The ZMODEM Inter-Application File Transfer Protocol

			     Chuck Forsberg

			  Omen Technology Inc

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

		 Please distribute as widely as possible.

	This document edited and typeset by J.G.Harston, 08-08-96.


		      Omen Technology Incorporated
		     The High Reliability Software

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


 1.  INTENDED AUDIENCE................................................  4

 2.  WHY DEVELOP ZMODEM?..............................................  4

 3.  ZMODEM Protocol Design Criteria..................................  6
	3.1	Ease of Use............................................  6
	3.2	Throughput.............................................  6
	3.3	Integrity and Robustness...............................  7
	3.4	Ease of Implementation.................................  7

 4.  EVOLUTION OF ZMODEM..............................................  8

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

 6.  ZMODEM REQUIREMENTS.............................................. 10
	6.1	File Contents.......................................... 11

 7.  ZMODEM BASICS.................................................... 11
	7.1	Packetization.......................................... 11
	7.2	Link Escape Encoding................................... 12
	7.3	Header................................................. 12
	7.4	Binary Data Subpackets................................. 14
	7.5	ASCII Encoded Data Subpacket........................... 15

 8.  PROTOCOL TRANSACTION OVERVIEW.................................... 15
	8.1	Session Startup........................................ 15
	8.2	File Transmission...................................... 16
	8.3	Session Cleanup........................................ 17
	8.4	Session Abort Sequence................................. 17

 9.  STREAMING TECHNIQUES / ERROR RECOVERY............................ 18
	9.1	Full Streaming with Sampling........................... 18
	9.2	Full Streaming with Reverse Interrupt.................. 19
	9.3	Full Streaming with Sliding Window..................... 19
	9.4	Full Streaming over Error Free Channels................ 20
	9.5	Segmented Streaming.................................... 20

10.  ATTENTION SEQUENCE............................................... 20

11.  FRAME TYPES...................................................... 21
	11.1	ZRQINIT................................................ 21
	11.2	ZRINIT................................................. 21
	11.3	ZSINIT................................................. 21
	11.4	ZACK................................................... 21
	11.5	ZFILE.................................................. 21
	11.6	ZSKIP.................................................. 23
	11.7	ZNAK................................................... 23
	11.8	ZABORT................................................. 23
	11.9	ZFIN................................................... 23
	11.10	ZRPOS.................................................. 23
	11.11	ZDATA.................................................. 23
	11.12	ZEOF................................................... 23
	11.13	ZFERR.................................................. 24
	11.14	ZCRC................................................... 24
	11.15	ZCHALLENGE............................................. 24
	11.16	ZCOMPL................................................. 24
	11.17	ZCAN................................................... 24
	11.18	ZFREECNT............................................... 24
	11.19	ZCOMMAND............................................... 24

				   - i -

12.  SESSION TRANSACTION EXAMPLES..................................... 25
	12.1	A simple file transfer................................. 25
	12.2	Challenge and Command Download......................... 25

13.  ZFILE FRAME FILE INFORMATION..................................... 25

14.  PERFORMANCE RESULTS.............................................. 27
	14.1	Compatibility.......................................... 27
	14.2	Throughput............................................. 27
	14.3	Error Recovery......................................... 28

15.  PACKET SWITCHED NETWORK CONSIDERATIONS........................... 28

16.  PERFORMANCE COMPARISON TABLES.................................... 29

17.  FUTURE EXTENSIONS................................................ 33

18.  REVISIONS........................................................ 33

19.  MORE INFORMATION................................................. 34
	19.1	TeleGodzilla Bulletin Board............................ 34
	19.2	Unix UUCP Access....................................... 34

20.  ZMODEM PROGRAMS.................................................. 35
	20.1	Adding ZMODEM to DOS Programs.......................... 36

21.  YMODEM PROGRAMS.................................................. 36

22.  ACKNOWLEDGMENTS.................................................. 37

23.  RELATED FILES.................................................... 37


Figure 1.  Order of Bytes in Header................................... 13

Figure 2.  16 Bit CRC Binary Header................................... 13

Figure 3.  32 Bit CRC Binary Header................................... 13

Figure 4.  HEX Header................................................. 14

Figure 5.  Transmission Time Comparison............................... 30


TABLE 1.  Network and Flow Control Compatibility...................... 29

TABLE 2.  Protocol Overhead Information............................... 30

TABLE 3.  Local Timesharing Computer Download Performance............. 30

TABLE 4.  File Transfer Speeds........................................ 31

TABLE 5.  Protocol Checklist.......................................... 32

				 - ii -

	   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 name.

