High Voltage Shareware/high1.zip/high1/DIR10/WXYZMODM.ZIP/ZMODEM.DOC

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       The ZMODEM Inter Application    File Transfer Protocol

                  Chuck Forsberg

               Omen    Technology Inc


      A overview of    this document is available as ZMODEM.OV
                 (in ZMDMOV.ARC)
















               Omen Technology Incorporated
              The High Reliability Software

           17505-V Northwest Sauvie Island Road
              Portland Oregon 97231
            VOICE: 503-621-3406 :VOICE
        Modem: 503-621-3746 Speed 1200,2400,19200
             Compuserve:70007,2304  GEnie:CAF
            UUCP: ...!tektronix!reed!omen!caf
























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Chapter    0             ZMODEM Protocol                 2



1.  INTENDED AUDIENCE

This document is intended for telecommunications managers, systems
programmers, and others    who choose and implement asynchronous file
transfer protocols over    dial-up    networks and related environments.


2.  WHY    DEVELOP    ZMODEM?

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,    now called XMODEM.

Advances in computing, modems and networking have spread the XMODEM
protocol far beyond the    micro to micro environment for which it    was
designed.  These application have exposed some weaknesses:

   + The awkward user interface    is suitable for    computer hobbyists.
     Multiple commands must be keyboarded to transfer each file.

   + Since commands must be given to both programs, simple menu    selections
     are not possible.

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

   + The 8 bit checksum    and unprotected    supervison allow undetected errors
     and disrupted file    transfers.

   + Only one file can be sent per command.  The file name has to be given
     twice, first to the sending program and then again    to the receiving
     program.

   + The transmitted file accumulates as many as 127 bytes of garbage.

   + The modification date and other file attributes are lost.

   + XMODEM requires complete 8    bit transparency, all 256 codes.  XMODEM
     will not operate over some    networks that use ASCII    flow control or
     escape codes.  Setting network transparency disables important
     control functions for the duration    of the call.

A number of other protocols have been developed    over the years,    but none
have proven satisfactory.

   + Lack of public domain documentation and example programs have kept
     proprietary protocols such    as Relay, Blast, and others tightly bound
     to    the fortunes of    their suppliers.  These    protocols have not
     benefited from public scrutiny of their design features.



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   + Link level    protocols such as X.25,    X.PC, and MNP do not manage
     application to application    file transfers.

   + Link Level    protocols do not eliminate end-to-end errors.  Interfaces
     between error-free    networks are not necessarily error-free.
     Sometimes,    error-free networks aren't.

   + The Kermit    protocol was developed to allow    file transfers in
     environments hostile to XMODEM.  The performance compromises
     necessary to accommodate traditional mainframe environments limit
     Kermit's efficiency.  Even    with completely    transparent channels,
     Kermit control character quoting limits the efficiency of binary file
     transfers to about    75 per cent.[1]

     A number of submodes are used in various Kermit programs, including
     different methods of transferring binary files.  Two Kermit programs
     will mysteriously fail to operate with each other if the user has not
     correctly specified these submodes.

     Kermit Sliding Windows ("SuperKermit") improves throughput    over
     networks at the cost of increased complexity.  SuperKermit    requires
     full duplex communications    and the    ability    to check for the presence
     of    characters in the input    queue, precluding its implementation on
     some operating systems.

     SuperKermit state transitions are encoded in a special language
     "wart" which requires a C compiler.

     SuperKermit sends an ACK packet for each data packet of 96    bytes
     (fewer if control characters are present).     This reduces throughput
     on    high speed modems, from    1350 to    177 characters per second in one
     test.

A number of extensions to the XMODEM protocol have been    made to    improve
performance and    (in some cases)    the user interface.  They provide useful
improvements in    some applications but not in others.  XMODEM's unprotected
control    messages compromise their reliability.    Complex    proprietary
techniques such    as Cybernetic Data Recovery(TM)[2] improve reliability,
but are    not universally    available.  Some of the    XMODEM mutant protocols
have significant design    flaws of their own.

 + XMODEM-k uses 1024 byte blocks to reduce the    overhead from transmission
   delays by 87    per cent compared to XMODEM, but network delays    still


__________

 1. Some Kermit    programs support run length encoding.

 2. Unique to DSZ, ZCOMM, Professional-YAM and PowerCom




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   degrade performance.     Some networks cannot transmit 1024 byte packets
   without flow    control, which is difficult to apply without impairing the
   perfect transparency    required by XMODEM.  XMODEM-k adds garbage to
   received files.

 + YMODEM sends    the file name, file length, and    creation date at the
   beginning of    each file, and allows optional 1024 byte blocks    for
   improved throughput.     The handling of files that are    not a multiple of
   1024    or 128 bytes is    awkward, especially if the file    length is not
   known in advance, or    changes    during transmission.  The large    number of
   non conforming and substandard programs claiming to support YMODEM
   further complicates its use.

 + YMODEM-g provides efficient batch file transfers, preserving    exact file
   length and file modification    date.  YMODEM-g    is a modification to
   YMODEM wherein ACKs for data    blocks are not used.  YMODEM-g is
   essentially insensitive to network delays.  Because it does not support
   error recovery, YMODEM-g must be used hard wired or with a reliable
   link    level protocol.     Successful application    at high    speed requires
   cafeful attention to    transparent flow control.  When    YMODEM-g detects a
   CRC error, data transfers are aborted.  YMODEM-g is easy to implement
   because it closely resembles    standard YMODEM.

 + WXMODEM, SEAlink, and MEGAlink have applied a subset    of ZMODEM's
   techniques to "Classic XMODEM" to improve upon their    suppliers'
   previous offerings.    They provide good performance under ideal
   conditions.

Another    XMODEM "extension" is protocol cheating, such as Omen Technology's
OverThruster(TM) and OverThruster II(TM).  These improve XMODEM    throughput
under some conditions by compromising error recovery.

The ZMODEM Protocol corrects the weaknesses described above while
maintaining as much of XMODEM/CRC's simplicity and prior art as    possible.



3.  ZMODEM Protocol Design Criteria

The design of a    file transfer protocol is an engineering compromise
between    conflicting requirements:

3.1  Ease of Use

 + ZMODEM allows either    program    to initiate file transfers, passing
   commands and/or modifiers to    the other program.

 + File    names need be entered only once.

 + Menu    selections are supported.




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 + Wild    Card names may be used with batch transfers.

 + Minimum keystrokes required to initiate transfers.

 + ZRQINIT frame sent by sending program can trigger automatic downloads.

 + ZMODEM can step down    to YMODEM if the other end does    not support
   ZMODEM.[1]

3.2  Throughput

All file transfer protocols make tradeoffs between throughput,
reliability, universality, and complexity according to the technology and
knowledge base available to their designers.

In the design of ZMODEM, three applications deserve special attention.

  + Network applications with significant delays (relative to character
    transmission time) and low error rate

  + Timesharing    and buffered modem applications    with significant delays
    and    throughput that    is quickly degraded by reverse channel traffic.
    ZMODEM's economy of    reverse    channel    bandwidth allows modems    that
    dynamically    partition bandwidth between the    two directions to operate
    at optimal speeds.    Special    ZMODEM features    allow simple, efficient
    implementation on a    wide variety of    timesharing hosts.

  + Direct modem to modem communications with high error rate

Unlike Sliding Windows Kermit, ZMODEM is not optimized for optimum
throughput when    error rate and delays are both high.  This tradeoff
markedly reduces code complexity and memory requirements.  ZMODEM
generally provides faster error    recovery than network compatible XMODEM
implementations.

In the absence of network delays, rapid    error recovery is possible, much
faster than MEGAlink and network compatible versions of    YMODEM and XMODEM.

File transfers begin immediately regardless of which program is    started
first, without the 10 second delay associated with XMODEM.







__________

 1. Provided the transmission medium accommodates X/YMODEM.




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3.3  Integrity and Robustness

Once a ZMODEM session is begun,    all transactions are protected with 16 or
32 bit CRC.[2] Complex proprietary techniques such as Cybernetic Data
Recovery(TM)[3]    are not    needed for reliable transfers.

An optional 32-bit CRC used as the frame check sequence    in ADCCP (ANSI
X3.66, also known as FIPS PUB 71 and FED-STD-1003, the U.S. versions of
CCITT's    X.25) is used when available.  The 32 bit CRC reduces undetected
errors by at least five    orders of magnitude when properly applied (-1
preset,    inversion).

A security challenge mechanism guards against "Trojan Horse" messages
written    to mimic legitimate command or file downloads.

3.4  Ease of Implementation

ZMODEM accommodates a wide variety of systems:

 + Microcomputers that cannot overlap disk and serial i/o

 + Microcomputers that cannot overlap serial send and receive

 + Computers and/or networks requiring XON/XOFF    flow control

 + Computers that cannot check the serial input    queue for the presence of
   data    without    having to wait for the data to arrive.

Although ZMODEM    provides "hooks" for multiple "threads", ZMODEM    is not
intended to replace link level protocols such as X.25.

ZMODEM accommodates network and    timesharing system delays by continuously
transmitting data unless the receiver interrupts the sender to request
retransmission of garbled data.     ZMODEM    in effect uses the entire file as
a window.[4] Using the entire file as a    window simplifies buffer
management, avoiding the window    overrun    failure    modes that affect
MEGAlink, SuperKermit, and others.

ZMODEM provides    a general purpose application to application file transfer
protocol which may be used directly or with with reliable link level


__________

 2. Except for the CAN-CAN-CAN-CAN-CAN abort sequence which requires five
    successive CAN characters.

 3. Unique to Professional-YAM and PowerCom

 4. Streaming strategies are discussed in coming chapters.




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protocols such as X.25,    MNP, Fastlink, etc.  When used with X.25, MNP,
Fastlink, etc.,    ZMODEM detects and corrects errors in the interfaces
between    error controlled media and the remainder of the    communications
link.

ZMODEM was developed for the public domain under a Telenet contract.  The
ZMODEM protocol    descriptions and the Unix rz/sz    program    source code are
public domain.    No licensing, trademark, or copyright restrictions apply
to the use of the protocol, the    Unix rz/sz source code and the ZMODEM
name.


4.  EVOLUTION OF ZMODEM

In early 1986, Telenet funded a    project    to develop an improved public
domain application to application file transfer    protocol.  This    protocol
would alleviate    the throughput problems    network    customers were
experiencing with XMODEM and Kermit file transfers.

In the beginning, we thought a few modifications to XMODEM would allow
high performance over packet switched networks while preserving    XMODEM's
simplicity.

The initial concept would add a    block number to    the ACK    and NAK    characters
used by    XMODEM.     The resultant protocol    would allow the    sender to send
more than one block before waiting for a response.

But how    to add the block number    to XMODEM's ACK    and NAK?  WXMODEM,
SEAlink, MEGAlink and some other protocols add binary byte(s) to indicate
the block number.

Pure binary was    unsuitable for ZMODEM because binary code combinations
won't pass bidirectionally through some    modems,    networks and operating
systems.  Other    operating systems may not be able to recognize something
coming back[1] unless a    break signal or    a system dependent code    or
sequence is present.  By the time all this and other problems with the
simple ACK/NAK sequences mentioned above were corrected, XMODEM's simple
ACK and    NACK characters    had evolved into a real    packet.     The Frog was
riveting.

