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XMODEM/YMODEM PROTOCOL REFERENCE
A compendium of documents describing the
XMODEM and YMODEM
File Transfer Protocols
Edited by Chuck Forsberg
Please distribute as widely as possible.
Questions to Chuck Forsberg
Omen Technology Inc
17505-V Sauvie Island Road
Portland Oregon 97231
Voice: 503-621-3406
Modem (Telegodzilla): 503-621-3746 Speed 1200,300
Compuserve: 70715,131
UUCP: ...!tektronix!reed!omen!caf
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1. ROSETTA STONE
Here are some definitions which reflect the current vernacular in the
computer media. The attempt here is identify the file transfer protocol
rather than specific programs.
XMODEM refers to the original 1979 file transfer etiquette introduced by
Ward Christensen's 1979 MODEM2 program. It's also called the
MODEM or MODEM2 protocol. Some who are unaware of MODEM7's
unusual batch file mode call it MODEM7. Other aliases include
"CP/M Users's Group" and "TERM II FTP 3". This protocol is
supported by every serious communications program because of its
universality, simplicity, and reasonable performance.
XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte Cyclical
Redunancy Check (CRC-16), giving modern error detection
protection.
YMODEM refers to the XMODEM/CRC protocol with the throughput and/or batch
transmission enhancements described below.
2. YET ANOTHER PROTOCOL?
Since its development half a decade ago, the Ward Christensen modem
protocol has enabled a wide variety of computer systems to interchange
data. There is hardly a communications program that doesn't at least
claim to support this protocol.
Recent advances in computing, modems and networking have revealed a number
of weaknesses in the original protocol:
+ The short block length caused throughput to suffer when used with
timesharing systems, packet switched networks, satellite circuits,
and buffered (error correcting) modems.
+ The 8 bit arithemetic checksum and other aspects allowed line
impairments to interfere with dependable, accurate transfers.
+ Only one file could be sent per command. The file name had to be
given twice, first to the sending program and then again to the
receiving program.
+ The transmitted file could accumulate as many as 127 extraneous
bytes.
+ The modification date of the file was lost.
A number of other protocols have been developed over the years, but none
have displaced XMODEM to date:
Chapter 2
X/YMODEM Protocol Reference 10-10-85 3
+ Lack of public domain documentation and example programs have kept
proprietary protocols such as MNP, Blast, and others tightly bound to
the fortunes of their suppliers.
+ Complexity discourages the widespread application of BISYNC, SDLC,
HDLC, X.25, and X.PC protocols.
+ Performance compromises and moderate complexity have limited the
popularity of the Kermit protocol, which was developed to allow file
transfers in environments hostile to XMODEM.
The YMODEM Protocol extensions were developed as a means of addressing the
weaknesses described above while maintaining XMODEM's simplicity as much
as possible.
YMODEM is supported by the public domain programs YAM (CP/M),
YAM(CP/M-86), YAM(CCPM-86), IMP (CP/M), KMD (CP/M), MODEM76.ASM (CP/M),
rb/sb (Unix, VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus)
as well as Professional-YAM.1 These programs have been in use since 1981.
The 1k packet length capability described below may be used in conjunction
with the Batch Protocol, or with single file transfers identical to the
XMODEM/CRC protocol except for the minimal changes to support 1k packets.
Another extension is simply called the g option. It provides maximum
throughput when used with end to end error correcting media, such as X.PC
and error correcting modems, including the emerging 9600 bps units by
Electronic Vaults and others.
To complete this tome, Ward Christensen's original protocol document and
John Byrns's CRC-16 document are included for reference.
References to the MODEM or MODEM7 protocol have been changed to XMODEM to
accommodate the vernacular. In Australia, it is properly called the
Christensen Portocol.
Watch for an article describing the YMODEM protocol in a more coherent
fashion later this year. The article will include some interesting
history on the development of microcomputer file transfers.
__________
1. Available for IBM PC,XT,AT, Unix and Xenix
Chapter 2
X/YMODEM Protocol Reference 10-10-85 4
2.1 Some Messages from the Pioneer
#: 130940 S0/Communications 25-Apr-85 18:38:47
Sb: my protocol
Fm: Ward Christensen 76703,302 (EDITED)
To: all
Be aware the article2 DID quote me correctly in terms of the phrases like
"not robust", etc.
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.
I think its time for me to
(1) document it; (people call me and say "my product is going to include
it - what can I 'reference'", or "I'm writing a paper on it, what do I put
in the bibliography") and
(2) propose an "incremental extension" to it, which might take "exactly"
the form of Chuck Forsberg's YAM protocol. He wrote YAM in C for CP/M and
put it in the public domain, and wrote a batch protocol for Unix3 called
rb and sb (receive batch, send batch), which was basically XMODEM with
(a) a record 0 containing filename date time and size
(b) a 1K block size option
(c) CRC-16.
He did some clever programming to detect false ACK or EOT, but basically
left them the same.
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!
