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Xmodem, CRC Xmodem, Wxmodem
File Transfer Protocols
Please circulate this document anyway that you see
fit without alteration except on the page at the
end titled: "Notes and Comments". It is requested
that anyone using these protocols within a commer-
cial product not charge for them as an option or
surcharge, but include XMODEM and its derivations
as part of the basic product.
Peter Boswell
June 20, 1986
People/Link email: TOPPER
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 2
----------------------------------------------------------------------
TABLE OF CONTENTS
1. PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. XMODEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Xmodem Hardware Level Protocol . . . . . . . . . . . . . 7
4.2. Xmodem Initiation . . . . . . . . . . . . . . . . . . . . 7
4.3. Xmodem Data Transmission . . . . . . . . . . . . . . . . 8
4.4. Xmodem Cancellation . . . . . . . . . . . . . . . . . . . 9
4.5. Xmodem Error Recovery and Timing . . . . . . . . . . . . 9
5. CRC XMODEM . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. CRC Calculation Rules . . . . . . . . . . . . . . . . . . 13
5.2. CRC Xmodem Initiation . . . . . . . . . . . . . . . . . . 14
6. WINDOWED XMODEM (WXMODEM) . . . . . . . . . . . . . . . . . . 15
6.2. Transparency and Flow Control Rules (Byte Level Rules) . 16
6.3. Initial Handshake Rules . . . . . . . . . . . . . . . . . 18
6.4. Window Packet Transmission Rules . . . . . . . . . . . . 18
6.5. Notes for X.25 Hosts . . . . . . . . . . . . . . . . . . 22
7. APPENDIX A - CRC CALCULATION RULES . . . . . . . . . . . . . . 23
7.1. IBM PC - 8088/8086 Data Structure . . . . . . . . . . . . 23
7.2. BASIC Implementation of Bit Shift Method . . . . . . . . 23
7.3. BASIC Implementation of the Table Method . . . . . . . . 26
8. NOTES AND COMMENTS . . . . . . . . . . . . . . . . . . . . . . 28
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 3
----------------------------------------------------------------------
1. PREFACE
In the years that have past since Xmodem was first developed as a file
transfer protocol, many thousands of people have been involved in
finding reasonable ways to move data via asynchronous telephone commun-
ications. I appreciate the opportunity that I have had to meet and
learn from many of these people. There is nothing in this document
that did not actually come from someone else. Indeed, whether it is
WXMODEM, X.PC, Synchronous dial-up X.25, SNA, ZMODEM, Blast, Kermit or
any other protocol that becomes the dominant dial-up file transfer
protocol for personal and home computers is just not important. What
is important is that the public domain have a high speed file transfer
protocol that is reasonably popular and commonly available for many
types of personal computers, for bulletin boards and for services such
as People/Link, Delphi, CompuServe, GEnie and The Source.
Here are a few people that all of us should thank and I would espec-
ially like to recognize:
Ward Christensen Ward, a true pioneer in the microcomputer
communications area, is the author of the original Checksum
Xmodem protocol. Thanks for reminding me to "keep it simple
stupid".
Chuck Forsberg Chuck has edited perhaps the best work on
Xmodem and has provided both YMODEM (1K Xmodem) and ZMODEM
(Windowed YMODEM) to the public domain. Thanks for showing
me a protocol which would deal with the X-On/X-Off problem
and reminding me that there is such a thing as a DLE char-
acter.
Richard (Scott) McGinnis Scott is the architect, the moving
force, for the People/Link software system. His ideas,
comments and encouragement have been wonderful. Wait until
you see his visual conference program for the IBM PC!
Thanks for showing me how to use a DLE.
Gene Plantz Gene operates a major IBM PC bulletin board in
the Chicago area and has been active in the National SYSOP
Association. Thanks for pushing me to do something about
performance.
In a historical perspective, there seems to be a common pattern in all
computer systems development that can shed some light on where we stand
and how we got here. The pattern is function first, then integrity and
finally performance.
Any kind of software must first do something worthwhile. There is no
point in being error free, or inexpensive to operate if we do not want
the function. Back in 1977, Ward Christensen had a need to move data
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 4
----------------------------------------------------------------------
between microcomputers. Within a year it became obvious that the
function Xmodem provided met a real need to many microcomputer users.
Once we have a new function and we accept it, there is a normal desire
for the function to be correct. No one can't count the times that new
software users have pointed out ... "that new function is super, but
the results are wrong". The effort changes from providing new function
to providing integrity. The development of CRC Xmodem is a clear
response to the integrity phase of a service as it reduced undetected
transmission errors by many orders of magnitude.
After the integrity has been accepted, people begin to look toward cost
and performance. XMODEM entered this phase in 1984-1985. Chuck
Forsberg's YMODEM is a major step in this effort providing larger
block sizes, batch mode and more. His ZMODEM is a major step toward
making XMODEM derivative protocols work effectively with Public Data
Networks and most importantly, provides for restart of a file transfer
at the point of failure. WXMODEM, presented here, is an alternate
solution to ZMODEM which is, hopefully, an easier solution to the most
important performance problems.
