1 This document is meant to printed on a 66 line/page printer.

2 IÕve included the first page so that youÕll be able to test print it

3 out to make sure the rest of the document comes out currectly. You can

4 then cut the first page out where indicated below. If youÕre uploading or

5 spreading this document around, please include this first page. IÕve gone

6 through unbelivable pains in order to make this thing print out right.

7 For those Mac owners whoÕve gotten this file instead of the MacWriteII file,

8 use the follwing info to make your task alot simpler.

9 page setup => options / larger print area

10 font = Courier 10

11 line spacing = 0.158Ó (Format/Paragraph menuitem)

12 top margin = bottom margin = 0.25Ó (Format/Page menuitem)

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66  ---- THIS IS THE LAST LINE OF THE FIRST PAGE ---




                                      - 1 -




                         XMODEM/YMODEM PROTOCOL REFERENCE

                     A compendium of documents describing the


                                XMODEM and YMODEM


                             File Transfer Protocols





                      This document was formatted 10-27-87.








                             Edited by Chuck Forsberg













                     Please distribute as widely as possible.


                           Questions to Chuck Forsberg






                               Omen Technology Inc

                          The High Reliability Software

                            17505-V Sauvie Island Road

                              Portland Oregon 97231

                            VOICE: 503-621-3406 :VOICE

    Modem (TeleGodzilla): 503-621-3746 Speed 19200(Telebit PEP),2400,1200,300

                              CompuServe: 70007,2304

                                    GEnie: CAF

                        UUCP: É!tektronix!reed!omen!caf















                                      - 2 -




    1.  TOWER OF BABEL


    A "YMODEM Tower of Babel" has descended on the microcomputing community

    bringing with it confusion, frustration, bloated phone bills, and wasted

    man hours.  Sadly, I (Chuck Forsberg) am partly to blame for this mess.


    As author of the early 1980s batch and 1k XMODEM extensions, I assumed

    readers of earlier versions of this document would implement as much of

    the YMODEM protocol as their programming skills and computing environments

    would permit.  This proved a rather naive assumption as programmers

    motivated by competitive pressure implemented as little of YMODEM as

    possible.  Some have taken whatever parts of YMODEM that appealed to them,

    applied them to MODEM7 Batch, Telink, XMODEM or whatever, and called the

    result YMODEM.


    Jeff Garbers (Crosstalk package development director) said it all: "With

    protocols in the public domain, anyone who wants to dink around with them

    can go ahead." [1]


    Documents containing altered examples derived from YMODEM.DOC have added

    to the confusion.  In one instance, the heading in YMODEM.DOC's Figure 1

    has mutated from "1024 byte Packets" to "YMODEM/CRC File Transfer

    Protocol".  None of the XMODEM and YMODEM examples shown in that document

    were correct.


    To put an end to this confusion, we must make "perfectly clear" what

    YMODEM stands for, as Ward Christensen defined it in his 1985 coining of

    the term.


    To the majority of you who read, understood, and respected Ward's

    definition of YMODEM, I apologize for the inconvenience.


    1.1  Definitions


    ARC     ARC is a program that compresses one or more files into an archive

            and extracts files from such archives.


    XMODEM  refers to the file transfer etiquette introduced by Ward

            Christensen's 1977 MODEM.ASM program.  The name XMODEM comes from

            Keith Petersen's XMODEM.ASM program, an adaptation of MODEM.ASM

            for Remote CP/M (RCPM) systems.  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'

            Group" and "TERM II FTP 3".  The name XMODEM caught on partly

            because it is distinctive and partly because of media interest in



    __________


     1. Page C/12, PC-WEEK July 12, 1987





    Chapter 1








    X/YMODEM Protocol Reference      10-27-87                                3




            bulletin board and RCPM systems where it was accessed with an

            "XMODEM" command.  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

            Redundancy Check (CRC-16), giving modern error detection

            protection.


    XMODEM-1k Refers to the XMODEM/CRC protocol with 1024 byte data blocks.


    YMODEM  Refers to the XMODEM/CRC (optional 1k blocks) protocol with batch

            transmission as described below.  In a nutshell, YMODEM means

            BATCH.


    YMODEM-g Refers to the streaming YMODEM variation described below.