Consider the PC-NET group back in '77 or so - documenting to beat the band
- THEY had a protocol, but it was "extremely complex", because it tried to
be "all things to all people" - i.e. send binary files on a 7-bit system,
etc. I was not that "benevolent". I (emphasize > I < ) had an 8-bit UART,
__________
2. Infoworld April 29 p. 16
3. VAX/VMS versions of these programs are also available.
Chapter 2
X/YMODEM Protocol Reference 10-10-85 5
so "my protocol was an 8-bit protocol", and I would just say "sorry" to
people who were held back by 7-bit limitations. ...
Block size: Chuck Forsberg created an extension of my protocol, called
YAM, which is also supported via his public domain programs for UNIX
called rb and sb - receive batch and send batch. They cleverly send a
"block 0" which contains the filename, date, time, and size.
Unfortunately, its UNIX style, and is a bit weird4 - octal numbers, etc.
BUT, it is a nice way to overcome the kludgy "echo the chars of the name"
introduced with MODEM7. Further, chuck uses CRC-16 and optional 1K
blocks. Thus the record 0, 1K, and CRC, make it a "pretty slick new
protocol" which is not significantly different from my own.
Also, there is a catchy name - YMODEM. That means to some that it is the
"next thing after XMODEM", and to others that it is the Y(am)MODEM
protocol. I don't want to emphasize that too much - out of fear that
other mfgrs might think it is a "competitive" protocol, rather than an
"unaffiliated" protocol. Chuck is currently selling a much-enhanced
version of his CP/M-80 C program YAM, calling it Professional Yam, and its
for the PC - I'm using it right now. VERY slick! 32K capture buffer,
script, scrolling, previously captured text search, plus built-in commands
for just about everything - directory (sorted every which way), XMODEM,
YMODEM, KERMIT, and ASCII file upload/download, etc. You can program it
to "behave" with most any system - for example when trying a number for
CIS it detects the "busy" string back from the modem and substitutes a
diff phone # into the dialing string and branches back to try it.
3. XMODEM PROTOCOL ENHANCEMENTS
This chapter discusses the protocol extensions to Ward Christensen's 1982
XMODEM protocol description document.
The original document recommends the user be asked wether to continue
trying or abort after 10 retries. Most programs no longer ask the
operator whether he wishes to keep retrying. Virtually all correctable
errors are corrected within the first few retransmissions. If the line is
so bad that ten attempts are insufficient, there is a significant danger
of undetected errors. If the connection is that bad, it's better to
redial for a better connection, or mail a floppy disk.
__________
4. The file length, time, and file mode are optional. The pathname and
file length may be sent alone if desired.
Chapter 3 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 6
3.1 Graceful Abort
YAM and Professional-YAM recognize a sequence of two consecutive CAN (Hex
18) characters without modem errors (overrun, framing, etc.) as a transfer
abort command.1 The check for two consecutive CAN characters virtually
eliminates the possibility of a line hit aborting the transfer. YAM sends
five CAN characters when it aborts an XMODEM or YMODEM protocol file
transfer, followed by five backspaces to delete the CAN characters from
the remote's keyboard input buffer (in case the remote had already aborted
the transfer).
3.2 CRC-16 Option
The XMODEM protocol uses an optional two character CRC-16 instead of the
one character arithmetic checksum used by the original protocol and by
most commercial implementations. CRC-16 guarantees detection of all
single and double bit errors, all errors with an odd number of error
bits, all burst errors of length 16 or less, 99.9969% of all 17-bit error
bursts, and 99.9984 per cent of all possible longer error bursts. By
contrast, a double bit error, or a burst error of 9 bits or more can sneak
past the XMODEM protocol arithmetic checksum.
The XMODEM/CRC protocol is similar to the XMODEM protocol, except that the
receiver specifies CRC-16 by sending C (Hex 43) instead of NAK when
requesting the FIRST packet. A two byte CRC is sent in place of the one
byte arithmetic checksum.
YAM's c option to the r command enables CRC-16 in single file reception,
corresponding to the original implementation in the MODEM7 series
programs. This remains the default because many commercial communications
programs and bulletin board systems still do not support CRC-16,
especially those written in Basic or Pascal.
XMODEM protocol with CRC is accurate provided both sender and receiver
both report a successful transmission. The protocol is robust in the
presence of characters lost by buffer overloading on timesharing systems.
The single character ACK/NAK responses generated by the receiving program
adapt well to split speed modems, where the reverse channel is limited to
ten per cent or less of the main channel's speed.
XMODEM and YMODEM are half duplex protocols which do not attempt to
transmit information and control signals in both directions at the same
__________
1. This is recognized when YAM is waiting for the beginning of a packet
or for an acknowledge to one that has been sent.
Chapter 3 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 7
time. This avoids buffer overrun problems that have been reported by
users attempting to exploit full duplex aynchronous file transfer
protocols such as Blast.
Professional-YAM adds several proprietary logic enhancements to XMODEM's
error detection and recovery. These compatible enhancements eliminate
most of the bad file transfers other programs make when using the XMODEM
protocol under less than ideal conditions.