No one really knows where XMODEM and the file transfer function will go
in the coming years. Perhaps X.PC from Tymnet, MNP from Microcom or
Synchronous X.25 will slowly push XMODEM, et. al, into history. I
think this will happen, but not for maybe 5 to 10 years. Perhaps when
50% of the households outgrow the Commodore 64, or when modem manufac-
turers can provide a $50 synchronous modem we will see the beginning of
the end for XMODEM, but not today.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 5
----------------------------------------------------------------------
2. INTRODUCTION
XMODEM and its derivatives have become the primary method for file
transfer for personal computers. Hopefully this document will help
people to understand these protocols and to implement them on their
own. In particular, this document presents an additional XMODEM
derivation to the public domain: WXMODEM.
Why develop another file transfer protocol?
After working with bulletin boards, Public Data Networks such as Tymnet
and Telenet, and commercial host systems such as People/Link, Delphi,
CompuServe and others, a number of people came to believe that hobby-
ist, home and business users would benefit significantly from a new,
conceptually simple file transfer protocol which would provide improved
performance and fully support the public data networks such as Tymnet,
Telenet and Datapac.
But before WXMODEM can be presented, XMODEM and CRC XMODEM must be
described in detail.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 6
----------------------------------------------------------------------
3. TERMINOLOGY
I've elected to use two special terms: transmitter and receiver. The
transmitter is the computer/software which is transmitting data packets
and receiving acknowledgement characters. The receiver is the com-
puter/software receiving the data packets and transmitting acknowledge-
ment characters.
Here is a table of special ASCII characters that are used throughout
this paper:
Name Decimal Hexadecimal Description
SOH 01 H001 Start Of Header
EOT 04 H004 End Of Transmission
ACK 06 H006 Acknowledge (positive)
DLE 16 H010 Data Link Escape
X-On (DC1) 17 H011 Transmit On
X-Off(DC3) 19 H013 Transmit Off
NAK 21 H015 Negative Acknowledge
SYN 22 H016 Synchronous idle
CAN 24 H018 Cancel
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 7
----------------------------------------------------------------------
4. XMODEM
Xmodem is a popular error recovery type protocol for transferring files
between computers via serial, asynchronous communications. Before
learning more about Xmodem, it is important to hear what its author has
to say:
"It was a quick hack I threw together, very unplanned (like
everything I do), to satisfy a personal need to communicate
with some other people. ONLY the fact that it was done in
8/77, and that I put it in the public domain immediately,
made it become the standard that it is"....."People who
suggest I make SIGNIFICANT changes to the protocol, such as
'full duplex', 'multiple outstanding blocks', 'multiple
destinations', etc etc don't understand that the incredible
simplicity of the protocol is one of the reasons it survived
to this day in as many machines and programs as it may be
found in!"a
4.1. Xmodem Hardware Level Protocol
The protocol is Asynchronous, 8 data bits, no parity bit, one stop
bit. Modems which are commonly used are AT&T 103 (300 baud), AT&T
212A (1200 baud) and CCITT V.22 (2400 baud).
Typically, the data in a file is transmitted without change (if a
7 bit machine, the left most, high order, bit is always zero)
except that CP/M and MS/DOS operating systems want a ^Z (decimal
26) to represent end-of-file.
4.2. Xmodem Initiation
Prior to entering the protocol, both the transmitting and receiv-
ing computer must know where to get the data (what file is to be
transmitted) and where to put the data (file to store the data or
buffer area). In Xmodem one side of the file transmission is
always in charge (local computer), asking the other side (remote
computer) to either transmit a file or to accept a file. Through
a dialog outside of Xmodem the local computer (your PC) first
sends commands to the remote computer to select a file name
to prepare to transmit or receive a file via XMODEM. Once this is
completed the remote computer enters the XMODEM protocol. Now the
local computer must be told what file to transmit or receive and
it enters the XMODEM protocol, and hopefully data starts moving.
a Ward Christensen, quoted from a message posted on CompuServe
in 1985. Edited by Chuck Forsberg, "X/Ymodem Protocol
Reference", unpublished, 10/20/1985.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 8
----------------------------------------------------------------------
Upon entering the Xmodem protocol, the transmitting computer waits
between 10 seconds and a minute to receive an NAK character from
the receiving computer. The receiving computer is said to drive
the protocol. The transmitter may retry any number of times. If
any character other than a NAK or CAN is read by the transmitter,
it is ignored. The CAN character implies cancellation of the
Xmodem file transfer and that the transmitter should leave the
Xmodem protocol. Once the receiver has sent a NAK, it will wait
10 seconds for data to begin to arrive. If none arrives in 10
seconds, the receiver will send another NAK and continue to repeat
10 times at which point the receiver will leave the Xmodem
protocol (typically with a super cryptic error message such as
"aborted", "NAK retry maximum exceeded").
Transmitter Receiver
[wait for one minute] < [NAK]
[begin block transmission] >
4.3. Xmodem Data Transmission
The transmitter takes the data, divides it into 128, 8 bit byte
pieces and places it in an Xmodem Packet.
The Xmodem Packet looks like this:
[SOH] [seq] [cmpl [seq] [128 data bytes] [csum]
SOH Start of header character (decimal 1).
seq one byte sequence number which starts at 1, and
increments by one until it reaches 255 and then
wraps around to zero.
cmpl seq one byte 1's complement of seq. This can be
calculated as cmpl = 255 - (255 and seq) or using
xor as cmpl = (255 and seq) xor 255.
data 128, 8 bit bytes of data. Note than when sending
CP/M and MS/DOS files a ^Z (decimal 26) must be
added to then end of the file. If the last block
of data is less than 128 bytes, the Xmodem packet
must be padded with characters, usually ^Z's.
csum one byte sum of all of the data bytes where any
overflow or carry is discarded immediately. For
example, if the first 3 bytes are 255, 5 and 6 the
checksum after the first 3 bytes will be 10.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 9
----------------------------------------------------------------------
Once Xmodem Initiation has completed, the transmitter sends the
first Xmodem packet and then waits. After the receiver has the
full packet, it will compare its own checksum calculation with the
checksum that was sent by the transmitter. If the checksums
match, the receiver will send an ACK. If the checksums are
different, the receiver will send a NAK.
