to do 8th-bit quoting, since most
senders will
not normally do it by default.
The repeat prefix is followed immediately by a single-character repeat
count,
encoded printably via tochar(), followed by the character itself
(perhaps
prefixed by control or 8th bit prefixes, as explained below). The repeat
count
may express values from 0 to 94. If a character appears more than 94
times in
a row, it must be "cut off" at 94, emitted with all appropriate
prefixes, and
"restarted".
The following table should clarify Kermit's prefixing
mechanism
(the final line shows how a sequence of 120 consecutive NULs would be
encoded):
Character
A
^A
'A
'^A
#
'#
&
'&
~
'~
NUL
Prefixed
Representation
A
#A
&A
&#A
##
&##
#&
&#&
#~
&#~
#@
With
Repeat Count for 8
~(A
["(" is ASCII 40 - 32 = 8]
~(#A
~(&A
~(&#A
~(##
~(&##
~(#&
~(&#&
~(#~
~(&#~
~~#@~:#@ [120 NULs]
A represents any printable character, ^A represents any control
character, 'x
represents any character with the 8th bit set. The # character is
used for
control-character prefixing, and the & character for 8-bit prefixing.
The
repeat count must always precede any other prefix character. The repeat
count
is taken literally (after transformation by unchar(); for instance "#"
and "&"
immediately following a "~" denote repeat counts, not control
characters or
8-bit characters. The control prefix character "#" is most closely
bound to
the data character, then the 8-bit prefix, then the repeat count; in
other
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words, the order is: repeat prefix and count, 8-bit prefix, control
prefix, and
the data character itself. To illustrate, note that &#A is not
equivalent to
#&A.
When the parity bit is available for data, then 8th-bit prefixing should
not be
done, and the 8th bit of the prefixed character will have the same value
as the
8th bit of the original data byte. In that case, the table looks like
this:
Character
'A
'^A
'#
'&
'~
Prefixed
Representation
'A
#'A
#'#
'&
#'~
With
Repeat Count for 8
~('A
~(#'A
~(#'#
~('&
~(#'~
Note that since 8th bit prefixing is not being done, "&" is not being
used as
an 8th bit prefix character, so it does not need to be prefixed
with "#".
Also, note that the 8th bit is set on the final argument of the
repeat sequence, no matter how long, and not on any of the prefix characters.
Finally, remember the following rules:
- Prefixed sequences must not be broken across packets.
- Control, 8th-bit, and repeat count prefixes must be distinct.
- Data fields of all packets must pass through the prefix
encoding
mechanism, except for S, I, and A packets, and ACKs to those
packets,
whose data fields must not be encoded.
In the first rule above, note that a prefixed sequence means a single
character
and all its prefixes, like ~%&#X, not a sequence like #M#J, which
is two
prefixed sequences.
8.2. Server Operation
A Kermit server is a Kermit program running remotely with no "user
interface".
All commands to the server arrive in packets from the local Kermit.
SERVER
operation is much more convenient than basic operation, since the
user need
never again interact directly with the remote Kermit program after once
starting it up in server mode, and therefore need not issue complementary
SEND and
RECEIVE commands on the two sides to get a file transfer started;
rather, a
single command (such as SEND or GET) to the local Kermit suffices.
Kermit servers can also provide services beyond file transfer.
Between transactions, a Kermit server waits for packets containing server
commands. The packet sequence number is always set back to 0 after a
transaction.
A Kermit server in command wait should be looking for packet 0, and
command
packets sent to servers should also be packet 0. Certain server
commands will
result in the exchange of multiple packets. Those operations proceed
exactly
like file transfer.
A Kermit server program waiting for a command packet is said to be in
"server
command wait". Once put into server command wait, the server should
never
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leave it until it gets a command packet telling it to do so. This
means that
after any transaction is terminated, either normally or by any kind of
error,
the server must go back into command wait. While in command wait, a
server may
elect to send out periodic NAKs for packet 0, the expected command
packet.
Since the user may be disconnected from the server for long periods
of time
(hours), the interval between these NAKs should be significantly longer
than
the normal timeout interval (say, 30-60 seconds, rather than 5-10). The
periodic NAKs are useful for breaking the deadlock that would occur if a
local
program was unable to time out, and sent a command that was lost. On the
other
hand, they can cause problems for local Kermit programs that cannot clear
their
input buffers, or for systems that do XON/XOFF blindly, causing the
NAKs to
buffered in the server's host system output buffer, to be suddenly
released en
masse when an XON appears. For this reason, servers should have an
option to
set the command-wait wakeup interval, or to disable it altogher.
Server operation must be implemented in two places: in the server
itself, and
in any Kermit program that will be communicating with a server. The
server
must have code to read the server commands from packets and respond to
them.
The user Kermit must have code to parse the user's server-related
commands, to
form the server command packets, and to handle the responses to those
server
commands.
8.2.1. Server Commands
Server commands are listed below. Not all of them have been
implemented, and
some may never be, but their use should be reserved.
Although
server-mode
operation is optional, certain commands should be implemented in every
server.
These include Send-Init (S), Receive-Init (R), and the Generic Logout
(GL)
and/or Finish (GF) commands. If the server receives a command it does
not understand, or cannot execute, it should respond with an Error (E) packet
containing a message like "Unimplemented Server Command" and both sides
should set
the packet sequence number back to 0, and the server should remain in
server
command wait. Only a GL or GF command should terminate server operation.
Server commands are as follows:
S
Send Initiate (exchange parameters, server waits for a file).
R
Receive Initiate (ask the server to send the specified files).
I
Initialize (exchange parameters).
X
Text header. Allows transfer of text to the user's screen in
response to a
generic or host command. This works just like file transfer except
that
the destination "device" is the screen rather than a file. Data
field may
contain a filename, title, or other heading.
C
Host Command. The data field contains a string to be executed as a
command
by the host system command processor.
K
Kermit Command.
The data field contains a string in the
interactive command language of the Kermit server (normally a SET command) to be
executed
as if it were typed in at command level.
G
Generic Kermit Command. Single character in data field (possibly
followed
by operands, shown in {braces}, optional fields in [brackets])
specifies
the command:
I
C
L
Login [{*user[*password[*account]]}]
CWD, Change Working Directory [{*directory[*password]}]
Logout, Bye
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F
D
U
E
T
R
K
W
M
H
Q
P
J
V
Finish (Shut down the server, but don't logout).
Directory [{*filespec}]
Disk Usage Query [{*area}]
Erase (delete) {*filespec}
Type {*filespec}
Rename {*oldname*newname}
Copy {*source*destination}
Who's logged in? (Finger) [{*user ID or network host[*options]}]
Send a short Message {*destination*text}
Help [{*topic}]
Server Status Query
Program {*[program-filespec][*program-commands]}
Journal {*command[*argument]}
Variable {*command[*argument[*argument]]}
Asterisk as used above ("*") represents a single-character length
field,
encoded using tochar(), for the operand that follows it; thus lengths
from
0 to 94 may be specified. This allows multiple operands to be
clearly
delimited regardless of their contents.
Note that field length encoding is used within the data field of all
Generic
command packets, but not within the data fields of the other packets,
such as
S, I, R, X, K, and C.
All server commands that send arguments in their data fields should
pass
through the prefix encoding mechanism.
Thus if a data character or
length
field happens to correspond to an active prefix character, it must
itself be
prefixed.
The field length denotes the length of the field before
prefix encoding and (hopefully) after prefix decoding. For example, to send a
generic
command with two fields, "ABC" and "ZZZZZZZZ", first each field
would be
prefixed by tochar() of its length, in this case tochar(3) and
tochar(8),
giving "#ABC(ZZZZZZZZ".
But "#" is the normal control prefix character
so it
must be prefixed itself, and the eight Z's can be condensed to 3
characters
using a repeat prefix (if repeat counts are in effect), so the result
after en-
coding would be "##ABC(~(Z" (assuming the repeat prefix is tilde ("~").
The
recipient would decode this back into the original "#ABC(ZZZZZZZZ"
before attempting to extract the two fields.
Since a generic command must fit into a single packet, the program
sending the
command should ensure that the command actually fits, and should not
include
length fields that point beyond the end of the packet.
Servers,
however,
should be defensive and not attempt to process any characters beyond the
end of
the data field, even if the argument length field would lead them to do
so.
8.2.2. Timing
Kermit does not provide a mechanism for suspending and continuing a
transaction.
This means that text sent to the user's screen should not be
frozen
for long periods (i.e. not longer than the timeout period times the
retry
threshold).
Between transactions, when the server has no tasks pending, it may
send out
periodic NAKs (always with type 1 checksums) to prevent a deadlock in
case a
command was sent to it but was lost. These NAKs can pile up in the
local
"user" Kermit's input buffer (if it has one), so the user Kermit
should be
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prepared to clear its input buffer before sending a command to a
server.
Meanwhile, servers should recognize that some systems provide no function
to do
this (or even when they do, the process can be foiled by system flow
control
firmware) and should therefore provide a way turn off or slow down the
commandwait NAKs.
8.2.3. The R Command
The R packet, generally sent by a local Kermit program whose user typed
a GET
command, tells the server to send the files specified by the name in the
data
field of the R packet. Since we can't assume that the two Kermits are
running
on like systems, the local (user) Kermit must parse the file
specification as a
character string, send it as-is (but encoded) to the server, and let the
server
take care of validating its syntax and looking up the file. If the
server can
open and read the specified file, it sends a Send-Init (S) packet -not an
acknowledgement! -- to the user, and then completes the file-sending
transaction, as described above.
If the server cannot send the file, it should respond with an error (E)
packet
containing a reason, like "File not found" or "Read access required".
Thus, the only two valid responses to a successfully received R packet
are an S
packet or an E packet. The R packet is not ACK'd.
8.2.4. The K Command
The K packet can contain a character string which the server
interprets as a
command in its own interactive command language. This facility is
useful for
achieving the same effect as a direct command without having to shut
down the
server, connect back to the remote system, continue it (or start a new
one),
and issue the desired commands. The server responds with an ACK if the
command
was executed successfully, or an error packet otherwise. The most
likely use
for the K packet might be for transmitting SET commands, e.g. for
switching between text and binary file modes.
8.2.5. Short and Long Replies
Any request made of a server may be answered in either of two ways,
and any
User Kermit that makes such a request should be prepared for either
kind of
reply:
- A short reply. This consists of a single ACK packet, which may
contain text in its data field. For instance, the user might send
a
disk space query to the server, and the server might ACK the
request
with a short character string in the data field, such as "12K
bytes
free". The user Kermit should display this text on the screen.
- A long reply.
This proceeds exactly like a
file
transfer
(and
in
some
cases
it
may
be a file transfer).
It begins with one of
the
following:
* A File-Header (F) packet (optionally followed by one or more
Attributes packets; these are discussed later);
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* A Text-Header (X) packet.
* A Send-Init (S) Packet, followed by an X or F packet.
After the X or F packet comes an arbitrary number of Data (D)
packets, then an End-Of-File (Z) packet, and finally a BreakTransmission
(B) packet, as for ordinary file transfer.
A long reply should begin with an S packet unless an I-packet exchange
has already taken place, and the type 1 (single-character) block check is being
used.
8.2.6. Additional Server Commands
The following server commands request the server to perform tasks other
than
sending or receiving files. Almost any of these can have either short
or long
replies. For instance, the Generic Erase (GE) command may elicit a
simple ACK,
or a stream of packets containing the names of all the files it
erased (or
didn't erase). These commands are now described in more detail;
arguments are
as provided in commands typed to the user Kermit (subject to prefix
encoding);
no transformations to any kind of normal or canonic form are done -filenames
and other operands are in the syntax of the server's host system.
I
Login. For use when a Kermit server is kept perpetually running on a
dedicated line. This lets a new user obtain an identity on the server's
host
system.
If the data field is empty, this removes the user's
identity, so
that the next user does not get access to it.
L
Logout, Bye. This shuts down the server entirely, causing the
server itself to log out its own job. This is for use when the server
has been
started up manually by the user, who then wishes to shut it down
remotely.
For a perpetual, dedicated server, this command simply removes the
server's
access rights to the current user's files, and leaves
waiting
for a new login command.
the
server
F
Finish.
This is to allow the user to shut down the server,
putting its
terminal back into normal (as opposed to binary or raw) mode, and
putting
the server's job back at system command level, still logged in, so
that the
user can connect back to the job. For a perpetual, dedicated server,
this
command behaves as the L (BYE) command.
C
CWD.
Change Working Directory. This sets the default directory
or area
for file transfer on the server's host. With no operands, this
command
sets the default area to be the user's own default area.
D
Directory.
Send a directory listing to the user. The user
program can
display it on the terminal or store it in a file, as it chooses.
The
directory listing should contain file sizes and creation dates as
well as
file names, if possible. A wildcard or other file-group designator
may be
specified to ask the server list only those files that match.
If no
operand is given, all files in the current area should be shown.
U
Disk Usage Query.
used and
the amount left
should be
specified).
The server responds with the amount of
space
free to use, in K bytes (or other units, which
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Page 33
E
Erase (delete).
Delete the specified file or file group.
T
Type. Send the specified file or file group, indicating (by starting
with
an X packet rather than an F packet, or else by using the Type
attribute)
that the file is to be displayed on the screen, rather than stored.
R
Rename. Change the name of the file or files as indicated. The
string indicating the new name may contain other attributes, such as
protection
code, permitted in file specifications by the host.
K
Copy. Produce a new copy of the file or file group, as indicated,
leaving
the source file(s) unmodified.
W
Who's logged in? (Finger). With no arguments, list all the users
who are
logged in on the server's host system.
If an argument is
specified,
provide more detailed information on the specified user or network
host.
M
Short Message.
Send
the indicated user's screen.
the given short (single-packet) message to
P
Program. This command has two arguments, program name
(filespec), and
command(s) for the program. The first field is required, but may
be left
null (i.e. zero length). If it is null, the currently loaded
program is
"fed" the specified command. If not null, the specified program is
loaded
and started; if a program command is given it is fed to the program
as an
initial command (for instance, as a command line argument on
systems that
support that concept). In any case, the output of the program is
sent back
in packets as either a long or short reply, as described above.
J
Journal. This command controls server transaction logging.
field
contains one of the following:
The data
+
close
Begin/resume logging transactions. If a filename is given,
any
currently open transaction and then open the specified file as
the new
transaction log. If no name given, but a log file was already
open,
resume logging to that file. If no filename was given and no
log was
open, the server should open a log with a default name,
like
TRANSACTION.LOG.
Stop logging transactions, but don't close the current
transaction log
file.
C
S
first.
Stop logging and close the current log.