3.3 1024 Byte Packet Option
The choice to use 1024 byte packets is expressed to the sending program on
its command line or selection menu.
Programs using the Hoff protocol use a two character sequence emitted by
the receiver (CK) to automatically trigger the use of 1024 byte packets as
an alternative to specifying this option on this command line. Although
this two character sequence works well on single process micros in direct
communication, timesharing systems and packet switched networks can
separate the successive characters by several seconds, rendering this
method unreliable.
An STX (02) replaces the SOH (01) at the beginning of the transmitted
block to notify the receiver of the longer packet length. The transmitted
packet contains 1024 bytes of data. The receiver should be able to accept
any mixture of 128 and 1024 byte packets. The packet number is
incremented by one for each packet regardless of the packet length.
The sender must not change between 128 and 1024 byte packet lengths if it
has not received a valid ACK for the current packet. Failure to observe
this restriction allows certain transmission errors to pass undetected.
If 1024 byte packets are being used, it is possible for a file to "grow"
up to the next multiple of 1024 bytes. This does not waste disk space if
the allocation granularity is 1k or greater. When 1024 byte packets are
used with YMODEM batch transmission, the file length transmitted in the
file name packet allows the receiver to discard the padding, preserving
the exact file length and contents.
CRC-16 should be used with the k option to preserve data integrity over
phone lines.2 1024 byte packets may be used with batch file transmission
or with single file transmission.
__________
2. Some programs enforce this recommendation.
Chapter 3 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 8
Figure 1. 1024 byte Packets
SENDER RECEIVER
"s -k foo.bar"
"foo.bar open x.x minutes"
C
STX 01 FE Data[1024] CRC CRC
ACK
STX 02 FD Data[1024] CRC CRC
ACK
STX 03 FC Data[1000] CPMEOF[24] CRC CRC
ACK
EOT
ACK
Figure 2. Mixed 1024 and 128 byte Packets
SENDER RECEIVER
"s -k foo.bar"
"foo.bar open x.x minutes"
C
STX 01 FE Data[1024] CRC CRC
ACK
STX 02 FD Data[1024] CRC CRC
ACK
SOH 03 FC Data[128] CRC CRC
ACK
SOH 04 FB Data[100] CPMEOF[28] CRC CRC
ACK
EOT
ACK
4. YMODEM Batch File Transmission
The YMODEM Batch protocol is an extension to the XMODEM/CRC protocol that
allows 0 or more files to be transmitted with a single command. (Zero
files may be sent if none of the requested files is accessible.) The
design approach of the YMODEM Batch protocol is to use the normal routines
for sending and receiving XMODEM packets in a layered fashion similar to
packet switching methods.
Why was it necessary to design a new batch protocol when one already
existed in MODEM7?1 The batch file mode used by MODEM7 is unsuitable
__________
1. The MODEM7 batch protocol transmitted CP/M FCB bytes f1...f8 and
t1...t3 one character at a time. The receiver echoed these bytes as
received, one at a time.
Chapter 4 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 9
because it does not permit full pathnames, file length, file date, or
other attribute information to be transmitted. Such a restrictive design,
hastily implemented with only CP/M in mind, would not have permitted
extensions to current areas of personal computing such as Unix, DOS, and
object oriented systems. In addition, the MODEM7 batch file mode is
somewhat susceptible to transmission impairments.
As in the case of single a file transfer, the receiver initiates batch
file transmission by sending a "C" character (for CRC-16).
The sender opens the first file and sends packet number 0 with the
following information.2
Only the pathname (file name) part is required for batch transfers.
To maintain upwards compatibility, all unused bytes in packet 0 must be
set to null.
Pathname The pathname (conventionally, the file name) is sent as a null
terminated ASCII string. This is the filename format used by the
handle oriented MSDOS(TM) functions and C library fopen functions.
An assembly language example follows:
DB 'foo.bar',0
No spaces are included in the pathname. Normally only the file name
stem (no directory prefix) is transmitted unless the sender has
selected YAM's f option to send the full pathname. The source drive
(A:, B:, etc.) is never sent.
Filename Considerations:
+ File names should be translated to lower case unless the sending
system supports upper/lower case file names. This is a
convenience for users of systems (such as Unix) which store
filenames in upper and lower case.
+ The receiver should accommodate file names in lower and upper
case.
+ The rb(1) program on Unix systems normally translates the
filename to lower case unless one or more letters in the
filename are already in lower case.
+ When transmitting files between different operating systems,
file names must be acceptable to both the sender and receiving
operating systems.
__________
2. Only the data part of the packet is described here.
Chapter 4 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 10
If directories are included, they are delimited by /; i.e.,
"subdir/foo" is acceptable, "subdir\foo" is not.
Length The file length and each of the succeeding fields are optional.3
The length field is stored in the packet as a decimal string counting
the number of data bytes in the file. The file length does not
include any CPMEOF (^Z) characters used to pad the last packet.
If the file being transmitted is growing during transmission, the
length field should be set to at least the final expected file
length, or not sent.
