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The ZMODEM Inter Application File Transfer Protocol

	   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)

This file may be redistributed without restriction provided the text is
not altered.

Please distribute as widely as possible.

		       Omen Technology Incorporated
		      The High Reliability Software

		   17505-V Northwest Sauvie Island Road
			  Portland Oregon 97231
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Chapter 0                    ZMODEM Protocol                             2


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.


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

   + 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, DART, 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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Chapter 2                    ZMODEM Protocol                             3

   + 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

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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Chapter 2                    ZMODEM Protocol                             4

   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-1k.

 + 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

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.

 + The sender can pass commands and/or modifiers to the receiving program.

 + File names need be entered only once.

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Chapter 3                    ZMODEM Protocol                             5

 + Menu selections are supported.

 + 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 optionally 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.

  + Traditional 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

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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Chapter 3                    ZMODEM Protocol                             6

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 Omen Technology's
Cybernetic Data Recovery(TM)[3] are not needed for reliable transfers.
This complete protection of data and supervisory information accounts for
most of ZMODEM's high reliability compared to XMODEM derived protocols
including WXMODEM, SEAlink, MEGAlink, etc.

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 available.  The 32 bit CRC reduces undetected errors by
at least five orders of magnitude when properly applied (-1 preset,

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.


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

 3. Unique to Professional-YAM, ZCOMM, and PowerCom

 4. Streaming strategies are discussed in coming chapters.

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Chapter 3                    ZMODEM Protocol                             7

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

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


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

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

Managing the window[2] was another problem.  Experience gained in


 1. Without stopping for a response

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

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Chapter 4                    ZMODEM Protocol                             8

debugging The Source's SuperKermit protocol indicated a window size of
about 1000 characters was needed at 1200 bps.  High speed modems require a
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

Another sore point with XMODEM and WXMODEM is the garbage added to files.
This was acceptable with the old CP/M files which had no exact length, but
not with newer systems such as PC-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.


 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

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Chapter 4                    ZMODEM Protocol                             9

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

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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Chapter 5                    ZMODEM Protocol                            10


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]


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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Chapter 6                    ZMODEM Protocol                            11

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.  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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Chapter 6                    ZMODEM Protocol                            12


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

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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Chapter 7                    ZMODEM Protocol                            13

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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Chapter 7                    ZMODEM Protocol                            14

		   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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Chapter 7                    ZMODEM Protocol                            15

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

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

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


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

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

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

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, containing either the serial


 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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Chapter 8                    ZMODEM Protocol                            18

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.

       The receiver has a file with the same name and length, may
       respond with a ZCRC header with a byte count, which
       requires the sender to perform a 32 bit CRC on the
       specified number of bytes in the file and transmit the
       complement of the CRC in an answering ZCRC header.[4] The
       receiver uses this information to determine whether to
       accept the file or skip it.  This sequence may be 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


 3. See below, under ZFILE header type.

 4. The crc is initialized to 0xFFFFFFFF.  A byte count of 0 implies the
    entire file.

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

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Chapter 8                    ZMODEM Protocol                            19

one or more data subpackets.

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.


 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

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

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[] = {

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Chapter 8                    ZMODEM Protocol                            21


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

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.


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


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.


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

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.  A ZCBIN from the sender overrides any other Conversion
Option given to the receiver except ZCRESUM.  A ZCBIN from the
receiver overrides any other Conversion Option sent by the sender.

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


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

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Chapter 11                   ZMODEM Protocol                            27

     corresponding to the receiver's end of file.  This
     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

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

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Chapter 11                   ZMODEM Protocol                            28

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

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

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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Chapter 11                   ZMODEM Protocol                            29

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

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.


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.


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.


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

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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Chapter 11                   ZMODEM Protocol                            30

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

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.


12.1  A simple file transfer

A simple transaction, one file, no errors, no CHALLENGE, overlapped

Sender         Receiver

ZDATA data ...

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Chapter 12                   ZMODEM Protocol                            31

12.2  Challenge and Command Download

Sender              Receiver

		    (Performs Command)


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     '',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.