Send the transaction log as a file.
If it was open, close it
Transaction logging is the recording of the progress of file
transfers. It
should contain entries showing the name of each file transferred,
when the
transfer began and ended, whether it completed successfully, and if
not,
why.
V
Set or Query a variable. The command can be S or Q. The first
argument is
the variable name. The second argument, if any, is the value.
S
value
Set the specified variable to the specified value.
If the
is
null, then undefine the variable.
If the variable is null
then do
Optional Features
Page 34
nothing.
If the variable did not exist before, create it.
The
server
should respond with an ACK if successful, and Error packet
otherwise.
Q
Query the value of the named variable. If no variable is
supplied,
display the value of all active variables. The server responds
with
either a short or long reply, as described above. If a queried
variable does not exist, a null value is returned.
Variables are named by character strings, and have character string
values,
which may be static or dynamic. For instance, a server might have
built-in
variables like "system name" which never changes, or others like
"mail
status" which, when queried, cause the server to check to see if
the user
has any new mail.
8.2.7. Host Commands
Host commands are conceptually simple, but may be hard to implement on
some
systems. The C packet contains a text string in its data field which is
simply
fed to the server's host system command processor; any output from the
processor is sent back to the user in Kermit packets, as either a short
or long
reply.
Implementation of this facility under UNIX, with its forking process
structure
and i/o redirection via pipes, is quite natural. On other systems, it
could be
virtually impossible.
8.2.8. Exchanging Parameters Before Server Commands
In basic Kermit, the Send-Init exchange is always sufficient to
configure the
two sides to each other.
During server operation, on the other
hand, some
transactions may not begin with a Send-Init packet.
For instance,
when the
user sends an R packet to ask the server to send a file, the server
chooses
what block check option to use. Or if the user requests a directory
listing,
the server does not know what packet length to use.
The solution to this problem is the "I" (Init-Info) packet. It is
exactly like
a Send-Init packet, and the ACK works the same way too. However, receipt
of an
I packet does not cause transition to file-send state. The I-packet
exchange
simply allows the two sides to set their parameters, in preparation
for the
next transaction.
Servers should be able to receive and ACK "I" packets when in server
command
wait. User Kermits need not send "I" packets, however; in that case, the
server will assume all the defaults for the user listed on page 26, or
whatever
parameters have been set by other means (e.g. SET commands typed to the
server
before it was put in server mode).
User Kermits which send I packets should be prepared to receive and
ignore an
Error packet in response. This could happen if the server has not
implemented
I packets.
The I packet, together with its ACK, constitute a complete
transaction,
separate from the S-packet or other exchange that follows it. The packet
number remains at zero after the I-packet exchange.
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8.3. Alternate Block Check Types
There are two optional kinds of block checks:
Type 2
A two-character checksum based on the low order 12 bits of the
arithmetic
sum of the characters in the packet (from the LEN field through
the last
data character, inclusive) as follows:
1
2
--------+----------------+---------------+
...data | tochar(b6-b11) | tochar(b0-b5) |
--------+----------------+---------------+
For instance, if the 16-bit result is 154321 (octal), then the 2
character
block check would be "C1".
Type 3
Three-character 16-bit CRC-CCITT. The CRC calculation treats the
data it
operates upon as a string of bits with the low order bit of the
first
character first and the high order bit of the last character last.
The initial value of the CRC is taken as 0; the 16-bit CRC is the remainder
after
16 12 5
dividing the data bit string by the polynomial X +X +X +1 (this
calculation can actually be done a character at a time, using a simple
table
lookup algorithm). The result is represented as three printable
characters
at the end of the packet, as follows:
1
2
3
--------+-----------------+----------------+---------------+
...data | tochar(b12-b15) | tochar(b6-b11) | tochar(b0-b5) |
--------+-----------------+----------------+---------------+
For instance, if the 16-bit result is 154321 (octal), then the 3
character
block check would be "-C1". The CRC technique chosen here agrees
with many
hardware implementations (e.g. the VAX CRC instruction).
Here is an algorithm for Kermit's CRC-CCITT calculation:
A:
crc = 0
i = <position of LEN field>
Start CRC off at 0
First byte to include
c = <byte at position i>
if (parity not NONE) then c = c AND 127;
q = (crc XOR c) AND 15;
crc = (crc / 16) XOR (q * 4225);
q = (crc XOR (c / 16)) AND 015;
crc = (crc / 16) XOR (q * 4225);
i = i + 1
LEN = LEN - 1
if (LEN > 0) goto A
Get current byte
Mask off any parity bit
Do low-order 4 bits
And high 4 bits
Position of next byte
Decrement packet length
Loop till done
At this point, the crc variable contains the desired quantity.
Thanks to Andy Lowry of Columbia's CS department for this "tableless" CRC
algorithm (actually, it uses a table with one entry -- 4225). AND is the
bitwise
Optional Features
Page 36
AND operation, XOR the bitwise exclusive OR, "*" is multiplication,
and "/"
signifies integer division ("crc / 16" is equivalent to shifting the crc
quantity 4 bits to the right).
The single-character checksum has proven quite adequate in practice. The
other
options can be used only if both sides agree to do so via Init packet (S
or I)
exchange.
The 2 and 3 character block checks should only be used
under conditions of severe line noise and packet corruption.
Since type 2 and 3 block checks are optional, not all Kermits can be
expected
to understand them. Therefore, during initial connection,
communication must
begin using the type 1 block check. If type 2 or 3 block checks are
agreed to
during the "I" or "S" packet exchange, the switch will occur only
after the
Send-Init has been sent and ACK'd with a type 1 block check. This means
that
the first packet with a type 2 or 3 block check must always be an "F"
or "X"
packet. Upon completion of a transaction, both sides must switch back to
type
1 (to allow for the fact that neither side has any way of knowing
when the
other side has been stopped and restarted). The transaction is over
after a
"B" or "E" packet has been sent and ACK'd, or after any error that
terminates
the transaction prematurely or abnormally.
A consequence of the foregoing rule is that if a type 2 or 3 block check
is to
be used, a long reply sent by the server must begin with a SendInit (S)
packet, even if an I packet exchange had already occurred.
If type 1
block
checks are being used, the S packet can be skipped and the transfer can
start
with an X or F packet.
A server that has completed a transaction and is awaiting a new
command may
send out periodic NAKs for that command (packet 0). Those NAKs must
have type
1 block checks.
The use of alternate block check types can cause certain complications.
For
instance, if the server gets a horrible error (so bad that it doesn't
even send
an error packet) and reverts to command wait, sending NAKs for packet 0
using a
type 1 block check, while a transfer using type 2 or 3 block checks
was in
progress, neither side will be able to read the other's packets.
Communication
can also grind to a halt if A sends a Send-Init requesting, say, type 3
block
checks, B ACKs the request, switches to type 3 and waits for the X or F
packet
with a type 3 block check, but the ACK was lost, so A resends the S
packet with
a type 1 block check. Situations like this will ultimately resolve
themselves
after the two sides retransmit up to their retry threshhold, but can
be rectified earlier by the use of two heuristics:
- The packet reader can assume that if the
the
block check type is 1.
packet
type
is
"S",
- A NAK packet never has anything in its data field. Therefore,
the
block check type can always be deduced by the packet reader from
the
length field of a NAK. In fact, it is the value of the length
field
minus 2. A NAK can therefore be thought of as a kind of
"universal
synchronizer".
These heuristics tend to violate the layered nature of the protocol,
since the
packet reader should normally be totally unconcerned with the packet
type
(which is of interest to the application level which invokes the
packet
reader). A better design would have had each packet include an
indicator of
Optional Features
Page 37
the type of its own block check; this would have allowed the block
check type
to be changed dynamically during a transaction to adapt to changing
conditions.
But it's too late for that now...
8.4. Interrupting a File Transfer
This section describes an optional feature of the Kermit protocol to
allow
graceful interruption of file transfer. This feature is unrelated to
server
operation.
To interrupt sending a file, send an EOF ("Z") packet in place of the
next data
packet, including a "D" (for Discard) in the data field.
The recipient
ACKs
the Z packet normally, but does not retain the file. This does not
interfere
with older Kermits on the receiving end; they will not inspect the data
field
and will close the file normally. The mechanism can be triggered by
typing an
interrupt character at the console of the sending Kermit program.
If a
(wildcard) file group is being sent, it is possible to skip to the next
file or
to terminate the entire batch; the protocol is the same in either case,
but the
desired action could be selected by different interrupt characters, e.g.
CTRL-X
to skip the current file, CTRL-Z to skip the rest of the batch.
To interrupt receiving a file, put an "X" in the data field of an ACK
for a
Data packet. To interrupt receiving an entire file group, use a "Z".
The user
could trigger this mechanism by typing an interrupt character, say, CTRLX and
CTRL-Z, respectively, at the receiving Kermit's console. A sender
that was
aware of the new feature, upon finding one of these codes, would
act as
described above, i.e. send a "Z" packet with a "D" code; a sender that
did not
implement this feature would simply ignore the codes and continue
sending.
In
this case, and if the user wanted the whole batch to be cancelled (or
only one
file was being sent), the receiving Kermit program, after determining
that the
sender had ignored the "X" or "Z" code, could send an Error (E) packet
to stop
the transfer.
The sender may also choose to send a Z packet containing the D code
when it
detects that the file it is sending cannot be sent correctly and
completely -for instance, after sending some packets correctly, it gets an i/o error
reading the file. Or, it notices that the "8th bit" of a file byte is set
when the
file is being sent as a text file and no provision has been made for
transmitting the 8th bit.
8.5. Transmitting File Attributes
The optional Attributes (A) packet provides a mechanism for the
sender of a
file to provide additional information about it. This packet can be
sent if
the receiver has indicated its ability to process it by setting the
Attributes
bit in the capability mask.
If both sides set this bit in the
Kermit
capability mask, then the sender, after sending the filename in the "F"
packet
and receiving an acknowledgement, may (but does not have to) send an "A"
packet
to provide file attribute information.
Setting the Attributes bit in the capability mask does not indicate
support for
any particular attributes, only that the receiver is prepared to accept
the "A"
packet.
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Page 38
The attributes are given in the data field of the "A" packet.
field
consists of 0 or more subfields, which may occur in any order.
subfield
is of the following form:
The data
Each
+-----------+----------------+------+
| ATTRIBUTE | tochar(LENGTH) | DATA |
+-----------+----------------+------+
where
ATTRIBUTE is a single printable character other than space,
LENGTH
added to
is
the
length
of
the
data characters (0 to 94), with 32
produce a single printable character, and
DATA
is length characters worth of data, all printable characters.
No quoting or prefixing is done on any of this data.
More than one attribute packet may be sent. The only requirement is
that all
the A packets for a file must immediately follow its File header (or X)
packet,
and precede the first Data packet.
There may be 93 different attributes, one for each of the 93 printable
ASCII
characters other than space. These are assigned in ASCII order.
! (ASCII 33)
Length. The data field gives the length in K (1024) bytes,
as a
printable decimal number, e.g. "!#109". This will allow the
receiver
to determine in advance whether there is sufficient room
for the
file, and/or how long the transfer will take.
" (ASCII 34)
Type. The data field can contain some indicator of the nature
of the
file.
Operands are enclosed in {braces}, optional
items in
[brackets]. The braces and brackets do not actually appear
in the
packet.
A[{xx}] ASCII
text,
containing no 8-bit quantities, logical
records
(lines) delimited by the (quoted) control character
sequence
{xx},
represented
here
by
its printable counterpart
(MJ =
CRLF, J = LF, etc).
the
For instance
AMJ
means
that
appearance
of
#M#J
(the
normal prefixed CRLF
sequence) in a
file data packet indicates the end of a record,
assuming
the
current
control
prefix is "#".
If {xx} is omitted,
MJ will
be assumed.
B[{xx}] Binary.
{xx} indicates in what manner the file is
binary:
8
(default) The file is a sequence of
8-bit
bytes,
which
must
be saved as is.
The 8th bit may be sent
"bare", or
prefixed according to
the
Send-Init
negotiation
about
8th-bit prefixing.
36
The
file
is
a PDP-10 format binary file, in
which five
7-bit bytes are fit into one 36-bit word, with the
final
Optional Features
Page 39
bit of each word being represented as the "parity
bit" of
every 5th character (perhaps prefixed).
D{x}
Moved from here to FORMAT attribute
F{x}
Moved from here to FORMAT attribute
I[{x}]
represented
Image.
on
the
The file is being sent exactly as it
system
of
origin.
is
For use between like
systems.
There are {x} usable bits per
character,
before
prefixing.
For
instance,
to
send binary data from a system with
9-bit
bytes, it might be convenient to send three 6-bit
characters
for every two 9-bit bytes.
Default {x} is 8.
# (ASCII 35)
Creation Date, expressed as "[yy]yymmdd[ hh:mm[:ss]]" (ISO
standard
date format), e.g. 831009 23:59. The time is optional; if
given, it
should be in 24-hour format, and the seconds may be omitted,
and a
single space should separate the time from the date.
$ (ASCII 36)
Creator's ID, expressed as a character string of the given
length.
% (ASCII 37)
Account to charge the file to, character string.
& (ASCII 38)
Area in which to store the file, character string.
' (ASCII 39)
Password for above, character string.
( (ASCII 40)
Block Size.
block
size.
) (ASCII 41)
Access:
The file has, or is to be stored with, the
given
N
New, the normal case -- create a new file of the given
S
A
Supersede (overwrite) any file of the same name.
Append to file of the given name.
name.
* (ASCII 42)
Encoding:
A
ASCII, normal ASCII encoding with any necessary prefixing,
etc.
H
Hexadecimal "nibble" encoding.
E
EBCDIC (sent as if it were a binary file).
X
Encrypted.
Q{x}
Huffman Encoded for compression. First x bytes of the
file
are
the key.
# (ASCII 43)
Disposition (operands are specified in the syntax of
receiver's
the
Optional Features
Page 40
host system):
M{user(s)}
Send the file as Mail to the specified user(s).
O{destination}
Send
the
file
as
a
lOng
terminal message
to the
specified destination (terminal, job, or user).
S[{options}]
Submit the file as a batch job,
with
any
specified
options.
P[{options}]
with
Print
the
file
on
a
system
printer,
any
specified options, which
may
specify
a
particular
printer, forms, etc.
T
L[{aaa}]
address, if
Type the file on the screen.
Load
the
file
into memory at the given
any.
address
X[{aaa}]
and
Load the file into memory at the
given
eXecute it.
A
Archive the file; save the file together with
the attribute packets that preceded it, so that it
can
be
sent
back
to
the system of origin with all
its attributes intact.