				 - iii -

Chapter 1		     ZMODEM Protocol				4


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:

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

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

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

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

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

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

    o+ The modification date and other file attributes are lost.

    o+ 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.

    o+ 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.

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

    o+ 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.

Chapter 2	     Rev 14-Oct-88  Typeset 08-08-96			4

Chapter 2		     ZMODEM Protocol				5

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

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

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

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

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

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

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

    o+ 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.

    o+ 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
 1. Some Kermit programs support run length encoding.
 2. Unique to DSZ, ZCOMM, Professional-YAM and PowerCom

Chapter 2	     Rev 14-Oct-88  Typeset 08-08-96			5

Chapter 2		     ZMODEM Protocol				6

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

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

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

3.  ZMODEM Protocol Design Criteria

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

3.1  Ease of Use

    o+ ZMODEM allows either program to initiate file transfers.

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

    o+ File names need be entered only once.

    o+ Menu selections are supported.

    o+ Wild Card names may be used with batch transfers.

    o+ Minimum keystrokes required to initiate transfers.

    o+ ZRQINIT frame sent by sending program can trigger automatic

    o+ 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.

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

    o+ 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.

 1. Provided the transmission medium accommodates X/YMODEM.

Chapter 3	     Rev 14-Oct-88  Typeset 08-08-96			6

Chapter 3		     ZMODEM Protocol				7

    o+ Traditional direct modem to modem communications with high error

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.

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:

    o+ Microcomputers that cannot overlap disk and serial i/o

    o+ Microcomputers that cannot overlap serial send and receive

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

    o+ 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.

Chapter 3	     Rev 14-Oct-88  Typeset 08-08-96			7

Chapter 3		     ZMODEM Protocol				8

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 name.


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
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.

 1. Without stopping for a response
 2. The WINDOW is the data in transit between sender and receiver.

Chapter 4	     Rev 14-Oct-88  Typeset 08-08-96			8

Chapter 4		     ZMODEM Protocol				9

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.

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 ZMODEM 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.
 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

Chapter 4	     Rev 14-Oct-88  Typeset 08-08-96			9

Chapter 4		     ZMODEM Protocol			       10

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 suffering 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.


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.

 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

Chapter 6	     Rev 14-Oct-88  Typeset 08-08-96		       10

Chapter 6		     ZMODEM Protocol			       11

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.

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]


7.1  Packetization

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

    o+ Block numbers are limited to 256

    o+ No provision for variable length blocks

    o+ 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.

    o+ 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.

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

Chapter 7	     Rev 14-Oct-88  Typeset 08-08-96		       11

Chapter 7		     ZMODEM Protocol			       12

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
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:

 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.
 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			       13

    o+ A type byte[2] [3]

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

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.

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

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Chapter 7		     ZMODEM Protocol			       14

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.)

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.
 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.

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Chapter 7		     ZMODEM Protocol			       15

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
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.
 1. Special considerations apply when sending commands.

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Chapter 8		     ZMODEM Protocol			       16

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

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

The receiver sends a ZACK header in response, containing either the
serial number of the receiving program, or 0.

8.2  File Transmission

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

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

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

	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
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]
 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.
 3. See below, under ZFILE header type.
 4. The crc is initialized to 0xFF.  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			       17

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

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

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

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

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

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

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

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

	static char canistr[] = {
	24,24,24,24,24,24,24,24,8,8,8,8,8,8,8,8,8,8,0 };
 6. If the ZMSPARS option is used, the receiver instead seeks to the
    position given in the ZDATA header.

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Chapter 9		     ZMODEM Protocol			       18


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]

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
 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			       19

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

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

Unix sz versions beginning with 9 May 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 9 May 1987 control the window size with
"zmodem pwN".  This is compatible with previous versions of these

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

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.

 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
    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			       20

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.

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

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

Two meta-characters perform special functions:

    o+ \335 (octal) Send a break signal

    o+ \336 (octal) Pause one second

 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 11		     ZMODEM Protocol			       21


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

11.2  ZRINIT

Sent by the receiving program. ZF0 and ZF1 contain the bitwise or of the
receiver capability flags:
 #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

/* 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 */

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 ZCCC
CRCW 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.

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Chapter 11		     ZMODEM Protocol			       22

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

ZCBIN "Binary" transfer - inhibit conversion unconditionally

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

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

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.

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

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Chapter 11		     ZMODEM Protocol			       23

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

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

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

ZTRLE Run Length encoding, Details to be determined.