Managing the window[2] was another problem.  Experience    gained in
debugging The Source's SuperKermit protocol indicated a    window size of
about 1000 characters was needed at 1200 bps.  High speed modems require a


__________

 1. Without stopping for a response

 2. The    WINDOW is the data in transit between sender and receiver.




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window of 20000    or more    characters for full throughput.     Much of the
SuperKermit's inefficiency, complexity and debugging time centered around
its ring buffering and window management.  There had to    be a better way    to
get the    job done.

A sore point with XMODEM and its progeny is error recovery.  More to the
point, how can the receiver determine whether the sender has responded,    or
is ready to respond, to    a retransmission request?  XMODEM attacks the
problem    by throwing away characters until a certain period of silence.
Too short a time allows    a spurious pause in output (network or timesharing
congestion) to masquerade as error recovery.  Too long a timeout
devastates throughput, and allows a noisy line to lock up the protocol.
SuperKermit solves the problem with a distinct start of    packet character
(SOH).    WXMODEM    and ZMODEM use unique character    sequences to delineate the
start of frames.  SEAlink and MEGAlink do not address this problem.

A further error    recovery problem arises    in streaming protocols.     How does
the receiver know when (or if) the sender has recognized its error signal?
Is the next packet the correct response    to the error signal?  Is it
something left over "in    the queue"?  Or    is this    new subpacket one of many
that will have to be discarded because the sender did not receive the
error signal?  How long    should this continue before sending another error
signal?     How can the protocol prevent this from    degenerating into an
argument about mixed signals?

SuperKermit uses selective retransmission, so it can accept any    good
packet it receives.  Each time the SuperKermit receiver    gets a data
packet,    it must    decide which outstanding packet    (if any) it "wants most"
to receive, and    asks for that one.  In practice, complex software "hacks"
are needed to attain acceptable    robustness.[3]

For ZMODEM, we decided to forgo    the complexity of SuperKermit's    packet
assembly scheme    and its    associated buffer management logic and memory
requirements.

Another    sore point with    XMODEM and WXMODEM is the garbage added    to files.
This was acceptable with old CP/M files    which had no exact length, but not
with modern systems such as DOS    and Unix.  YMODEM uses file length
information transmitted    in the header block to trim the    output file, but
this causes data loss when transferring    files that grow    during a transfer.
In some    cases, the file    length may be unknown, as when data is obtained
from a process.     Variable length data subpackets solve both of these


__________

 3. For    example, when SuperKermit encounters certain errors, the wndesr
    function is    called to determine the    next block to request.    A burst    of
    errors generates several wasteful requests to retransmit the same
    block.




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problems.

Since some characters had to be    escaped    anyway,    there wasn't any point
wasting    bytes to fill out a fixed packet length    or to specify a    variable
packet length.    In ZMODEM, the length of data subpackets is denoted by
ending each subpacket with an escape sequence similar to BISYNC    and HDLC.

The end    result is a ZMOEM header containing a "frame type", four bytes of
supervisory information, and its own CRC.  Data    frames consist of a header
followed by 1 or more data subpackets.    In the absence of transmission
errors,    an entire file can be sent in one data frame.

Since the sending system may be    sensitive to numerous control characters
or strip parity    in the reverse data path, all of the headers sent by the
receiver are sent in hex.  A common lower level    routine    receives all
headers, allowing the main program logic to deal with headers and data
subpackets as objects.

With equivalent    binary (efficient) and hex (application    friendly) frames,
the sending program can    send an    "invitation to receive"    sequence to
activate the receiver without crashing the remote application with
unexpected control characters.

Going "back to scratch"    in the protocol    design presents    an opportunity to
steal good ideas from many sources and to add a    few new    ones.

From Kermit and    UUCP comes the concept of an initial dialog to exchange
system parameters.

ZMODEM generalizes Compuserve B    Protocol's host    controlled transfers to
single command AutoDownload and    command    downloading.  A    Security Challenge
discourages password hackers and Trojan    Horse authors from abusing
ZMODEM's power.

We were    also keen to the pain and $uffering of legions of
telecommunicators whose    file transfers have been ruined    by communications
and timesharing    faults.     ZMODEM's file transfer    recovery and advanced file
management are dedicated to these kindred comrades.

After ZMODEM had been operational a short time,    Earl Hall pointed out the
obvious: ZMODEM's user friendly    AutoDownload was almost    useless    if the
user must assign transfer options to each of the sending and receiving
programs.  Now,    transfer options may be    specified to/by    the sending
program, which passes them to the receiving program in the ZFILE header.










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5.  ROSETTA STONE

Here are some definitions which    reflect    current    vernacular in the computer
media.    The attempt here is identify the file transfer protocol    rather
than specific programs.

FRAME    A ZMODEM frame consists    of a header and    0 or more data subpackets.

XMODEM    refers to the original 1977 file transfer etiquette introduced by
    Ward Christensen's MODEM2 program.  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's
    Group" and "TERM II FTP    3".  This protocol is supported    by most
    communications programs    because    it is easy to implement.

XMODEM/CRC replaces XMODEM's 1 byte checksum with a two    byte Cyclical
    Redundancy Check (CRC-16), improving error detection.

XMODEM-1k Refers to XMODEM-CRC with optional 1024 byte blocks.

YMODEM    refers to the XMODEM/CRC protocol with batch transmission and
    optional 1024 byte blocks as described in YMODEM.DOC.[1]


6.  ZMODEM REQUIREMENTS

ZMODEM requires    an 8 bit transfer medium.[1] ZMODEM escapes network
control    characters to allow operation with packet switched networks.  In
general, ZMODEM    operates over any path that supports XMODEM, and over many
that don't.

To support full    streaming,[2] the transmission path should either assert
flow control or    pass full speed    transmission without loss of data.
Otherwise the ZMODEM sender must manage    the window size.

6.1  File Contents

6.1.1  Binary Files
ZMODEM places no constraints on    the information    content    of binary files,
except that the    number of bits in the file must    be a multiple of 8.



__________

 1. Available on TeleGodzilla as part of YZMODEM.ZOO

 1. The    ZMODEM design allows encoded packets for less transparent media.

 2. With XOFF and XON, or out of band flow control such    as X.25    or CTS




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6.1.2  Text Files
Since ZMODEM is    used to    transfer files between different types of computer
systems, text files must meet minimum requirements if they are to be
readable on a wide variety of systems and environments.

Text lines consist of printing ASCII characters, spaces, tabs, and
backspaces.

6.1.2.1     ASCII End of Line
The ASCII code definition allows text lines terminated by a CR/LF (015,
012) sequence, or by a NL (012)    character.  Lines logically terminated by
a lone CR (013)    are not    ASCII text.

A CR (013) without a linefeed implies overprinting, and    is not acceptable
as a logical line separator.  Overprinted lines    should print all important
characters in the last pass to allow CRT displays to display meaningful
text.  Overstruck characters may be generated by backspacing or    by
overprinting the line with CR (015) not    followed by LF.

Overstruck characters generated    with backspaces    should be sent with the
most important character last to accommodate CRT displays that cannot
overstrike.  The sending program may use the ZCNL bit to force the
receiving program to convert the received end of line to its local end of
line convention.[3]






















__________

 3. Files that have been translated in such a way as to    modify their
    length cannot be updated with the ZCRECOV Conversion Option.




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7.  ZMODEM BASICS

7.1  Packetization

ZMODEM frames differ somewhat from XMODEM blocks.  XMODEM blocks are not
used for the following reasons:

 + Block numbers are limited to    256

 + No provision    for variable length blocks

 + Line    hits corrupt protocol signals, causing failed file transfers.  In
   particular, modem errors sometimes generate false block numbers, false
   EOTs    and false ACKs.     False ACKs are    the most troublesome as    they cause
   the sender to lose synchronization with the receiver.

   State of the    art programs such as Professional-YAM and ZCOMM    overcome
   some    of these weaknesses with clever    proprietary code, but a    stronger
   protocol is desired.

 + It is difficult to determine    the beginning and ends of XMODEM blocks
   when    line hits cause    a loss of synchronization.  This precludes rapid
   error recovery.

7.2  Link Escape Encoding

ZMODEM achieves    data transparency by extending the 8 bit character set
(256 codes) with escape    sequences based    on the ZMODEM data link    escape
character ZDLE.[1]

Link Escape coding permits variable length data    subpackets without the
overhead of a separate byte count.  It allows the beginning of frames to
be detected without special timing techniques, facilitating rapid error
recovery.

Link Escape coding does    add some overhead.  The    worst case, a file
consisting entirely of escaped characters, would incur a 50% overhead.

The ZDLE character is special.    ZDLE represents    a control sequence of some
sort.  If a ZDLE character appears in binary data, it is prefixed with
ZDLE, then sent    as ZDLEE.

The value for ZDLE is octal 030    (ASCII CAN).  This particular value was
chosen to allow    a string of 5 consecutive CAN characters to abort a ZMODEM


__________

 1. This and other constants are defined in the    zmodem.h include file.
    Please note    that constants with a leading 0    are octal constants in C.




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session, compatible with YMODEM    session    abort.

Since CAN is not used in normal    terminal operations, interactive
applications and communications    programs can monitor the data flow for
ZDLE.  The following characters    can be scanned to detect the ZRQINIT
header,    the invitation to automatically    download commands or files.

Receipt    of five    successive CAN characters will abort a ZMODEM session.
Eight CAN characters are sent.

The receiving program decodes any sequence of ZDLE followed by a byte with
bit 6 set and bit 5 reset (upper case letter, either parity) to    the
equivalent control character by    inverting bit 6.  This allows the
transmitter to escape any control character that cannot    be sent    by the
communications medium.    In addition, the receiver recognizes escapes for
0177 and 0377 should these characters need to be escaped.

ZMODEM software    escapes    ZDLE, 020, 0220, 021, 0221, 023, and 0223.  If
preceded by 0100 or 0300 (@), 015 and 0215 are also escaped to protect the
Telenet    command    escape CR-@-CR.     The receiver ignores 021, 0221, 023, and
0223 characters    in the data stream.

The ZMODEM routines in zm.c accept an option to    escape all control
characters, to allow operation with less transparent networks.    This
option can be given to either the sending or receiving program.

7.3  Header

All ZMODEM frames begin    with a header which may    be sent    in binary or HEX
form.  ZMODEM uses a single routine to recognize binary    and hex    headers.
Either form of the header contains the same raw    information:

 + A type byte[2] [3]

 + Four    bytes of data indicating flags and/or numeric quantities depending
   on the frame    type







__________

 2. The    frame types are    cardinal numbers beginning with    0 to minimize
    state transition table memory requirements.

 3. Future extensions to ZMODEM    may use    the high order bits of the type
    byte to indicate thread selection.




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           Figure 1.  Order of Bytes in    Header

           TYPE:  frame    type
           F0: Flags least significant byte
           P0: file Position least significant
           P3: file Position most significant

               TYPE     F3 F2 F1 F0
               -------------------
               TYPE     P0 P1 P2 P3

7.3.1  16 Bit CRC Binary Header
A binary header    is sent    by the sending program to the receiving    program.
ZDLE encoding accommodates XON/XOFF flow control.