After receiving an ACK the transmitter will send the next Xmodem
packet. If a NAK is received, the transmitter will resend the
same XMODEM packet again.
Once the transmitter has sent the last Xmodem packet and has
received an ACK, the transmitter will send an EOT and then wait
for a final ACK from the receiver before leaving the Xmodem
protocol. When the receiver sees an EOT instead of an SOH (the
first character the next packet), the receiver transmits an ACK
character, closes its files and leaves the Xmodem protocol.
Let's look at a three block file transfer:
Transmitter Receiver
<<<<< [NAK]
[SOH][001][255][...][csum] >>>>>
<<<<< [ACK]
[SOH][002][254][...][csum] >>>>>
<<<<< [ACK]
[SOH][003][253][...][csum] >>>>>
<<<<< [ACK]
[EOT] >>>>>
<<<<< [ACK]
Seems easy, right? And it is, until something goes wrong.
4.4. Xmodem Cancellation
It has become a defacto standard that the receiver may cancel the
file transfer by sending a CAN character and then leaving the
protocol. If the transmitter receives a CAN character when
expecting either a NAK or ACK, the transmitter is to termin-
ate and leave the protocol. Likewise, if the receiver sees a CAN
when expecting an SOH (start of packet) it should terminate the
file transfer. Many implementations now require two CAN char-
acters before recognizing a cancel condition.
4.5. Xmodem Error Recovery and Timing
Error detection and recovery are the primary purposes of the
Xmodem protocol. The transmitter and receiver should continue to
retry until 10 errors in a row have occurred. Some of the common
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 10
----------------------------------------------------------------------
error conditions are listed below:
4.5.1. Complement Error
If the sequence number does not match the complement
sequence number, the packet must be discarded and a NAK
sent to the transmitter.
4.5.2. Duplicate packet condition
If the sequence number is the same as the sequence
number of the last packet received, the packet should be
discarded and an ACK sent to the transmitter.
4.5.3. Out of sequence error
If the sequence number matches the complement sequence
number and it is neither the expected sequence number
nor the last sequence number, the receiver should send
two CAN characters and leave the Xmodem protocol
(e. g. abort the file transfer).
4.5.4. Receive timeout errors
When expecting data, if 10 seconds ever pass without
receipt of a character, the receiver should send another
NAK. This should be repeated 10 times. Some implement-
ations will timeout after 10 seconds waiting for the
first character of a packet, SOH, and then reduce the
timeout for characters in a packet. The timeout should
not go below 5 seconds if the protocol is to be used
with public data networks.
4.5.5. Transmit timeout errors
In the original protocol, the transmitter would wait 10
seconds for an ACK, NAK or CAN and then retransmit the
last Xmodem packet as if a NAK had been received. Most
implementations either have the transmitter wait for a
very long time (30 seconds to a minute) and then
terminate the file transfer if an ACK, NAK or CAN has
not been receive or wait about 30 seconds and retransmit
the last packet.
4.5.6. Packet synchronization errors
Since extraneous characters are frequently generated
when using asynchronous communications, the receiver
should not count on receiving exactly 132 characters for
each Xmodem packet. One algorithm for re-synchroniz-
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 11
----------------------------------------------------------------------
ation goes as follows:
Assume that the checksum algorithm will cause re-trans-
mission of Xmodem packets which contain extraneous
characters.
If the character received when expecting the start of a
packet is not a SOH then ignore the character and
continue to search for a SOH.
Once a SOH is found, assume that the next two characters
will be a valid sequence number and complement. If they
are complements then assume that the packet has begun.
If they are not complements, continue to search for a
SOH.
Send a NAK if a timeout occurs while attempting to
re-synchronize (e.g. continue to process timeouts as
described above).
If no re-synchronization occurs within 135 characters
then send a NAK character and retry receiving the
packet.
4.5.7. False EOT condition
When the receiver sees an EOT (which was not sent by the
transmitter, but generated out of a communications error)
instead of a SOH character, the receiver assumes incorrectly
that the complete file has been transmitted. This is
typically an unrecoverable error and it does occur especdally
when the transmitting and receiving UARTs are clocked
slightly differently. An algorithm to detect false EOT might
return a NAK for the first EOT received and only assume true
end of transmission after receiving two EOT's.
Transmitter Receiver
[last block .. ] >>>>>
<<<<< [ACK]
[EOT] >>>>>
<<<<< [NAK]
[EOT] >>>>>
<<<<< [ACK]
Just in case the transmitter was not prepared to resend the
EOT, it might be wise to set the timeout to about 3 seconds
and retransmit the NAK up to 3 times and then issue a warning
message but assume end of transmission.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 12
----------------------------------------------------------------------
4.5.8. False CAN condition
Some Xmodem implementations will terminate on a single CAN
character. Occasionally a CAN character will be generated by
a communications error and if this occurs and is seen by the
receiver between packets or is ever seen by the transmitter,
the file transfer will be incorrectly canceled. Many
implementations now require two CAN characters in a row
before assuming that the file transfer is to be aborted.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 13
----------------------------------------------------------------------
5. CRC XMODEM
CRC Xmodem is very similar to Checksum Xmodem. The protocol initiation
has changed and the 8 bit checksum has been replaced by a 16 bit CRC.