    True YMODEM(TM) In an attempt to sort out the YMODEM Tower of Babel, Omen

            Technology has trademarked the term True YMODEM(TM) to represent

            the complete YMODEM protocol described in this document, including

            pathname, length, and modification date transmitted in block 0.

            Please contact Omen Technology about certifying programs for True

            YMODEM(TM) compliance.


    ZMODEM  uses familiar XMODEM/CRC and YMODEM technology in a new protocol

            that provides reliability, throughput, file management, and user

            amenities appropriate to contemporary data communications.


    ZOO     Like ARC, ZOO is a program that compresses one or more files into

            a "zoo archive".  ZOO supports many different operating systems

            including Unix and VMS.
























    Chapter 1








    X/YMODEM Protocol Reference      10-27-87                                4




    2.  YMODEM MINIMUM REQUIREMENTS


    All programs claiming to support YMODEM must meet the following minimum

    requirements:


       + The sending program shall send the pathname (file name) in block 0.


       + The pathname shall be a null terminated ASCII string as described

         below.


       + The receiving program shall use this pathname for the received file

         name, unless explicitly overridden.


       + The sending program shall use CRC-16 in response to a "C" pathname

         nak, otherwise use 8 bit checksum.


       + The receiving program must accept any mixture of 128 and 1024 byte

         blocks within each file it receives.  Sending programs may switch

         between 1024 and 128 byte blocks at the end of file(s), and when the

         frequency of retransmissions so suggests.


       + The sending program must not change the length of an unacknowledged

         block.


       + At the end of each file, the sending program shall send EOT up to ten

         times until it receives an ACK character.  (This is part of the

         XMODEM spec.)


       + The end of a transfer session shall be signified by a null (empty)

         pathname.


    Programs not meeting all of these requirements are not YMODEM compatible,

    and shall not be described as supporting YMODEM.


    Meeting these MINIMUM requirements does not guarantee reliable file

    transfers under stress.  Particular attention is called to XMODEM's single

    character supervisory messages that are easily corrupted by transmission

    errors.

















    Chapter 2








    X/YMODEM Protocol Reference      10-27-87                                5




    3.  WHY YMODEM?


    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.


    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 arithmetic 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:


       + Lack of public domain documentation and example programs have kept

         proprietary protocols such as Blast, Relay, 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 complexity have limited the popularity of

         the Kermit protocol, which was developed to allow file transfers in

         environments hostile to XMODEM.


    The XMODEM protocol extensions and YMODEM Batch address some of these

    weaknesses while maintaining most of XMODEM's simplicity.


    YMODEM is supported by the public domain programs YAM (CP/M),

    YAM(CP/M-86), YAM(CCPM-86), IMP (CP/M), KMD (CP/M), rz/sz (Unix, Xenix,

    VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus).  Commercial

    implementations include MIRROR, and Professional-YAM.[1] Communications








    Chapter 3








    X/YMODEM Protocol Reference      10-27-87                                6




    programs supporting these extensions have been in use since 1981.


    The 1k block length (XMODEM-1k) described below may be used in conjunction

    with YMODEM Batch Protocol, or with single file transfers identical to the

    XMODEM/CRC protocol except for minimal changes to support 1k blocks.


    Another extension is the YMODEM-g protocol.  YMODEM-g provides batch

    transfers with maximum throughput when used with end to end error

    correcting media, such as X.PC and error correcting modems, including 9600

    bps units by TeleBit, U.S.Robotics, Hayes, Electronic Vaults, Data Race,

    and others.


    To complete this tome, edited versions of 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 Protocol.



    3.1  Some Messages from the Pioneer


    #: 130940 S0/Communications 25-Apr-85  18:38:47

    Sb: my protocol

    Fm: Ward Christensen 76703,302 [2]

    To: all


    Be aware the article[3] 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. Available for IBM PC,XT,AT, Unix and Xenix


     2. Edited for typesetting appearance


     3. Infoworld April 29 p. 16





    Chapter 3








    X/YMODEM Protocol Reference      10-27-87                                7




    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[4] 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.  É


    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 weird[5] - 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



    __________


     4. VAX/VMS versions of these programs are also available.


     5. The file length, time, and file mode are optional.  The pathname and

        file length may be sent alone if desired.





