The Hacker's Encyclopedia 1998

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81 By popular request, I am posting this document describing the XMODEM protocol, plus the XMODEM/CRC and YMODEM (Batch transmission, etc.) protocol extensions. Ward Christensen's article on this subject will appear late this year in Byte Magazine. .pa 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 CONTENTS 1. ROSETTA STONE..................................................... 2 2. TOWER OF BABEL ??................................................. 3 2.1 Some Messages from the Pioneer............................... 3 3. XMODEM PROTOCOL ENHANCEMENTS...................................... 6 3.1 CAN-CAN Abort................................................ 6 3.2 CRC-16 Option................................................ 6 3.3 1024 Byte Packet Option...................................... 7 4. YMODEM Batch File Transmission.................................... 8 6. XMODEM PROTOCOL OVERVIEW.......................................... 13 6.1 Definitions.................................................. 13 6.2 Transmission Medium Level Protocol........................... 13 6.3 File Level Protocol.......................................... 14 6.4 Programming Tips............................................. 15 7. XMODEM/CRC Overview............................................... 17 7.1 CRC Calculation.............................................. 17 7.2 CRC File Level Protocol Changes.............................. 18 7.3 Data Flow Examples with CRC Option........................... 19 8. MORE INFORMATION.................................................. 21 9. YMODEM Programs................................................... 22 - 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......... 11 Figure 6. XMODEM Message Block Level Protocol....................... 14 Figure 7. Data flow including Error Recovery........................ 15 Figure 8. Message Block Level Protocol, CRC mode.................... 17 Figure 9. Example of CRC Calculation written in C................... 18 Figure 10. Data Flow: Receiver has CRC Option, Sender Doesn't........ 19 Figure 11. Receiver and Sender Both have CRC Option.................. 20 - ii - X/YMODEM Protocol Reference 5-30-85 2 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 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 CRC-16, giving modern error detection protection. YMODEM refers to the XMODEM/CRC protocol with the throughput and batch transmission enhancements described below. - 2 - X/YMODEM Protocol Reference 5-30-85 3 2. TOWER OF BABEL ?? In the interest of fostering compatibility among communications programs, part of of the Professional-YAM manual is reproduced here to minimize the Electronic Tower of Babel. The YMODEM Protocol is supported by the public domain programs YAM (CP/M), YAM(CP/M-86), YAM(CCPM-86), MODEM76.ASM (CP/M), rb/sb (Unix, VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus) as well as Professional-YAM. These programs have been in use since 1981. The protocols described below are enhancements to Ward Christensen's XMODEM protocol, and are public domain. 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. 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. Watch for an article describing the YMODEM protocol in a more coherent fashion later this year. This article will include some interesting history on the development of microcomputer file transfers. 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 article1 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. __________ 1. Infoworld April 29 p. 16 - 3 - X/YMODEM Protocol Reference 5-30-85 4 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 Unix 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, 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 weird2 - 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 __________ 2. The Unix style stuff (time, file mode) is optional. The pathname and file length may be sent alone if desired. - 4 - .pa X/YMODEM Protocol Reference 5-30-85 5 "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. - 5 - .pa X/YMODEM Protocol Reference 5-30-85 6 3. XMODEM PROTOCOL ENHANCEMENTS Professional-YAM uses several compatible extensions and logic enhancements to the widely used Ward Christensen XMODEM protocol. This chapter discusses the protocol extensions to Ward Christensen's 1982 XMODEM protocol description document. YAM does not ask the operator whether he wishes to keep retrying after 10 attempts have failed to correctly transfer a packet. Virtualeectable errors are corrected within th few retransmissions. If the line is so bad that ten attempts are insufficient, there is a significant danger of undetected errors. In that case, it's better to redial for a better connection. 3.1 CAN-CAN Abort YAM recognizes 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 a XMODEM 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 __________ 1. This is recognized when YAM is waiting for the beginning of a packet or for an acknowledge to one that has been sent. - 6 - X/YMODEM Protocol Reference 5-30-85 7 the MODEM7 series programs. 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. 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 If YAM is sending with the k option, the transmitted packet contains 1024 bytes of data. An STX (02) replaces the SOH (01) at the beginning of the transmitted block to notify the receiver of the longer packet length. 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.2 If 1024 byte packets are being used, it is possible for a 128 byte file to "grow" to 1024 bytes on CP/M. This does not waste disk space on CP/M because the allocation granularity is 1k. When 1024 byte packets are used with batch transmission, the file length transmitted in the file name packet allows the receiver to discard the padding. CRC-16 should be used with the k option to preserve data integrity over phone lines. The k option may be used with batch file transmission, or with single file transmission. __________ 2. The displayed sector number should be the number of bytes divided by 128. This maintains a familiar unit of measurement for the progress report and avoids ambiguity caused by rollover of the sector number. - 7 - X/YMODEM Protocol Reference 5-30-85 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 YAM 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. - 8 - X/YMODEM Protocol Reference 5-30-85 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. 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 not sent. Filename Considerations: o + 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. o + The receiver should accommodate file names in lower and upper case. o + 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. o + When transmitting files between different operating systems, file names must be acceptable to both the sender and receiving operating systems. If directories are included, they are delimited by /; i.e., __________ 2. Only the data part of the packet is described here. - 9 - X/YMODEM Protocol Reference 5-30-85 10 "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. The file length does not include any CPMEOF (^Z) characters the communications program might use 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. Mod 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 happened to write 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. YAM gets around this problem by using the z parameter which is originate from a different timezone, the -zT option may be used use T as the timezone for an individual file transfer. 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 __________ 3. Fields may not be skipped. 4. Not necessarily that of the transmitting system! - 10 - X/YMODEM Protocol Reference 5-30-85 11 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.QQQ (rbsb.sh) should answer questions about YAM's 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. - 11 - X/YMODEM Protocol Reference 5-30-85 12 Figure 3. Batch Transmission Session SENDER RECEIVER "sb foo.*" "sending in batch mode etc." C 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 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 Figure 5. Header Information used by YMODEM Implementations Program Name Length Date Mode S/N 1k Blocks Unix rb/sb yes yes yes yes no yes VMS rb/sb yes yes no no no yes Pro-YAM yes yes yes no yes yes cp/M YAM yes no no no no yes MODEM76 yes no no no no no - 12 - X/YMODEM Protocol Reference 5-30-85 13 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 smply 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. 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: o + ASCII tabs used (09H); tabs set every 8. o + Lines terminated by CR/LF (0DH 0AH) o + End-of-file indicated by ^Z, 1AH. (one or more) o + 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. o + 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. - 13 - X/YMODEM Protocol Reference 5-30-85 14 o + The last block sent is no different from others, i.e. there is no "short block". Figure 6. 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 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. 6.3.2 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 - 14 - X/YMODEM Protocol Reference 5-30-85 15 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 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. Figure 7. 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> ---> <--- <ack> 6.4 Programming Tips o + 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. - 15 - X/YMODEM Protocol Reference 5-30-85 16 o + 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. o + 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 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. __________ 1. These times should be adjusted for use with timesharing systems. - 16 - X/YMODEM Protocol Reference 5-30-85 17 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 8. 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. See the next section for CRC calculation. 7.1 CRC Calculation 7.1.1 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 + 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 - 17 - X/YMODEM Protocol Reference 5-30-85 18 block is the coefficient of X^0 in the message polynomial. Figure 9. 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 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 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. - 18 - X/YMODEM Protocol Reference 5-30-85 19 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 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 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 e