A file stored in this way
should be
specially
marked
so
that
the Kermit that
sends it
back will recognize the attribute information
as distinct from the file data.
, (ASCII 44)
Protection. Protection code for the file, in the syntax
of the
receiver's host file system. With no operand, store according
to the
system's default protection for the destination area.
- (ASCII 45)
Protection.
Protection
code
for
the
file
with
respect
to the
"public" or "world", expressed generically in a 6-bit quantity
(made
printable by tochar()), in which the bits have the following
meaning:
b0:
b1:
b2:
b3:
b4:
b5:
A
Read Access
Write Access
Execute Access
Append Access
Delete Access
Directory Listing
one
in the bit position means allow the corresponding type
of access, a zero means prohibit it.
For example, the letter "E" in
this
field
would
allow
read,
execute,
and
directory
listing
access
(unchar("E") = 69-32 = 37 = 100101 binary).
. (ASCII 46)
Machine and operating system of origin. This is useful in
conjunction with the archive disposition attribute. It allows a file,
once
archived, to be transferred among different types of systems,
retain-
Optional Features
Page 41
ing its archive status, until it finds its way to a machine
with
the
right
characteristics
to de-archive it.
The systems are
denoted by
codes; the first character is the major system designator, the
second
designates the specific model or operating system.
A third
character
may be added to make further
distinctions,
for
instance
operating
system version. The systems below do not form a complete
collection;
many more can and probably will be added.
A
Apple microcomputers
1
2
3
4
B
Sperry (Univac) mainframes
1
2
C
Cyber
Cyber
Cyber
Cyber
series,
series,
series,
series,
NOS
NOS-BE
NOS-VE
SCOPE
DEC Systems
1
2
3
4
5
6
7
8
9
A
B
C
D
E
1100 series, EXEC
9080, VS9
CDC mainframes
1
2
3
4
D
Apple II, DOS
Apple III
Macintosh
Lisa
DECsystem-10/20, TOPS-10
DECsystem-10/20, TOPS-20
DECsystem-10/20, TENEX
DECsystem-10/20, ITS
DECsystem-10/20, WAITS
DECsystem-10/20, MAXC
VAX-11, VMS
PDP-11, RSX-11
PDP-11, IAS
PDP-11, RSTS/E
PDP-11, RT-11
Professional-300, P/OS
Word Processor (WPS or DECmate), WPS
Honeywell mainframes
1
2
3
4
F
MULTICS systems
DPS series, running CP-6
DPS series, GCOS
DTSS
Data General machines
1
2
3
RDOS
AOS
AOS/VS
Optional Features
Page 42
G
PR1ME machines, PRIMOS
H
Hewlett-Packard machines
1
2
I
HP-1000, RTE
HP-3000, MPE
IBM 370-series and compatible mainframes
1
2
3
4
5
6
VM/CMS
MVS/TSO
DOS
MUSIC
GUTS
MTS
J
Tandy microcomputers, TRSDOS
K
Atari computers
1
2
L
Commodore micros
1
2
3
M
Home computers, DOS
ST series
Pet
64
Amiga
Miscellaneous mainframes and
minis
with
proprietary
operation
systems:
1
2
3
4
5
6
7
8
9
A
B
N
Gould/SEL minis, MPX
Harris, VOS
Perkin-Elmer minis, OS/32
Prime, Primos
Tandem, Nonstop
Cray, CTSS
Burroughs (subtypes may be necessary here)
GEC 4000, OS4000
ICL machines
Norsk Data, Sintran III
Nixdorf machines
Miscellaneous micros and workstations:
1
2
3
Acorn BBC Micro
Alpha Micro
Apollo Aegis
4
Convergent, Burroughs, and similar systems with CTOS,
5
6
7
8
9
A
Corvus, CCOS
Cromemco, CDOS
Intel x86/3x0, iRMX-x86
Intel MDS, ISIS
Luxor ABC-800, ABCDOS
Perq
BTOS
Optional Features
Page 43
B
Motorola, Versados
O-T Reserved
U
Portable Operating or File Systems
1
2
3
4
5
6
7
8
9
A
B
C
D
UNIX
Software Tools
CP/M-80
CP/M-86
CP/M-68K
MP/M
Concurrent CP/M
MS-DOS
UCSD p-System
MUMPS
LISP
FORTH
OS-9
/ (ASCII 47)
Format of the data within the packets.
A{xx}
by
Variable length delimited records, terminated
the
character
sequence {xx}, where xx is a string
of one
or more control characters, represented here by
their
unprefixed
printable
equivalents,
e.g. MJ
for ^M^J
(CRLF).
D{x}
Variable length undelimited records.
Each
logical
record
begins
with
an
{x}-character ASCII
decimal
length field (similar to ANSI tape format "D").
For
example,
"D$"
would indicate 4-digit length
fields,
like "0132".
F{xxxx}
Fixed-length
undelimited
records.
Each
logical
record is {xxxx} bytes long.
R{x}
combina-
For record-oriented transfers, to be used in
tion with one of
the
formats
given
above.
Each
record
begins
(in
the
case of D format,
after the
length field) with an x-character long position
field
indicating the byte position within the file at
which
this record is to be stored.
M{x}
For record-oriented transfers, to be used in
combination
with
one
of the formats given above.
Maximum
record length for a variable-length record.
0 (ASCII 48)
Special system-dependent parameters for storing the file on
the system of origin, for specification of exotic attributes not
covered explicitly by any of the Kermit attribute descriptors. These are
given
as a character string in the system's own language, for
example a
list of DCB parameters in IBM Job Control Language.
Optional Features
Page 44
[email protected] (ASCII 49)
Exact byte count of the file as it is stored on the sender's
system,
before any conversions (e.g. to canonic form). Of limited
usefulness
when transferring text files between systems that represent
text
boundaries differently.
[email protected] (ASCII 50-64)
Reserved
Other attributes can be imagined, and can be added later if needed.
However,
two important points should be noted:
- The receiver may have absolutely no way of honoring, or even
recording, a given attribute. For instance, CP/M-80 has no slot for
creation date or creator's ID in its FCB; the DEC-20 has no concept
of
block size, etc.
- The sender may have no way of determining the correct values of
any
of the attributes. This is particularly true when sending files
of
foreign origin.
The "A" packet mechanism only provides a way to send certain information
about
a file to the receiver, with no provision or guarantee about what the
receiver
may do with it. That information may be obtained directly from the
file's
directory entry (FCB, FDB, ...), or specified via user command.
The ACK to the "A" packet may in turn have information in its data
field.
However, no complicated negotiations about file attributes may take
place, so
the net result is that the receiver may either refuse the file or
accept it.
The receiver may reply to the "A" packet with any of the following codes
in the
data field of the ACK packet:
<null>
(empty data field) I accept the file, go ahead and send it.
N[{xxx}]
I refuse the file as specified, don't send it; {xxx} is a
of
zero or more of the attribute characters listed above, to
specify what
attributes I object to (e.g. "!" means it's too long, "&" means I
don't
have write access to the specified area, etc).
string
Y[{xxx}]
I agree to receive the file, but I cannot honor attributes {xxx},
so
I
will store the file according to my own defaults.
Y
(degenerate case of Y{xxx}, equivalent to <null>, above)
How the receiver actually replies is an implementation decision. A
NAK in
response to the "A" packet means, of course, that the receiver did not
receive
the "A" correctly, not that it refuses to receive the file.
Optional Features
Page 45
8.6. Advanced Kermit Protocol State Table
The simple table presented previously is sufficient for a basic
Kermit implementation. The following is a state table for the full Kermit
protocol, including both server mode and sending commands to a server Kermit. It
does not
include handling of the file attributes packet (A).
Note that states
whose
names start with "Send" always send a packet each time they are entered
(even
when the previous state was the same). States whose name starts with
"Rec",
always wait for a packet to be received (up to the timeout value), and
process
the received packet. States whose names do not include either send or
receive
do not process packets directly. These are states which perform some
local
operation and then change to another state.
The initial state is determined by the user's command.
A "server"
command
enters at Rec_Server_Idle. A "send" command enters at Send_Init. A
"receive"
command (the old non-server version, not a "get" command) enters at
Rec_Init.
Any generic command, the "get" command, and the "host" command enter at
either
Send_Server_Init or Send_Gen_Cmd, depending upon the expected response.
Under "Rec'd Msg", the packet type of the incoming message is shown,
followed
by the packet number in parentheses; (n) means the current packet number,
(n-1)
and (n+1) mean the previous and next packet numbers (modulo 64), (0)
means
packet number zero. Following the packet number may be slash and a
letter, indicating some special signal in the data field. For instance Z(n)/D
indicates
a Z (EOF) packet, sequence number n, with a "D" in the data field.
Under "Action", "r+" means that the retry count is incremented and
compared
with a threshhold; if the threshhold is exceeded, an Error packet is
sent and
the state changes to "Abort".
"n+" means that the packet number is
incre-
mented, modulo 64, and the retry count, r, is set back to zero.
State
Rec'd Msg
Action
Next state
Rec_Server_Idle -- Server idle, waiting for a message
Set n and r to 0
I(0)
S(0)
R(0)
K, C or G(0)
Send ACK
Process params,
ACK with params, n+
Save file name
Short reply:
ACK(0)/reply
Long reply:
init needed
init not needed, n+
Rec_Server_Idle
Rec_File
Send_Init
Rec_Server_Idle
Send_Init
Open_File
Timeout
Send NAK(0)
Rec_Server_Idle
Other
Send E
Rec_Server_Idle
Rec_Init -- Entry point for non-server RECEIVE command
Set n and r to 0
S(0)
Process params, send
ACK with params, n+
Rec_File
Optional Features
Page 46
Timeout
Send NAK(0), r+
Rec_Init
Other
Send E
Abort
Rec_File -- Look for a file header or EOT message
F(n)
X(n)
Open file, ACK, n+
Prepare to type on
screen, ACK, n+
B(n)
ACK
S(n-1)
ACK with params, r+
Z(n-1)
ACK, r+
Timeout
Resend ACK(n), r+
Other
Send E
Rec_Data -- Receive data up to end of file
Store data, ACK, n+;
If interruption wanted
include X or Z in ACK
D(n-1)
Send ACK, r+
Z(n)
Close file, ACK, n+
Z(n)/D
Discard file, ACK, n+
F(n-1)
Send ACK, r+
X(n-1)
Send ACK, r+
Timeout
Send ACK(n-1), r+
Other
Send E
Send_Init -- Also entry for SEND command
Rec_Data
Rec_Data
Complete
Rec_File
Rec_File
Rec_File
Abort
D(n)
Rec_Data
Rec-Data
Rec_File
Rec_File
Rec_Data
Rec_Data
Rec_Data
Abort
Set n and r to 0, send S(0) with parameters
Y(0)
Process params, n+
N, Timeout
r+
Other
r+
Open_File -- Open file or set up text to send
Open_File
Send_Init
Send_Init
Send_File
Send_File -- Send file or text header
Send F or X(n)
Y(n), N(n+1)
Get first buffer of
data, n+
r+
Send_Data or Send_Eof if
empty file or text
N, Timeout
Send_File
Other
Abort
Send_Data -- Send contents of file or textual information
Send D(n) with current buffer
Y(n), N(n+1)
n+, Get next buffer
Y(n)/X or Z
N, Timeout
n+
r+
Send_Data or Send_Eof if
at end of file or text
Send_Eof
Send_Data
Other
Send_Eof -- Send end of file indicator
Abort
Send Z(n); if interrupting send Z(n)/D
Y(n), N(n+1)
Open next file, n+
Send_File if more, or
Optional Features
Page 47
N, Timeout
r+
Other
Send_Break -- End of Transaction
Send_Break if no more
or if interrupt "Z".
Send_Eof
Abort
Send B(n)
Y(n), N(0)
Complete
N(n), Timeout
Send_Break
Other
Abort
Send_Server_Init - Entry for Server commands which expect large response.
Send I(0) with parameters
Y(0)
Process params
Send_Gen_Cmd
N, Timeout
r+
Send_Server_Init
E
Use default params
Send_Gen_Cmd
Other
Abort
Send_Gen_Cmd - Entry for Server commands which expect short response
(ACK)
Send G, R or C(0)
S(0)
X(1)
Process params,
ACK with params, n+
Setup to type on
terminal, n+
Type data on TTY
r+
Rec_File
Rec_Data
Y(0)
Complete
N, Timeout
Send_Gen_Cmd
Other
Abort
Complete -- Successful Completion of Transaction
Set n and r to 0;
If server, reset params, enter Rec_Server_Idle
otherwise exit
Abort -- Premature Termination of Transaction
Reset any open file, set n and r to 0
If server, reset params, enter Rec_Server_Idle
otherwise exit
Exit, Logout states
Exit or Logout
Note that the generic commands determine the next state as follows:
1. If the command is not supported, an error packet
the
next state is "Abort".
is
sent
and
2. If the command generates a response which can be fit into the
data
portion of an ACK, an ACK is sent with the text (quoted
as
necessary) in the data portion.
3. If the command generates a large response or must send a file,
nothing is sent from the Rec_Server_Idle state, and the next state
is
either Send_Init (if either no I message was received or if
alter-
Optional Features
Page 48
nate block check types are to be used), or Open_File (if an
I
message
was
received
and
the
single character block check is to
be
used).
4. If the command is Logout, an ACK
is
sent
and
the
new
state
is
Logout.
5. If the command is Exit, an ACK is sent and the new state is Exit.
Performance Extensions
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9. Performance Extensions
The material in this chapter was added in 1985-86 to address the
inherent performance problems of a stop-and-wait protocol like Kermit.
9.1. Long Packets
A method is provided to allow the formation of long Kermit packets.
Questions
as to the desirability or appropriateness of this extension to the
Kermit
protocol are not addressed. All numbers are in decimal (base 10)
notation, all
arithmetic is integer arithmetic.
In order for long packets to be exchanged, the sender must set the
bit for
Capability #5 (the LONGP bit) in the CAPAS field of the Send-Init (S
or I)
packet,
bit5 bit4 bit3 bit2 bit1 bit0
+----+----+----+----+----+----+
| #1 | #2 | #3 | #4 | #5 | 0 |
+----+----+----+----+----+----+
^
|
LONGP
and also furnish
fields, as
follows:
the
MAXLX1 and MAXLX2 (extended length 1 and 2)
10
CAPAS+1 CAPAS+2 CAPAS+3
---+-------+-+--------+--------+--------+
| CAPAS | ... | WINDO | MAXLX1 | MAXLX2 |
---+-------+-+--------+--------+--------+
^
|
(currently field 11, because CAPAS is still 1 byte)
where WINDO is the window size (a separate Kermit protocol
extension), and
MAXLX1 and MAXLX2 are each a printable ASCII character in the range SP
(space,
ASCII 32) to ~ (tilde, ASCII 126), formed as follows:
MAXLX1 = tochar(m / 95)
MAXLX2 = tochar(m MOD 95)
(where m is the intended maximum length, / signifies integer division,
and MOD
is the modulus operator), to indicate the longest extended-length
packet it
will accept as input. The receiver responds with an ACK packet having
the same
bit also set in the CAPAS field, and with the MAXLX1 and MAXLX2 fields
set to
indicate the maximum length packet it will accept.