A binary header    begins with the    sequence ZPAD, ZDLE, ZBIN.

The frame type byte is ZDLE encoded.

The four position/flags    bytes are ZDLE encoded.

A two byte CRC of the frame type and position/flag bytes is ZDLE encoded.

0 or more binary data subpackets with 16 bit CRC will follow depending on
the frame type.

The function zsbhdr transmits a    binary header.    The function zgethdr
receives a binary or hex header.

           Figure 2.  16 Bit CRC Binary    Header
        * ZDLE A TYPE F3/P0    F2/P1 F1/P2 F0/P3 CRC-1    CRC-2


7.3.2  32 Bit CRC Binary Header
A "32 bit CRC" Binary header is    similar    to a Binary Header, except the
ZBIN (A) character is replaced by a ZBIN32 (C) character, and four
characters of CRC are sent.  0 or more binary data subpackets with 32 bit
CRC will follow    depending on the frame type.

The common variable Txfcs32 may    be set TRUE for    32 bit CRC iff the
receiver indicates the capability with the CANFC32 bit.     The zgethdr,
zsdata and zrdata functions automatically adjust to the    type of    Frame
Check Sequence being used.
           Figure 3.  32 Bit CRC Binary    Header
      *    ZDLE C TYPE F3/P0 F2/P1    F1/P2 F0/P3 CRC-1 CRC-2    CRC-3 CRC-4


7.3.3  HEX Header
The receiver sends responses in    hex headers.  The sender also uses hex
headers    when they are not followed by binary data subpackets.




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Hex encoding protects the reverse channel from random control characters.
The hex    header receiving routine ignores parity.

Use of Kermit style encoding for control and paritied characters was
considered and rejected    because    of increased possibility of interacting
with some timesharing systems' line edit functions.  Use of HEX    headers
from the receiving program allows control characters to    be used    to
interrupt the sender when errors are detected.    A HEX header may be used
in place of a binary header wherever convenient.  If a data packet follows
a HEX header, it is protected with CRC-16.

A hex header begins with the sequence ZPAD, ZPAD, ZDLE,    ZHEX.  The zgethdr
routine    synchronizes with the ZPAD-ZDLE    sequence.  The extra ZPAD
character allows the sending program to    detect an asynchronous header
(indicating an error condition)    and then call zgethdr to receive the
header.

The type byte, the four    position/flag bytes, and the 16    bit CRC    thereof
are sent in hex    using the character set    01234567890abcdef.  Upper case hex
digits are not allowed;    they false trigger XMODEM and YMODEM programs.
Since this form    of hex encoding    detects    many patterns of errors,
especially missing characters, a hex header with 32 bit    CRC has    not been
defined.

A carriage return and line feed    are sent with HEX headers.  The    receive
routine    expects    to see at least    one of these characters, two if    the first
is CR.    The CR/LF aids debugging from printouts, and helps overcome
certain    operating system related problems.

An XON character is appended to    all HEX    packets    except ZACK and    ZFIN.  The
XON releases the sender    from spurious XOFF flow    control    characters
generated by line noise, a common occurrence.  XON is not sent after ZACK
headers    to protect flow    control    in streaming situations.  XON is not sent
after a    ZFIN header to allow clean session cleanup.

0 or more data subpackets will follow depending    on the frame type.

The function zshhdr sends a hex    header.

              Figure 4.  HEX Header

      *    * ZDLE B TYPE F3/P0 F2/P1 F1/P2    F0/P3 CRC-1 CRC-2 CR LF    XON

(TYPE, F3...F0,    CRC-1, and CRC-2 are each sent as two hex digits.)










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Chapter    7             ZMODEM Protocol                16



7.4  Binary Data Subpackets

Binary data subpackets immediately follow the associated binary    header
packet.     A binary data packet contains 0 to 1024 bytes of data.
Recommended length values are 256 bytes    below 2400 bps,    512 at 2400 bps,
and 1024 above 4800 bps    or when    the data link is known to be relatively
error free.[4]

No padding is used with    binary data subpackets.     The data bytes    are ZDLE
encoded    and transmitted.  A ZDLE and frameend are then sent, followed by
two or four ZDLE encoded CRC bytes.  The CRC accumulates the data bytes
and frameend.

The function zsdata sends a data subpacket.  The function zrdata receives
a data subpacket.

7.5  ASCII Encoded Data    Subpacket

The format of ASCII Encoded data subpackets is not currently specified.
These could be used for    server commands, or main transfers in 7    bit
environments.


8.  PROTOCOL TRANSACTION OVERVIEW

As with    the XMODEM recommendation, ZMODEM timing is receiver driven.  The
transmitter should not time out    at all,    except to abort    the program if no
headers    are received for an extended period of time, say one minute.[1]


8.1  Session Startup

To start a ZMODEM file transfer    session, the sending program is    called
with the names of the desired file(s) and option(s).

The sending program may    send the string    "rz\r" to invoke the receiving
program    from a possible    command    mode.  The "rz"    followed by carriage
return activates a ZMODEM receive program or command if    it were    not
already    active.

The sender may then display a message intended for human consumption, such


__________

 4. Strategies for adjusting the subpacket length for optimal results
    based on real time error rates are still evolving.    Shorter    subpackets
    speed error    detection but increase protocol    overhead slightly.

 1. Special considerations apply when sending commands.




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Chapter    8             ZMODEM Protocol                17



as a list of the files requested, etc.

Then the sender    may send a ZRQINIT header.  The    ZRQINIT    header causes a
previously started receive program to send its ZRINIT header without
delay.

In an interactive or conversational mode, the receiving    application may
monitor    the data stream    for ZDLE.  The following characters may    be scanned
for B00    indicating a ZRQINIT header, a command to download a command or
data.

The sending program awaits a command from the receiving    program    to start
file transfers.     If a "C", "G",    or NAK is received, an XMODEM or YMODEM
file transfer is indicated, and    file transfer(s) use the YMODEM    protocol.
Note: With ZMODEM and YMODEM, the sending program provides the file name,
but not    with XMODEM.

In case    of garbled data, the sending program can repeat    the invitation to
receive    a number of times until    a session starts.

When the ZMODEM    receive    program    starts,    it immediately sends a ZRINIT
header to initiate ZMODEM file transfers, or a ZCHALLENGE header to verify
the sending program.  The receive program resends its header at    response
time (default 10 second) intervals for a suitable period of time (40
seconds    total) before falling back to YMODEM protocol.

If the receiving program receives a ZRQINIT header, it resends the ZRINIT
header.     If the    sending    program    receives the ZCHALLENGE    header,    it places
the data in ZP0...ZP3 in an answering ZACK header.

If the receiving program receives a ZRINIT header, it is an echo
indicating that    the sending program is not operational.

Eventually the sending program correctly receives the ZRINIT header.

The sender may then send an optional ZSINIT frame to define the    receiving
program's Attn sequence, or to specify complete    control    character
escaping.[2]

If the ZSINIT header specifies ESCCTL or ESC8, a HEX header is used, and
the receiver activates the specified ESC modes before reading the
following data subpacket.

The receiver sends a ZACK header in response, optionally containing the


__________

 2. If the receiver specifies the same or higher level of escaping, the
    ZSINIT frame need not be sent unless an Attn sequence is needed.




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serial number of the receiving program,    or 0.

8.2  File Transmission

The sender then    sends a    ZFILE header with ZMODEM Conversion, Management,
and Transport options[3] followed by a ZCRCW data subpacket containing the
file name, file    length,    modification date, and other information identical
to that    used by    YMODEM Batch.

The receiver examines the file name, length, and date information provided
by the sender in the context of    the specified transfer options,    the
current    state of its file system(s), and local security    requirements.  The
receiving program should insure    the pathname and options are compatible
with its operating environment and local security requirements.

The receiver may respond with a    ZSKIP header, which makes the sender
proceed    to the next file (if any) in the batch.

       If the receiver has a file with the same    name and length,
       it may respond with a ZCRC header, which    requires the
       sender to perform a 32 bit CRC on the file and transmit the
       complement of the CRC in    a ZCRC header.[4] The receiver
       uses this information to    determine whether to accept the
       file or skip it.     This sequence is triggered by the ZMCRC
       Management Option.

A ZRPOS    header from the    receiver initiates transmission    of the file data
starting at the    offset in the file specified in    the ZRPOS header.
Normally the receiver specifies    the data transfer to begin begin at
offset 0 in the    file.
       The receiver may    start the transfer further down    in the
       file.  This allows a file transfer interrupted by a loss
       or carrier or system crash to be    completed on the next
       connection without requiring the    entire file to be
       retransmitted.[5] If downloading    a file from a timesharing
       system that becomes sluggish, the transfer can be
       interrupted and resumed later with no loss of data.

The sender sends a ZDATA binary    header (with file position) followed by
one or more data subpackets.



__________

 3. See    below, under ZFILE header type.

 4. The    crc is initialized to 0xFFFFFFFF.

 5. This does not apply    to files that have been    translated.




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The receiver compares the file position    in the ZDATA header with the
number of characters successfully received to the file.     If they do not
agree, a ZRPOS error response is generated to force the    sender to the
right position within the file.[6]

A data subpacket terminated by ZCRCG and CRC does not elicit a response
unless an error    is detected; more data subpacket(s) follow immediately.

       ZCRCQ data subpackets expect a ZACK response with the
       receiver's file offset if no error, otherwise a ZRPOS
       response    with the last good file    offset.     Another data
       subpacket continues immediately.     ZCRCQ subpackets are
       not used    if the receiver    does not indicate FDX ability
       with the    CANFDX bit.

ZCRCW data subpackets expect a response    before the next    frame is sent.
If the receiver    does not indicate overlapped I/O capability with the
CANOVIO    bit, or    sets a buffer size, the    sender uses the    ZCRCW to allow
the receiver to    write its buffer before    sending    more data.

       A zero length data frame    may be used as an idle
       subpacket to prevent the    receiver from timing out in
       case data is not    immediately available to the sender.

In the absence of fatal    error, the sender eventually encounters    end of
file.  If the end of file is encountered within    a frame, the frame is
closed with a ZCRCE data subpacket which does not elicit a response
except in case of error.

The sender sends a ZEOF    header with the    file ending offset equal to
the number of characters in the    file.  The receiver compares this
number with the    number of characters received.    If the receiver    has
received all of    the file, it closes the    file.  If the file close was
satisfactory, the receiver responds with ZRINIT.  If the receiver has
not received all the bytes of the file,    the receiver ignores the ZEOF
because    a new ZDATA is coming.    If the receiver    cannot properly    close
the file, a ZFERR header is sent.

       After all files are processed, any further protocol
       errors should not prevent the sending program from
       returning with a    success    status.





__________

 6. If the ZMSPARS option is used, the receiver    instead    seeks to the
    position given in the ZDATA    header.




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Chapter    8             ZMODEM Protocol                20



8.3  Session Cleanup

The sender closes the session with a ZFIN header.  The receiver
acknowledges this with its own ZFIN header.

When the sender    receives the acknowledging header, it sends two
characters, "OO" (Over and Out)    and exits to the operating system or
application that invoked it.  The receiver waits briefly for the "O"
characters, then exits whether they were received or not.