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

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

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

11.6  ZSKIP

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

11.7  ZNAK

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

11.8  ZABORT

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

11.9  ZFIN

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

11.10  ZRPOS

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

11.11  ZDATA

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

11.12  ZEOF

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

 2. Or ZCOMPL in case of server mode.

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Chapter 11		     ZMODEM Protocol			       24

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.

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

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

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

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

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

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Chapter 11		     ZMODEM Protocol			       25

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


12.1  A simple file transfer

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

	Sender				Receiver
	~~~~~~				~~~~~~~~
	ZDATA data ...

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.

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Chapter 13		     ZMODEM Protocol			       26

Filename Considerations

    o+ 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.

    o+ The receiver should accommodate file names in lower and upper case.

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

 1. Fields may not be skipped.

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Chapter 13		     ZMODEM Protocol			       27

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

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

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.

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

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Chapter 14		     ZMODEM Protocol			       28

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 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

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

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

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Chapter 15		     ZMODEM Protocol			       29

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
	rst? 5:1

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

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

	TABLE 1.  Network and Flow Control Compatibility

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

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


"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. 

Chapter 16	     Rev 14-Oct-88  Typeset 08-08-96		       29

Chapter 16		     ZMODEM Protocol			       30

ZMODEM uses 256 byte data subpackets for this example.  SuperKermit uses
maximum standard packet size, 8 bit transparent transmission, no run
length compression.  The 4 block WXMODEM window is too small to span the 5
second delay in this example; the resulting thoughput degradation is

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

	TABLE 3.  Local Timesharing Computer Download Performance

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

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

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

Chapter 16	     Rev 14-Oct-88  Typeset 08-08-96		       30

Chapter 16		     ZMODEM Protocol			       31

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

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

	TABLE 4. File Transfer Speeds

|Protocol  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
 PTL	 Portland Oregon network node
 SK	 Sliding Window Kermit (SuperKermit) w=15

Chapter 16	     Rev 14-Oct-88  Typeset 08-08-96		       31

Chapter 16		     ZMODEM Protocol			       32

 XN	 XMODEM protocol implemented in network modes
 X/ovth XMODEM, Omen Technology OverThruster(TM)

	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
(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)

Chapter 16	     Rev 14-Oct-88  Typeset 08-08-96		       32

Chapter 16		     ZMODEM Protocol			       33

(8) More then 3% from protected network control characters
(9) With Segmented Streaming
(10) With Pro-YAM extensions


Future extensions include:

    o+ Compatibility with 7 bit networks

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

    o+ Multiple independent threads

    o+ Bidirectional transfers (STEREO ZMODEM)

    o+ Encryption

    o+ Compression

    o+ File Comparison

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

    o+ 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

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

05-30-87 Added ZMNEW and ZMSKNOLOC

05-14-87 Window management, ZACK zshhdr XON removed, control character
escaping, ZMSPARS changed to ZXPARS, editorial changes.

Chapter 18	     Rev 14-Oct-88  Typeset 08-08-96		       33

Chapter 18		     ZMODEM Protocol			       34

04-13-87  The ZMODEM file transfer protocol's public domain status is

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 recognition.


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.

Chapter 19	     Rev 14-Oct-88  Typeset 08-08-97		       34

Chapter 19		     ZMODEM Protocol			       35

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 account.

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:

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

  o+ ZCOMMDOC.ARC "Universal Line Printer Edition" Manual

  o+ 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

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

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.

Chapter 20	     Rev 14-Oct-88  Typeset 08-08-96		       35

Chapter 20		     ZMODEM Protocol			       36

The LAN modem sharing program Line Plus has announced ZMODEM support.

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 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 as
       dsz port 2 sz file1 file2
to send files, or as
      dsz port 2 rz
to receive zero or more file(s), or as
       dsz port 2 rz filea fileb
to receive two files, the first to filea and the second (if sent) to
fileb. This form of dsz may be used to control the pathname of incoming
file(s).  In this example, if the sending program attempted to send a
third file, the transfer would be terminated.

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

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


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

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

Irv Hoff has added 1k packets and YMODEM transfers to the KMD and IMP
series programs, which replace the XMODEM and MODEM7/MDM7xx series
respectively.  Overlays are available for a wide variety of CP/M systems.

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

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

Chapter 21	     Rev 14-Oct-88  Typeset 08-08-96		       36

Chapter 21		     ZMODEM Protocol			       37

	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

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 as

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.

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

Chapter 23	     Rev 14-Oct-88  Typeset 08-08-96		       37