The maximum length expressible by this construct is 95 x 94 + 94, or
9024.
Since the sender can not know in advance whether the receiver is capable
of extended headers, the Send-Init MAXL field must also be set in the normal
manner
for compatibility.
If the receiver responds favorably to an extended-length packet bid
(that is,
Performance Extensions
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if its ACK has the LONGP bit set in the CAPAS field), then the combined
value
of its MAXLX1,MAXLX2 fields is used.
If the LONGP bit is set
but the
MAXLX1,MAXLX2 pair is missing, then the value 500 will be used by
default.
If the response is unfavorable (the LONGP bit is not set in the
receiver's
CAPAS field), then extended headers will not be used and the MAXL field
will
supply the maximum packet length.
After the Send-Init has been sent and acknowledged with agreement to
allow extended headers, all packets up to and including the B or E packet which
terminates the transaction (and its acknowledgement) are allowed -- but
not required -- to have extended headers; extended and normal packets may be
freely
mixed by both Kermits.
The normal Kermit packet length field (LEN) specifies the number of
bytes to
follow, up to and including the block check. Since at least 3 bytes must
follow (SEQ, TYPE, and CHECK), a value of 0, 1, or 2 is never encountered
in the
LEN field of a valid unextended Kermit packet. When extended packets
have been
negotiated, the LEN field is treated as follows for the duration of the
transaction:
- If unchar(LEN) > 2 then the packet is a normal, unextended packet.
- If unchar(LEN) = 0 then the packet has a "Type 0" extended header.
- If unchar(LEN) = 1 or 2, the packet is invalid and should cause
an
Error.
"Lengths" of 1 and 2 are
extended
headers, yet to be specified.
reserved for future use in Type 1 and 2
A Type 0 extended packet has the following layout:
+------+-----+-----+------+-------+-------+--------+-----+
----+-----
| MARK |
| SEQ | TYPE | LENX1 | LENX2 | HCHECK | DATA ....
|
CHECK |
+------+-----+-----+------+-------+-------+--------+-------+------+
| Extended Header
|
The blank length field (SP = tochar(0)) indicates that the first 3
bytes of
what is normally the data field is now an extended header of Type 0, in
which
the number of bytes remaining in the packet, up to and including the
block
check, is
extended-length = (95 x unchar(LENX2)) + unchar(LENX2)
and HCHECK is a header checksum, formed exactly like a Type-1 Kermit
block
check, but from the sum of the ASCII values of the SEQ, TYPE, LENX1, and
LENX2
fields:
s = LEN + SEQ + TYPE + LENX1 + LENX2
HCHECK = tochar((s + ((s & 192)/64)) & 63)
where & is the bitwise AND operator.
Since the value of the extended length field must be known accurately in
order
to locate the end of the packet and the packet block check, it is vital
that
this information not be corrupted before it is used. The header
checksum
Performance Extensions
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prevents this.
The extended header, like the normal header itself, is not prefixencoded.
This is because it is used at datalink level, before decoding takes
place.
Therefore the entity responsible for building packets must leave 3
spaces at
the beginning of the data field, and the datalink function (spack)
fills in
LENX1, LENX2, and HCHECK based upon the data actually entered into the
packet,
after encoding. The packet receiving datalink function (rpack) behaves
accordingly.
The packet block check is formed in the usual manner, based on all packet
bytes
beginning with LEN and ending with the last character in the data
field. The
block check may be Type 1, 2, or 3, depending upon what was
negotiated, but
longer packets are more likely to be corrupted than shorter ones and
should
therefore have higher-order block checks if possible. This proposal
does not
change the way block check type is negotiated, and does not require that
Type 2
or 3 block check be implemented.
With long packets, the possibility exists that the arithmetic sum
of the
characters in a packet will exceed 2^15, and will overflow a 16-bit
word, or
become negative. The checksum function would have to be modified to
guard
against this, for instance by always setting the high four bits of the
sum to
zero before adding in the next byte.
Implementation can be a bit tricky. The Kermit program should be set up
to use
normal, untextended packets by default -- that is, to mimic the
behavior of
original, "classic" Kermit. Even when the program believes itself
to be
capable of sending and receiving long packets, it has no knowledge
of what
devices may lie along the communication path, whose buffers might not be
long
enough to accommodate bursts of data of the desired length. Long
packets
should be elected when the user has explicitly elected them with a SET
command.
The current SET SEND PACKET-LENGTH <n> command will do; if the number is
larger
than 94, then the program will -- transparently to the user -- try to
negotiate
long packets.
A finer degree of control can be accomplished by
included SET
commands to explicitly enable or disable the use of long packets.
Once long packets are successfully negotiated, the program should be
prepared
to back off when errors occur, since the very size of the packets may
be the
cause of the errors. Upon timeout or receipt of a NAK (or extra copies
of the
previous packet), the sender should be prepared to reconstruct the
current
packet at, say, half its size, down to some reasonable minimum,
before
retransmission.
Even when the size itself is not the problem, this
makes
retransmission less painful under noisy conditions.
Long packets and sliding windows may be used at the same time,
though the
benefits from doing so may not be worth the trouble of coding the dynamic
buffer allocation required (for n buffers of size m, negotiated at Send-Init
time).
It's also worth noting that the benefit/cost ratio of long packets
declines
after a length of about 1000, at which point the benefit of additional
length
is less than 1%, and the cost of retransmission is very high.
Performance Extensions
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9.2. Sliding Windows
The sliding window extension to Kermit was proposed and developed by a
group at
The Source Telecomputing in McLean, Virginia, led by Leslie Spira and
including
Hugh Matlock and John Mulligan, who wrote the following material. Like
other
extensions, this one is designed for "upward compatibility" with Kermits
that
do not support this extension.
The windowing protocol as defined for the Kermit file transfer
protocol is
based on the main premise of continuously sending data packets up to the
number
defined by a set window size. These data packets are continuously
acknowledged
by the receive side and the ideal transfer occurs as long as they are
transmitted with good checksums, they are transmitted in sequential order and
there
are no lost data packets or acknowledgements.
The various error
conditions
define the details of the windowing protocol and are best examined on
a case
basis.
There are five stages that describe the overall sequence of events in the
Kermit protocol. Three of these stages deviate from the original protocol
in order to add the windowing feature. Stages 1 through 5 are briefly
described on
the following page.
The three stages (1, 3 and 4) which deviate
from the
original protocol are then described in greater detail in the pages that
follow.
9.2.1. Overall Sequence of Events
STAGE 1 - Propose and Accept Windowing
The send side requests windowing in the transmission of the SendInitiate
(S) packet. The receive side accepts windowing by sending an
acknowledgement (ACK packet) for the Send-Initiate packet.
STAGE 2 - Send and Accept File-Header Packet
The send side transmits the File-Header (F) packet and waits
for the
receive side to acknowledge it prior to transmitting any data.
STAGE 3 - Transfer Data
The sending routine transmits Data (D) packets one after the other
until
the protocol window is closed. The receiving side ACKs good data,
stores
data to disk as necessary and NAKs bad data.
When the sender receives an ACK, the window may be rotated and
the next
packet sent. If the sender receives a NAK, the data packet
concerned is
retransmitted.
STAGE 4 - Send and Accept End_of_File Packet
As the sender is reading the file for data to send, it will
eventually
reach the end of the file. It then waits until all outstanding data
packets have been acknowledged, and then sends an End-of_File (Z) packet.
When the receive side gets the End-of-File packet it stores the rest
of the
data to disk, closes the file, and ACKs the End-of_File packet.
The protocol then returns to Stage 2, sending and acknowledging any
further
File-Header (F) packets.
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STAGE 5 - End of Transmission
Once the End-of-File packet has been sent and acknowledged and there
are no
more files to send, the sender transmits the End-of-Transmission (B)
packet
in order to end the ongoing transaction. Once the receiver
ACKs this
packet, the transaction is ended and the logical connection closed.
Stage 1 - Propose and Accept Windowing
The initial connection as currently defined for the Kermit protocol
need
to change only in terms of the contents of the Send-Initiate
packet. The
receiving Kermit waits for the sending Kermit to transmit the SendInitiate (S)
packet and the sending packet does not proceed with any additional
transmission
until the ACK has been returned by the receiver.
will
The contents of the Send-Init packet, however, will be slightly revised.
The
data field of the Send-Init packet currently contains all of the
configuration
parameters. The first six fields of the Send-Init packet are fixed as
follows:
1
2
3
4
5
6
+--------+--------+--------+--------+--------+--------+
| MAXL
| TIME
| NPAD
| PADC
| EOL
| QCTL
|
+--------+--------+--------+--------+--------+--------+
Fields 7 through 10 are optional features of Kermit and fields 7 through
9 will
also remain unchanged as defined for the existing protocol:
7
8
9
10
+--------+--------+--------+--------+
| QBIN
| CHKT
| REPT
| CAPAS |
+--------+--------+--------+--------+
The windowing capability constitutes a fourth capability and the fourth
bit of
the capability field will be set to 1 if the Kermit implementation can
handle
windowing:
bit5 bit4 bit3 bit2 bit1 bit0
+----+----+----+----+----+----+
| #1 | #2 | #3 | #4 | #5 | 0 |
+----+----+----+----+----+----+
^
|
SWC (sliding window capability)
The remaining fields of the Send-Init packet are either reserved for
future use
by the standard Kermit protocol or reserved for local site
implementations.
The four fields following the capability field are reserved for the
standard
Kermit protocol. The field following the capability mask is used to
specify
the "Window Size":
10
CAPAS+1 CAPAS+2 CAPAS+3
---+-------+-+--------+--------+--------+
| CAPAS | ... | WINDO | MAXLX1 | MAXLX2 |
---+-------+-+--------+--------+--------+
^
|
Performance Extensions
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(currently field 11, because CAPAS is still 1 byte)
WINDO is the window size to be used, encoded printably using the tochar()
function. The window size may range from 1 to 31 inclusive.
The sender will specify the window size it wishes to use and the receiver
will
reply (in the ACK packet) with the window size it wishes to use. The
window
size actually used will be the minimum of the two.
If the receiver
replies
with a window size of 0 then no windowing will be done.
Stage 3 - Transfer Data
The sequence of events required for the transmission of data packets
and confirmation of receipts constitute the main functions of the windowing
protocol.
There are four main functions which can be identified within this stage.
These
are:
-
the
the
the
the
sender's processing of the data packets,
receiver's handling of incoming packets,
sender's handling of the confirmations,
error handling on both sides.
The following discussion details the specific actions required for
each of
these functions. Refer to the state table at the end of this document
for the
specific action taken on a "received message" basis for the full
protocol.
The Sender's Processing of Data Packets
The sender instigates the transmission by sending the first data
packet and
then operating in a cyclical mode of sending data until the defined
window is
closed.
Data to be sent must be read from the
Data
packet, and saved in a Send-Table.
the data
file,
encoded
into
the
Kermit
A Send-Table entry consists of
packet itself (which makes convenient the re-send of a NAK'd packet),
a bit
which keeps track of whether the packet has been ACK'd (the ACK'd bit),
and a
retry counter. The table is large enough to hold all the packets
for the
protocol window.
Before each transmission, the input buffer is checked and input is
processed,
as described below. Transmission is stopped if the protocol window
"closes",
that is, if the Send-Table is full.
The Receiver's Handling of Incoming Packets
The receiver keeps its own table as it receives incoming data packets.
This
allows the receiver to receive subsequent packets while it is waiting
for a
re-send of an erroneous or lost packet. In other words, the incoming
packets
do not have to be received in sequential order and can still be written
to disk
in order.
A Receive-Table entry consists of the data packet, a bit which keeps
track of
whether a good version of the packet has been received (the ACK'd bit),
and a
retry counter for the NAKs we send to request retransmissions of the
packet.
The table is large enough to hold all the packets for the protocol
window.
Performance Extensions
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The different possibilities for a received packet are:
1.
2.
3.
4.
5.
A new packet, the next sequential one (the usual case)
A new packet, not the next sequential one (some were lost)
An old packet, retransmitted
An unexpected data packet
Any packet with a bad checksum
These are now discussed separately:
1. The next new packet has sequence number <one past the latest
table
entry>.
The packet is ACK'd, and the Receive-Table is checked
for
space. If it is full (already contains window_size entries)
then
the oldest entry is written to disk. (This entry should have
the
ACK'd bit set. If not, the receiver aborts the file transfer.)
The
received packet is then stored in the Receive-Table, with the
ACK'd
bit set.
2. If the packet received has sequence number in the range <two
past
the latest table entry> to <window_size past the latest table
entry>
then it is a new packet, but some have been lost. (The upper
limit
here represents the highest packet the sender could send within
its
protocol window. Note that the requirement to test for this case
is
what limits the maximum window_size to half of the range of
possible
sequence numbers) We ACK the packet, and NAK all packets that
were
skipped.
(The skipped packets are those from <one past the
latest
table entry> to <one before the received packet>) The ReceiveTable
is then checked. The table may have to be rotated to accomodate
the
packet, as with case 1. (This time, several table entries may
have
to be written to disk. As before, if any do not have the ACK'd
bit
set, they will trigger an abort.) The packet is then stored in
the
table, and the ACK'd bit set.
3. A retransmitted packet will have sequence number in the range
<the
oldest table entry> to <the latest table entry>.
The packet
is
ACK'd, then placed in the table, setting the ACK'd bit.
4. A packet with sequence number outside of the range from <the
oldest
table entry> to <window_size past the latest table entry> is
ignored.
5. If the packet received has a bad checksum, we must decide whether
to
generate a NAK, and if so, with what sequence number.
The best
action
may
depend
on the configuration and channel error rate.
For
now, we adopt the following heuristic: If there are unACK'd
entries
in our Receive-Table, we send a NAK for the oldest one.
Otherwise
we ignore the packet. (Notice that this will occur in a
common
case:
when things have been going smoothly and one packet
gets
garbled. In this case, when we later receive the next packet
we
will NAK for this one as described under Case 2 above.)
