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	 A compendium of documents describing the
		 File Transfer Protocols
		 Edited	by Chuck Forsberg
	 Please	distribute as widely as	possible.
	       Questions to Chuck Forsberg
		   Omen	Technology Inc
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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

YMODEM refers to the XMODEM/CRC protocol with the throughput and/or batch

transmission enhancements described below.


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
 + 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 © 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.


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" " 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" " 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      '',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
+ 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

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 ( 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
					"sb foo.*<CR>"
"sending in batch mode etc..."
					G (command:rb -g)
SOH 00 FF foo.c	NUL[123] CRC CRC
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


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


		      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>


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 +

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;
	   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


		<---		    <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


		  <---		       <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>


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


1. ROSETTA STONE…………………………………………….. 2

2. YET ANOTHER PROTOCOL?……………………………………… 2

  2.1	 Some Messages from the	Pioneer...............................	 4


  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 -

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 -
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