    Chapter 3








    X/YMODEM Protocol Reference      10-27-87                                8




    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.












































    Chapter 3








    X/YMODEM Protocol Reference      10-27-87                                9




    4.  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 whether 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.



    4.1  Graceful Abort


    The YAM and Professional-YAM X/YMODEM routines recognize a sequence of two

    consecutive CAN (Hex 18) characters without modem errors (overrun,

    framing, etc.) as a transfer abort command.  This sequence is recognized

    when is waiting for the beginning of a block or for an acknowledgement to

    a block that has been sent.  The check for two consecutive CAN characters

    reduces the number of transfers aborted by line hits.  YAM sends eight CAN

    characters when it aborts an XMODEM, YMODEM, or ZMODEM protocol file

    transfer.  Pro-YAM then sends eight backspaces to delete the CAN

    characters from the remote's keyboard input buffer, in case the remote had

    already aborted the transfer and was awaiting a keyboarded command.



    4.2  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 block.  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




    Chapter 4                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               10




    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

    time.  This avoids buffer overrun problems that have been reported by

    users attempting to exploit full duplex asynchronous 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.



    4.3  XMODEM-1k 1024 Byte Block


    Disappointing throughput downloading from Unix with YMODEM[1] lead to the

    development of 1024 byte blocks in 1982.  1024 byte blocks reduce the

    effect of delays from timesharing systems, modems, and packet switched

    networks on throughput by 87.5 per cent in addition to decreasing XMODEM's

    per byte overhead 3 per cent on long files.


    The choice to use 1024 byte blocks is expressed to the sending program on

    its command line or selection menu.[2] 1024 byte blocks improve throughput

    in many applications, but some environments cannot accept 1024 byte

    bursts, especially minicomputers running 19.2kb ports.


    An STX (02) replaces the SOH (01) at the beginning of the transmitted

    block to notify the receiver of the longer block length.  The transmitted

    block contains 1024 bytes of data.  The receiver should be able to accept

    any mixture of 128 and 1024 byte blocks.  The block number (in the second

    and third bytes of the block) is incremented by one for each block

    regardless of the block length.


    The sender must not change between 128 and 1024 byte block lengths if it

    has not received a valid ACK for the current block.  Failure to observe

    this restriction allows transmission errors to pass undetected.




    __________


     1. The name hadn't been coined yet, but the protocol was the same.


     2. See "KMD/IMP Exceptions to YMODEM" below.





    Chapter 4                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               11




    If 1024 byte blocks 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.  With YMODEM batch transmission,

    the optional file length transmitted in the file name block allows the

    receiver to discard the padding, preserving the exact file length and

    contents.


    1024 byte blocks may be used with batch file transmission or with single

    file transmission.  CRC-16 should be used with the k option to preserve

    data integrity over phone lines.  If a program wishes to enforce this

    recommendation, it should cancel the transfer, then issue an informative

    diagnostic message if the receiver requests checksum instead of CRC-16.


    Under no circumstances may a sending program use CRC-16 unless the

    receiver commands CRC-16.


              Figure 1.  XMODEM-1k Blocks


              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 Blocks


              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








    Chapter 4                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               12




    5.  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 blocks 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

    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 block 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 block 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:




    __________


     1. The MODEM7 batch protocol transmitted CP/M FCB bytes f1Éf8 and

        t1Ét3 one character at a time.  The receiver echoed these bytes as

        received, one at a time.


     2. Only the data part of the block is described here.





    Chapter 5                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               13




            + File names are forced 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.


            + 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.,

         "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 block as a decimal string counting

         the number of data bytes in the file.  The file length does not

         include any CPMEOF (^Z) or other garbage characters used to pad the

         last block.


         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 block.


    Modification Date The mod date is optional, and the filename and length

         may be sent without requiring the mod date to be sent.


         Iff the modification date is sent, a single space separates the

         modification date from the file length.


         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.






    __________


     3. Fields may not be skipped.





    Chapter 5                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               14




    Mode Iff the file mode is sent, 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 Iff the serial number is sent, 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.



    Other Fields YMODEM was designed to allow additional header fields to be

         added as above without creating compatibility problems with older

         YMODEM programs.  Please contact Omen Technology if other fields are

         needed for special application requirements.