8.4  Session Abort Sequence

If the receiver    is receiving data in streaming mode, the Attn
sequence is executed to    interrupt data transmission before the Cancel
sequence is sent.  The Cancel sequence consists    of eight CAN
characters and ten backspace characters.  ZMODEM only requires five
Cancel characters, the other three are "insurance".

The trailing backspace characters attempt to erase the effects of the
CAN characters if they are received by a command interpreter.

       static char canistr[] = {
    24,24,24,24,24,24,24,24,8,8,8,8,8,8,8,8,8,8,0
       };






























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Chapter    8             ZMODEM Protocol                21



9.  STREAMING TECHNIQUES / ERROR RECOVERY

It is a    fact of    life that no single method of streaming    is applicable
to a majority of today's computing and telecommunications
environments.  ZMODEM provides several data streaming methods
selected according to the limitations of the sending environment,
receiving environment, and transmission    channel(s).


9.1  Full Streaming with Sampling

If the receiver    can overlap serial I/O with disk I/O, and if the
sender can sample the reverse channel for the presence of data
without    having to wait,    full streaming can be used with    no Attn
sequence required.  The    sender begins data transmission    with a ZDATA
header and continuous ZCRCG data subpackets.  When the receiver
detects    an error, it executes the Attn sequence    and then sends a
ZRPOS header with the correct position within the file.

At the end of each transmitted data subpacket, the sender checks for
the presence of    an error header    from the receiver.  To do this,    the
sender samples the reverse data    stream for the presence    of either a
ZPAD or    CAN character.[1] Flow control characters (if present) are
acted upon.

Other characters (indicating line noise) increment a counter which is
reset whenever the sender waits    for a header from the receiver.     If
the counter overflows, the sender sends    the next data subpacket    as
ZCRCW, and waits for a response.

ZPAD indicates some sort of error header from the receiver.  A CAN
suggests the user is attempting    to "stop the bubble machine" by
keyboarding CAN    characters.  If    one of these characters    is seen, an
empty ZCRCE data subpacket is sent.  Normally, the receiver will have
sent an    ZRPOS or other error header, which will    force the sender to
resume transmission at a different address, or take other action.  In
the unlikely event the ZPAD or CAN character was spurious, the
receiver will time out and send    a ZRPOS    header.[2]

Then the receiver's response header is read and    acted upon.[3]


__________

 1. The    call to    rdchk()    in sz.c    performs this function.

 2. The    obvious    choice of ZCRCW    packet,    which would trigger an ZACK from
    the    receiver, is not used because multiple in transit frames could
    result if the channel has a    long propagation delay.

 3. The    call to    getinsync() in sz.c performs this function.



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Chapter    9             ZMODEM Protocol                22



A ZRPOS    header resets the sender's file    offset to the correct
position.  If possible,    the sender should purge    its output buffers
and/or networks    of all unprocessed output data,    to minimize the
amount of unwanted data    the receiver must discard before receiving
data starting at the correct file offset.  The next transmitted    data
frame should be    a ZCRCW    frame followed by a wait to guarantee
complete flushing of the network's memory.

If the receiver    gets a ZACK header with    an address that    disagrees
with the sender    address, it is ignored,    and the    sender waits for
another    header.     A ZFIN, ZABORT, or TIMEOUT terminates the session; a
ZSKIP terminates the processing    of this    file.

The reverse channel is then sampled for    the presence of    another
header from the    receiver.[4] if    one is detected, the getinsync()
function is again called to read another error header.    Otherwise,
transmission resumes at    the (possibly reset) file offset with a    ZDATA
header followed    by data    subpackets.


9.1.1  Window Management
When sending data through a network, some nodes    of the network store
data while it is transferred to    the receiver.  7000 bytes and more of
transient storage have been observed.  Such a large amount of storage
causes the transmitter to "get ahead" of the reciever.    This can be
fatal with MEGAlink and    other protocols    that depend on timely
notification of    errors from the    receiver.  This    condition is not
fatal with ZMODEM, but it does slow error recovery.

To manage the window size, the sending program uses ZCRCQ data
subpackets to trigger ZACK headers from    the receiver.  The returning
ZACK headers inform the    sender of the receiver's progress.  When the
window size (current transmitter file offset - last reported receiver
file offset) exceeds a specified value,    the sender waits for a
ZACK[5]    packet with a receiver file offset that    reduces    the window
size.

Unix sz    versions beginning with    May 9 1987 control the window size
with the "-w N"    option,    where N    is the maximum window size.  Pro-YAM,
ZCOMM and DSZ versions beginning with May 9 1987 control the window
size with "zmodem pwN".     This is compatible with previous versions of
these programs.[6]


__________

 4. If sampling    is possible.

 5. ZRPOS and other error packets are handled normally.

 6. When used with modems or networks that simultaneously assert flow



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Chapter    9             ZMODEM Protocol                23



9.2  Full Streaming with Reverse Interrupt

The above method cannot    be used    if the reverse data stream cannot be
sampled    without    entering an I/O    wait.  An alternate method is to
instruct the receiver to interrupt the sending program when an error
is detected.

The receiver can interrupt the sender with a control character,    break
signal,    or combination thereof,    as specified in    the Attn sequence.
After executing    the Attn sequence, the receiver    sends a    hex ZRPOS
header to force    the sender to resend the lost data.

When the sending program responds to this interrupt, it    reads a    HEX
header (normally ZRPOS)    from the receiver and takes the    action
described in the previous section.  The    Unix sz.c program uses a
setjmp/longjmp call to catch the interrupt generated by    the Attn
sequence.  Catching the    interrupt activates the    getinsync() function
to read    the receiver's error header and    take appropriate action.

When compiled for standard SYSTEM III/V    Unix, sz.c uses    an Attn
sequence of Ctrl-C followed by a 1 second pause    to interrupt the
sender,    then give the sender (Unix) time to prepare for    the
receiver's error header.


9.3  Full Streaming with Sliding Window

If none    of the above methods is    applicable, hope is not    yet lost.  If
the sender can buffer responses    from the receiver, the sender can use
ZCRCQ data subpackets to get ACKs from the receiver without
interrupting the transmission of data.    After a    sufficient number of
ZCRCQ data subpackets have been    sent, the sender can read one of the
headers    that should have arrived in its    receive    interrupt buffer.

A problem with this method is the possibility of wasting an excessive
amount of time responding to the receiver's error header.  It may be
possible to program the    receiver's Attn    sequence to flush the
sender's interrupt buffer before sending the ZRPOS header.







__________________________________________________________________________

    control with XON and XOFF characters and pass XON characters that
    violate flow control, the receiving    program    should have a revision
    date of May    9 or later.




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Chapter    9             ZMODEM Protocol                24



9.4  Full Streaming over Error Free Channels

File transfer protocols    predicated on the existence of an error    free
end to end communications channel have been proposed from time to
time.  Such channels have proven to be more readily available in
theory than in actuality.  The frequency of undetected errors
increases when modem scramblers    have more bits than the    error
detecting CRC.

A ZMODEM sender    assuming an error free channel with end    to end flow
control    can send the entire file in one    frame without any checking of
the reverse stream.  If    this channel is    completely transparent,    only
ZDLE need be escaped.  The resulting protocol overhead for average
long files is less than    one per    cent.[7]

9.5  Segmented Streaming

If the receiver    cannot overlap serial and disk I/O, it uses the
ZRINIT frame to    specify    a buffer length    which the sender will not
overflow.  The sending program sends a ZCRCW data subpacket and    waits
for a ZACK header before sending the next segment of the file.

If the sending program supports    reverse    data stream sampling or
interrupt, error recovery will be faster (on average) than a protocol
(such as YMODEM) that sends large blocks.

A sufficiently large receiving buffer allows throughput    to closely
approach that of full streaming.  For example, 16kb segmented
streaming adds about 3 per cent    to full    streaming ZMODEM file
transfer times when the    round trip delay is five seconds.


10.  ATTENTION SEQUENCE

The receiving program sends the    Attn sequence whenever it detects an
error and needs    to interrupt the sending program.

The default Attn string    value is empty (no Attn    sequence).  The
receiving program resets Attn to the empty default before each
transfer session.

The sender specifies the Attn sequence in its optional ZSINIT frame.
The Attn string    is terminated with a null.



__________

 7. One    in 256 for escaping ZDLE, about    two (four if 32    bit CRC    is used)
    in 1024 for    data subpacket CRC's




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Chapter    10             ZMODEM Protocol                25



Two meta-characters perform special functions:

   + \335 (octal) Send a break signal

   + \336 (octal) Pause    one second


11.  FRAME TYPES

The numeric values for the values shown    in boldface are    given in
zmodem.h.  Unused bits and unused bytes    in the header (ZP0...ZP3) are
set to 0.

11.1  ZRQINIT

Sent by    the sending program, to    trigger    the receiving program to send
its ZRINIT header.  This avoids    the aggravating    startup    delay
associated with    XMODEM and Kermit transfers.  The sending program may
repeat the receive invitation (including ZRQINIT) if a response    is
not obtained at    first.

ZF0 contains ZCOMMAND if the program is    attempting to send a command,
0 otherwise.

11.2  ZRINIT

Sent by    the receiving program.    ZF0 and    ZF1 contain the     bitwise or
of the receiver    capability flags:
#define    CANCRY        8    /* Receiver can    decrypt    */
#define    CANFDX       01    /* Rx can send and receive true    FDX */
#define    CANOVIO       02    /* Rx can receive data during disk I/O */
#define    CANBRK       04    /* Rx can send a break signal */
#define    CANCRY      010    /* Receiver can    decrypt    */
#define    CANLZW      020    /* Receiver can    uncompress */
#define    CANFC32      040    /* Receiver can    use 32 bit Frame Check */
#define    ESCCTL     0100    /* Receiver expects ctl    chars to be escaped
*/
#define    ESC8     0200    /* Receiver expects 8th    bit to be escaped */

ZP0 and    ZP1 contain the    size of    the receiver's buffer in bytes,    or 0
if nonstop I/O is allowed.

11.3  ZSINIT

The Sender sends flags followed    by a binary data subpacket terminated
with ZCRCW.

/* Bit Masks for ZSINIT    flags byte ZF0 */
#define    TESCCTL    0100   /* Transmitter expects ctl chars    to be escaped
*/
#define    TESC8    0200   /* Transmitter expects 8th bit to be escaped



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Chapter    11             ZMODEM Protocol                26



*/

The data subpacket contains the    null terminated    Attn sequence,
maximum    length 32 bytes    including the terminating null.

11.4  ZACK

Acknowledgment to a ZSINIT frame, ZCHALLENGE header, ZCRCQ or ZCRCW
data subpacket.     ZP0 to    ZP3 contain file offset.  The response to
ZCHALLENGE contains the    same 32    bit number received in the ZCHALLENGE
header.

11.5  ZFILE

This frame denotes the beginning of a file transmission    attempt.
ZF0, ZF1, and ZF2 may contain options.    A value    of 0 in    each of    these
bytes implies no special treatment.  Options specified to the
receiver override options specified to the sender with the exception
of ZCBIN which overrides any other Conversion Option given to the
sender or receiver.