Only theses changes are presented.
One of the earliest and most persistent problems with Xmodem were
transmission errors which were not caught by the checksum algorithm.
Assuming that there is no bias in asynchronous communications errors,
we would expect that 1 out of every 256 erroneous complete or oversized
Xmodem packets to have a valid checksum. With the same assumption, if
the checksum were 16 bits, we would expect 1 out of every 65,536
erroneous complete or oversized packets would have a valid checksum.
5.1. CRC Calculation Rules
Considerable theoretical research has shown that a 16 bit cyclical
redundancy check character (CRC/16) will detect a much higher
percent of errors such that it would only allow 1 undetected
bit in error for every 10^14 bits transmitted. That's 1 un-
detected error per 30 years of constant transmission at 1 mega-
bit per second. However, my personal experience indicates that
something around 10^9 to 10^10 is more realistic. Why such a vast
improvement over the checksum algorithm? It is caused by the
unique properties that prime numbers have when being divided into
integers. Simply stated, if an integer is divided by a prime
number, the remainder is unique. The CRC/16 algorithm treats all
integer by 2^16 and then divides that 1040 bit number by a 17 bit
prime number. The low order 16 bits of the remainder becomes the
16 bit CRC.
The 17 bit prime number in CRC Xmodem is 2^16 + 2^12 + 2^5 + 1 or
65536 + 4096 + 32 + 1 = 69665. So calculating the CRC is simple,
just multiply the 128 byte data number by 65536, divide by 69665
and the low order 16 bits of the remainder are the CRC. The only
problem is, I've never seen a computer which has instructions to
support 130 byte integer arithmetic! Fortunately for us, Seephan
Satchell, Satchell Evaluations, published a specification a very
efficient algorithm to calculate the CRC without either 130 byte
arithmetic or bit manipulation. Appendix A contains the source
code, in IBM/PC BASIC, for the calculation of a CRC.
Other methods of calculating CRC's for Xmodem involve bit level
logic. a.
a Chuck Forsberg, "X/Ymodem Protocol Reference", available on
many bulletin boards.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 14
----------------------------------------------------------------------
5.2. CRC Xmodem Initiation
The initiation of CRC Xmodem was designed to provide for automatic
fall back to Checksum Xmodem if the transmitter does not support
the CRC version.
The receiver requests CRC Xmodem by sending the letter C (decimal
67) instead of a NAK. If the transmitter supports CRC Xmodem, it
will begin transmission of the first Xmodem packet upon receipt of
the C. If the transmitter does not support CRC Xmodem, it will
ignore the C. The receiver should timeout after 3 seconds and
repeat sending the C. After 3 timeouts, the receiver should fall
back to the checksum Xmodem protocol and send a NAK.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 15
----------------------------------------------------------------------
6. WINDOWED XMODEM (WXMODEM)a
First, Xmodem provided the basic file transfer function, then CRC
Xmodem improved the data integrity, now we come to WXmodem which
provides better cost/performance.
6.1. WXmodem Design Criteria
A few people began discussing improvements to Xmodem with me in
late 1985, over time we developed the following criteria:
6.1.1. The protocol must be as similar as possible to the
XMODEM originally developed by Ward Christensen.
The popularity of XMODEM, I believe, is based on
its conceptual simplicity. More software writers
are familiar with this protocol than any other.
More files are transferred everyday by this
protocol than any other asynchronous protocol.
Simplicity here implies a limited number of rules
for timing, error recovery and initiation.
6.1.2. The protocol must overcome the propagation delay
that is characteristic of the public data net-
works. While the cost of long distance communi-
cation is 50 to 90% less via the public data
networks than via the public voice networks, the
propagation delays inherent in public data networks
both reduces the cost savings and increases the
aggravation that occurs while watching a computer
slowly perform a file transfer.
6.1.3. The protocol must overcome the flow control
problems which are characteristic in many public
data network situations. Basically, in most
situations, the X-On and X-Off characters must
always be used for flow control and only for flow
control when using public data networks.
6.1.4. The protocol should improve error recovery by
simplifying the manner in which a programmer can
determine the beginning of an XMODEM block. Since
the Start of Header character (SOH) can appear in
the data or CRC, the programmer must use a rela-
tively sophisticated method to determine if a SOH
actually represents the beginning of a XMODEM
block.
a This section assumes that the reader is already familiar with
Xmodem and CRC Xmodem presented above.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 16
----------------------------------------------------------------------
6.2. Transparency and Flow Control Rules (Byte Level Rules)
This protocol provides special public data network support for
non-X.25 hosts and PC-Pursuit access to bulletin boards. In order
to accomplish this, the transmitter is not permitted to transmit
the X-On and X-Off characters in the Xmodem packets. The reason
for this restriction is simple:
By the very nature of X.25 public data networks, without
flow control, buffer overruns and lost data are inevit-
able from time to time at any baud rate.
To avoid data loss public data networks must always
assume that any X-Off and X-On character is a flow
control character when supporting PC-Pursuit for
bulletin boards and when supporting non-X.25 host
systems.