    The rest of the block is set to nulls.  This is essential to preserve

    upward compatibility.[4]


    If the filename block is received with a CRC or other error, a

    retransmission is requested.  After the filename block has been received,

    it is ACK'ed if the write open is successful.  If the file cannot be

    opened for writing, the receiver cancels the transfer with CAN characters

    as described above.


    The receiver then initiates transfer of the file contents according to the

    standard XMODEM/CRC protocol.


    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.




    __________


     4. If, perchance, this information extends beyond 128 bytes (possible

        with Unix 4.2 BSD extended file names), the block should be sent as a

        1k block as described above.





    Chapter 5                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               15




    The YMODEM receiver requests CRC-16 by default.


    The Unix programs sz(1) and rz(1) included in the source code file

    RZSZ.ZOO should answer other questions about YMODEM batch protocol.


              Figure 3.  YMODEM 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 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.  YMODEM Batch Transmission Session-1k Blocks


            SENDER                                  RECEIVER

                                                    "sb -k foo.*<CR>"

            "sending in batch mode etc."

                                                    C (command:rb)

            SOH 00 FF foo.c NUL[123] CRC CRC

                                                    ACK

                                                    C

            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

                                                    NAK

            EOT

                                                    ACK

                                                    C

            SOH 00 FF NUL[128] CRC CRC

                                                    ACK





    Chapter 5                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               16




           Figure 5.  YMODEM Filename block transmitted by sz


           -rw-rÑrÑ6347 Jun 17 1984 20:34 bbcsched.txt


           00 0100FF62 62637363 6865642E 74787400   |Ébbcsched.txt.|

           10 36333437 20333331 34373432 35313320   |6347 3314742513 |

           20 31303036 34340000 00000000 00000000   |100644ÉÉÉ.|

           30 00000000 00000000 00000000 00000000

           40 00000000 00000000 00000000 00000000

           50 00000000 00000000 00000000 00000000

           60 00000000 00000000 00000000 00000000

           70 00000000 00000000 00000000 00000000

           80 000000CA 56


                Figure 6.  YMODEM Header Information and Features


    _____________________________________________________________

    | Program   | Length | Date | Mode | S/N | 1k-Blk | YMODEM-g |

    |___________|________|______|______|_____|________|__________|

    |Unix rz/sz | yes    | yes  | yes  | no  | yes    | sb only  |

    |___________|________|______|______|_____|________|__________|

    |VMS rb/sb  | yes    | no   | no   | no  | yes    | no       |

    |___________|________|______|______|_____|________|__________|

    |Pro-YAM    | yes    | yes  | no   | yes | yes    | yes      |

    |___________|________|______|______|_____|________|__________|

    |CP/M YAM   | no     | no   | no   | no  | yes    | no       |

    |___________|________|______|______|_____|________|__________|

    |KMD/IMP    | ?      | no   | no   | no  | yes    | no       |

    |___________|________|______|______|_____|________|__________|


    5.1  KMD/IMP Exceptions to YMODEM


    KMD and IMP use a "CK" character sequence emitted by the receiver to

    trigger the use of 1024 byte blocks as an alternative to specifying this

    option to the sending program.  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.


    Sending programs may detect the CK sequence if the operating enviornment

    does not preclude reliable implementation.


    Instead of the standard YMODEM file length, KMD and IMP transmit the CP/M

    record count in the last two bytes of the header block.











    Chapter 6                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               17




    6.  YMODEM-g 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, nearly

    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 block, sending

    succeeding blocks 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

    close files as necessary.


    If an error is detected in a YMODEM-g transfer, the receiver aborts the

    transfer with the multiple CAN abort sequence.  The ZMODEM protocol should

    be used in applications that require both streaming throughput and error

    recovery.


            Figure 7.  YMODEM-g Transmission Session


            SENDER                                  RECEIVER

                                                    "sb foo.*<CR>"

            "sending in batch mode etcÉ"

                                                    G (command:rb -g)

            SOH 00 FF foo.c NUL[123] CRC CRC

                                                    G

            SOH 01 FE Data[128] CRC CRC

            STX 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











    Chapter 6                                     XMODEM Protocol Enhancements








    X/YMODEM Protocol Reference      10-27-87                               18




    7.  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.