11.5.1    ZF0: Conversion    Option
If the receiver    does not recognize the Conversion Option, an
application dependent default conversion may apply.

ZCBIN "Binary" transfer    - inhibit conversion unconditionally

ZCNL Convert received end of line to local end of line
     convention.  The supported    end of line conventions    are
     CR/LF (most ASCII based operating systems except Unix
     and Macintosh), and NL (Unix).  Either of these two end
     of    line conventions meet the permissible ASCII
     definitions for Carriage Return and Line Feed/New Line.
     Neither the ASCII code nor    ZMODEM ZCNL encompass lines
     separated only by carriage    returns.  Other    processing
     appropriate to ASCII text files and the local operating
     system may    also be    applied    by the receiver.[1]

ZCRECOV    Recover/Resume interrupted file    transfer.  ZCREVOV is
     also useful for updating a    remote copy of a file that
     grows without resending of    old data.  If the destination
     file exists and is    no longer than the source, append to
     the destination file and start transfer at    the offset
     corresponding to the receiver's end of file.  This


__________

 1. Filtering RUBOUT, NULL, Ctrl-Z, etc.




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     option does not apply if the source file is shorter.
     Files that    have been converted (e.g., ZCNL) or subject
     to    a single ended Transport Option    cannot have their
     transfers recovered.

11.5.2    ZF1: Management    Option
If the receiver    does not recognize the Management Option, the
file should be transferred normally.

The ZMSKNOLOC bit instructs the    receiver to bypass the
current    file if    the receiver does not have a file with the
same name.

Five bits (defined by ZMMASK) define the following set of
mutually exclusive management options.

ZMNEWL Transfer    file if    destination file absent.  Otherwise,
     transfer file overwriting destination if the source file
     is    newer or longer.

ZMCRC Compare the source and destination files.     Transfer if
     file lengths or file polynomials differ.

ZMAPND Append source file contents to the end of the existing
     destination file (if any).

ZMCLOB Replace existing    destination file (if any).

ZMDIFF Transfer    file if    destination file absent.  Otherwise,
     transfer file overwriting destination if files have
     different lengths or dates.

ZMPROT Protect destination file    by transferring    file only if
     the destination file is absent.

ZMNEW Transfer file if destination file    absent.     Otherwise,
     transfer file overwriting destination if the source file
     is    newer.

11.5.3    ZF2: Transport Option
If the receiver    does not implement the particular transport
option,    the file is copied without conversion for later
processing.

ZTLZW Lempel-Ziv compression.  Transmitted data    will be
     identical to that produced    by compress 4.0    operating on
     a computer    with VAX byte ordering,    using 12 bit
     encoding.

ZTCRYPT    Encryption.  An    initial    null terminated    string
     identifies    the key.  Details to be    determined.



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Chapter    11             ZMODEM Protocol                28



ZTRLE Run Length encoding, Details to be determined.

A ZCRCW    data subpacket follows with file name, file length,
modification date, and other information described in a    later
chapter.

11.5.4    ZF3: Extended Options
The Extended Options are bit encoded.

ZTSPARS    Special    processing for sparse files, or    sender managed
     selective retransmission.    Each file segment is transmitted as
     a separate    frame, where the frames    are not    necessarily
     contiguous.  The sender should end    each segment with a ZCRCW
     data subpacket and    process    the expected ZACK to insure no data
     is    lost.  ZTSPARS cannot be used with ZCNL.

11.6  ZSKIP

Sent by    the receiver in    response to ZFILE, makes the sender skip to
the next file.

11.7  ZNAK

Indicates last header was garbled.  (See also ZRPOS).

11.8  ZABORT

Sent by    receiver to terminate batch file transfers when    requested by
the user.  Sender responds with    a ZFIN sequence.[2]

11.9  ZFIN

Sent by    sending    program    to terminate a ZMODEM session.    Receiver
responds with its own ZFIN.

11.10  ZRPOS

Sent by    receiver to force file transfer    to resume at file offset
given in ZP0...ZP3.








__________

 2. Or ZCOMPL in case of server    mode.




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11.11  ZDATA

ZP0...ZP3 contain file offset.    One or more data subpackets follow.

11.12  ZEOF

Sender reports End of File.  ZP0...ZP3 contain the ending file
offset.

11.13  ZFERR

Error in reading or writing file, protocol equivalent to ZABORT.

11.14  ZCRC

Request    (receiver) and response    (sender) for file polynomial.
ZP0...ZP3 contain file polynomial.

11.15  ZCHALLENGE

Request    sender to echo a random    number in ZP0...ZP3 in a ZACK frame.
Sent by    the receiving program to the sending program to    verify that
it is connected    to an operating    program, and was not activated by
spurious data or a Trojan Horse    message.

11.16  ZCOMPL

Request    now completed.

11.17  ZCAN

This is    a pseudo frame type returned by    gethdr() in response to    a
Session    Abort sequence.

11.18  ZFREECNT

Sending    program    requests a ZACK    frame with ZP0...ZP3 containing    the
number of free bytes on    the current file system.  A value of 0
represents an indefinite amount    of free    space.

11.19  ZCOMMAND

ZCOMMAND is sent in a binary frame.  ZF0 contains 0 or ZCACK1 (see
below).

A ZCRCW    data subpacket follows,    with the ASCII text command string
terminated with    a NULL character.  If the command is intended to be
executed by the    operating system hosting the receiving program
(e.g., "shell escape"),    it must    have "!" as the    first character.
Otherwise the command is meant to be executed by the application
program    which receives the command.



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If the receiver    detects    an illegal or badly formed command, the
receiver immediately responds with a ZCOMPL header with    an error
code in    ZP0...ZP3.

If ZF0 contained ZCACK1, the receiver immediately responds with    a
ZCOMPL header with 0 status.

Otherwise, the receiver    responds with a    ZCOMPL header when the
operation is completed.     The exit status of the    completed command is
stored in ZP0...ZP3.  A    0 exit status implies nominal completion of
the command.

If the command causes a    file to    be transmitted,    the command sender
will see a ZRQINIT frame from the other    computer attempting to send
data.

The sender examines ZF0    of the received    ZRQINIT    header to verify it
is not an echo of its own ZRQINIT header.  It is illegal for the
sending    program    to command the receiving program to send a command.

If the receiver    program    does not implement command downloading,    it
may display the    command    to the standard    error output, then return a
ZCOMPL header.



12.  SESSION TRANSACTION EXAMPLES

12.1  A    simple file transfer

A simple transaction, one file,    no errors, no CHALLENGE, overlapped
I/O:

Sender           Receiver

"rz\r"
ZRQINIT(0)
           ZRINIT
ZFILE
           ZRPOS
ZDATA data ...
ZEOF
           ZRINIT
ZFIN
           ZFIN
OO








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Chapter    12             ZMODEM Protocol                31



12.2  Challenge    and Command Download


Sender            Receiver

"rz\r"
ZRQINIT(ZCOMMAND)
            ZCHALLENGE(random-number)
ZACK(same-number)
            ZRINIT
ZCOMMAND, ZDATA
            (Performs Command)
            ZCOMPL
ZFIN
            ZFIN
OO


13.  ZFILE FRAME FILE INFORMATION

ZMODEM sends the same file information with the    ZFILE frame data
that YMODEM Batch sends    in its block 0.

N.B.: The pathname (file name) field is    mandatory.

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 absolute or
     relative pathname.     The source drive designator (A:, B:, etc.)
     usually is    not sent.

             Filename Considerations

    + File names should be translated to lower case    unless the
      sending system supports upper/lower case file    names.    This
      is a convenience for users of    systems    (such as Unix) which
      store    filenames in upper and lower case.

    + 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 not, transformations
      should be applied to make the    file names acceptable.    If
      the transformations are unsuccessful,    a new file name    may



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Chapter    13             ZMODEM Protocol                32



      be invented be the receiving program.

     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.[1] The length field is stored as    a decimal string
     counting the number of data bytes in the file.

     The ZMODEM    receiver uses the file length as an estimate only.
     It    may be used to display an estimate of the transmission time,
     and may be    compared with the amount of free disk space.  The
     actual length of the received file    is determined by the data
     transfer.    A file may grow    after transmission commences, and
     all the data will be sent.

Modification Date A single space separates the modification date
     from the file length.

     The mod date is optional, and the filename    and length may be
     sent without requiring the    mod date to be sent.

     The mod date is sent as an    octal number giving the    time the
     contents of the file were last changed measured in    seconds    from
     Jan 1 1970    Universal Coordinated Time (GMT).  A date of 0
     implies the modification date is unknown and should be left as
     the date the file is received.

     This standard format was chosen to    eliminate ambiguities
     arising from transfers between different time zones.


File Mode A single space separates the file mode from the
     modification date.     The file mode is stored as an octal string.
     Unless the    file originated    from a Unix system, the    file mode is
     set to 0.    rz(1) checks the file mode for the 0x8000 bit which
     indicates a Unix type regular file.  Files    with the 0x8000    bit
     set are assumed to    have been sent from another Unix (or
     similar) system which uses    the same file conventions.  Such
     files are not translated in any way.


Serial Number A    single space separates the serial number from the
     file mode.     The serial number of the transmitting program is
     stored as an octal    string.     Programs which    do not have a serial


__________

 1. Fields may not be skipped.




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Chapter    13             ZMODEM Protocol                33



     number should omit    this field, or set it to 0.  The receiver's
     use of this field is optional.

The file information is    terminated by a    null.  If only the pathname
is sent, the pathname is terminated with two nulls.  The length    of
the file information subpacket,    including the trailing null, must
not exceed 1024    bytes; a typical length    is less    than 64    bytes.


14.  PERFORMANCE RESULTS

14.1  Compatibility

Extensive testing has demonstrated ZMODEM to be    compatible with
satellite links, packet    switched networks, microcomputers,
minicomputers, regular and error correcting buffered modems at 75 to
19200 bps.  ZMODEM's economy of    reverse    channel    bandwidth allows
modems that dynamically    partition bandwidth between the    two
directions to operate at optimal speeds.

14.2  Throughput

Between    two single task    PC-XT computers    sending    a program image    on
an in house Telenet link, SuperKermit provided 72 ch/sec throughput
at 1200    baud.  YMODEM-k    yielded    85 chars/sec, and ZMODEM provided
113 chars/sec.    XMODEM was not measured, but would have    been much
slower based on    observed network propagation delays.

Recent tests downloading large binary files to an IBM PC (4.7 mHz
V20) running YAMK 16.30    with table driven 32 bit CRC calculation
yielded    a throughput of    1870 cps on a 19200 bps    direct connection.

Tests with TELEBIT TrailBlazer modems have shown transfer rates
approaching 1400 characters per    second for long    files.    When files
are compressed,    effective transfer rates of 2000 characters per
second are possible.


14.3  Error Recovery

Some tests of ZMODEM protocol error recovery performance have been
made.  A PC-AT with SCO    SYS V Xenix or DOS 3.1 was connected to    a PC
with DOS 2.1 either directly at    9600 bps or with unbuffered dial-up
1200 bps modems.  The ZMODEM software was configured to    use 1024
byte data subpacket lengths above 2400 bps, 256    otherwise.