Performance Extensions
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The Sender's Handling of Confirmations
The sender's receipt of confirmations controls the rotation of the
Send-Table
and normally returns the sender to a sending state.
The sender's
action
depends on the packet checksum, the type of confirmation (ACK or
NAK), and
whether the confirmation is within the high and low boundaries
of the
Send-Table.
If the checksum is bad the packet is ignored.
When the sender receives an ACK, the sequence number is examined. If
the sequence number is outside of the current table boundaries, then the ACK is
also
ignored. If the sequence number is inside of the current table
boundaries then
the ACK'd bit for that packet is marked. If the entry is at the low
boundary,
this enables a "rotation" of the table. The low boundary is changed
to the
next sequential entry for which the ACK'd bit is not set. This frees
space in
the table to allow further transmissions.
When the sender receives a NAK, the table boundaries are checked. A
NAK outside of the table boundary is ignored and a NAK inside the table
boundary indicates that the sender must re-send the packet. The sender first
tests the
packet's retry counter against the retry threshold. If the threshold has
been
reached, then the transfer is stopped (by going to the Abort state).
Otherwise, the retry counter is incremented and the packet re-sent.
Error Handling for Both Sides
Three situations are discussed here:
and invalid packets.
Sender timeout, Receiver timeout,
If certain packets are lost, each side may "hang", waiting for the
other. To
get things moving when this happens each may have a "timeout
limit", the
longest they will wait for something from the other side.
If the sender's timeout condition is triggered, then it will send the
oldest
unACK'd packet. This will be the first one in the Send-Table.
If the receiver's timeout condition is triggered, then it will send a
NAK for
the "most desired packet".
This is defined as either the oldest
unACK'd
packet, or if none are unACK'd, then the next packet to be received
(sequence
number <latest table entry plus one>). The packet retry count is not
incremented by this NAK; instead we depend on the timeout retry count,
discussed
next.
For either the sender or receiver, the timeout retry count is
incremented each
time a timeout occurs. If the timeout retry limit is exceeded then the
side
aborts the file transfer. Each side resets the retry count to zero
whenever
they receive a packet.
In addition, as with the existing Kermit, any invalid packet types
received by
either side will cause an Error packet and stop the file transfer.
Performance Extensions
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Stage 4 - Send and Accept End of File Packet
There are several ways to end the file transfer. The first is the
normal way,
when the sender encounters an end-of-file condition when reading the
file to
get a packet for transmission. The second is because of a sender side
user interrupt. The third is because of a receiver side user interrupt.
Both of
these cause the received file to be discarded. In addition either
side may
stop the transfer with an Error packet if an unrecoverable error is
encountered.
Normal End of File Handling
When the sender reaches the end of file, it must wait until all data
packets
have been acknowledged before sending the End-of-File (Z) packet. To do
this
it must be able to check the end-of-file status when it processes ACKs.
If the
ACK causes the Send-Table to be emptied and the end-of-file has been
reached,
then a transition is made to the Send_Eof state which sends the
End_of_File
packet.
When the receiver gets the End_of_File packet, it writes the contents
of the
Receive-Table to the file (suitably decoded) and closes the file.
(If any
entries do not have the ACK'd bit set, or if errors occur in writing the
file,
the receiver aborts the file transfer.) If the operation is
successful, the
receiver sends an ACK. It then sets its sequence number to the
End_of_File
packet sequence number and goes to Rcv_File state.
File Transfer Interruptions
Sender User Interrupt
Whenever the sender checks for input from the data communications
line, it
should also check for user input. If that indicates that the file
transfer
should be stopped, the sender goes directly to the Send_Eof state and
sends
an End_of_File packet with the Discard indication. It will not
have to
wait for outstanding packets to be ACK'd.
When the receiver gets the End_of_File packet with the Discard
indication
it discards the file, sets its sequence number to the End_of_File
packet
sequence number, and goes to RcvFile state.
Receiver User Interrupt
Whenever the receiver checks for input from the data communications
line,
it also should check for user input.
If that indicates that the
file
transfer should be stopped, the receiver sets an "interrupt
indication" of
X (for "stop this file transfer") or of Z (for "stop the batch of
file
transfers").
When the receiver later sends an ACK, it places an X
or Z in
the data field.
When the sender gets this ACK, it goes to the Send_Eof state and
sends the
End_of_File packet with the Discard indication, as above.
When the receiver gets the End_of_File packet with the Discard
indication,
it discards the file, sets its sequence number to the End_of_File
packet
sequence number, and goes to RcvFile state.
Performance Extensions
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Low Level Protocol Requirements
The windowing protocol makes certain assumptions about the underlying
transmission and reception mechanism.
First, it must provide a full-duplex channel so that messages may be
sent and
received simultaneously.
Second, it will prove advantageous to be able to buffer several
received messages at the low level before processing them at the Kermit level. This
is for
two reasons. The first is that the Kermit windowing level of the
protocol may
take a while to process one input, and meanwhile several others may
arrive.
The second reason is to support XON/XOFF flow control. If Kermit
receives an
XOFF from the data communications line, it must wait for an XON before
sending
its packet. While it is waiting, the low level receive must be able to
accept
input. Otherwise a deadlock situation could arise with each side flow
controlled, waiting for the other.
Kermit Windowing Protocol State Table
The following table shows the inputs expected, the actions performed,
and the
succeeding states for the Send_Data_Windowing and Rcv_Data_Windowing
states.
If both sides agree on windowing in the Send Init exchange, then
instead of
entering the old Send_Data or Rcv_Data states from Send_File or
Rcv_File, we
enter the new Send_Data_Windowing or Rcv_Data_Windowing.
Performance Extensions
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SEND_DATA_WINDOWING (SDW)
Rec'd Msg
Action
Next State
No input/Window closed
No input/Window open
(1) Wait for input
(2) Read file, encode packet,
Place in table, mark unACK'd,
Send packet
SDW
SDW
ACK/ X or Z
ACK/outside table
ACK/inside table
(3) set interrupt indicator (X/Z) Send_Eof
-ignoreSDW
(4) mark pkt ACK'd,
SDW or Send_Eof
if low rotate table,
if file eof & table empty
then goto Send_Eof
NAK/outside table
NAK/inside table
-ignore(5) test retry limit,
re-send DATA packet
SDW
SDW
Bad checksum
-ignore-
SDW
Timeout
(6) re-send oldest unACK'd pkt
SDW
User interrupt
(7) set interrupt indicator (X/Z)
Other
(8) send Error
Send_Eof
Quit
RCV_DATA_WINDOWING (RDW)
Rec'd Msg
DATA/new
Action
Next State
(1) send ACK
if table full: file & rotate
store new pkt in table
(2) send ACK, store in table
-ignore-
RDW
RDW
Z/discard
Z/
(3) discard file
(4) write table to file & close
if OK send ACK, else Error
Rcv_File
Rcv_File
or Quit
Bad checksum
(5) send NAK for oldest unACK'd
RDW
Timeout
(6) send NAK for most desired pkt
RDW
User Interrupt
(7) Set interrupt indicator X or Z
RDW
Other
(8) send Error pkt
DATA/old
DATA/unexpected
RDW
Quit
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9.2.2. Questions and Answers about Sliding Windows
Q.
What is the purpose of the "windowing" extension?
A. The object is to speed up file transfers using Kermit. The increase
will
be especially noticeable over the data networks (such as
Telenet and
Tymnet) and over connections using satellite links. This is because
there
are long communications delays over these connections.
Q.
How does it work?
A. Basically, it allows you to send several packets out in a row
before getting the first acknowledgment back. The number of packets that can
be sent
out is set by the "window size", hence the name windowing.
Q.
Could you explain in more detail?
A. Right now, a system sending a file transmits one packet of data,
then does
nothing more until it gets back an acknowledgment that the packet has
been
received.
Once it gets an acknowledgment, it sends the next
packet of
data. Over standard direct-dial land-based phone lines, the
transmission
delays are relatively small. However, the public data networks or
satellite links can introduce delays of up to several seconds round trip.
As a
result, the sending system ends up spending much more time waiting
than actually sending data.
With the new windowing enhancement, the sending system will be able
to keep
sending data continuously, getting the acknowledgments back later.
It only
has to stop sending data if it reaches the end of the current
"window"
without getting an acknowledgment for the first packet in the
current
"window".
Q.
What size is the "window"?
A. The window size can vary depending on what the two ends of the
connection
agree on. The suggested standard window size will be 8 packets.
The maximum is 31 packets.
The Kermit sequence numbering is modulo 64 (it "wraps" back to the
sequence number after the 64th sequence number). It is helpful to
limit the
maximum window size to 31 to avoid problems (ambiguous sequence
numbers)
under certain error conditions.
1st
Q.
Is windowing in effect throughout a Kermit session?
A. No, it is only in effect during the actual data transfer (data
packets)
portion of a file transfer. Windowing begins with the first data
packet (D
packet type), and stops when you get an End-of-File packet (Z packet
type).
Q.
Why does it stop when you get to the End-of-File packet?
A. This is done primarily to avoid having more than one file open at
once.
Q. Why will windowing be especially helpful at higher baud rates
over communications paths that have delays?
Performance Extensions
Page 61
A. As you increase the baud rate, the transmission speed of the
data increases, but you do not change the delay caused by the communications
path.
As a result, the delay becomes more and more significant.
Assume, for example, that your communications path introduces a delay
1
second each way for packets, for a total delay of 2 seconds round
trip.
Assume also that your packets have 900 bits in them so it takes
you 3
seconds to send a packet at 300 baud (this is roughly equivalent to a
typical Kermit packet).
of
WITHOUT windowing, here is what happens:
If at 300 baud you transmitted data for 3 seconds (sending 900 bits),
then
waited 2 seconds for each acknowledgment, your throughput would be
roughly
180 baud. (Total time for each transmission = 5 seconds. 900/5 =
180).
However, if you went to 2400 baud, you would transmit data for 3/8
second,
then wait 2 seconds for an acknowledgment. (Total time for each
transmission = 2 and 3/8 seconds). The throughput would increase only to
about 378
baud. (900 / 2.375 = 378).
The delay becomes the limiting factor; in this case, with this packet
size,
the delay sets an outside limit of 450 baud (900 / 2 second delay =
450),
no matter how fast the modem speed.
WITH windowing, the throughput should be close to the actual
transmission
speed. It should be possible to send data nearly continuously.
exact
speed will depend on the window size, length of transmission
delays, and
error rate.
Q.
Are there any new packet types introduced by this extension?
The
A. No, the only change is to the contents of the Send-Init packet, to
arrange
for windowing if both sides can do it. If either side cannot,
Kermit will
work as it does now. Adding an extension such as this was provided
for in
the original Kermit definition. See section 3 of the windowing
definition
for details.
Q. On the receive side, in section 4.2, why does the definition say that
writing to disk is done when the Receive-Table becomes full rather than
as soon
as you get a good packet?
A.
of
The definition was phrased this way because it
the
receive side clearer and simpler to implement.
makes
the
logic
Actually, you could also write a packet to disk when it is a good
packet
and it is the earliest entry in the receive table. This approach
has the
disadvantage that you don't know at this point that the sender has
received
your ACK, so you have to be prepared to handle the same packet later
on if
the sender never gets the ACK, times out, and sends the same packet
again.
Thus you have to be prepared to deal with packets previous to the
current
window; you will have to ACK such a packet if it has been received
properly
before.
By writing packets to disk only when the receive table becomes
full,
(the
oldest
(otherwise
packet)
you
know that the sender has received your ACK
Performance Extensions
Page 62
the sender could not have rotated the window to the n+1
position
to
send
the current packet, where n is the window size). This makes it
very easy
to stay in synch with the sender. The disadvantage of this
approach is
that when you receive the End-of-File packet, you have to take the
time to
write all the remaining packets in the Receive-Table to disk.
Q. Could you briefly explain what happens if a single packet gets
corrupted?
A. In essence, the receiver will ignore the bad packet. When it gets
the next
good packet, it will realize (because packets are numbered) that
one or
more packets were lost, and NAK those packets. The receiver
continues to
accept good data.
As long as the sender's window does not become "blocked", the only
loss of
throughput will be the time it takes to transmit the NAK'd packets.
Q. There are currently two proposals for Kermit extensions: the
Windowing extension and a proposal for extended packet lengths. What are the
relative
advantages and disadvantages of sliding windows and extended
packet
lengths?
A. What is best depends on the exact conditions and systems involved in
a particular file transfer. There are some general rules however.
Windowing helps more and more as the communications path delays get
longer.
Windowing is also more and more helpful as the baud rate goes up.
Increased packet length is most helpful on circuits with low error
rates.
If the error rate is high, it is difficult for a long packet to get
through
uncorrupted. Also, it then takes longer to re-transmit the
corrupted
packet.
On some machines, the CPU time to process a packet is relatively
constant
no matter what the packet length, so longer packets can reduce CPU
time.
Q.
Are extended packet lengths and sliding windows mutually exlusive?
A. No, there is no real reason that they would have to be.
As a
practical
matter, it is slightly easier to implement windowing if you know
the maximum packet size ahead of time, since you can then just use an
array to
store your data.
In standard Kermit, you know automactically
that your
maximum packet length is 94, so you can just go ahead and dimension
an array at 94 by Window-size.
If you are going to use both extended packet length and windowing,
you need
to select the maximum packet length and window-size so that the
combination
does not exceed the available memory for each side of the transfer.
In addition, it is possible to see the desired relationship between
packet
size and windowing for various baud rates and communications delays.
For
the common case of an error corrected by one retransmission of
the corrupted packet, the minimum window size needed for continuous
throughput
(the window never gets "blocked") can be calculated by:
Performance Extensions
Page 63
WS
>
1 +
4 x delay x baud rate
-----------------------packet-size x 10
Windowing always helps
size is
always greater than 1).
(the minimal continuous throughput window
In the above equation, the "4" derives
corrupted
packet has 4 transit times involved:
-
(this is the # of bits)
from
the
fact
that
a
Original (bad checksum) packet
NAK for the packet
Retransmission of packet
ACK for retransmission.
All of this must happen before the window becomes blocked.
The "delay" is the effective maximum one-way communications path
delay,
which includes any CPU delays.
the
Strictly speaking, the "packet-size" should have
ACK
packets added to it.
the
length
of
As an example, if you assume a 2-second (one-way) delay, at 1200
baud, with
a packet size of 94, the minimum window size for continuous
throughput
would be:
WS
>
4 x 2 x 1200
-----------94 x 10
Under these
chosen,
if possible.