    7.1  Definitions


      <soh> 01H

      <eot> 04H

      <ack> 06H

      <nak> 15H

      <can> 18H

      <C>   43H



    7.2  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.


    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.






    Chapter 7                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               19




       + The last block sent is no different from others, i.e.  there is no

         "short block".

                  Figure 8.  XMODEM Message Block Level Protocol


    Each block of the transfer looks like:

          <SOH><blk #><255-blk #><Ñ128 data bytesÑ><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.


    7.3  File Level Protocol


    7.3.1  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 aborting transmission.


    The protocol may be considered "receiver driven", that is, the sender need

    not automatically re-transmit, although it does in the current

    implementations.



    7.3.2  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




    Chapter 7                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               20




    that looked like an <ack>.  Abort the transmission, sending a <can>



    7.3.3  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.


                  Figure 9.  Data flow including Error Recovery


    SENDER                                  RECEIVER

                                  times out after 10 seconds,

                                  <Ñ-              <nak>

    <soh> 01 FE -data- <xx>Ñ->

                                  <Ñ-              <ack>

    <soh> 02 FD -data- xxÑ->       (data gets line hit)

                                  <Ñ-              <nak>

    <soh> 02 FD -data- xxÑ->

                                  <Ñ-              <ack>

    <soh> 03 FC -data- xxÑ->

    (ack gets garbaged)           <Ñ-              <ack>

    <soh> 03 FC -data- xxÑ->              <ack>

    <eot>Ñ->

                                  <Ñ-       <anything except ack>

    <eot>Ñ->

                                  <Ñ-              <ack>

    (finished)


    7.4  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.




    Chapter 7                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               21




       + 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

         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.





    Chapter 7                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               22




    8.  XMODEM/CRC Overview


    Original 1/13/85 by John ByrnsÑCRC 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.[1]


    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 10.  Message Block Level Protocol, CRC mode


    Each block of the transfer in CRC mode looks like:

         <SOH><blk #><255-blk #><Ñ128 data bytesÑ><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.


    8.1  CRC Calculation


    8.1.1  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 +



    __________


     1. This reliability figure is misleading because XMODEM's critical

        supervisory functions are not protected by this CRC.





    Chapter 8                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               23




    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 11.  Example of CRC Calculation written in C


    The following XMODEM crc routine is taken from "rbsb.c".  Please refer to

    the source code for these programs (contained in RZSZ.ZOO) for usage.  A

    fast table driven version is also included in this file.


    /* update CRC */

    unsigned short

    updcrc(c, crc)

    register c;

    register unsigned crc;

    {

            register count;


            for (count=8;Ñcount>=0;) {

                    if (crc & 0x8000) {

                            crc <<= 1;

                            crc += (((c<<=1) & 0400)  !=  0);

                            crc ^= 0x1021;

                    }

                    else {

                            crc <<= 1;

                            crc += (((c<<=1) & 0400)  !=  0);

                    }

            }

            return crc;

    }


    8.2  CRC File Level Protocol Changes


    8.2.1  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




    Chapter 8                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               24




    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.

    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.



    8.2.2  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.



    8.2.3  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 8                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               25




    8.3  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 12.  Data Flow: Receiver has CRC Option, Sender Doesn't


    SENDER                                        RECEIVER

                            <Ñ-                <C>

                                    times out after 3 seconds,

                            <Ñ-                <C>

                                    times out after 3 seconds,

                            <Ñ-                <C>

                                    times out after 3 seconds,

                            <Ñ-                <C>

                                    times out after 3 seconds,

                            <Ñ-                <nak>

    <soh> 01 FE -data- <xx>Ñ->

                            <Ñ-                <ack>

    <soh> 02 FD -data- <xx>Ñ->        (data gets line hit)

                            <Ñ-                <nak>

    <soh> 02 FD -data- <xx>Ñ->

                            <Ñ-                <ack>

    <soh> 03 FC -data- <xx>Ñ->

       (ack gets garbaged)  <Ñ-                <ack>

                                    times out after 10 seconds,

                            <Ñ-                <nak>

    <soh> 03 FC -data- <xx>Ñ->

                            <Ñ-                <ack>

    <eot>Ñ->

                            <Ñ-                <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 8                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               26