The receiver stores the specified number of characters, discarding
any padding added by the sender to fill up the last packet.
Modification Date A single space separates the modification date from the
file length.
The mod date is optional, and the filename and length may be sent
without requiring the mod date to be sent.
The mod date is sent as an octal number giving the time the contents
of the file were last changed measured in seconds from Jan 1 1970
Universal Coordinated Time (GMT). A date of 0 implies the
modification date is unknown and should be left as the date the file
is received.
This standard format was chosen to eliminate ambiguities arising from
transfers between different time zones.
Two Microsoft blunders complicate the use of modification dates in
file transfers with MSDOS(TM) systems. The first is the lack of
timezone standardization in MS-DOS. A file's creation time can not
be known unless the timezone of the system that wrote the file4 is
known. Unix solved this problem (for planet Earth, anyway) by
stamping files with Universal Time (GMT). Microsoft would have to
include the timezone of origin in the directory entries, but does
not. Professional-YAM gets around this problem by using the z
parameter which is set to the number of minutes local time lags GMT.
For files known to originate from a different timezone, the -zT
option may be used to specify T as the timezone for an individual
file transfer.
__________
3. Fields may not be skipped.
4. Not necessarily that of the transmitting system!
Chapter 4 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 11
The second problem is the lack of a separate file creation date in
DOS. Since some backup schemes used with DOS rely on the file
creation date to select files to be copied to the archive, back-
dating the file modification date could interfere with the safety of
the transferred files. For this reason, Professional-YAM does not
modify the date of received files with the header information unless
the d parameter is non zero.
Mode A single space separates the file mode from the modification date.
The file mode is stored as an octal string. Unless the file
originated from a Unix system, the file mode is set to 0. rb(1)
checks the file mode for the 0x8000 bit which indicates a Unix type
regular file. Files with the 0x8000 bit set are assumed to have been
sent from another Unix (or similar) system which uses the same file
conventions. Such files are not translated in any way.
Serial Number A single space separates the serial number from the file
mode. The serial number of the transmitting program is stored as an
octal string. Programs which do not have a serial number should omit
this field, or set it to 0. The receiver's use of this field is
optional.
The rest of the packet is set to nulls. This is essential to preserve
upward compatibility.5 After the filename packet has been received, it is
ACK'ed if the write open is successful. The receiver then initiates
transfer of the file contents according to the standard XMODEM/CRC
protocol. If the file cannot be opened for writing, the receiver cancels
the transfer with CAN characters as described above.
After the file contents have been transmitted, the receiver again asks for
the next pathname. Transmission of a null pathname terminates batch file
transmission. Note that transmission of no files is not necessarily an
error. This is possible if none of the files requested of the sender
could be opened for reading.
In batch transmission, the receiver automatically requests CRC-16.
The Unix programs sb(1) and rb(1) included in the source code file
RBSB.SHQ (rbsb.sh) should answer other questions about YMODEM batch
protocol.
__________
5. If, perchance, this information extends beyond 128 bytes (possible
with Unix 4.2 BSD extended file names), the packet should be sent as a
1k packet as described above.
Chapter 4 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 12
Figure 3. Batch Transmission Session
SENDER RECEIVER
"sb foo.*<CR>"
"sending in batch mode etc."
C (command:rb)
SOH 00 FF foo.c NUL[123] CRC CRC
ACK
C
SOH 01 FE Data[128] CRC CRC
ACK
SOH 02 FD Data[1024] CRC CRC
ACK
SOH 03 FC Data[128] CRC CRC
ACK
SOH 04 FB Data[100] CPMEOF[28] CRC CRC
ACK
EOT
NAK
EOT
ACK
C
SOH 00 FF NUL[128] CRC CRC
ACK
Figure 4. Filename packet transmitted by sb
-rw-r--r-- 6347 Jun 17 1984 20:34 bbcsched.txt
00 0100FF62 62637363 6865642E 74787400 |...bbcsched.txt.|
10 36333437 20333331 34373432 35313320 |6347 3314742513 |
20 31303036 34340000 00000000 00000000 |100644..........|
30 00000000 00000000 00000000 00000000
80 000000CA 56
Chapter 4 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 13
Figure 5. Header Information used by YMODEM Implementations
_____________________________________________________________________
| Program | Batch | Length | Date | Mode | S/N | 1k-Blk | g-Option |
|___________|_______|________|______|______|_____|________|__________|
|Unix rb/sb | yes | yes | yes | yes | no | yes | sb only |
|___________|_______|________|______|______|_____|________|__________|
|VMS rb/sb | yes | yes | no | no | no | yes | no |
|___________|_______|________|______|______|_____|________|__________|
|Pro-YAM | yes | yes | yes | no | yes | yes | yes |
|___________|_______|________|______|______|_____|________|__________|
|CP/M YAM | yes | no | no | no | no | yes | no |
|___________|_______|________|______|______|_____|________|__________|
|KMD/IMP | yes | no | no | no | no | yes | no |
|___________|_______|________|______|______|_____|________|__________|
|MEX | no | no | no | no | no | yes | no |
|___________|_______|________|______|______|_____|________|__________|
4.1 IMP/KMD Record Count
Due to programming constraints, these programs do not send the file length
as described above. Instead, they send (and look for) the CP/M record
count stored in the last two bytes of the header packet. The least
significant bits are stored in the penultimate byte.