=
10.2
circumstances, a window size of at least 11 should be
9.2.3. More Q-and-A About Windows
While reading the following questions and answers, keep in mind that the
Kermit
windowing definiton was developed to handle a common situation of long
circuit
delays with possible moderate error rates. Kermit does not need this
type of
extension for clean lines with insignificant delays - Kermit could
be left
alone, or use Extended Packet Lengths, in such environments.
Long delays with significant error rates will occur under two obvious and
common conditions:
1. Local phone line
Networks
(such as Telenet).
(of
uncertain
quality) to Public Data
2. Satellite phone links. These often occur with the lowerpriced
phone services, which often also have noisier lines. In
addition,
satellite links will increase as more people need to transfer
data
overseas.
The above conditions will become more common, as well increased baud
rates,
which make the delays more significant.
Performance Extensions
Page 64
As an aside, note that the benefit of Extended Packet Lengths over the
Public
Data Networks is limited by the number of outstanding bytes the PDN
allows.
(Internally, the PDNs require end-to-end acknowledgement. They use
their own
windowing system within the network.) I don't currently know the exact
impact
of this.
Now on to the questions...
Q. Can sliding windows be done on half-duplex channels?
modifications
to the proposal required?
Are any
A. An underlying assumption in the development of windowing was that
there was
a full-duplex channel.
The intent of windowing is to try to keep the sender continuously
sending
data.
Obviously, this is not possible on a half-duplex channel. A
better
solution for half-duplex channels would be to use an extended
packet
length.
An attempt to use windowing on half-duplex really is just a way of
doing
extended packet lengths. The sender would send out a group of
packets,
then wait and get a group of ACKS. It would be better to simply send
out a
large packet, which would have less overhead.
Q. Is the cost in complexity for sliding windows worth the increase in
performance?
A. Under the conditions described above (long delays and possibly
significant
error rates) windowing can increase performance by a factor of 2,
3, or
more, especially at higher baud rates. This increase is necessary
to make
Kermit viable under some conditions. With classic Kermit over the
Public
Data Networks, I have had througput as low as 250 baud over a
1200 baud
circuit (with a negligible error rate).
throughput
close to the maximum baud rate.
Windowing should allow
Windowing is most helpful when the delay is significant in relation
to data
sending time. Any delay becomes more significant as users move to
higher
baud rates (2400 baud and beyond).
The complexity of implementing windowing has yet to be fully
evaluated.
The first implementation (for the IBM PC using C-Kermit) proved
to be
fairly manageable. It appears that the windowing logic can be
implemented
so that Kermit Classic uses the same code, but with a window size
of 1,
which should avoid having to keep separate sections of code.
The windowing definiton was developed with the idea of keeping
changes to
Kermit to a minimum. No new packet types were developed, ACKs
NAKS
were kept the same, and windowing is in effect only during
actual data
transfer (D packets). We tried to define the protocol so that
window
size of 1 was the same as the current classic Kermit.
These factors should help reduce the complexity of implementing
windowing.
We currently have a working implementation of Kermit for the IBM
going
through testing.
It's fun to see the modem "Send" light stay on constantly!
and
a
PC
Performance Extensions
Page 65
Q.
Why doesn't the Windowing proposal use a "bulk ACK"?
A. There are a couple of possibilities for ways to use some sort of
"bulk" or
combined ACK. We looked at them when developing the Windowing
definition.
We did not see any advantages that outweighed the disadvantages.
Here are two possible ways of changing how ACKs would work:
1. An
ACK for any packet would also ACK all previous packets.
The
concept that an ACK would also ACK all
previous
packets
seems
attractive
at
first, since it would appear to reduce
overhead.
However, it has a major drawback in that you then must
re-
synch
when
you
get errors.
This is because, once you have an
error,
you have to send a NAK, then stop and wait for a retransmission
of the NAK'd packet, before you send out any more ACKs. (If
you
sent out an ACK for a later packet, it would imply that you
had
received the NAK'd packet.
Not until you safely get
the
re-transmission can you go ahead.) This would negate one of
the
nicest parts of windowing as it is defined now, which is
that
the sender can transmit continuously, including during
error
recovery, as long as the window does not become blocked.
It
does not appear to us that the reduction in the number of
ACKs
sent is worth this penalty. In addition, this is a
departure
from the way ACKs in Kermit work now. It seemed best to make
as
few changes to Kermit as possible. If this facility turns
out
to be useful, it would be better to introduce a new packet
type
(or other means of distinguishing regular ACKs from
"Bulk
ACKS").
2. A new "Bulk ACK" packet type could be developed.
This
did
not
seem
to
us to be a good idea, since it required defining a
new
packet type.
We were trying to fit windowing
in
with
as
few
changes
to
Kermit
as
possible.
A "Bulk ACK", in which
one
packet could contain a whole string of ACKs and NAKs, also
seems
like
a
good
idea at first.
The penalty here is a little
more
subtle.
First, if you lose a "Bulk ACK" packet, you
lose
more
information
and
it takes longer to get things flowing
smoothly
again.
Second, and probably more importantly, efficient
windowing
depends
on
the window never becoming "blocked" (i.e.,
the
sender can always keep sending).
A "Bulk ACK"
interferes
with
this to some extent, because if you have a long delay, the
"Bulk
ACK" with its multiple individual ACKs may not get back
to
the
sender
in
time
to
prevent
the window from becoming
blocked.
With the current definition of windowing, returning an
ACK
for
each
packet
gets
the
ACKs (or NAKs) to the sender as soon
as
possible.
This provides the best chance for keeping the
window
open
so
that the sender can transmit continually.
Once
again,
remember the conditions under which windowing
is
most
useful:
long
delays
with
significant
error
rates.
Under these
conditions, individual ACKs have advantages.
If
these
conditions
don't apply, it may not be necessary to use windowing, or it
may
be better to use extended packet lengths.
Kermit Commands
Page 66
10. Kermit Commands
The following list of Kermit commands and terms is suggested. It is
not intended to recommend a particular style of command parsing, only to
promote a
consistent vocabulary, both in documentation and in choosing the names
for commands.
10.1. Basic Commands
SEND
This verb tells a Kermit program to send one or more files from
its own
file structure.
RECEIVE This verb should tell a Kermit program to expect one or more
files to
arrive.
GET
Some
This
verb
should
tell a user Kermit to send one or more files.
Kermit implementations have separate RECEIVE and GET
commands;
others
use RECEIVE for both purposes, which creates confusion.
Since it can be useful, even necessary, to specify different names for
source
and destination files, these commands should take operands as follows
(optional
operands in [brackets]):
SEND local-source-filespec [remote-destination-filespec]
If the destination file specification is included, this will go
in the
file header packet, instead of the file's local name.
RECEIVE [local-destination-filespec]
If the destination filespec is given, the incoming file will be
stored
under that name, rather than the one in the file header pakcet.
GET remote-source-filespec [local-destination-filespec]
If the destination filespec is given, the incoming file will be
stored
under that name, rather than the one in the file header packet.
If a file group is being sent or received, alternate names should not be
used.
It may be necessary to adopt a multi-line syntax for these commands
when
filespecs may contain characters that are also valid command field
delimiters.
10.2. Program Management Commands
EXIT
Leave the Kermit program, doing whatever cleaning up must be
done -deassigning of devices, closing of files, etc.
QUIT
as to
Leave the Kermit program without cleaning up, in such a
manner
allow further manipulation of the files and devices.
PUSH
Preserve the
command
processor.
TAKE
current
Kermit environment and enter the system
Read and execute Kermit program commands from a local file.
LOG
Specify a log for file transfer transactions, or for
session
logging.
terminal
Kermit Commands
Page 67
10.3. Terminal Emulation Commands
CONNECT This verb, valid only for a local Kermit, means to go into
terminal
emulation mode; present the illusion of being directly connected
as a
terminal to the remote system. Provide an "escape character" to
allow
the user to "get back" to the local system. The escape
character, when
typed, should take a single-character argument; the following
are suggested:
0
B
C
(zero) Transmit a NUL
Transmit a BREAK
Close the connection, return to local Kermit command
level
P
Push to system command processor
Q
Quit logging (if logging is being done)
R
Resume logging
S
Show status of connection
?
Show the available arguments to the escape character
(a second copy of the escape character): Transmit the
escape
character itself
Lower case equivalents should be accepted.
argument is
typed, issue a beep.
If any invalid
Also see the SET command.
10.4. Special User-Mode Commands
These commands are used only by Users of Servers.
BYE
out,
This command sends a message to the remote server to
log
itself
and upon successful completion, terminate the local Kermit
program.
FINISH This command causes the remote server to shut itself down
gracefully
without logging out its job, leaving the local Kermit at Kermit
command
level, allowing the user to re-CONNECT to the remote job.
10.5. Commands Whose Object Should Be Specified
Some Kermit implementations include various local file management
services and
commands to invoke them. For instance, an implementation might have
commands
to let you get directory listings, delete files, switch disks, and
inquire
about free disk space without having to exit and restart the program.
In addition, remote servers may also provide such services. A user Kermit
must be
able to distinguish between commands aimed at its own system and those
aimed at
the remote one. When any confusion is possible, such a command may be
prefixed
by one of the following "object prefixes":
REMOTE
Ask the remote Kermit server to provide this service.
LOCAL
Perform the service locally.
If the "object prefix" is omitted, the command should be executed
locally. The
services include:
Kermit Commands
Page 68
LOGIN
This should be used in its timesharing sense, to create an
identity
("job", "session", "access", "account") on the system.
LOGOUT
To terminate a session that was initiated by LOGIN.
COPY
Make a new copy of the specified file with the specified name.
CWD
like
Change Working Directory.
This is ugly, but more
CONNECT and ATTACH are too imprecise.
natural
verbs
CWD is the ARPAnet file
transfer
standard command to invoke this function.
DIRECTORY
Provide a list of the names, and possibly other attributes,
of the
files in the current working directory (or the specified
directory).
DELETE
Delete the specified files.
ERASE
This could be a synomym for DELETE, since its meaning is clear.
(It doesn't seem wise to include UNDELETE
or
UNERASE
in
the
standard
list; most systems don't support such a function,
and
users' expectations should not be toyed with...)
KERMIT
com-
Send a command to the remote Kermit server in its own interactive
mand syntax.
RENAME
Change the name of the specified file.
TYPE
Display the contents of the specified file(s) at the terminal.
SPACE
Tell how much space is used and available for storing files in
the current working directory (or the specified directory).
SUBMIT
Submit the specified file(s) for background (batch) processing.
PRINT
Print the specified file(s) on a printer.
MOUNT
Request a mount of the specified tape, disk, or other removable
storage
medium.
WHO
infor-
Show
who
is
logged in (e.g. to a timesharing system), or give
mation about a specified user or network host.
MAIL
Send electronic mail to the specified user(s).
MESSAGE Send a terminal message (on a network or timesharing system).
HELP
Give brief information about how to use Kermit.
SET
mode,
Set various parameters relating to debugging, transmission, file
and so forth.
SHOW
Display settings of SET parameters, capabilities in force, etc.
STATISTICS
Give information about the performance of the most recent file
transfer
Kermit Commands
Page 69
-- elapsed time, effective baud rate, various counts, etc.
HOST
local)
Pass
the
given command string to the specified (i.e. remote or
host for execution in its own command language.
LOGGING Open or close a transaction or debugging log.
10.6. The SET Command
A SET command should be provided to allow the user to tailor a
connection to
the peculiarities of the communication path, the local or remote file
system,
etc. Here are some parameters that should be SET-able:
BLOCK-CHECK
Specify the type of block check to be used: single character
checksum,
two-character checksum, 3-character CRC.
DEBUGGING
Display or log the packet traffic, packet numbers, and/or
program
states. Useful for debugging new versions of Kermit, novel
combinations of Kermit programs, etc.
DELAY
send-
How
many seconds a remote (non-server) Kermit should wait before
ing the Send-Init packet, to give the user time to escape back
to
the
local Kermit and type a RECEIVE command.
DISPLAY Style of file transfer display (NONE, SERIAL, SCREEN, etc).
DUPLEX
For terminal emulation, specify FULL or HALF duplex echoing.
END-OF-LINE
Specify any line terminator that must be used after a packet.
ESCAPE
Specify the escape character for terminal emulation.
FILE attributes
Almost any of the attributes listed above in the Attributes
section
(8.5). The most common need is to tell the Kermit program
whether an
incoming or outbound file is text or binary.
FLOW-CONTROL
Specify the flow control mechanism for the line, such as
XON/XOFF,
ENQ/ACK, DTR/CTS, etc. Allow flow control to be turned off
(NONE) as
well as on. Flow control is done only on full-duplex
connections.
HANDSHAKE
Specify any line-access negotiation that must be used or
simulated
during file transfer. For instance, a half duplex system will
often
need to "turn the line around" after sending a packet, in order
to give
you permission to reply. A common handshake is XON (^Q); the
current
user of the line transmits an XON when done transmitting data.
LINE
is for
Specify
the line or device designator for the connection.
use in a Kermit program that can run in either remote
or
This
local
mode;
the
operation).
default
line
is the controlling terminal (for remote
Kermit Commands
Page 70
If an external device is used, local operation is presumed.
LOG
There
Specify a local file in which to keep a log of the transaction.
may
transitions,
etc)
disposition
each
be logs for debugging purposes (packet traffic, state
and for auditing purposes (to record the name and
of
file transferred).
MARKER Change the start-of-packet marker from the default of SOH
(CTRL-A) to
some other control character, in case one or both systems has
problems
using CTRL-A for this purpose.
PACKET-LENGTH
The maximum length for a packet. This should normally be no less
than
30 or 40, and can be greater than 94 only if the long-packet
protocol
extension is available, in which case it can be a much larger
number,
up to the maximum size allowed for the particular Kermit
program (but
never greater than 9024). Short packets can be an advantage on
noisy
lines; they reduce the probabily of a particular packet
being corrupted, as well as the retransmission overhead when corruption
does occur. Long packets boost performance on clean lines.
PADDING The number of padding characters that should be sent
before each
packet, and what the padding character should be. Rarely
necessary.
PARITY
con-
Specify the parity (ODD, EVEN, MARK, SPACE, NONE) of the physical
nection.
If other than none, the "8th bit" cannot be used to
transmit
data and must not be used by either side in block check
computation.
PAUSE
How many seconds to pause after receiving a packet before
sending the
next packet. Normally 0, but when a system communication
processor or
front end has trouble keeping up with the traffic, a short
between packets may allow it to recover its wits; hopefully,
something
under a second will suffice.
pause
PREFIX Change the default prefix for control characters, 8-bit
characters, or
repeated quantities.
PROMPT Change the program's prompt. This is useful when running
Kermit between two systems whose prompt is the same, to eliminate
confusion
about which Kermit you are talking to.
