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    X/YMODEM Protocol Reference      08-03-87                               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 #><--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.

    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







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    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,
                                  <---              <nak>
    <soh> 01 FE -data- <xx>       --->
                                  <---              <ack>
    <soh> 02 FD -data- xx         --->       (data gets line hit)
                                  <---              <nak>
    <soh> 02 FD -data- xx         --->
                                  <---              <ack>
    <soh> 03 FC -data- xx         --->
    (ack gets garbaged)           <---              <ack>
    <soh> 03 FC -data- xx         --->              <ack>
    <eot>                         --->
                                  <---       <anything except ack>
    <eot>                         --->
                                  <---              <ack>
    (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      08-03-87                               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







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    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.

                Figure 10.  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.

    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







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    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; --count>=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



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    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      08-03-87                               25