ymodem.doc

XMODEM传输协议。 通信用代码。 C语言实现

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                         XMODEM/YMODEM PROTOCOL REFERENCE
                     A compendium of documents describing the

                                XMODEM and YMODEM

                             File Transfer Protocols




                       This document was formatted 8-4-87.







                             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
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    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 one 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      08-03-87                                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.

    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      08-03-87                                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 it receives.

       + 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.

    Meeting the minimum requirements does not guarantee reliable file
    transfers under stress.





















    Chapter 2







    X/YMODEM Protocol Reference      08-03-87                                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.

    Recent advances in computing, modems and networking have revealed a number
    of weaknesses in the original protocol:

       + The short block length caused throughput to suffer when used with
         timesharing systems, packet switched networks, satellite circuits,
         and buffered (error correcting) modems.

       + The 8 bit 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      08-03-87                                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 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.

    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


    __________________________________________________________________________

     1. Available for IBM PC,XT,AT, Unix and Xenix

     2. Edited for typesetting appearance

     3. Infoworld April 29 p. 16




    Chapter 3