               Figure 13.  Receiver and Sender Both have CRC Option


    SENDER                                       RECEIVER

                              <Ñ-                 <C>

    <soh> 01 FE -data- <xxxx>Ñ->

                              <Ñ-                 <ack>

    <soh> 02 FD -data- <xxxx>Ñ->         (data gets line hit)

                              <Ñ-                 <nak>

    <soh> 02 FD -data- <xxxx>Ñ->

                              <Ñ-                 <ack>

    <soh> 03 FC -data- <xxxx>Ñ->

    (ack gets garbaged)       <Ñ-                 <ack>

                                         times out after 10 seconds,

                              <Ñ-                 <nak>

    <soh> 03 FC -data- <xxxx>Ñ->

                              <Ñ-                 <ack>

    <eot>Ñ->

                              <Ñ-                 <ack>





































    Chapter 8                                         Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               27




    9.  MORE INFORMATION


    Please contact Omen Technology for troff source files and typeset copies

    of this document.



    9.1  TeleGodzilla Bulletin Board


    More information may be obtained by calling TeleGodzilla at 503-621-3746.

    Speed detection is automatic for 1200, 2400 and 19200(Telebit PEP) bps.

    TrailBlazer modem users may issue the TeleGodzilla trailblazer command to

    swith to 19200 bps once they have logged in.


    Interesting files include RZSZ.ZOO (C source code), YZMODEM.ZOO (Official

    XMODEM, YMODEM, and ZMODEM protocol descriptions), ZCOMMEXE.ARC,

    ZCOMMDOC.ARC, and ZCOMMHLP.ARC (PC-DOS shareware comm program with XMODEM,

    True YMODEM(TM), ZMODEM, Kermit Sliding Windows, Telink, MODEM7 Batch,

    script language, etc.).



    9.2  Unix UUCP Access


    UUCP sites can obtain the current version of this file with

                     uucp omen!/u/caf/public/ymodem.doc /tmp

    A continually updated list of available files is stored in

    /usr/spool/uucppublic/FILES.  When retrieving these files with uucp,

    remember that the destination directory on your system must be writeable

    by anyone, or the UUCP transfer will fail.


    The following L.sys line calls TeleGodzilla (Pro-YAM in host operation).

    TeleGodzilla determines the incoming speed automatically.


    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 2400 1-503-621-3746 se:Ñse: link ord: Giznoid in:Ñin: uucp




    10.  REVISIONS


    10-27-87 Optional fields added for number of files remaining to be sent

    and total number of bytes remaining to be sent.

    10-18-87 Flow control discussion added to 1024 byte block descritpion,

    minor revisions for clarity per user comments.

    8-03-87 Revised for clarity.

    5-31-1987 emphasizes minimum requirements for YMODEM, and updates




    Chapter 10                                        Xmodem Protocol Overview








    X/YMODEM Protocol Reference      10-27-87                               28




    information on accessing files.

    9-11-1986 clarifies nomenclature and some minor points.

    The April 15 1986 edition clarifies some points concerning CRC

    calculations and spaces in the header.



    11.  YMODEM Programs


    ZCOMM, A shareware little brother to Professional-YAM, is available as

    ZCOMMEXE.ARC on TeleGodzilla and other bulletin board systems.  ZCOMM may

    be used to test YMODEM amd ZMODEM implementations.


    Unix programs supporting YMODEM are available on TeleGodzilla in RZSZ.ZOO.

    This ZOO archive includes a ZCOMM/Pro-YAM/PowerCom script ZUPL.T to upload

    a bootstrap program MINIRB.C, compile it, and then upload the rest of the

    files using the compiled MINIRB.  Most Unix like systems are supported,

    including V7, Xenix, Sys III, 4.2 BSD, SYS V, Idris, Coherent, and

    Regulus.


    A version for VAX-VMS is available in VRBSB.SHQ.


    Irv Hoff has added 1k blocks and basic 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.