KMD and IMP use the record count to allow the receiving program to display
the file size and estimated transmission time; the file length is
determined by the actual number of records sent.
5. g Option File Transmission
Developing technology is providing phone line data transmission at ever
higher speeds using very specialized techniques. These high speed modems,
as well as session protocols such as X.PC, provide high speed, error free
communications at the expense of considerably increased delay time.
This delay time is moderate compared to human interactions, but it
cripples the throughput of most error correcting protocols.
The g option to YMODEM has proven effective under these circumstances.
The g option is driven by the receiver, which initiates the batch transfer
by transmitting a G instead of C. When the sender recognizes the G, it
bypasses the usual wait for an ACK to each transmitted packet, sending
succeeding packets at full speed, subject to XOFF/XON or other flow
control exerted by the medium.
The sender expects an inital G to initiate the transmission of a
particular file, and also expects an ACK on the EOT sent at the end of
each file. This synchronization allows the receiver time to open and
Chapter 5 XMODEM Protocol Enhancements
X/YMODEM Protocol Reference 10-10-85 14
close files as necessary.
Figure 6. g Option Transmission Session
SENDER RECEIVER
"sb foo.*<CR>"
"sending in batch mode etc..."
G (command:rb -g)
SOH 00 FF foo.c NUL[123] CRC CRC
G
SOH 01 FE Data[128] CRC CRC
SOH 02 FD Data[1024] CRC CRC
SOH 03 FC Data[128] CRC CRC
SOH 04 FB Data[100] CPMEOF[28] CRC CRC
EOT
ACK
G
SOH 00 FF NUL[128] CRC CRC
6. XMODEM PROTOCOL OVERVIEW
8/9/82 by Ward Christensen.
I will maintain a master copy of this. Please pass on changes or
suggestions via CBBS/Chicago at (312) 545-8086, CBBS/CPMUG (312) 849-1132
or by voice at (312) 849-6279.
6.1 Definitions
<soh> 01H
<eot> 04H
<ack> 06H
<nak> 15H
<can> 18H
<C> 43H
6.2 Transmission Medium Level Protocol
Asynchronous, 8 data bits, no parity, one stop bit.
The protocol imposes no restrictions on the contents of the data being
transmitted. No control characters are looked for in the 128-byte data
messages. Absolutely any kind of data may be sent - binary, ASCII, etc.
The protocol has not formally been adopted to a 7-bit environment for the
transmission of ASCII-only (or unpacked-hex) data , although it could be
simply by having both ends agree to AND the protocol-dependent data with
7F hex before validating it. I specifically am referring to the checksum,
and the block numbers and their ones- complement.
Chapter 6 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 15
Those wishing to maintain compatibility of the CP/M file structure, i.e.
to allow modemming ASCII files to or from CP/M systems should follow this
data format:
+ ASCII tabs used (09H); tabs set every 8.
+ Lines terminated by CR/LF (0DH 0AH)
+ End-of-file indicated by ^Z, 1AH. (one or more)
+ Data is variable length, i.e. should be considered a continuous
stream of data bytes, broken into 128-byte chunks purely for the
purpose of transmission.
+ A CP/M "peculiarity": If the data ends exactly on a 128-byte
boundary, i.e. CR in 127, and LF in 128, a subsequent sector
containing the ^Z EOF character(s) is optional, but is preferred.
Some utilities or user programs still do not handle EOF without ^Zs.
+ The last block sent is no different from others, i.e. there is no
"short block".
Figure 7. XMODEM Message Block Level Protocol
Each block of the transfer looks like:
<SOH><blk #><255-blk #><--128 data bytes--><cksum>
in which:
<SOH> = 01 hex
<blk #> = binary number, starts at 01 increments by 1, and
wraps 0FFH to 00H (not to 01)
<255-blk #> = blk # after going thru 8080 "CMA" instr, i.e.
each bit complemented in the 8-bit block number.
Formally, this is the "ones complement".
<cksum> = the sum of the data bytes only. Toss any carry.
6.3 File Level Protocol
6.3.1 Common_to_Both_Sender_and_Receiver
All errors are retried 10 times. For versions running with an operator
(i.e. NOT with XMODEM), a message is typed after 10 errors asking the
operator whether to "retry or quit".
Some versions of the protocol use <can>, ASCII ^X, to cancel transmission.
This was never adopted as a standard, as having a single "abort" character
makes the transmission susceptible to false termination due to an <ack>
<nak> or <soh> being corrupted into a <can> and cancelling transmission.
The protocol may be considered "receiver driven", that is, the sender need
not automatically re-transmit, although it does in the current
implementations.