Since many non-X.25 hosts, bulletin boards and communications
programs use X-On and X-Off as flow control characters, public
data networks must support those X-Off and X-On requests at the
point of connection where the X-Off is received by the public data
network. Otherwise, as many as several hundred characters backed
up in the network would be transmitted by the public data network
before the X-Off used for flow control reached the transmitter.
The public data network has no way to know whether an X-On/X-Off
protocol or Xmodem is operational at any point in time. Therefore
a Xmodem packet which contains an X-Off character and no succeed-
ing X-On character will cause the public data network to stop
forwarding the ACK or NAK.
In addition, error recovery requires sophisticated programming for
the receiver to determine the start of an XMODEM packet. This
protocol simplifies this task by dedicating a special character as
an indicator that an XMODEM packet is about to begin. The
SYN (synch, decimal 22) character is used for this purpose.
The presence of one or more SYN characters in a data stream always
indicates that the next non SYN character is the beginning of an
XMODEM packet (e.g. SOH).
The method used here to handle these situations is through the
insertion of the DLE character (H010, decimal 16, data link escape
character) as an indicator that the character following the DLE is
in fact a modified DLE, SYN, X-On, or X-Off character.
Rules:
6.2.1. Whenever an X-On, X-Off, SYN or DLE character is
about to be transmitted as any part of an actual
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 17
----------------------------------------------------------------------
XMODEM packet including the CRC, the transmitter
will transmit instead a DLE character followed by
the original character which has been modified by
exclusive or'ing it with 64 to its value. 1
6.2.2. The inserted DLE characters are not counted in the
128 byte data length of the data portion of the
XMODEM packet. Indeed, it would be possible to
have a packet which is physically 264 bytes in
length because the Xmodem block sequence number
(or its complement), all of the 128 data characters
and two CRC characters are all either X-On, X-Off,
SYN or DLE characters.
6.2.3. Neither the DLE nor the adjusted characters are
used in the CRC calculation, rather the original
character is always used in the CRC calculation.
6.2.4. When the receiver sees a DLE character, it does not
count it in the XMODEM block length calculation,
nor compute it in the CRC calculation but discards
it and then remembers to exclusive or the next
character with 64 and to verify that the result
character is either a DLE, SYN, X-On or X-Off (the
receiver will reject the packet unconditionally, if
not one of those four characters) and then include
the character as part of the packet.
6.2.5. Prior to transmission of a XMODEM packet, the
transmitter will send one or more SYN characters
(recommend two) as a positive indicator to the
receiver of the beginning of a Xmodem packet.2
6.2.6. Except for the character received after a DLE, the
receiver will test each incoming character to see
if it is a SYN character. If it is, it will
discard the character and assume that the next
character will be another SYN or SOH. If a SYN
character is received in the middle of a packet,
the receiver will NAK that packet. The purpose of
the SYN character is to simplify recognition of the
beginning of a XMODEM packet by the receiver. Once
an out of synch condition occurs on incoming
data, the receiver can just ignore every incoming
character until it sees a SYN. Existing XMODEM
code which already properly deals with this
situation could just always discard any SYN
character at time of receipt with no further
action.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 18
----------------------------------------------------------------------
6.2.7. The transmitter must support flow control char-
acters (X-On, and X-Off) during transmission of
packets. Upon receipt of an X-Off it will wait 10
seconds for an X-On and will start transmission
again after 10 seconds or an X-On is received,
whichever occurs first. Any extraneous X-On
characters received by the transmitter will be
ignored and discarded. (Note that this does NOT
apply to X.25 host computers which use X.25 L2 and
L3 windows for flow control.)
6.3. Initial Handshake Rules
An initial handshake is provided to permit the receiver to
indicate to the transmitter whether it can support checksum
Xmodem, CRC Xmodem, or Windowed Xmodem:
6.3.1. WXMODEM - The receiver will send a character W
(decimal 87) and wait 3 seconds for the beginning
of a Xmodem packet. This will be repeated 3 times
and then the receiver will drop down to CRC Xmodem.
6.3.2. CRC XMODEM - The receiver will send a character C
(decimal 67) and wait 3 seconds for the beginning
of a Xmodem packet. This will be repeated 3 times
and then the receiver will drop down to Checksum
Xmodem.
6.3.3. Checksum XMODEM - The receivers will send a NAK
and wait up to 3 seconds for the beginning of a
Xmodem packet. This will be repeated 4 times and
if no valid SOH is received, the receiver will
abort the file transfer request.
6.4. Window Packet Transmission Rules
In order to overcome the propagation delays inherent with public
data networks such as Tymnet, Telenet, Datapac, IPSS, Transpac and
dozens more, the protocol must permit the transmitter to send more
than one packet before receiving an acknowledgement from the
receiver. The number of packets that the transmitter will send
before stopping transmission if an acknowledgement has not been
received is called the "window". WXmodem uses a window of 4
packets for several reasons. Most importantly, it uses a single
set of timing rules which would deal reasonably well with a wide
range of baud rates (that implied keeping the window fairly
small). Secondly, the window sequence number is directly related
to the Xmodem packet sequence number which, hopefully, will
simplify implementation of windowing.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 19
----------------------------------------------------------------------
Rules:
6.4.1. The window is always 4 Xmodem packets. That is,
the transmitter will send 4 unacknowledged pack-
ets. Transmission will not cease and the time out
interval will not begin until 4 unacknowledged
packets have been transmitted. Note that the
window may be less than 4 Xmodem packets for short
files or at end-of-file.
