xymodem.htm

X / Y Modem 协议 XMODEM/YMODEM PROTOCOL REFERENCE

    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



    Chapter 6                                     XMODEM Protocol Enhancements







    X/YMODEM Protocol Reference    June 18 1988                             19



    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 8.  YMODEM-g Transmission Session

            SENDER                                  RECEIVER
                                                    "sb foo.*<cr>"
            "sending in batch mode etc..."
                                                    G (command:rb -g)
            SOH 00 FF foo.c NUL[123] CRC CRC
                                                    G
            SOH 01 FE Data[128] CRC CRC
            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    June 18 1988                             20



    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 [phone], CBBS/CPMUG [phone]
    or by voice at [phone].

    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    June 18 1988                             21



       + The last block sent is no different from others, i.e.  there is no
         "short block".
                  Figure 9.  XMODEM Message Block Level Protocol

    Each block of the transfer looks like:
          <soh><blk #="">&lt;255-blk #&gt;&lt;--128 data bytes--&gt;<cksum>
    in which:
    <soh>         = 01 hex
    <blk #="">       = binary number, starts at 01 increments by 1, and
                    wraps 0FFH to 00H (not to 01)
    &lt;255-blk #&gt;   = 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    June 18 1988                             22



    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 10.  Data flow including Error Recovery

    SENDER                                  RECEIVER
                                  times out after 10 seconds,
                                  &lt;---              <nak>
    <soh> 01 FE -data- <xx>       ---&gt;
                                  &lt;---              <ack>
    <soh> 02 FD -data- xx         ---&gt;       (data gets line hit)
                                  &lt;---              <nak>
    <soh> 02 FD -data- xx         ---&gt;
                                  &lt;---              <ack>
    <soh> 03 FC -data- xx         ---&gt;
    (ack gets garbaged)           &lt;---              <ack>
    <soh> 03 FC -data- xx         ---&gt;              <ack>
    <eot>                         ---&gt;
                                  &lt;---       <anything except="" ack="">
    <eot>                         ---&gt;
                                  &lt;---              <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    June 18 1988                             23



       + 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    June 18 1988                             24



    8.  XMODEM/CRC Overview

    Original 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 [phone], 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.