Number of Files Remaining Iff the number of files remaining is sent,
     a single space separates this field from the previous field.
     This field is coded as a decimal number, and includes the
     current file.  This field is an estimate, and incorrect values
     must not be allowed to cause loss of data.  The receiver's use
     of this field is optional.

Number of Bytes Remaining Iff the number of bytes remaining is sent,
     a single space separates this field from the previous field.
     This field is coded as a decimal number, and includes the
     current file.  This field is an estimate, and incorrect values
     must not be allowed to cause loss of data.  The receiver's use
     of this field is optional.

File Type Iff the file type is sent, a single space separates this
     field from the previous field.  This field is coded as a
     decimal number.  Currently defined values are:

     0    Sequential file - no special type

     1    Other types to be defined.
     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.


14.1  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.

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Chapter 14                   ZMODEM Protocol                            34

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.

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

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Chapter 14                   ZMODEM Protocol                            35

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.


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

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

	set 5:1<ENTER>
	rst? 5:1<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.

Chapter 15           Rev Oct-14-88  Typeset 10-14-88                    35

Chapter 15                   ZMODEM Protocol                            36

	     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
(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.


"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.
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

Chapter 16           Rev Oct-14-88  Typeset 10-14-88                    36

Chapter 16                   ZMODEM Protocol                            37

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

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   |

		 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

Chapter 16           Rev Oct-14-88  Typeset 10-14-88                    37

Chapter 16                   ZMODEM Protocol                            38

	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

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

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

Chapter 16           Rev Oct-14-88  Typeset 10-14-88                    38

Chapter 16                   ZMODEM Protocol                            39

		      TABLE 4.  File Transfer Speeds

| Prot         file        bytes    bps     ch/sec             Notes         |
|X        jancol.c         18237    2400   53          Tymnet PTL 5/3/87     |
|X       6143     2400   56          Source PTL 5/29/87    |
|X        jancol.c         18237    2400   64          Tymnet PTL            |
|XN       tsrmaker.arc     25088    1200   94          GEnie PTL             |
|B/ovth   emaibm.arc       51200    1200   101         CIS PTL MNP           |
|UUCP     74 files, each   >7000    1200   102         (Average)             |
|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           |
|ZM       frombyte87.txt   62506    1200   117         BIX                   |
|SK       6143     2400   170         Source PTL 5/29/87    |
|ZM       jancol.c         18237    2400   221         Tymnet PTL upl/dl     |
|B/ovth   destro.gif       33613    2400   223         CIS/PTL 9-12-87       |
|ZM       jancol.c         18237    2400   224         Tymnet PTL            |
|ZM       tp40kerm.arc     112640   2400   224         BIX 6/88              |
|ZM       readme.lis       9466     2400   231         BIX 6/88              |
|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          |
|QB       gifeof.arc       32187    2400   232         CIS PTL node          |
|ZM       pcpbbs.txt       38423    2400   534         MNP Level 5           |
|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 TK upl      |
|ZM       mbox -a          511893   38k    3860        386 Xenix 2.2 Beta    |
|ZM       c.doc            218823   57k    5611        AT Clone & 386        |

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.
 B     Compuserve B Protocol
 QB    Compuserve Quick-B/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

Chapter 16           Rev Oct-14-88  Typeset 10-14-88                    39

Chapter 16                   ZMODEM Protocol                            40

 PTL   Portland Oregon network node
 SK    Sliding Window Kermit (SuperKermit) w=15
 XN    XMODEM protocol implemented in network modes
 X/ovth                 XMODEM, Omen Technology OverThruster(TM)