REPEAT-COUNT-PROCESSING
Change the default for repeat count processing.
will be
done if both Kermit programs agree to do it.
Normally, it
RETRY
The maximum number of times to attempt to send or receive a
packet before giving up. The normal number is about 5, but the user
should be
able to adjust it according to the condition of the line, the
load on
the systems, etc.
TIMEOUT Specify the length of the timer to set when waiting for a packet
to arrive.
WINDOW-SIZE
Maximum number of unacknowledged packets outstanding, when the
sliding
window option is available, usually between 4 and 31.
Kermit Commands
Page 71
10.7. Macros, the DEFINE Command
In addition to the individual set commands, a "macro" facility is
recommended
to allow users to combine the characteristics of specific systems into a
single
SET option. For example:
DEFINE IBM = PARITY ODD, DUPLEX HALF, HANDSHAKE XON
DEFINE UNIX = PARITY NONE, DUPLEX FULL
DEFINE TELENET = PARITY MARK
This could be done by providing a fancy runtime parser for commands
like this
(which could be automatically TAKEn from the user's Kermit initialization
file
upon program startup), or simply hardwired into the SET command table.
With these definitions in place, the user would simply type "SET
IBM", "SET
UNIX", and so forth, to set up the program to communication to the remote
system.
Terminal Emulation
Page 72
11. Terminal Emulation
The local system must be able to act as a terminal so that the user can
connect
to the remote system, log in, and start up the remote Kermit.
Terminal emulation should be provided by any Kermit program that runs
locally,
so that the user need not exit and restart the local Kermit program in
order to
switch between terminal and protocol operation. On smaller systems,
this is
particularly important for various reasons -- restarting the program and
typing
in all the necessary SET commands is too inconvenient and timeconsuming; in
some micros, switching in and out of terminal emulation may cause
carrier to
drop, etc.
Only bare-bones terminal emulation need be supplied by Kermit; there is
no need
to emulate any particular kind of "smart" terminal. Simple "dumb"
terminal
emulation is sufficient to do the job. Emulation of fancier terminals is
nice
to have, however, to take advantage of the remote system's editing and
display
capabilities. In some cases, microcomputer firmware will take care of
this.
To build emulation for a particular type of terminal into the program,
you must
interpret and act upon escape sequences as they arrive at the port.
No error checking is done during terminal emulation.
It is
"outside the
protocol"; characters go back and forth "bare". In this sense, terminal
emulation through Kermit is no better than actually using a real terminal.
Some Kermit implementations may allow logging of the terminal emulation
session
to a local file. Such a facility allows "capture" of remote
typescripts and
files, again with no error checking or correction.
When this
facility is
provided, it is also desirable to have a convenient way of "toggling"
the logging on and off.
If the local system does not provide system- or firmware-level flow
control,
like XON/XOFF, the terminal emulation program should attempt to
simulate it,
especially if logging is being done.
The terminal emulation facility should be able to handle either remote or
local
echoing (full or half duplex), any required handshake, and it should be
able to
transmit any parity required by the remote side or the communication
medium.
A terminal emulator works by continuously sampling both console input
from the
local terminal and input from the communication line. Simple input and
output
functions will not suffice, however, since if you ask for input from a
certain
device and there is none available, you will generally block until
input does
become available, during which time you will be missing input from the
other
device.
Thus you must have a way to bounce back and forth
regardless of
whether input is available. Several mechanisms are commonly used:
- Continuously jump back and forth between the port status register
and
the console status register, checking the status bits for
input
available. This is only practical on single-user, singleprocess
systems, where the CPU has nothing else to do.
- Issue an ordinary blocking input request for the port, but enable
interrupts on console input, or vice versa.
- Handle port input in one process and console input in another,
paral-
Terminal Emulation
Page 73
lel process.
The UNIX Kermit program listed in this manual uses
this
method.
Any input at the port should be displayed immediately on the screen. Any
input
from the console should be output immediately to the port. In addition,
if the
connection is half duplex, console input should also be sent immediately
to the
screen.
The terminal emulation code must examine each console character to
determine
whether it is the "escape character". If so, it should take the next
character
as a special command, which it executes. These commands are described
above,
in section 10.3.
The terminal emulator should be able to send every ASCII character, NUL
through
DEL, and it should also be able to transmit a BREAK signal (BREAK is
not a
character, but an "escape" from ASCII transmission in which a 0 is put
on the
line for about a quarter of a second, regardless of the baud rate,
with no
framing bits). BREAK is important when communicating with various
systems,
such as IBM mainframes.
Finally, it is sometimes necessary to perform certain transformations on
the CR
character that is normally typed to end a line of input. Some systems
use LF,
EOT, or other characters for this function. To complicate matters,
intervening
communications equipment (particularly the public packet-switched
networks) may
have their own independent requirements. Thus if using Kermit to
communicate
over, say, TRANSPAC with a system that uses LF for end-of-line, it
may be
necessary to transform CR into LFCR (linefeed first -- the CR tells the
network
to send the packet, which will contain the LF, and the host uses the
LF for
termination). The user should be provided with a mechanism for
specifying this
transformation, a command like "SET CR sequence".
Writing a Kermit Program
Page 74
12. Writing a Kermit Program
Before writing a new implementation of Kermit or modifying an old one,
first be
sure to contact the Kermit Distribution center at Columbia University
to make
sure that you're not duplicating someone else's effort, and that you
have all
the latest material to work from. If you do write or significantly
modify (or
document) a Kermit program, please send it back to Columbia so that it
can be
included in the standard Kermit distribution and others can benifit
from it.
It is only through this kind of sharing that Kermit has grown from its
modest
beginnings to its present scale.
The following sections provide some hints on Kermit programming.
12.1. Program Organization
A basic Kermit implementation can usually be written as a relatively
small
program, self-contained in a single source file. However, it is often
the case
that a program written to run on one system will be adapted to run on
other
systems as well. In that case, it is best to avoid having totally
divergent
sources, because when new features are added to (or bugs fixed in) the
systemindependent parts of the program -- i.e. to the protocol itself -- only
one implementation will reap the benefits initially, and the other will require
painful, error-prone "retrofitting" to bring it up to the same level.
Thus, if there is any chance that a Kermit program will run on more
than one
machine, or under more than one operating system, or support more than
one kind
of port or modem, etc, it is desirable to isolate the system-dependent
parts in
a way that makes the common parts usable by the various implementations.
There
are several approaches:
1. Runtime support. If possible, the program can inspect the
hardware
or inquire of the system about relevant parameters, and
configure
itself dynamically at startup time. This is hardly ever possible.
2. Conditional compilation (or assembly).
If the number of systems
or
options
to
be supported is small, the system dependent code can
be
enclosed in conditional compilation brackets
(like
IF
IBMPC
....
ENDIF).
However,
as the number of system dependencies to be
supported grows, this method becomes unwieldy and
error-prone
--
installing
support
for system X tends to break the pre-existing
support for system Y.
3. Modular composition.
When there is a potentially
large
number
of
options
a
program
should
support,
it
should
be broken up
into
separate modules (source
files),
with
clearly
specified,
simple
calling conventions. This allows people with new options to
provide
their own support for them in an easy way, without endangering
any
existing support. Suggested modules for a Kermit program are:
4.
- System-Indendent
protocol
handling:
state
switching,
packet
formation, encoding (prefixing) and decoding, etc.
- User Interface: the command parser.
separate
Putting this in a
Writing a Kermit Program
Page 75
module
allows
plug-in
of
command parsers to suit the
user's
taste, to mimic the style of the host system command parser
or
some popular application, etc.
- Screen i/o: This module would contain the screen control
codes,
cursor positioning routines, etc.
- Port i/o: Allows support of various port hardware.
This
module
can
define the port status register location, the status
bits,
and so forth, and can implement the functions to read and
write
characters at the port.
- Modem
control:
This
module
would
support
any
kind
of
"intelligent" modem, which is not simply a
transparent
extension
of
the communications port.
Such modems may accept
special commands to perform functions like dialing out,
redialing
a
recent
number,
hanging
up, etc., and may need special
initialization (for instance, setting modem signals like DTR).
- Console input: This module would supply
the
function
to
get
characters
from
the
console;
it would know about the
status
register
locations
and
bits,
interrupt
structure,
key-
tocharacter
mappings,
etc.,
and
could
also
implement
key
redefinitions, keystroke macros,
programmable
function
keys,
expanded control and meta functions, etc.
- Terminal
Emulation:
This
module
would
interpret escape
sequences in the incoming character
stream
(obtained
from
the
port i/o module) for the particular type of terminal being
emulated and interpret them by making appropriate
the
calls
the
screen i/o module, and it would send user typein (obtained
from
the console input module) out the serial port (again using
the
port
i/o module).
Ideally, this module could be replacable
by
other modules to emulate different
kinds
of
terminals
(e.g.
ANSI, VT52, ADM3A, etc).
- File i/o: This module contains all the knowledge about the
host
system's file structure; how to open and close
files,
perform
"get next file" operations, read and write files, determine
and
set their attributes, detect the end of a file, and
so
forth,
and
provides
the
functions,
including
buffering,
to get
a
character from a file and put a character
to
a
file.
This
module
may
also
provide
file
management services for
local
files -- directory listings, deleting, renaming,
copying,
and
so forth.
- Definitions
and
Data: Separate modules might also be kept
for
compile-time parameter definitions and for global runtime
data.
Writing a Kermit Program
Page 76
12.2. Programming Language
The language to be used in writing a Kermit program is more than a
matter of
taste.
The primary consideration is that the language provide the
necessary
functionality and speed. For instance, a microcomputer implementation of
BASIC
may not allow the kind of low-level access to device registers needed
to do
terminal emulation, or to detect console input during file transfer, or
even if
it can do these things, it might not be able to run fast enough do
drive the
communication line at the desired baud rate.
The second consideration in choosing a language is portability. This is
used
in two senses: (1) whether the language is in the public domain (or,
equivalently, provided "free" as part of the basic system), and (2) whether
it is
well standardized and in wide use on a variety of systems. A language
that is
portable in both senses is to be preferred.
Whatever programming language is selected, it is important that all
lines in
the program source be kept to 80 characters or less (after expansion of
tabs).
This is because Kermit material must often be shipped over RJE and other
cardformat communication links.
In addition, it is important that the names of all files used in
creating and
supporting a particular Kermit implementation be (possibly a subset)
of the
form NAME.TYPE, where NAME is limited to six characters, and TYPE is
limited to
three, and where the NAME of each file begin with a common 2 or 3
character
prefix.
This is so that all related files will be grouped together in
an alphabetic directory listing, and so when all of the hundreds of Kermit
related
files are placed together on a tape, all names will be both legal and
unique,
especially on systems (like PDP-11 operating systems) with restrictive
file
naming conventions.
12.3. Documentation
A new Kermit program should be thoroughly documented; one of the
hallmarks of
Kermit is its documentation. The documentation should be at both the
user
level (how to use the program, what the commands are, etc, similar
to the
documentation presently found in the Kermit Users Guide), and the
implementation level (describe system dependencies, give pointers for adapting
to new
systems, and so forth).
In addition, programs themselves should
contain
copious commentary.
Like program source, documentation should be kept
within
80-character lines.
If possible, a section for the implementation should be written for the
Kermit
User Guide using the UNILOGIC Scribe formatting language (subsets of
which are
also to be found in some microcomputer text processing software such as
Perfect
Writer or Final Word), using the same general conventions as the
existing
Scribe-format implementation sections.
Kermit programs should also contain a revision history, in which each
change is
briefly explained, assigned an "edit number", and the programmer and
site are
identified. The lines or sections comprising the edit should be marked
with
the corresponding edit number, and the Kermit program, upon startup,
should announce its version and edit numbers, so that when users complain of
problems we
will know what version of the program is in question.
Writing a Kermit Program
Page 77
The version number changes when the functionality has been changed
sufficiently
to require major revisions of user documentation. The edit number
should increase (monotonically, irrespective of version number) whenever a
change is
made to the program. The edit numbers are very important for program
management; after shipping out a version of, say, CP/M Kermit-80, we often
receive
many copies of it, each containing its own set of changes, which we must
reconcile in some manner. Edit numbers help a great deal here.
12.4. Bootstrapping
Finally, a bootstrap procedure should be provided. Kermit is
generally distributed on magnetic tape to large central sites; the users at those
sites need
ways of "downloading" the various implementations to their micros and
other local systems. A simple bootstrap procedure would consist of precise
instructions on how to accomplish an "unguarded" capture of the program.
Perhaps a
simple, short program can be written for each each end that will do the
job;
listings and instructions can be provided for the user to type in and run
these
programs.
Packet Format and Types
Page 78
I. Packet Format and Types
Basic Kermit Packet Layout
|<------Included in CHECK------>|
|
|
+------+-----+-----+------+------ - - -+-------+
| MARK | LEN | SEQ | TYPE | DATA
| CHECK |<terminator>
+------+-----+-----+------+------ - - -+-------+
|
|
|<--------LEN-32 characters------>|
MARK
LEN
SEQ
TYPE
CHECK
A real control character, usually CTRL-A.
One character, length of remainder of packet + 32, max 95
One character, packet sequence number + 32, modulo 64
One character, an uppercase letter
One, two, or three characters, as negotiated.
<terminator>
Any control character required for reading the packet.
Kermit Extended Packet Layout
|<-------------------------Included in CHECK------------->|
|
|
|<-------Included in HCHECK------->|
|
|
|
|
+------+-----+-----+------+-------+-------+--------+----- - - - -+------+
| MARK |
| SEQ | TYPE | LENX1 | LENX2 | HCHECK | DATA
| CHECK
|
+------+-----+-----+------+-------+-------+--------+----- - - - -+------+
blank
|
|
|<------------------>|
LX1=LENX1-32, LX2=LX2-32
95 x LX1 + LX2 chars
HCHECK is a single-character type 1 checksum
Initialization String
1
2
3
4
5
6
7
8
9
10
+-------+-------+-------+-------+-------+-------+-------+-------+------+- | MAXL | TIME | NPAD | PADC | EOL
| QCTL | QBIN | CHKT | REPT |
+-------+-------+-------+-------+-------+-------+-------+-------+------+- -
10
CAPAS+1 CAPAS+2 CAPAS+3
- --+-------+-+--------+--------+--------+- | CAPAS
... 0| WINDO | MAXLX1 | MAXLX1 |
- --+-------+-+--------+--------+--------+- MAXL
TIME
NPAD
EOL
QCTL
QBIN
CHKT
REPT
CAPAS
Maximum length (0-94) +32
Timeout, seconds (0-94) +32
Number of pad characters (0-94) +32
Packet terminator (0-63) +32
Control prefix, literal
8th bit prefix, literal
Block check type {1,2,3}, literal
Repeat count prefix, literal
Extendable capabilities mask, ends when value-32 is even
Packet Format and Types
Page 79
WINDO
MAXLX1
MAXLX2
Window size (0-31) +32
High part of extended packet maximum length (int(max/95)+32)
Low part of extended packet maximum length (mod(max,95)+32)
Packet Types
Y
Acknowledgment (ACK). Data according to what kind of packet is being
acknowledged.