Chapter 6 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 16
6.3.2 Receive_Program_Considerations
The receiver has a 10-second timeout. It sends a <nak> every time it
times out. The receiver's first timeout, which sends a <nak>, signals the
transmitter to start. Optionally, the receiver could send a <nak>
immediately, in case the sender was ready. This would save the initial 10
second timeout. However, the receiver MUST continue to timeout every 10
seconds in case the sender wasn't ready.
Once into a receiving a block, the receiver goes into a one-second timeout
for each character and the checksum. If the receiver wishes to <nak> a
block for any reason (invalid header, timeout receiving data), it must
wait for the line to clear. See "programming tips" for ideas
Synchronizing: If a valid block number is received, it will be: 1) the
expected one, in which case everything is fine; or 2) a repeat of the
previously received block. This should be considered OK, and only
indicates that the receivers <ack> got glitched, and the sender re-
transmitted; 3) any other block number indicates a fatal loss of
synchronization, such as the rare case of the sender getting a line-glitch
that looked like an <ack>. Abort the transmission, sending a <can>
6.3.3 Sending_program_considerations
While waiting for transmission to begin, the sender has only a single very
long timeout, say one minute. In the current protocol, the sender has a
10 second timeout before retrying. I suggest NOT doing this, and letting
the protocol be completely receiver-driven. This will be compatible with
existing programs.
When the sender has no more data, it sends an <eot>, and awaits an <ack>,
resending the <eot> if it doesn't get one. Again, the protocol could be
receiver-driven, with the sender only having the high-level 1-minute
timeout to abort.
Here is a sample of the data flow, sending a 3-block message. It includes
the two most common line hits - a garbaged block, and an <ack> reply
getting garbaged. <xx> represents the checksum byte.
Chapter 6 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 17
Figure 8. Data flow including Error Recovery
SENDER RECEIVER
times out after 10 seconds,
<--- <nak>
<soh> 01 FE -data- <xx> --->
<--- <ack>
<soh> 02 FD -data- xx ---> (data gets line hit)
<--- <nak>
<soh> 02 FD -data- xx --->
<--- <ack>
<soh> 03 FC -data- xx --->
(ack gets garbaged) <--- <ack>
<soh> 03 FC -data- xx ---> <ack>
<eot> --->
<--- <anything except ack>
<eot> --->
<--- <ack>
(finished)
6.4 Programming Tips
+ The character-receive subroutine should be called with a parameter
specifying the number of seconds to wait. The receiver should first
call it with a time of 10, then <nak> and try again, 10 times.
After receiving the <soh>, the receiver should call the character
receive subroutine with a 1-second timeout, for the remainder of the
message and the <cksum>. Since they are sent as a continuous stream,
timing out of this implies a serious like glitch that caused, say,
127 characters to be seen instead of 128.
+ When the receiver wishes to <nak>, it should call a "PURGE"
subroutine, to wait for the line to clear. Recall the sender tosses
any characters in its UART buffer immediately upon completing sending
a block, to ensure no glitches were mis- interpreted.
The most common technique is for "PURGE" to call the character
receive subroutine, specifying a 1-second timeout,1 and looping back
to PURGE until a timeout occurs. The <nak> is then sent, ensuring
the other end will see it.
+ You may wish to add code recommended by John Mahr to your character
receive routine - to set an error flag if the UART shows framing
error, or overrun. This will help catch a few more glitches - the
__________
1. These times should be adjusted for use with timesharing systems.
Chapter 6 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 18
most common of which is a hit in the high bits of the byte in two
consecutive bytes. The <cksum> comes out OK since counting in 1-byte
produces the same result of adding 80H + 80H as with adding 00H +
00H.
7. XMODEM/CRC Overview
1/13/85 by John Byrns -- CRC option.
Please pass on any reports of errors in this document or suggestions for
improvement to me via Ward's/CBBS at (312) 849-1132, or by voice at (312)
885-1105.
The CRC used in the Modem Protocol is an alternate form of block check
which provides more robust error detection than the original checksum.
Andrew S. Tanenbaum says in his book, Computer Networks, that the CRC-
CCITT used by the Modem Protocol will detect all single and double bit
errors, all errors with an odd number of bits, all burst errors of length
16 or less, 99.997% of 17-bit error bursts, and 99.998% of 18-bit and
longer bursts.
The changes to the Modem Protocol to replace the checksum with the CRC are
straight forward. If that were all that we did we would not be able to
communicate between a program using the old checksum protocol and one
using the new CRC protocol. An initial handshake was added to solve this
problem. The handshake allows a receiving program with CRC capability to
determine whether the sending program supports the CRC option, and to
switch it to CRC mode if it does. This handshake is designed so that it
will work properly with programs which implement only the original
protocol. A description of this handshake is presented in section 10.
Figure 9. Message Block Level Protocol, CRC mode
Each block of the transfer in CRC mode looks like:
<SOH><blk #><255-blk #><--128 data bytes--><CRC hi><CRC lo>
in which:
<SOH> = 01 hex
<blk #> = binary number, starts at 01 increments by 1, and
wraps 0FFH to 00H (not to 01)
<255-blk #> = ones complement of blk #.