Chapter 16           Rev Oct-14-88  Typeset 10-14-88                    40

Chapter 16                   ZMODEM Protocol                            41

		       TABLE 5.  Protocol Checklist
Etc: Relay, BLAST, DART

|Item                      XMODEM  YMDM-k   YMDM-g  ZMODEM  SK      Etc.|
|IN SERVICE             |  1977  | 1982   | 1985  | 1986  | 1985  | ?   |
|VENDORS                |  ??    | ??     | >20   | >20   | ??    | 1   |
|HOST AVAILABILITY      |        |        |       |       |       |     |
|Compuserve             |  YES   | -      | -     | YES   | -     | -   |
|BIX                    |  YES   | -      | -     | YES   | YES   | -   |
|Portal                 |        |        | YES   | -     | -     | SOON|
|The Source             |  YES   | -      | -     | -     | YES   | -   |
|USER FEATURES          |        |        |       |       |       |     |
|User Friendly          |  -     | -      | -     | YES   | (10)  | -   |
|Commands/batch         |  2*N   | 2      | 2     | 1     | 1(1)  |     |
|Commands/file          |  2     | 0      | 0     | 0     | 0     |     |
|Command Download       |  -     | -      | -     | YES   | YES(6)| -   |
|Menu Compatible        |  -     | -      | -     | YES   | -     | -   |
|Crash Recovery         |  -     | -      | -     | YES   | -     | ??  |
|File Management        |  -     | -      | -     | YES   | -     | some|
|Security Check         |  -     | -      | -     | YES   | -     | -   |
|COMPATIBILITY          |        |        |       |       |       |     |
|Dynamic Files          |  YES   | -      | -     | YES   | YES   | ?   |
|Packet SW NETS         |  -     | -      | -     | YES   | YES   | ?   |
|7 bit PS NETS          |  -     | -      | -     | (3)   | YES   | ?   |
|Old Mainframes         |  -     | -      | -     | (3)   | YES   | ?   |
|ATTRIBUTES             |        |        |       |       |       |     |
|Reliability(5)         |  fair  | fair(5)| none  | BEST  | good  | ?   |
|Streaming              |  -     | -      | YES   | YES   | YES   |     |
|Overhead(2)            |  7%    | 1%     | 1%    | 4%(8) | 30%   |     |
|Faithful Xfers         |  -     | YES(7) | YES(7)| YES   | YES   | ?   |
|Preserve Date          |  -     | YES    | YES   | YES   | -     | ?   |
|COMPLEXITY             |        |        |       |       |       |     |
|No-Wait Sample         |  -     | -      | -     | opt   | REQD  | REQD|
|Ring Buffers           |  -     | -      | -     | opt   | REQD  | REQD|
|Complexity             |  LOW(5)| LOW(5) | LOW   | MED   | HIGH  | ?   |
|EXTENSIONS             |        |        |       |       |       |     |
|Server Operation       |  -     | -      | -     | YES(4)| YES   | ?   |
|Multiple Threads       |  -     | -      | -     | future| -     | -   |

(1) Server mode or Omen Technology Kermit AutoDownload
(2) Character count, binary file, transparent channel

Chapter 16           Rev Oct-14-88  Typeset 10-14-88                    41

Chapter 16                   ZMODEM Protocol                            42

(3) Future enhancement provided for
(4) AutoDownload operation
(5) Omen Technology's Cybernetic Data Recovery(TM) improves XMODEM
and YMODEM reliability with complex proprietary logic.
(6) Server commands only
(7) Only with True YMODEM(TM)
(8) More then 3% from protected network control characters
(9) With Segmented Streaming
(10) With Pro-YAM extensions

Chapter 16           Rev Oct-14-88  Typeset 10-14-88                    42

Chapter 16                   ZMODEM Protocol                            43


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

   + Bidirectional transfers (STEREO ZMODEM)

   + Encryption

   + Compression

   + File Comparison

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

   + Selective Retransmission for error correction


10-14-88 Pascal source code now available in Phil Burn's PibTerm
v4.2.  6-24-88  An exception to the previously unconditional ZCBIN
override: a ZCRESUM specified by the receiver need not be overridden
by the sender's ZCBIN.

11-18-87 Editorial improvements

10-27-87 Optional fields added for number of files remaining to be
sent and total number of bytes remaining to be sent.

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

Chapter 18           Rev Oct-14-88  Typeset 10-14-88                    43

Chapter 18                   ZMODEM Protocol                            44

1979 instead of the actual August 1977.  More performance data was

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

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
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.


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 19200
(Telebit PEP), 2400 and 1200 bps callers with automatic speed

Chapter 19           Rev Oct-14-88  Typeset 10-14-88                    44

Chapter 19                   ZMODEM Protocol                            45


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
	   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
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

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


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:

Chapter 20           Rev Oct-14-88  Typeset 10-14-88                    45

Chapter 20                   ZMODEM Protocol                            46

  + 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

C 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.