N
Negative Acknowledgment (NAK). Data field always empty.
S
Send Initiation.
Data field contains unencoded initialization
string.
Tells receiver to expect files. ACK to this packet also contains
unencoded
initialization string.
I
Initialize.
Data field contains unencoded initialization string.
Sent to
server to set parameters prior to a command. ACK to this packet also
contains unencoded initialization string.
F
File Header.
Indicates file data about to arrive for named file.
Data
field contains encoded file name. ACK to this packet may contain
encoded
name receiver will store file under.
X
Text Header.
Indicates screen data about to arrive. Data field
contains
encoded heading for display.
A
File Attributes. Data field contains unencoded attributes. ACK may
contain unencoded corresponding agreement or refusal, per attribute.
D
Data Packet.
Data field contains encoded file or screen data.
ACK may
contain X to interrupt sending this file, Z to interrupt entire
transaction.
Z
End of file. Data field may contain D for Discard.
B
Break transmission.
E
Error. Data field contains encoded error message.
R
Receive Initiate. Data field contains encoded file name.
C
Host Command.
Data field contains encoded command for host's
command
processor.
K
Kermit Command. Data field contains encoded command for Kermit
command
processor.
T
Timeout psuedopacket, for internal use.
Q
Block check error psuedopacket, for internal use.
G
Generic Kermit Command. Data field contains a single character
subcommand,
followed by zero or more length-encoded operands, encoded after
formation:
I
C
L
F
D
U
E
T
R
K
W
M
H
Q
P
J
V
Login [<%user[%password[%account]]>]
CWD, Change Working Directory [<%directory[%password]>]
Logout, Bye
Finish (Shut down the server, but don't logout).
Directory [<%filespec>]
Disk Usage Query [<%area>]
Erase (delete) <%filespec>
Type <%filespec>
Rename <%oldname%newname>
Copy <%source%destination>
Who's logged in? [<%user ID or network host[%options]>]
Send a short Message <%destination%text>
Help [<%topic>]
Server Status Query
Program <%[program-filespec][%program-commands]>
Journal <%command[%argument]>
Variable <%command[%argument[%argument]]>
List of Features
Page 80
II. List of Features
There's no true linear scale along which to rate Kermit
implementations.
A
basic, minimal implementation provides file transfer in both
directions, and,
for microcomputers (PC's, workstations, other single user systems),
terminal
emulation. Even within this framework, there can be variations. For
instance,
can the program send a file group in a single command, or must a
command be
issued for each file? Can it time out? Here is a list of features that
may be
present; for any Kermit implementation, the documentation should show
whether
these features exist, and how to invoke them.
- File groups.
Can it send a group of files with a single
command,
using "wildcard", pattern, or list notation?
Can it
successfully
send or receive a group of files of mixed types? Can it recover
from
an error on a particular file and go on to the next one? Can it
keep
a log of the files involved showing the disposition of each?
- Filenames. Can it take action to avoid overwriting a local file
when
a new file of the same name arrives? Can it convert filenames to
and
from legal or "normal form"?
- File types.
- 8th-Bit
Can binary as well as text files be transmitted?
prefixing.
Can
it
send and receive 8-bit data through
a
7-bit channel using the prefixing mechanism?
- Repeat-Count processing. Can it send and receive data with
repeated
characters replaced by a prefix sequence?
- Terminal Emulation.
Does it have a terminal emulation
facility?
Does it emulate a particular terminal? To what extent?
Does
it
provide various communication options, such as duplex, parity,
and
handshake selection?
Can it transmit all ASCII characters?
Can
it
transmit BREAK?
Can it log the remote session locally?
- Communications Options. Can duplex, parity, handshake, and line
terminator be specified for file transfer?
- Block Check Options.
In addition to the basic singlecharacter
checksum, can the two-character checksum and the three-character
CRC
be selected?
- Basic Server. Can it run in server mode, accepting commands to
send
or receive files, and to shut itself down?
- Advanced Server.
Can it accept server commands to delete
files,
provide directory listings, send messages, and forth?
- Issue Commands to Server. Can it send
and
handle all possible responses?
- Host
Commands.
commands
to
a
server,
Can it parse and send remote "host commands"?
If
it
is a server, can it pass these commands to the host system
command
processor and return the results to the local user Kermit?
- Interrupt
a
File
Transfers.
Can it interrupt sending or receiving
List of Features
Page 81
file?
Can it respond to interrupt requests from the other side?
- Local File Management Services. Are there commands
local
directory listings, delete local files, and so forth?
to
get
- File Attributes. Can it send file attribute information about
local
files, and can deal with incoming file attribute information?
Can
alternate dispositions be specified. Can files be archived?
- Long Packets.
- Sliding
implemented?
Is the long packet protocol extension implemented?
Windows.
Is
- Debugging Capability.
examined,
single-stepped?
the sliding window protocol extension
Can
packet
traffic
be
logged,
- Frills.
Does it have login scripts? Raw download/upload?
DIAL
command and modem control? Phone directories?
A
The ASCII Character Set
Page 82
III. The ASCII Character Set
ASCII Code (ANSI X3.4-1968)
There are 128 characters in the ASCII (American national Standard Code
for Information Interchange) "alphabet". The characters are listed in order of
ASCII
value; the columns are labeled as follows:
Bit
ASCII Dec
ASCII Oct
ASCII Hex
EBCDIC Hex
tables.
Char
Remark
Even parity bit for ASCII character.
Decimal (base 10) representation.
Octal (base 8) representation.
Hexadecimal (base 16) representation.
EBCDIC hexadecimal equivalent for Kermit translate
Name or graphical representation of character.
Description of character.
The first group consists of nonprintable 'control' characters:
Bit
0
1
1
0
1
0
0
1
1
0
0
1
0
1
1
0
1
0
0
1
0
1
1
0
0
1
1
.....ASCII.... EBCDIC
Dec
Oct Hex Hex
000
000
00
00
001
001
01
01
002
002
02
02
003
003
03
03
004
004
04
37
005
005
05
2D
006
006
06
2E
007
007
07
2F
008
010
08
16
009
011
09
05
010
012
0A
25
011
013
0B
0B
012
014
0C
0C
013
015
0D
0D
014
016
0E
0E
015
017
0F
0F
016
020
10
10
017
021
11
11
018
022
12
12
019
023
13
13
020
024
14
3C
021
025
15
3D
022
026
16
32
023
027
17
26
024
030
18
18
025
031
19
19
026
032
1A
3F
Char
NUL
SOH
STX
ETX
EOT
ENQ
ACK
BEL
BS
HT
LF
VT
FF
CR
SO
SI
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN
EM
SUB
Remarks
^@, Null, Idle
^A, Start of heading
^B, Start of text
^C, End of text
^D, End of transmission
^E, Enquiry
^F, Acknowledge
^G, Bell, beep, or fleep
^H, Backspace
^I, Horizontal tab
^J, Line feed
^K, Vertical tab
^L, Form feed (top of page)
^M, Carriage return
^N, Shift out
^O, Shift in
^P, Data link escape
^Q, Device control 1, XON
^R, Device control 2
^S, Device control 3, XOFF
^T, Device control 4
^U, Negative acknowledge
^V, Synchronous idle
^W, End of transmission block
^X, Cancel
^Y, End of medium
^Z, Substitute
0
1
0
0
1
027
028
029
030
031
033
034
035
036
037
1B
1C
1D
1E
1F
27
1C
1D
1E
1F
ESC
FS
GS
RS
US
^[,
^\,
^],
^^,
^_,
Escape, prefix, altmode
File separator
Group separator
Record separator
Unit separator
The last four are usually associated with the control version of
backslash,
right square bracket, uparrow (or circumflex), and underscore,
respectively,
but some terminals do not transmit these control characters.
The ASCII Character Set
Page 83
The following characters are printable:
First, some punctuation characters.
Bit
1
0
0
1
0
1
1
0
0
1
1
0
1
0
0
1
.....ASCII.... EBCDIC
Dec
Oct Hex Hex
032
040
20
40
033
041
21
5A
034
042
22
7F
035
043
23
7B
036
044
24
5B
037
045
25
6C
038
046
26
50
039
047
27
7D
040
050
28
4D
041
051
29
5D
042
052
2A
5C
043
053
2B
4E
044
054
2C
6B
045
055
2D
60
046
056
2E
4B
047
057
2F
61
Char
SP
!
"
#
$
%
&
'
(
)
*
+
,
.
/
Remarks
Space, blank
Exclamation mark
Doublequote
Number sign, pound sign
Dollar sign
Percent sign
Ampersand
Apostrophe, accent acute
Left parenthesis
Right parenthesis
Asterisk, star
Plus sign
Comma
Dash, hyphen, minus sign
Period, dot
Slash
Char
0
1
2
3
4
5
6
7
8
9
Remarks
Zero
One
Two
Three
Four
Five
Six
Seven
Eight
Nine
Numeric characters:
Bit
0
1
1
0
1
0
0
1
1
0
.....ASCII.... EBCDIC
Dec
Oct Hex Hex
048
060
30
F0
049
061
31
F1
050
062
32
F2
051
063
33
F3
052
064
34
F4
053
065
35
F5
054
066
36
F6
055
067
37
F7
056
070
38
F8
057
071
39
F9
More punctuation characters:
Bit
0
1
0
1
1
0
1
.....ASCII.... EBCDIC
Dec
Oct Hex Hex
058
072
3A
7A
059
073
3B
5E
060
074
3C
4C
061
075
3D
7E
062
076
3E
6E
063
077
3F
6F
064
100
40
7C
Char
:
;
<
=
>
?
@
Remarks
Colon
Semicolon
Left angle bracket
Equal sign
Right angle bracket
Question mark
"At" sign
The ASCII Character Set
Page 84
Upper-case alphabetic characters (letters):
Bit
0
0
1
0
1
1
0
0
1
1
0
1
0
0
1
0
1
1
0
1
0
0
1
1
0
0
.....ASCII.... EBCDIC
Dec
Oct Hex Hex
065
101
41
C1
066
102
42
C2
067
103
43
C3
068
104
44
C4
069
105
45
C5
070
106
46
C6
071
107
47
C7
072
110
48
C8
073
111
49
C9
074
112
4A
D1
075
113
4B
D2
076
114
4C
D3
077
115
4D
D4
078
116
4E
D5
079
117
4F
D6
080
120
50
D7
081
121
51
D8
082
122
52
D9
083
123
53
E2
084
124
54
E3
085
125
55
E4
086
126
56
E5
087
127
57
E6
088
130
58
E7
089
131
59
E8
090
132
5A
E9
Char
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Remarks
More punctuation characters:
Bit
1
0
1
1
0
0
.....ASCII.... EBCDIC
Dec
Oct Hex Hex
091
133
5B
AD
092
134
5C
E0
093
135
5D
BD
094
136
5E
5F
095
137
5F
6D
096
140
60
79
Char
[
\
]
^
_
`
Remarks
Left square bracket
Backslash
Right square bracket
Circumflex, up arrow
Underscore, left arrow
Accent grave
The ASCII Character Set
Page 85
Lower-case alphabetic characters (letters):
Bit
1
1
0
1
0
0
1
1
0
0
1
0
1
1
0
1
0
0
1
0
1
1
0
0
1
1
.....ASCII.... EBCDIC
Dec
Oct Hex Hex
097
141
61
81
098
142
62
82
099
143
63
83
100
144
64
84
101
145
65
85
102
146
66
86
103
147
67
87
104
150
68
88
105
151
69
89
106
152
6A
91
107
153
6B
92
108
154
6C
93
109
155
6D
94
110
156
6E
95
111
157
6F
96
112
160
70
97
113
161
71
98
114
162
72
99
115
163
73
A2
116
164
74
A3
117
165
75
A4
118
166
76
A5
119
167
77
A6
120
170
78
A7
121
171
79
A8
122
172
7A
A9
Char
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
y
z
Remarks
More punctuation characters:
Bit
0
1
0
0
.....ASCII.... EBCDIC
Dec
Oct Hex Hex
123
173
7B
C0
124
174
7C
4F
125
175
7D
D0
126
176
7E
A1
Char
{
|
}
~
Remarks
Left brace (curly bracket)
Vertical bar
Right brace (curly bracket)
Tilde
Finally, one more nonprintable character:
0
127
177
7F
07
DEL
Delete, rubout
,Index
lxxxvi
Page
Index
8th Bit
Names
6, 27
NAK
8, 36
Normal Form for File
16
ACK
8
ASCII
7, 11, 82
Baud
9
Binary Files
Packet
Parity
Prefix
Prefixed
10, 11
8, 20
22, 26, 82
27, 30
Sequence
28
Binary Mode
9
Bit Positions
6
Block Check
21, 22
Bootstrap
77
BREAK
73
Printable Files
Program, Kermit
Protocol
4
Raw Mode
9
Records
11
Remote
6, 9
Repeat Prefix
Capabilies
25
CAPAS
25
Checksum
21
Control Character
7
Control Characters
20, 82
Control Fields
22
Ctl(x)
7
11
74
27
Send-Init
23
Sequence Number
13
Sequential Files
4
Server
6
Server Command Wait
28
Data Encoding
DEFINE
71
Duplex
9
22
EBCDIC
9, 11, 82
Edit Number
76
Encoding
27, 30
End-Of-Line (EOL)
Errors
15
Server Commands
32
Server Operation
28
Short Reply
31
Sliding Window
52
SOH
9
Tab Expansion
11
Text Files
11
Timeout
8
Tochar(x)
7
Transaction
13
Transaction Log
17
TTY
6
9, 21
Fatal Errors
15
File Names
16
Flow Control
9, 17
Full Duplex
9
GET Command
31
Unchar(x)
User
6
Half Duplex
Host
6
9
Window
XON/XOFF
Initial Connection
23
Interrupting a File Transfer
37
7
52
9, 17, 82
Kermit
4
Language, Programming
76
Line Terminator
21
Line Terminator (see End-Of-Line)
Local
6
Logical Record
11
Logical Records
11
Long Packet Extension
49
Long Reply
31
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