    Questions about Professional-YAM communications software may be directed

    to:

         Chuck Forsberg

         Omen Technology Inc

         17505-V Sauvie Island Road

         Portland Oregon 97231

         VOICE: 503-621-3406 :VOICE

         Modem: 503-621-3746 Speed: 19200(Telebit PEP),2400,1200,300

         Usenet: É!tektronix!reed!omen!caf

         CompuServe: 70007,2304

         GEnie: CAF


    Unlike ZMODEM and Kermit, XMODEM and YMODEM place obstacles in the path of

    a reliable high performance implementation, evidenced by poor reliability

    under stress of the industry leaders' XMODEM and YMODEM programs.  Omen

    Technology provides consulting and other services to those wishing to

    implement XMODEM, YMODEM, and ZMODEM with state of the art features and

    reliability.












    Chapter 11                                        Xmodem Protocol Overview












                                     CONTENTS



     1.  TOWER OF BABELÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ   2

         1.1  DefinitionsÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.   2


     2.  YMODEM MINIMUM REQUIREMENTSÉÉÉÉÉÉÉÉÉÉÉÉ..   4


     3.  WHY YMODEM?ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ   5

         3.1  Some Messages from the PioneerÉÉÉÉÉÉÉÉÉÉ   6


     4.  XMODEM PROTOCOL ENHANCEMENTSÉÉÉÉÉÉÉÉÉÉÉÉ.   9

         4.1  Graceful AbortÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.   9

         4.2  CRC-16 OptionÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ..   9

         4.3  XMODEM-1k 1024 Byte BlockÉÉÉÉÉÉÉÉÉÉÉ..  10


     5.  YMODEM Batch File TransmissionÉÉÉÉÉÉÉÉÉÉÉ..  12

         5.1  KMD/IMP Exceptions to YMODEMÉÉÉÉÉÉÉÉÉÉ..  16


     6.  YMODEM-g File TransmissionÉÉÉÉÉÉÉÉÉÉÉÉÉ  17


     7.  XMODEM PROTOCOL OVERVIEWÉÉÉÉÉÉÉÉÉÉÉÉÉ..  18

         7.1  DefinitionsÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.  18

         7.2  Transmission Medium Level ProtocolÉÉÉÉÉÉÉÉ..  18

         7.3  File Level ProtocolÉÉÉÉÉÉÉÉÉÉÉÉÉ..  19

         7.4  Programming TipsÉÉÉÉÉÉÉÉÉÉÉÉÉÉ..  20


     8.  XMODEM/CRC OverviewÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.  22

         8.1  CRC CalculationÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ  22

         8.2  CRC File Level Protocol ChangesÉÉÉÉÉÉÉÉÉ..  23

         8.3  Data Flow Examples with CRC OptionÉÉÉÉÉÉÉÉ..  25


     9.  MORE INFORMATIONÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.  27

         9.1  TeleGodzilla Bulletin BoardÉÉÉÉÉÉÉÉÉÉÉ  27

         9.2  Unix UUCP AccessÉÉÉÉÉÉÉÉÉÉÉÉÉÉ..  27


    10.  REVISIONSÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ..  27


    11.  YMODEM ProgramsÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ..  28
















                                      - i -















                                 LIST OF FIGURES



     Figure 1.  XMODEM-1k BlocksÉÉÉÉÉÉÉÉÉÉÉÉÉÉ  11


     Figure 2.  Mixed 1024 and 128 byte BlocksÉÉÉÉÉÉÉÉÉ.  11


     Figure 3.  YMODEM Batch Transmission SessionÉÉÉÉÉÉÉÉ.  15


     Figure 4.  YMODEM Batch Transmission Session-1k BlocksÉÉÉÉÉ  15


     Figure 5.  YMODEM Filename block transmitted by szÉÉÉÉÉÉ.  16


     Figure 6.  YMODEM Header Information and FeaturesÉÉÉÉÉÉ..  16


     Figure 7.  YMODEM-g Transmission SessionÉÉÉÉÉÉÉÉÉ..  17


     Figure 8.  XMODEM Message Block Level ProtocolÉÉÉÉÉÉÉ..  19


     Figure 9.  Data flow including Error RecoveryÉÉÉÉÉÉÉÉ  20


    Figure 10.  Message Block Level Protocol, CRC modeÉÉÉÉÉÉ..  22


    Figure 11.  Example of CRC Calculation written in CÉÉÉÉÉÉ.  23


    Figure 12.  Data Flow: Receiver has CRC Option, Sender Doesn'tÉÉ..  25


    Figure 13.  Receiver and Sender Both have CRC OptionÉÉÉÉÉÉ  26
























                                      - ii -