6.4.2. The receiver will transmit acknowledgements in the
form:
ACK[sequence]
The [sequence] field is an 8 bit number where the
high order or most significant 6 bits are always
zero and the low order or least significant 2 bits
are always the same as the low order 2 bits of the
XMODEM block sequence number of the XMODEM packet
being acknowledged (value in decimal may range
from 0 to 3).
6.4.3. The receiver does not have to acknowledge every
packet, but must acknowledge at minimum every
fourth packet. The transmitter will accept one
ACK[sequence] for multiple XMODEM packets. For
example, after an unknown number of packets:
Transmitter Receiver
....
....
....
[Block Sequence Number H0FE]
[Block Sequence Number H0FF] ACK[H002]
[Block Sequence Number H000] ACK[H003]
[Block Sequence Number H001]
[Block Sequence Number H002] ACK[H001]
.....
Since some transmitters must close the window and
cease all communications before doing disk I/O to
read more data, it is suggested that acknowledge-
ments be sent for every packet (except when the
receiver can easily determine that another packet
is already being received at the point in time that
the ACK[sequence] is about to be sent).3
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 20
----------------------------------------------------------------------
6.4.4. The receiver will reject a packet (request re-
transmission) by sending:
NAK[sequence]
Where [sequence] is then next window sequence
number (between H000 and H003) after the [sequence]
of the last good block. The receiver will discard
up to 3 Xmodem packets received after the NAK is
transmitted until it receives the packet with the
sequence number that had previously been nak'ed by
the receiver. The receiver will not send a second
NAK until another packet with the same sequence
number is received which is also invalid or a
timeout has occurred.
6.4.5. When the transmitter receives a NAK[sequence], it
will complete transmission of any XMODEM block
currently being transmitted and then begin re-
transmission starting with the block which was
nak'ed.
6.4.6. The receiver will discard duplicate packets but
count them in the window for purposes of deter-
mining the maximum receive window without an ACK in
response. For example, if the receiver gets packet
sequence number 127 four times in a row, it must
send an ACK H003 even if the receiver has previous-
ly acked that block.
6.4.7. The timeout intervals at various points in process-
ing are:
Waiting for a character on receive, start of packet
not yet recognized: 15 seconds
Waiting for a character on receive, start of packet
has been recognized: 15 seconds
Waiting for an Ack or Nak on transmit side after
the window has closed: 15 seconds4
Waiting for an X-On after receipt of an X-Off by
the transmitter: 10 seconds
6.4.8. When the transmitter times out waiting for an ACK
or NAK when the window is closed (e.g. four blocks
have been transmitted), the transmitter will
retransmit the last block transmitted and wait
again. Only after 10 consecutive timeouts have
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 21
----------------------------------------------------------------------
occurred will the transmitter cancel the trans-
mission.
6.4.9. Where possible, it is recommended that the receiver
return an ACK[sequence] for every packet or at
least 50% of the Xmodem packets. When the receiver
must wait for the window to close (e.g. receive 4
Xmodem packets without an acknowledgement),
some performance benefit will be lost.
If the receiver cannot overlap disk I/O and communications
I/O, the receiver can temporarily stop transmission by either:
"Closing the window" (e.g. receiving 4 blocks without sending
an ACK[sequence]) performing the disk I/O and then sending an
ACK[sequence].
Transmitting an X-Off followed by an X-On when the receiver
is ready to resume accepting data. Note that the receiver
should be prepared to accept data for about a 1/4 of a second
after the X-Off is sent to cover situations where satellite
propagation delay may occur. One possible implementation
would let the computer user set the "X-Off delay time" so
that in the normal case the X-Off delay could be set to 25
milleseconds. A sophisticated implementation might set the
initial X-Off delay at 250 milleseconds and then reduce it
based on experience during the file transfer.
Each approach has its advantages, but the X-Off approach will
provide the best performance in most cases especially when
using a public data network. Note, however, that some
computers, notably the Commodore 64 and the IBM PC Jr cannot
receive communications data while writing to disk.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 22
----------------------------------------------------------------------
6.5. Notes for X.25 Hosts
Host computer systems which utilize the X.25 protocol
(examples: People/Link, Delphi, CompuServe, The Source) to
interface with the various public data networks may send special
control packets which change the manner in which the network will
communicate with the remote personal computer, bulletin board or
terminal. For the purposes of this paper, it is assumed that the
X.25 host can already support CRC and/or Checksum Xmodem and
present only the changes for WXMODEM.
6.5.1. When an X.25 Host is the transmitter, it must be
sure to set the distant X.3 PAD parameters to
assure that the receiver can use X-Off/X-On for
flow control. This is accomplished by sending a
Q-Bit command packet to set X.3 parameter 12 to a 1
prior to the initial handshake. Note that if the
receiver cannot support WXMODEM, the X.25 Host must
send the appropriate Q-Bit packet to reset para-
meter 12 to a 0 before transmitting the first CRC
or Checksum Xmodem packet.