Because    no time    delays are necessary in    normal file transfers, per
file negotiations are much faster than with YMODEM, the    only
observed delay being the time required by the program(s) to update
logging    files.




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During a file transfer,    a short    line hit seen by the receiver
usually    induces    a CRC error.  The interrupt sequence is    usually    seen
by the sender before the next data subpacket is    completely sent, and
the resultant loss of data throughput averages about half a data
subpacket per line hit.     At 1200 bps this is would be about .75
second lost per    hit.  At 10-5 error rate, this would degrade
throughput by about 9 per cent.

The throughput degradation increases with increasing channel delay,
as more    data subpackets    in transit through the channel are discarded
when an    error is detected.

A longer noise burst that affects both the receiver and    the sender's
reception of the interrupt sequence usually causes the sender to
remain silent until the    receiver times out in 10 seconds.  If the
round trip channel delay exceeds the receiver's    10 second timeout,
recovery from this type    of error may become difficult.

Noise affecting    only the sender    is usually ignored, with one common
exception.  Spurious XOFF characters generated by noise    stop the
sender until the receiver times    out and    sends an interrupt sequence
which concludes    with an    XON.

In summation, ZMODEM performance in the    presence of errors resembles
that of    X.PC and SuperKermit.  Short bursts cause minimal data
retransmission.     Long bursts (such as pulse dialing noises) often
require    a timeout error    to restore the flow of data.


15.  PACKET SWITCHED NETWORK CONSIDERATIONS

Flow control is    necessary for printing messages    and directories, and
for streaming file transfer protocols.    A non transparent flow
control    is incompatible    with XMODEM and    YMODEM transfers.  XMODEM
and YMODEM protocols require complete transparency of all 256 8    bit
codes to operate properly.

The "best" flow    control    (when X.25 or hardware CTS is unavailable)
would not "eat"    any characters at all.    When the PAD's buffer almost
fills up, an XOFF should be emitted.  When the buffer is no longer
nearly full, send an XON.  Otherwise, the network should neither
generate nor eat XON or    XOFF control characters.

On Telenet, this can be    met by setting CCIT X3 5:1 and 12:0 at both
ends of    the network.  For best throughput, parameter 64    (advance
ACK) should be set to something    like 4.     Packets should    be forwarded
when the packet    is a full 128 bytes, or    after a    moderate delay
(3:0,4:10,6:0).

With PC-Pursuit, it is sufficient to set parameter 5 to    1 at both
ends after one is connected to the remote modem.



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Chapter    15             ZMODEM Protocol                35



    <ENTER>@<ENTER>
    set 5:1<ENTER>
    rst? 5:1<ENTER>
    cont<ENTER>

Unfortunately, many PADs do not    accept the "rst?" command.

For YMODEM, PAD    buffering should guarantee that    a minimum of 1040
characters can be sent in a burst without loss of data or generation
of flow    control    characters.  Failure to    provide    this buffering will
generate excessive retries with    YMODEM.

         TABLE 1.  Network and Flow    Control    Compatibility

______________________________________________________________________________
|   Connectivity    |  Interactive|  XMODEM|  WXMODEM|    SUPERKERMIT|  ZMODEM |
______________________________________________________________________________
|___________________|_____________|________|_________|_____________|_________|
|Direct    Connect        |  YES      |  YES   |  YES    |    YES       |  YES    |
|___________________|_____________|________|_________|_____________|_________|
|Network, no FC        |  no      |  YES   |  (4)    |    (6)       |  YES (1)|
|___________________|_____________|________|_________|_____________|_________|
|Net, transparent FC|  YES      |  YES   |  YES    |    YES       |  YES    |
|___________________|_____________|________|_________|_____________|_________|
|Net, non-trans. FC |  YES      |  no       |  no (5) |    YES       |  YES    |
|___________________|_____________|________|_________|_____________|_________|
|Network, 7 bit        |  YES      |  no       |  no     |    YES (2)       |  YES (3)|
|___________________|_____________|________|_________|_____________|_________|

(1) ZMODEM can optimize    window size or burst length for    fastest
transfers.
(2) Parity bits    must be    encoded, slowing binary    transfers.
(3) Natural protocol extension possible    for encoding data to 7 bits.
(4) Small WXMODEM window size may may allow operation.
(5) Some flow control codes are    not escaped in WXMODEM.
(6) Kermit window size must be reduced to avoid    buffer overrun.


16.  PERFORMANCE COMPARISON TABLES


"Round Trip Delay Time"    includes the time for the last byte in a
packet to propagate through the    operating systems and network to the
receiver, plus the time    for the    receiver's response to that packet
to propagate back to the sender.

The figures shown below    are calculated for round trip delay times of
40 milliseconds    and 5 seconds.    Shift registers    in the two computers
and a pair of 212 modems generate a round trip delay time on the
order of 40 milliseconds.  Operation with busy timesharing computers
and networks can easily    generate round trip delays of five seconds.



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Chapter    16             ZMODEM Protocol                36



Because    the round trip delays cause visible interruptions of data
transfer when using XMODEM protocol, the subjective effect of these
delays is greatly exaggerated, especially when the user    is paying
for connect time.

A 102400 byte binary file with randomly    distributed codes is sent at
1200 bps 8 data    bits, 1    stop bit.  The calculations assume no
transmission errors.  For each of the protocols, only the per file
functions are considered.  Processor and I/O overhead are not
included.  YM-k    refers to YMODEM with 1024 byte    data packets.  YM-g
refers to the YMODEM "g" option.  ZMODEM uses 256 byte data
subpackets for this example.  SuperKermit uses maximum standard
packet size, 8 bit transparent transmission, no    run length
compression.  The 4 block WXMODEM window is too    small to span the 5
second delay in    this example; the resulting thoughput degradation is
ignored.

For comparison,    a straight "dump" of the file contents with no file
management or error checking takes 853 seconds.

         TABLE 2.  Protocol Overhead Information
       (102400 byte    binary file, 5 Second Round Trip)

____________________________________________________________________________
|      Protocol          |     XMODEM|  YM-k |  YM-g|     ZMODEM|  SKermit|  WXMODEM|
____________________________________________________________________________
|_____________________|________|_______|______|________|_________|_________|
|Protocol Round    Trips |     804   |  104  |  5   |     5     |  5     |  4       |
|_____________________|________|_______|______|________|_________|_________|
|Trip Time at 40ms    |     32s   |  4s   |  0   |     0     |  0     |  0       |
|_____________________|________|_______|______|________|_________|_________|
|Trip Time at 5s      |     4020s |  520s |  25s |     25s   |  25     |  20       |
____________________________________________________________________________
|_____________________|________|_______|______|________|_________|_________|
|Overhead Characters  |     4803  |  603  |  503 |     3600  |  38280     |  8000   |
____________________________________________________________________________
|_____________________|________|_______|______|________|_________|_________|
|Line Turnarounds     |     1602  |  204  |  5   |     5     |  2560     |  1602   |
____________________________________________________________________________
|_____________________|________|_______|______|________|_________|_________|
|Transfer Time at 0s  |     893s  |  858s |  857s|     883s  |  1172s     |  916s   |
|_____________________|________|_______|______|________|_________|_________|
|Transfer Time at 40ms|     925s  |  862s |  857s|     883s  |  1172s     |  916s   |
|_____________________|________|_______|______|________|_________|_________|
|Transfer Time at 5s  |     5766s |  1378s|  882s|     918s  |  1197s     |  936s   |
|_____________________|________|_______|______|________|_________|_________|








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Chapter    16             ZMODEM Protocol                37



         Figure    5.  Transmission Time Comparison
       (102400 byte    binary file, 5 Second Round Trip)

************************************************** XMODEM
************ YMODEM-K
********** SuperKermit (Sliding    Windows)
*******    ZMODEM 16kb Segmented Streaming
*******    ZMODEM Full Streaming
*******    YMODEM-G

    TABLE 3.  Local    Timesharing Computer Download Performance

__________________________________________________________________________
|    Command    |  Protocol|  Time/HD|    Time/FD|  Throughput|  Efficiency|
__________________________________________________________________________
|_______________|__________|_________|_________|____________|____________|
|kermit    -x    |  Kermit  |  1:49   |    2:03   |  327        |  34%     |
|_______________|__________|_________|_________|____________|____________|
|sz -Xa    phones.t|  XMODEM  |  1:20   |    1:44   |  343        |  36%     |
|_______________|__________|_________|_________|____________|____________|
|sz -a phones.t    |  ZMODEM  |   :39   |     :48   |  915        |  95%     |
|_______________|__________|_________|_________|____________|____________|


Times were measured downloading    a 35721    character text file at 9600
bps, from Santa    Cruz SysV 2.1.2    Xenix on a 9 mHz IBM PC-AT to DOS
2.1 on an IBM PC.  Xenix was in    multiuser mode but otherwise idle.
Transfer times to PC hard disk and floppy disk destinations are
shown.

C-Kermit 4.2(030) used server mode and file compression, sending to
Pro-YAM    15.52 using 0 delay and    a "get phones.t" command.

Crosstalk XVI 3.6 used XMODEM 8    bit checksum (CRC not available) and
an "ESC    rx phones.t" command.  The Crosstalk time does not include
the time needed    to enter the extra commands not    needed by Kermit and
ZMODEM.

Professional-YAM used ZMODEM AutoDownload.  ZMODEM times included a
security challenge to the sending program.














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Chapter    16             ZMODEM Protocol                38



              TABLE 4.    File Transfer Speeds

________________________________________________________________________________
| Prot          file     bytes      bps      ch/sec           Notes           |
________________________________________________________________________________
|X      jancol.c     18237      2400     53         Tymnet PTL    5/3/87           |
|X      source.xxx     6143      2400     56         Source/Telenet PTL    5/29/87|
|X      jancol.c     18237      2400     64         Tymnet PTL               |
|XN      c-format.arc     18432      1200     84         GEnie PTL               |
|B      jancol.c     18237      1200     87         DataPac (604-687-7144)    |
|B/ovth      emaibm.arc     51200      1200     101         CIS PTL MNP           |
|ZM      jancol.c     18237      1200     112         DataPac (604-687-7144)    |
|X/ovth      emaibm.arc     51200      1200     114         CIS PTL MNP           |
|ZM      emaibm.arc     51200      1200     114         CIS PTL MNP           |
|B      jancol.c     18237      2400     124         Tymnet PTL               |
|B      YI0515.87     9081      2400     157         CIS PTL node 5/29/87      |
|SK      source.xxx     6143      2400     170         Source/Telenet PTL    5/29/87|
|ZM      jancol.c     18237      2400     221         Tymnet PTL    upl/dl           |
|ZM      jancol.c     18237      2400     224         Tymnet PTL               |
|ZM      jancol.c     18237      2400     226/218     TeleGodzilla upl           |
|ZM      jancol.c     18237      2400     226         Tymnet PTL    5/3/87           |
|ZM      zmodem.ov     35855      2400     227         CIS PTL node           |
|C      jancol.c     18237      2400     229         Tymnet PTL    5/3/87           |
|ZM      jancol.c     18237      2400     229/221     TeleGodzilla           |
|ZM      zmodem.ov     35855      2400     229         CIS PTL node upl           |
|ZM      jancol.c     18237      2400     232         CIS PTL node           |
|ZM      mbox         473104      9600     948/942     TeleGodzilla upl           |
|ZM      zmodem.arc     318826      14k     1357/1345   TeleGodzilla           |
|ZM      mbox         473104      14k     1367/1356   TeleGodzilla upl           |
|ZM      c2.doc     218823      38k     3473         Xenix 386 Toolkit upl     |
|ZM      mbox         511893      38k     3860         386 Xenix 2.2 Beta    #      |
|ZM      c.doc         218823      57k     5611         **                   |
|______________________________________________________________________________|