Pascal source code for ZMODEM support is available in PibTerm v4.2
written by Phil Burns.

The PC-DOS EXEC-PC, QuickBBS, Opus and Nochange bulletin boards
support ZMODEM.  Integrated ZMODEM support for the Collie bulletin
board program is planned.  Most of the PC-DOS bulletin board
programs that lack integrated ZMODEM support ZMODEM with external
modules (DSZ, etc.).

The BinkleyTerm, Dutchie and D'Bridge email systems support ZMODEM
as their primary protocol.

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

The LAN modem sharing program Line Plus has announced ZMODEM

Many programs have added direct ZMODEM support, including Crosstalk
Mark IV, and Telix 3.

Most other PC-DOS communications programs support external ZMODEM
via Omen Technology's DSZ, including PibTerm, Qmodem SST and BOYAN.

The ZMDM communications program by Jwahar Bammi runs on Atari ST

The Online!  and A-Talk Gold programs for the Amiga support ZMODEM.

The Byte Information eXchange supports ZMODEM.  The Compuserve
Information Service has ported the Unix rz/sz ZMODEM programs to

Chapter 20           Rev Oct-14-88  Typeset 10-14-88                    46

Chapter 20                   ZMODEM Protocol                            47

DECSYSTEM 20 assembler, and has announced future support for ZMODEM.

20.1  Adding ZMODEM to DOS Programs

DSZ is a small shareware 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
		     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.


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

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.

Chapter 21           Rev Oct-14-88  Typeset 10-14-88                    47

Chapter 21                   ZMODEM Protocol                            48

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


The High Reliability Software(TM), TurboLearn Script Writer(TM),
Cybernetic Data Recovery(TM), AutoDownload(TM), Intelligent Crash
Recovery(TM), Error Containment(TM), Full Time Capture(TM), True
YMODEM(TM), OverThruster(TM), Password Guardian(TM),
CryptoScript(TM), and TurboDial(TM) are Omen Technology trademarks.

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.


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

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.

minirb.c A YMODEM bootstrap program, 178 lines.

Chapter 23           Rev Oct-14-88  Typeset 10-14-88                    48

Chapter 23                   ZMODEM Protocol                            49

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 Oct-14-88  Typeset 10-14-88                    49


 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................................................  34
     14.3   Error Recovery............................................  34

15.  PACKET SWITCHED NETWORK CONSIDERATIONS...........................  35

16.  PERFORMANCE COMPARISON TABLES....................................  36

17.  FUTURE EXTENSIONS................................................  43

18.  REVISIONS........................................................  43

19.  MORE INFORMATION.................................................  44
     19.1   TeleGodzilla Bulletin Board...............................  44
     19.2   Unix UUCP Access..........................................  45

20.  ZMODEM PROGRAMS..................................................  45
     20.1   Adding ZMODEM to DOS Programs.............................  47

21.  YMODEM PROGRAMS..................................................  47

22.  ACKNOWLEDGMENTS..................................................  48

23.  RELATED FILES....................................................  48


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


TABLE 1.  Network and Flow Control Compatibility......................  36

TABLE 2.  Protocol Overhead Information...............................  37

TABLE 3.  Local Timesharing Computer Download Performance.............  38

TABLE 4.  File Transfer Speeds........................................  39

TABLE 5.  Protocol Checklist..........................................  41

				 - iii -

	   The ZMODEM Inter Application File Transfer Protocol

			      Chuck Forsberg

			   Omen Technology Inc


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 protects against errors that continue to
sneak into even the most advanced networks.

Unlike traditional and many recently introduced protocols, ZMODEM
safeguards all data and supervisory information with effective error

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

User Friendliness is an important ZMODEM feature.  ZMODEM AutoDownload
(Automatic file Download initiated without user intervention) greatly
simplifies file transfers compared to the traditional protocols.

ZMODEM provides advanced file management features including Crash
Recovery, flexible control of 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.

ZMODEM is the only high performance high reliability public protocol that
does not require large buffer allocations for normal file transfers.

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

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