6.5.2. When an X.25 Host is the receiver and in WXMODEM
mode, it must be sure to set the distant X.3 PAD
parameters to assure that the network will use
X-Off/X-On for flow control between the network and
the transmitter to prevent its buffers from
overflowing. This is accomplished by sending a
Q-Bit command packet to set X.3 parameter 5 to a 1
prior to the initial handshake.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 23
----------------------------------------------------------------------
7. APPENDIX A - CRC CALCULATION RULES
The purpose of this appendix is to give non-technical and non mathema-
tical software writers a cook book approach to calculating the CRC-16
used in Xmodem. We have half accomplished that goal. The BASIC code
in the examples below has been tested on an IBM PC and found to work
effectively even at 9600 with compiled Basic. Some BASIC languages do
not offer an XOR function and others do not have MKI$ and CVI functions
which simplified the movement of data between data types. Someday we
hope to provide a Commodore C-64/C-128 implementation which simulates
XOR, but not today!
My thanks go to Chuck Forsberg, Joe Noonan, John Byrns and Stephen
Satchell. Without their help and public domain documents, this would
have never been possible.
7.1. IBM PC - 8088/8086 Data Structure
The Intel 8080 and upward has a feature, convenient only to some
electrical engineer somewhere, which places 2 byte (16) bit
integers in BYTE REVERSE order in memory. That is, the least
significant byte is placed in memory before the most significant
byte for integer operations. If A$ is one byte containing the
number 52 and it is assigned to I% using the ASC function, the
binary value (52) ends up in the first byte of I% and the second
byte is zero.
Result
I%=0 [x'0000']
I%=1 [x'0100']
A$="A" [x'41']
I%=ASC(A$) [x'4100']
B$=MKI$(I%) [x'4100'] letter "A" then binary zero
I%=CVI(CHR$(0)+A$) [x'0041']
A$=CHR$(65) [x'41']
Once this is understood, many problems with these algorithms goes
away.
7.2. BASIC Implementation of Bit Shift Method
The bit shift method here was converted from the "C" logic
presented in Chuck Forsberg's "Xmodem/Ymodem" protocol reference
and from an old IBM two page reference guide that Joe Noonan
carries with him in his appointment calendar!
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 24
----------------------------------------------------------------------
Chucks' "C" code:
/*
* This function calculates the CRC used by the XMODEM/CRC Protocol
* The first argument is a pointer to the message block.
* The second argument is the number of bytes in the message block.
* The function returns an integer which contains the CRC.
* The low order 16 bits are the coefficients of the CRC.
*/
int calcrc(ptr, count)
char *ptr;
int count;
{
int crc, i;
crc = 0;
while (--count >= 0) {
crc = crc ^ (int)*ptr++ << 8;
for (i = 0; i < 8; ++i)
if (crc & 0x8000)
crc = crc << 1 ^ 0x1021;
else
crc = crc << 1;
}
return (crc & 0xFFFF);
}
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 25
----------------------------------------------------------------------
But in IBM PC BASIC, our implementation looks like:
100 DEFINT A-Z 'DEFAULT IS TWO BYTE INTEGERS
2000 REM * V$ CONTAINS 133 CHARACTER COMPLETE XMODEM PACKET
2010 REM * CRC$ IS TWO BYTE CRC WITH MOST SIGNIFICANT BYTE FIRST
2020 CRC$=CHR$(0)+CHR$(0) 'START AT ZERO
2030 FOR I2=4 TO 131
2040 A$=MID$(V$,I2,1)
2050 GOSUB 4000
2060 NEXT I2
2070 REM * CRC$ CONTAINS CALCULATED CRC!
3000 IF CRC$=MID$(V$,132,2) THEN .... 'IT'S GOOD!!!
4000 REM * CRC BITWISE CALCULATION (WHAT A JOKE!)
4010 CRCH1=ASC(LEFT$(CRC$,1)) XOR ASC(A$)
4020 CRCL1=ASC(RIGHT$(CRC$,1))
4030 FOR I3 = 0 TO 7
4040 CARRY=0 : IF CRCH1 > 127 THEN CARRY=-1 'IS HIGH BIT ON IN CRC?
4050 CRCH1=(CRCH1*2) AND 255 'CRCH << 1 AND 255
4060 IF CRCL1>127 THEN CRCH1=CRCH1+1 'IF CRCL CARRIES THEN INCR CRCH
4070 CRCL1=(CRCL1*2) AND 255 'CRCL << 1 AND 255
4080 IF CARRY=0 THEN GOTO 4105 'IF HIGH BIT WAS ON,
4090 CRCH1=CRCH1 XOR 16 'XOR WITH &H1021
4100 CRCL1=CRCL1 XOR 33
4110 NEXT I3
4130 CRC$=CHR$(CRCH1)+CHR$(CRCL1)
4140 RETURN 'WHEW
That routine will execute 128 * 7 + 128 * 9 * 8 BASIC statements
for each Xmodem packet or 10112 statements per Xmodem packet! It
will work for low baud rates in compiled BASIC, but just is too
much for interpretive BASIC.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 26
----------------------------------------------------------------------
7.3. BASIC Implementation of the Table Method
This method is based on routine M4 in Steven Satchell's paper,
"Test of CRC Routines for CRC-CCITT", but has some very signifi-
cant differences. A table of 256 CRC's, originally calculated
with the bit shift method is used to avoid performing the bit
shift during communications. The table contains the CRC's for
each byte value from 0 to 255 when the original CRC is zero. The
result of this calculation is included in the DATA statements in
the code.
The comments are intended to show what is logically happening
rather than physically. Because of the "byte reverse" nature of
integers in the 8088, a logical shift of 8 bits to the left is a
physical shift of eight bits to the right!