Times are for downloads    unless noted.  Where two speeds    are noted,
the faster speed is reported by    the receiver because its transfer
time calculation excludes the security check and transaction log
file processing.  The TeleGodzilla computer is a 4.77 mHz IBM PC
with a 10 MB hard disk.     The 386 computer uses an Intel    motherboard
at 18 mHz 1ws.    The AT Clone (QIC) runs    at 8 mHz 0ws.
Abbreviations:
 B     Compuserve B Protocol
 B/ovth              CIS B with Omen Technology OverThruster(TM)
 C     Capture DC2/DC4 (no protocol)
 K     Kermit
 MNP   Microcom    MNP error correcting SX/1200 modem
 PTL   Portland    Oregon network node
 SK    Sliding Window Kermit (SuperKermit) w=15
 X     XMODEM
 XN    XMODEM protocol implemented in network modes
 X/ovth              XMODEM, Omen Technology OverThruster(TM)



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Chapter    16             ZMODEM Protocol                39



 ZM    ZMODEM
 Tk    Xenix 386 Toolkit, rz compiled -M3, dumb    serial port
 **    AT Clone    ramdisk    to 386 ramdisk,    or either ramdisk to nul
 #     On the fly format translation NL    to CR/LF
               TABLE 5.     Protocol Checklist

_________________________________________________________________________
|Item               XMODEM  WXMODEM    YMDM-k     YMDM-g     ZMODEM     SKermit|
_________________________________________________________________________
|IN SERVICE        |  1977  | 1986   |    1982   | 1985  | 1986  | 1985    |
|___________________|________|________|________|_______|_______|________|
|USER FEATURES        |         |          |           |       |       |    |
|User Friendly I/F  |  -     | -      |    -      | -     | YES   | -    |
|Commands/batch        |  2*N   | 2*N    |    2      | 2     | 1     | 1(1)    |
|Commands/file        |  2     | 2      |    0      | 0     | 0     | 0    |
|Command Download   |  -     | -      |    -      | -     | YES   | YES(6)    |
|Menu Compatible    |  -     | -      |    -      | -     | YES   | -    |
|Transfer Recovery  |  -     | -      |    -      | -     | YES   | -    |
|File Management    |  -     | -      |    -      | -     | YES   | -    |
|Security Check        |  -     | -      |    -      | -     | YES   | -    |
|YMODEM    Fallback    |  YES   | YES    |    YES    | YES   | YES   | -    |
|___________________|________|________|________|_______|_______|________|
|COMPATIBILITY        |         |          |           |       |       |    |
|Dynamic Files        |  YES   | YES    |    FAIL   | FAIL  | YES   | YES    |
|Packet    SW NETS        |  -     | YES    |    -      | -     | YES   | YES    |
|7 bit PS NETS        |  -     | -      |    -      | -     | (8)   | YES    |
|Old Mainframes        |  -     | -      |    -      | -     | (8)   | YES    |
|CP/M-80        |  YES   | YES    |    YES    | -     | YES(9)| -    |
|___________________|________|________|________|_______|_______|________|
|ATTRIBUTES        |         |          |           |       |       |    |
|Reliability(5)        |  fair  | poor   |    fair(5)| none  | BEST  | HIGH    |
|Streaming        |  -     | YES    |    -      | YES   | YES   | YES    |
|Overhead(2)        |  7%    | 7%     |    1%     | 1%    | 1%    | 30%    |
|Faithful Xfers        |  -     | -      |    YES    | YES   | YES   | YES    |
|Preserve Date        |  -     | -      |    YES    | YES   | YES   | -    |
|___________________|________|________|________|_______|_______|________|
|COMPLEXITY        |         |          |           |       |       |    |
|No-Wait Sample        |  -     | REQD   |    -      | -     | opt   | REQD    |
|Ring Buffers        |  -     | REQD   |    -      | -     | opt   | REQD    |
|XMODEM    Similar        |  YES   | LOW    |    HIGH   | HIGH  | LOW   | NONE    |
|Complexity        |  LOW(5)| MED    |    LOW(5) | LOW   | MED   | HIGH    |
|___________________|________|________|________|_______|_______|________|
|EXTENSIONS        |         |          |           |       |       |    |
|Server    Operation   |  -     | -      |    -      | -     | YES(4)| YES    |
|Multiple Threads   |  -     | -      |    -      | -     | future| -    |
_________________________________________________________________________

NOTES:
(1) Server mode    or Omen    Technology Kermit AutoDownload
(2) Character count, binary file, transparent channel
(3) 32 bit math    needed for accurate transfer (no garbage added)



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Chapter    16             ZMODEM Protocol                40



(4) AutoDownload operation
(5) Cybernetic Data Recovery(TM) improves XMODEM and YMODEM
reliability with complex proprietary logic.
(6) Server commands only
(7) No provision for transfers across time zones
(8) Future enhancement provided    for
(9) With Segmented Streaming
WXMODEM: XMODEM    derivative protocol with data encoding and windowing














































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Chapter    16             ZMODEM Protocol                41



17.  FUTURE EXTENSIONS

Future extensions include:

   + Compatibility with    7 bit networks

   + Server/Link Level operation: An END-TO-END    error corrected
     program to    program    session    is required for    financial and other
     sensitive applications.

   + Multiple independent threads

   + Encryption

   + Compression

   + File Comparison

   + Selective transfer    within a file (e.g., modified segments of a
     database file)

   + Selective Retransmission for error    correction


18.  REVISIONS

07-31-1987 The receiver    should ignore a    ZEOF with an offset that
does not match the current file    length.     The previous action of
responding with    ZRPOS caused transfers to fail if a CRC    error
occurred immediately before end    of file, because two retransmission
requests were being sent for each error.  This has been    observed
under exceptional conditions, such as data transmission    at speeds
greater    than the receiving computer's interrupt    response capabilitiy
or gross misapplication    of flow    control.

Discussion of the Tx backchannel garbage count and ZCRCW after error
ZRPOS was added.  Many revisions for clarity.
07-09-87 Corrected XMODEM's development    date, incorrectly stated as
1979 instead of    the actual August 1977.     More performance data was
added.
05-30-87 Added ZMNEW and ZMSKNOLOC
05-14-87 Window    management, ZACK zshhdr    XON removed, control
character escaping, ZMSPARS changed to ZXPARS, editorial changes.
04-13-87  The ZMODEM file transfer protocol's public domain status
is emphasized.
04-04-87: minor    editorial changes, added conditionals for overview
version.
03-15-87: 32 bit CRC added.
12-19-86: 0 Length ZCRCW data subpacket    sent in    response to ZPAD or
ZDELE detected on reverse channel has been changed to ZCRCE.  The
reverse    channel    is now checked for activity before sending each



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Chapter    18             ZMODEM Protocol                42



ZDATA header.
11-08-86: Minor    changes    for clarity.
10-2-86:  ZCNL definition expanded.
9-11-86:  ZMPROT file management option    added.
8-20-86:  More performance data    included.
8-4-86:     ASCII DLE (Ctrl-P, 020) now escaped; compatible with
previous versions.  More document revisions for    clarity.
7-15-86: This document was extensively edited to improve clarity and
correct    small errors.  The definition of the ZMNEW management option
was modified, and the ZMDIFF management    option was added.  The
cancel sequence    was changed from two to    five CAN characters after
spurious two character cancel sequences    were detected.


19.  MORE INFORMATION

Please contact Omen Technology for troff source    files and typeset
copies of this document.


19.1  TeleGodzilla Bulletin Board

More information may be    obtained by calling the    TeleGodzilla
bulletin board at 503-621-3746.     TeleGodzilla supports 2400 and    1200
bps callers with automatic speed recognition.  Once connected,
Telebit    TrailBlazer uses can keyboard
            trailblazer
to switch the modems to    high speed (19kb) operation.

Relevant files include YZMODEM.ZOO, YAMDEMO.ZOO, YAMHELP.ZOO,
ZCOMMEXE.ARC, ZCOMMDOC.ARC, ZCOMMHLP.ARC, and YMODEM.DQC.

Useful commands    for TeleGodzilla include "menu", "dir",    "sx file
(XMODEM)", "kermit sb file ...", and "sz file ...".

19.2  Unix UUCP    Access

UUCP sites can obtain the current version of this file with
          uucp omen!/u/caf/public/zmodem.doc /tmp
A continually updated list of available    files is stored    in
/usr/spool/uucppublic/FILES.
       uucp     omen!~uucp/FILES   /usr/spool/uucppublic

The following L.sys line allows    UUCP to    call site "omen" via Omen's
bulletin board system "TeleGodzilla".  TeleGodzilla is an instance
of Omen    Technology's Professional-YAM in host operation, acting    as a
bulletin board and front ending    a Xenix    system.

In response to TeleGodzilla's "Name Please:" (e:--e:), uucico gives
the Pro-YAM "link" command as a    user name.  Telegodzilla then asks
for a link password (d:).  The password    (Giznoid) controls access to



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Chapter    19             ZMODEM Protocol                43



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 sees the Xenix "Login:" message (n:--
n:), and logs in as "uucp".  No    password is used for the uucp
account.

omen Any ACU 2400 1-503-621-3746 e:--e:    link d:    Giznoid    n:--n: uucp



20.  ZMODEM PROGRAMS

A copy of this document, a demonstration version of
Professional-YAM, a flash-up tree structured help file and
processor, are available in YZMODEM.ZOO    on TeleGodzilla    and other
bulletin boards.  This file must be unpacked with LOOZ.EXE, also
available on TeleGodzilla.  YZMODEM.ZOO    may be distributed provided
none of    the files are deleted or modified without the written
consent    of Omen    Technology.

TeleGodzilla and other bulletin    boards also feature ZCOMM, a
shareware communications program.  ZCOMM includes Omen Technology's
TurboLearn(TM) Script Writer, ZMODEM, Omen's highly acclaimed XMODEM
and YMODEM protocol support, Sliding Windows Kermit, several
traditional protocols, a powerful script language, and the most
accurate VT100/102 emulation available in a usr    supported program.
The ZCOMM files    include:

  + ZCOMMEXE.ARC Executable files and beginner's telephone directory

  + ZCOMMDOC.ARC "Universal Line Printer Edition" Manual

  + ZCOMMHLP.ARC Tree structured Flash-UP help processor and
    database

Source code and    manual pages for the Unix/Xenix    rz and sz programs
are available on TeleGodzilla in RZSZ.ZOO.  This ZOO archive may be
unpacked with LOOZ.EXE,    also available on TeleGodzilla.     Most Unix
like systems are supported, including V7, Sys III, 4.x BSD, SYS    V,
Idris, Coherent, and Regulus.