<CRC hi> = byte containing the 8 hi order coefficients of the CRC.
<CRC lo> = byte containing the 8 lo order coefficients of the CRC.
7.1 CRC Calculation
7.1.1 Formal_Definition
To calculate the 16 bit CRC the message bits are considered to be the
coefficients of a polynomial. This message polynomial is first multiplied
by X^16 and then divided by the generator polynomial (X^16 + X^12 + X^5 +
Chapter 7 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 19
1) using modulo two arithmetic. The remainder left after the division is
the desired CRC. Since a message block in the Modem Protocol is 128 bytes
or 1024 bits, the message polynomial will be of order X^1023. The hi order
bit of the first byte of the message block is the coefficient of X^1023 in
the message polynomial. The lo order bit of the last byte of the message
block is the coefficient of X^0 in the message polynomial.
Figure 10. Example of CRC Calculation written in C
/*
* 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);
}
7.2 CRC File Level Protocol Changes
7.2.1 Common_to_Both_Sender_and_Receiver
The only change to the File Level Protocol for the CRC option is the
initial handshake which is used to determine if both the sending and the
receiving programs support the CRC mode. All Modem Programs should support
the checksum mode for compatibility with older versions. A receiving
program that wishes to receive in CRC mode implements the mode setting
handshake by sending a <C> in place of the initial <nak>. If the sending
program supports CRC mode it will recognize the <C> and will set itself
into CRC mode, and respond by sending the first block as if a <nak> had
been received. If the sending program does not support CRC mode it will
not respond to the <C> at all. After the receiver has sent the <C> it will
wait up to 3 seconds for the <soh> that starts the first block. If it
receives a <soh> within 3 seconds it will assume the sender supports CRC
mode and will proceed with the file exchange in CRC mode. If no <soh> is
received within 3 seconds the receiver will switch to checksum mode, send
Chapter 7 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 20
a <nak>, and proceed in checksum mode. If the receiver wishes to use
checksum mode it should send an initial <nak> and the sending program
should respond to the <nak> as defined in the original Modem Protocol.
After the mode has been set by the initial <C> or <nak> the protocol
follows the original Modem Protocol and is identical whether the checksum
or CRC is being used.
7.2.2 Receive_Program_Considerations
There are at least 4 things that can go wrong with the mode setting
handshake.
1. the initial <C> can be garbled or lost.
2. the initial <soh> can be garbled.
3. the initial <C> can be changed to a <nak>.
4. the initial <nak> from a receiver which wants to receive in checksum
can be changed to a <C>.
The first problem can be solved if the receiver sends a second <C> after
it times out the first time. This process can be repeated several times.
It must not be repeated too many times before sending a <nak> and
switching to checksum mode or a sending program without CRC support may
time out and abort. Repeating the <C> will also fix the second problem if
the sending program cooperates by responding as if a <nak> were received
instead of ignoring the extra <C>.
It is possible to fix problems 3 and 4 but probably not worth the trouble
since they will occur very infrequently. They could be fixed by switching
modes in either the sending or the receiving program after a large number
of successive <nak>s. This solution would risk other problems however.
7.2.3 Sending_Program_Considerations
The sending program should start in the checksum mode. This will insure
compatibility with checksum only receiving programs. Anytime a <C> is
received before the first <nak> or <ack> the sending program should set
itself into CRC mode and respond as if a <nak> were received. The sender
should respond to additional <C>s as if they were <nak>s until the first
<ack> is received. This will assist the receiving program in determining
the correct mode when the <soh> is lost or garbled. After the first <ack>
is received the sending program should ignore <C>s.
Chapter 7 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 21
7.3 Data Flow Examples with CRC Option
Here is a data flow example for the case where the receiver requests
transmission in the CRC mode but the sender does not support the CRC
option. This example also includes various transmission errors. <xx>
represents the checksum byte.
Figure 11. Data Flow: Receiver has CRC Option, Sender Doesn't
SENDER RECEIVER
<--- <C>
times out after 3 seconds,
<--- <C>
times out after 3 seconds,
<--- <C>
times out after 3 seconds,
<--- <C>
times out after 3 seconds,
<--- <nak>
<soh> 01 FE -data- <xx> --->
<--- <ack>
<soh> 02 FD -data- <xx> ---> (data gets line hit)
<--- <nak>
<soh> 02 FD -data- <xx> --->
<--- <ack>
<soh> 03 FC -data- <xx> --->
(ack gets garbaged) <--- <ack>
times out after 10 seconds,
<--- <nak>
<soh> 03 FC -data- <xx> --->
<--- <ack>
<eot> --->
<--- <ack>
Here is a data flow example for the case where the receiver requests
transmission in the CRC mode and the sender supports the CRC option. This
example also includes various transmission errors. <xxxx> represents the
2 CRC bytes.
Chapter 7 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 22
Figure 12. Receiver and Sender Both have CRC Option
SENDER RECEIVER
<--- <C>
<soh> 01 FE -data- <xxxx> --->
<--- <ack>
<soh> 02 FD -data- <xxxx> ---> (data gets line hit)
<--- <nak>
<soh> 02 FD -data- <xxxx> --->
<--- <ack>
<soh> 03 FC -data- <xxxx> --->
(ack gets garbaged) <--- <ack>
times out after 10 seconds,
<--- <nak>
<soh> 03 FC -data- <xxxx> --->
<--- <ack>
<eot> --->
<--- <ack>
8. MORE INFORMATION
More information may be obtained by calling Telegodzilla at 503-621-3746.