200 DEFINT A-Z 'ALL INTEGERS
210 DIM CRCTB(256)
300 GOSUB 9000 'INITIALIZE CRC TABLES
6200 REM * CRC CALCULATION USING TABLE METHOD, V$=XMODEM PACKET
6210 CRC$=CHR$(0)+CHR$(0) 'INITIALIZE TO ZERO
6220 FOR Q=4 TO 131
6230 CRCH1=ASC(LEFT$(CRC$,1)) 'CRC >> 8 AND 255
6240 CRCL2=CVI(CHR$(0)+RIGHT$(CRC$,1)) 'CRC << 8 AND 255
6250 CRC1$=MKI$(CRCTB(CRCH1 XOR ASC(MID$(V$,Q,1))) XOR CRCL2)
6260 CRC$=RIGHT$(CRC1$,1)+LEFT$(CRC1$,1) 'SET IT BACK!
6270 NEXT Q
6280 IF CRC$ <> MID$(V$,N,2) THEN ....... 'GOTO ERROR ROUTINE
6290 REM * END OF CRC CALC
9000 FOR I%=0 TO 255 ' INITIALIZE CRC TABLE
9010 READ CRCTB(I%)
9020 NEXT I%
9025 RETURN
9030 DATA 0, 4129, 8258, 12387, 16516, 20645, 24774, 28903
9040 DATA -32504,-28375,-24246,-20117,-15988,-11859,-7730,-3601
9050 DATA 4657, 528, 12915, 8786, 21173, 17044, 29431, 25302
9060 DATA -27847,-31976,-19589,-23718,-11331,-15460,-3073,-7202
9070 DATA 9314, 13379, 1056, 5121, 25830, 29895, 17572, 21637
9080 DATA -23190,-19125,-31448,-27383,-6674,-2609,-14932,-10867
9090 DATA 13907, 9842, 5649, 1584, 30423, 26358, 22165, 18100
9100 DATA -18597,-22662,-26855,-30920,-2081,-6146,-10339,-14404
9110 DATA 18628, 22757, 26758, 30887, 2112, 6241, 10242, 14371
9120 DATA -13876,-9747,-5746,-1617,-30392,-26263,-22262,-18133
9130 DATA 23285, 19156, 31415, 27286, 6769, 2640, 14899, 10770
9140 DATA -9219,-13348,-1089,-5218,-25735,-29864,-17605,-21734
9150 DATA 27814, 31879, 19684, 23749, 11298, 15363, 3168, 7233
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 27
----------------------------------------------------------------------
9160 DATA -4690,-625,-12820,-8755,-21206,-17141,-29336,-25271
9170 DATA 32407, 28342, 24277, 20212, 15891, 11826, 7761, 3696
9180 DATA -97,-4162,-8227,-12292,-16613,-20678,-24743,-28808
9190 DATA -28280,-32343,-20022,-24085,-12020,-16083,-3762,-7825
9200 DATA 4224, 161, 12482, 8419, 20484, 16421, 28742, 24679
9210 DATA -31815,-27752,-23557,-19494,-15555,-11492,-7297,-3234
9300 DATA 689, 4752, 8947, 13010, 16949, 21012, 25207, 29270
9310 DATA -18966,-23093,-27224,-31351,-2706,-6833,-10964,-15091
9320 DATA 13538, 9411, 5280, 1153, 29798, 25671, 21540, 17413
9330 DATA -22565,-18438,-30823,-26696,-6305,-2178,-14563,-10436
9340 DATA 9939, 14066, 1681, 5808, 26199, 30326, 17941, 22068
9350 DATA -9908,-13971,-1778,-5841,-26168,-30231,-18038,-22101
9360 DATA 22596, 18533, 30726, 26663, 6336, 2273, 14466, 10403
9370 DATA -13443,-9380,-5313,-1250,-29703,-25640,-21573,-17510
9380 DATA 19061, 23124, 27191, 31254, 2801, 6864, 10931, 14994
9390 DATA -722,-4849,-8852,-12979,-16982,-21109,-25112,-29239
9400 DATA 31782, 27655, 23652, 19525, 15522, 11395, 7392, 3265
9410 DATA -4321,-194,-12451,-8324,-20581,-16454,-28711,-24584
9420 DATA 28183, 32310, 20053, 24180, 11923, 16050, 3793, 7920
This method uses 128 * 6 BASIC statements per Xmodem packet or a
miserly 768 BASIC statements per packet. And, if you want, the
code can be tightened still more. Unfortunately, any further
tightening that we could see would eliminate most of the already
limited readability of the code.
Xmodem, CRC Xmodem, WXmodem
June 20, 1986 Page 28
----------------------------------------------------------------------
8. NOTES AND COMMENTS
Please add your notes and comments here or send them to me and I'll get
them added to the current copy on People/Link.
1. This was originally set up to ADD 32 to the character on transmit
and SUBTRACT 32 on receive. By using exclusive or with 64, the
logic is the same on transmit and receive.
2. The use of the SYN character was added at the request of several
people who have coded Xmodem routines and have struggled valiantly
to improve their error recovery routines. Peter Boswell 6/10/86
3. The suggestion that ACK[sequence] be sent for every block received
was added. Peter Boswell 6/10/86
4. The original value for the ACK/NAK timeout was 10 seconds. This
was changed to 15 seconds the situation where the receiver is
operating at 300 baud and using X-Off to stop receipt of characters
during disk I/O. Peter Boswell, 6/10/86

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