RZSZ.ZOO includes a ZCOMM/Pro-YAM/PowerCom script ZUPL.T to upload
the small (178 lines) YMODEM bootstrap program MINIRB.C    without    a
file transfer protocol.     MINIRB    uses the Unix stty(1) program to set
the required raw tty modes, and    compiles without special flags on
virtually all Unix and Xenix systems.  ZUPL.T directs the Unix
system to compile MINIRB, then uses it as a bootstrap to upload    the
rz/sz source and manual    files.




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Chapter    20             ZMODEM Protocol                44



The PC-DOS Opus    bulletin board supports    ZMODEM.     Integrated ZMODEM
support    for the    Collie bulletin    board program is planned.

A number of other bulletin board programs support ZMODEM with
external modules (DSZ, etc.).

The PC-DOS Teleconferencing system IN-SYNCH uses ZMODEM.

Other PC-DOS communications programs with ZMODEM support modules
include    QMODEM and BOYAN.  Crosstalk is    believed to be adding ZMODEM
to Mark    IV.

The ZMDM communications    program    by Jwahar Bammi    runs on    Atari ST
machines.

A program for the Amiga    is also    in development.

The Compuserve Information Service has ported the Unix rz/sz ZMODEM
programs to DECSYSTEM 20 assembler.

20.1  Adding ZMODEM to DOS Programs

DSZ is a small program that supports XMODEM, YMODEM, and ZMODEM    file
transfers.  DSZ    is designed to be called from a    bulletin board
program    or another communications program.  It may be called as
             dsz port 2    sz file1 file2
to send    files, or as
               dsz port 2 rz
to receive zero    or more    file(s), or as
             dsz port 2    rz filea fileb
to receive two files, the first    to filea and the second    (if sent) to
fileb.    This form of dsz may be    used to    control    the pathname of
incoming file(s).  In this example, if the sending program attempted
to send    a third    file, the transfer would be terminated.

Dsz uses DOS stdout for    messages (no direct CRT    access), acquires
the COMM port vectors with standard DOS    calls, and restores the    COMM
port's interrupt vector    and registers upon exit.

Further    information on dsz may be found    in dsz.doc and the ZCOMM or
Pro-YAM    user manuals.


21.  YMODEM PROGRAMS

The Unix rz/sz programs    support    YMODEM as well as ZMODEM.  Most    Unix
like systems are supported, including V7, Sys III, 4.2 BSD, SYS    V,
Idris, Coherent, and Regulus.

A version for VAX-VMS is available in VRBSB.SHQ, in the    same
directory.



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Chapter    21             ZMODEM Protocol                45



Irv Hoff has added 1k packets and YMODEM 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.

Many other programs, including MEX-PLUS    and Crosstalk Mark IV also
support    some of    YMODEM's features.

Questions about    YMODEM,    the Professional-YAM communications program,
and requests for evaluation copies may be directed to:

     Chuck Forsberg
     Omen Technology Inc
     17505-V Sauvie Island Road
     Portland Oregon 97231
     VOICE: 503-621-3406 :VOICE
     Modem (TeleGodzilla): 503-621-3746
     Usenet: ...!tektronix!reed!omen!caf
     Compuserve: 70007,2304
     Source: TCE022


22.  ACKNOWLEDGMENTS

ZMODEM was developed for the public domain under a Telenet contract.
The ZMODEM protocol descriptions and the Unix rz/sz program source
code are public    domain.     No licensing, trademark, or copyright
restrictions apply to the use of the protocol, the Unix    rz/sz source
code and the ZMODEM name.

Encouragement and suggestions by Thomas    Buck, Ward Christensen,    Earl
Hall, Irv Hoff,    Stuart Mathison, and John Wales, are gratefully
acknowledged.  32 bit CRC code courtesy    Gary S.    Brown.


23.  RELATED FILES

The following files may    be useful while    studying this document:

YMODEM.DOC Describes the XMODEM, XMODEM-1k, and    YMODEM batch file
    transfer protocols.  This file is available on TeleGodzilla
    as YMODEM.DQC.

zmodem.h Definitions for ZMODEM    manifest constants

rz.c, sz.c, rbsb.c Unix    source code for    operating ZMODEM programs.

rz.1, sz.1 Manual pages    for rz and sz (Troff sources).

zm.c    Operating system independent low level ZMODEM subroutines.




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Chapter    23             ZMODEM Protocol                46



minirb.c A YMODEM bootstrap program, 178 lines.

RZSZ.ZOO,rzsz.arc Contain the C    source code and    manual pages listed
    above, plus a ZCOMM script to upload minirb.c to a Unix    or
    Xenix system, compile it, and use the program to upload    the
    ZMODEM source files with error checking.

DSZ.ZOO,dsz.arc    Contains DSZ.COM, a shareware X/Y/ZMODEM subprogram,
    DESQview "pif" files for background operation in minimum
    memory,    and DSZ.DOC.

ZCOMM*.ARC Archive files for ZCOMM, a powerful shareware
    communications program.









































Chapter    23        Rev 8-3-87  Typeset 8-4-87            46











                  CONTENTS


 1.  INTENDED AUDIENCE................................................     2

 2.  WHY DEVELOP ZMODEM?..............................................     2

 3.  ZMODEM Protocol Design Criteria..................................     4
     3.1    Ease of Use...............................................     4
     3.2    Throughput................................................     5
     3.3    Integrity and Robustness..................................     6
     3.4    Ease of Implementation....................................     6

 4.  EVOLUTION OF ZMODEM..............................................     7

 5.  ROSETTA STONE....................................................    10

 6.  ZMODEM REQUIREMENTS..............................................    10
     6.1    File Contents.............................................    10

 7.  ZMODEM BASICS....................................................    12
     7.1    Packetization.............................................    12
     7.2    Link Escape    Encoding......................................    12
     7.3    Header....................................................    13
     7.4    Binary Data    Subpackets....................................    16
     7.5    ASCII Encoded Data Subpacket..............................    16

 8.  PROTOCOL TRANSACTION OVERVIEW....................................    16
     8.1    Session Startup...........................................    16
     8.2    File Transmission.........................................    18
     8.3    Session Cleanup...........................................    20
     8.4    Session Abort Sequence....................................    20

 9.  STREAMING TECHNIQUES / ERROR RECOVERY............................    21
     9.1    Full Streaming with    Sampling..............................    21
     9.2    Full Streaming with    Reverse    Interrupt.....................    23
     9.3    Full Streaming with    Sliding    Window........................    23
     9.4    Full Streaming over    Error Free Channels...................    24
     9.5    Segmented Streaming.......................................    24

10.  ATTENTION SEQUENCE...............................................    24

11.  FRAME TYPES......................................................    25
     11.1   ZRQINIT...................................................    25
     11.2   ZRINIT....................................................    25
     11.3   ZSINIT....................................................    25
     11.4   ZACK......................................................    26
     11.5   ZFILE.....................................................    26
     11.6   ZSKIP.....................................................    28
     11.7   ZNAK......................................................    28
     11.8   ZABORT....................................................    28



                  - i -











     11.9   ZFIN......................................................    28
     11.10  ZRPOS.....................................................    28
     11.11  ZDATA.....................................................    29
     11.12  ZEOF......................................................    29
     11.13  ZFERR.....................................................    29
     11.14  ZCRC......................................................    29
     11.15  ZCHALLENGE................................................    29
     11.16  ZCOMPL....................................................    29
     11.17  ZCAN......................................................    29
     11.18  ZFREECNT..................................................    29
     11.19  ZCOMMAND..................................................    29

12.  SESSION TRANSACTION EXAMPLES.....................................    30
     12.1   A simple file transfer....................................    30
     12.2   Challenge and Command Download............................    31

13.  ZFILE FRAME FILE INFORMATION.....................................    31

14.  PERFORMANCE RESULTS..............................................    33
     14.1   Compatibility.............................................    33
     14.2   Throughput................................................    33
     14.3   Error Recovery............................................    33

15.  PACKET SWITCHED NETWORK CONSIDERATIONS...........................    34

16.  PERFORMANCE COMPARISON TABLES....................................    35

17.  FUTURE EXTENSIONS................................................    41

18.  REVISIONS........................................................    41

19.  MORE INFORMATION.................................................    42
     19.1   TeleGodzilla Bulletin Board...............................    42
     19.2   Unix UUCP Access..........................................    42

20.  ZMODEM PROGRAMS..................................................    43
     20.1   Adding ZMODEM to DOS Programs.............................    44

21.  YMODEM PROGRAMS..................................................    44

22.  ACKNOWLEDGMENTS..................................................    45

23.  RELATED FILES....................................................    45


LIST OF    FIGURES


Figure 1.  Order of Bytes in Header...................................    14

Figure 2.  16 Bit CRC Binary Header...................................    14



                  - ii -











Figure 3.  32 Bit CRC Binary Header...................................    14

Figure 4.  HEX Header.................................................    15

Figure 5.  Transmission    Time Comparison...............................    37


LIST OF    TABLES


TABLE 1.  Network and Flow Control Compatibility......................    35

TABLE 2.  Protocol Overhead Information...............................    36

TABLE 3.  Local    Timesharing Computer Download Performance.............    37

TABLE 4.  File Transfer    Speeds........................................    38

TABLE 5.  Protocol Checklist..........................................    39



































                 - iii -









       The ZMODEM Inter Application    File Transfer Protocol

                  Chuck Forsberg

               Omen    Technology Inc


                 ABSTRACT



The ZMODEM file    transfer protocol provides reliable file and command
transfers with complete    END-TO-END data    integrity between application
programs.  ZMODEM's 32 bit CRC catches errors that continue to sneak into
even the most advanced networks.

ZMODEM rapidly transfers files,    particularly with buffered (error
correcting) modems, timesharing    systems, satellite relays, and wide area
packet switched    networks.

ZMODEM greatly simplifies file transfers compared to XMODEM.  In addition
to a friendly user interface, ZMODEM provides Personal Computer    and other
users an efficient, accurate, and robust file transfer method.

ZMODEM provides    advanced file management features including AutoDownload
(Automatic file    Download initiated without user    intervention), Crash
Recovery, selective file transfers, and    security verified command
downloading.

ZMODEM protocol    features allow implementation on a wide    variety    of systems
operating in a wide variety of environments.  A    choice of buffering and
windowing modes    allows ZMODEM to operate on systems that cannot    support
other streaming    protocols.  Finely tuned control character escaping allows
operation with real world networks without Kermit's high overhead.

Although ZMODEM    software is more complex than unreliable XMODEM    routines,
actual C source    code to    production programs allows developers to upgrade
their applications with    efficient, reliable ZMODEM file    transfers with a
minimum    of effort.

ZMODEM is carefully designed to    provide    these benefits using a minimum of
new software technology.  ZMODEM can be    implemented on all but the most
brain damaged computers.

ZMODEM was developed for the public domain under a Telenet contract.  The
ZMODEM protocol    descriptions and the Unix rz/sz    program    source code are
public domain.    No licensing, trademark, or copyright restrictions apply
to the use of the protocol, the    Unix rz/sz source code and the ZMODEM
name.