Hit RETURNs for baud rate detection.
A version this file with boldface, underlining, and superscripts for
printing on Epson or Gemini printers is available on Telegodzilla as
"YMODEME.DOC" or "YMODEME.DQC".
UUCP sites can obtain this file with
uucp omen!/usr/spool/uucppublic/ymodem.doc /tmp
The following L.sys line calls Telegodzilla (Pro-YAM in host operation).
Telegodzilla waits for carriage returns to determine the incoming speed.
If none is detected, 1200 bps is assumed and a greeting is displayed.
In response to "Name Please:" uucico gives the Pro-YAM "link" command as a
user name. The password (Giznoid) controls access to the Xenix system
connected to the IBM PC's other serial port. Communications between
Pro-YAM and Xenix use 9600 bps; YAM converts this to the caller's speed.
Finally, the calling uucico logs in as uucp.
omen Any ACU 1200 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp
Contact omen!caf if you wish the troff sources.
Chapter 9 Xmodem Protocol Overview
X/YMODEM Protocol Reference 10-10-85 23
9. YMODEM Programs
A demonstration version of Professional-YAM is available as YAMDEMO.LQR (A
SQueezed Novosielski library). This may be used to test YMODEM
implementations.
Unix programs supporting the YMODEM protocol are available on Telegodzilla
in the "upgrade" subdirectory as RBSB.SHQ (a SQueezed shell archive).
Most Unix like systems are supported, including V7, Sys III, 4.2 BSD, SYS
V, Idris, Coherent, and Regulus.
A version for VAX-VMS is available in VRBSB.SHQ, in the same directory.
A CP/M assembly version is available as MODEM76.AQM and MODEM7.LIB.
Irv Hoff has added YMODEM 1k packets and YMODEM batch transfers to the KMD
and IMP series programs, which replace the XMODEM and MODEM7/MDM7xx series
respectively. Overlays are available for a wide variety of CP/M systems.
MEX and MEX-PC also support some of the YMODEM extensions.
Questions about YMODEM, the Professional-YAM communications program, and
requests for evaluation copies may be directed to:
Chuck Forsberg
Omen Technology Inc
17505-V Sauvie Island Road
Portland Oregon 97231
Voice: 503-621-3406
Modem: 503-621-3746 Speed: 1200,300
Usenet: ...!tektronix!reed!omen!caf
Compuserve: 70715,131
Source: TCE022
Chapter 9 Xmodem Protocol Overview
CONTENTS
1. ROSETTA STONE..................................................... 2
2. YET ANOTHER PROTOCOL?............................................. 2
2.1 Some Messages from the Pioneer............................... 4
3. XMODEM PROTOCOL ENHANCEMENTS...................................... 5
3.1 Graceful Abort............................................... 6
3.2 CRC-16 Option................................................ 6
3.3 1024 Byte Packet Option...................................... 7
4. YMODEM Batch File Transmission.................................... 8
4.1 IMP/KMD Record Count......................................... 13
5. g Option File Transmission........................................ 13
6. XMODEM PROTOCOL OVERVIEW.......................................... 14
6.1 Definitions.................................................. 14
6.2 Transmission Medium Level Protocol........................... 14
6.3 File Level Protocol.......................................... 15
6.4 Programming Tips............................................. 17
7. XMODEM/CRC Overview............................................... 18
7.1 CRC Calculation.............................................. 18
7.2 CRC File Level Protocol Changes.............................. 19
7.3 Data Flow Examples with CRC Option........................... 21
8. MORE INFORMATION.................................................. 22
9. YMODEM Programs................................................... 23
- i -
LIST OF FIGURES
Figure 1. 1024 byte Packets......................................... 7
Figure 2. Mixed 1024 and 128 byte Packets........................... 7
Figure 3. Batch Transmission Session................................ 11
Figure 4. Filename packet transmitted by sb......................... 11
Figure 5. Header Information used by YMODEM Implementations......... 13
Figure 6. g Option Transmission Session............................. 14
Figure 7. XMODEM Message Block Level Protocol....................... 15
Figure 8. Data flow including Error Recovery........................ 17
Figure 9. Message Block Level Protocol, CRC mode.................... 18
Figure 10. Example of CRC Calculation written in C................... 19
Figure 11. Data Flow: Receiver has CRC Option, Sender Doesn't........ 21
Figure 12. Receiver and Sender Both have CRC Option.................. 22
- ii -