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The ZMODEM Inter Application File Transfer Protocol
Chuck Forsberg
Omen Technology Inc
A overview of this document is available as ZMODEM.OV
(in ZMDMOV.ARC)
Omen Technology Incorporated
The High Reliability Software
17505-V Northwest Sauvie Island Road
Portland Oregon 97231
VOICE: 503-621-3406 :VOICE
Modem: 503-621-3746 Speed 1200,2400,19200(Telebit PEP)
Compuserve:70007,2304 GEnie:CAF
UUCP: ...!tektronix!reed!omen!caf
Chapter 0 Rev 10-27-87 Typeset 10-27-87 1
Chapter 0 ZMODEM Protocol 2
1. INTENDED AUDIENCE
This document is intended for telecommunications managers, systems
programmers, and others who choose and implement asynchronous file
transfer protocols over dial-up networks and related environments.
2. WHY DEVELOP ZMODEM?
Since its development half a decade ago, the Ward Christensen MODEM
protocol has enabled a wide variety of computer systems to interchange
data. There is hardly a communications program that doesn't at least
claim to support this protocol, now called XMODEM.
Advances in computing, modems and networking have spread the XMODEM
protocol far beyond the micro to micro environment for which it was
designed. These application have exposed some weaknesses:
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, and others tightly bound
to the fortunes of their suppliers. These protocols have not
benefited from public scrutiny of their design features.
Chapter 2 Rev 10-27-87 Typeset 10-27-87 2
Chapter 2 ZMODEM Protocol 3
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.
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
__________
1. Some Kermit programs support run length encoding.
2. Unique to DSZ, ZCOMM, Professional-YAM and PowerCom
Chapter 2 Rev 10-27-87 Typeset 10-27-87 3
Chapter 2 ZMODEM Protocol 4
degrade performance. Some networks cannot transmit 1024 byte packets
without flow control, which is difficult to apply without impairing the
perfect transparency required by XMODEM. XMODEM-k adds garbage to
received files.
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 YMODEM.
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, passing
commands and/or modifiers to the other program.
o File names need be entered only once.
o Menu selections are supported.
Chapter 3 Rev 10-27-87 Typeset 10-27-87 4
Chapter 3 ZMODEM Protocol 5
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 downloads.
o ZMODEM can step down to YMODEM if the other end does not support
ZMODEM.[1]
3.2 Throughput
All file transfer protocols make tradeoffs between throughput,
reliability, universality, and complexity according to the technology and
knowledge base available to their designers.
In the design of ZMODEM, three applications deserve special attention.
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.
o Direct modem to modem communications with high error rate
Unlike Sliding Windows Kermit, ZMODEM is not optimized for optimum
throughput when error rate and delays are both high. This tradeoff
markedly reduces code complexity and memory requirements. ZMODEM
generally provides faster error recovery than network compatible XMODEM
implementations.
In the absence of network delays, rapid error recovery is possible, much
faster than MEGAlink and network compatible versions of YMODEM and XMODEM.
File transfers begin immediately regardless of which program is started
first, without the 10 second delay associated with XMODEM.
__________
1. Provided the transmission medium accommodates X/YMODEM.
Chapter 3 Rev 10-27-87 Typeset 10-27-87 5
Chapter 3 ZMODEM Protocol 6
3.3 Integrity and Robustness
Once a ZMODEM session is begun, all transactions are protected with 16 or
32 bit CRC.[2] Complex proprietary techniques such as Cybernetic Data
Recovery(TM)[3] are not needed for reliable transfers.
An optional 32-bit CRC used as the frame check sequence in ADCCP (ANSI
X3.66, also known as FIPS PUB 71 and FED-STD-1003, the U.S. versions of
CCITT's X.25) is used when available. The 32 bit CRC reduces undetected
errors by at least five orders of magnitude when properly applied (-1
preset, inversion).
A security challenge mechanism guards against "Trojan Horse" messages
written to mimic legitimate command or file downloads.
3.4 Ease of Implementation
ZMODEM accommodates a wide variety of systems:
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.
ZMODEM provides a general purpose application to application file transfer
protocol which may be used directly or with with reliable link level
__________
2. Except for the CAN-CAN-CAN-CAN-CAN abort sequence which requires five
successive CAN characters.
3. Unique to Professional-YAM and PowerCom
4. Streaming strategies are discussed in coming chapters.
Chapter 3 Rev 10-27-87 Typeset 10-27-87 6
Chapter 3 ZMODEM Protocol 7
protocols such as X.25, MNP, Fastlink, etc. When used with X.25, MNP,
Fastlink, etc., ZMODEM detects and corrects errors in the interfaces
between error controlled media and the remainder of the communications
link.
ZMODEM was developed for the public domain under a Telenet contract. The
ZMODEM protocol descriptions and the Unix rz/sz program source code are
public domain. No licensing, trademark, or copyright restrictions apply
to the use of the protocol, the Unix rz/sz source code and the ZMODEM
name.
4. EVOLUTION OF ZMODEM
In early 1986, Telenet funded a project to develop an improved public
domain application to application file transfer protocol. This protocol
would alleviate the throughput problems network customers were
experiencing with XMODEM and Kermit file transfers.
In the beginning, we thought a few modifications to XMODEM would allow
high performance over packet switched networks while preserving XMODEM's
simplicity.
The initial concept would add a block number to the ACK and NAK characters
used by XMODEM. The resultant protocol would allow the sender to send
more than one block before waiting for a response.
But how to add the block number to XMODEM's ACK and NAK? WXMODEM,
SEAlink, MEGAlink and some other protocols add binary byte(s) to indicate
the block number.
Pure binary was unsuitable for ZMODEM because binary code combinations
won't pass bidirectionally through some modems, networks and operating
systems. Other operating systems may not be able to recognize something
coming back[1] unless a break signal or a system dependent code or
sequence is present. By the time all this and other problems with the
simple ACK/NAK sequences mentioned above were corrected, XMODEM's simple
ACK and NACK characters had evolved into a real packet. The Frog was
riveting.
Managing the window[2] was another problem. Experience gained in
debugging The Source's SuperKermit protocol indicated a window size of
about 1000 characters was needed at 1200 bps. High speed modems require a
__________
1. Without stopping for a response
2. The WINDOW is the data in transit between sender and receiver.
Chapter 4 Rev 10-27-87 Typeset 10-27-87 7
Chapter 4 ZMODEM Protocol 8
window of 20000 or more characters for full throughput. Much of the
SuperKermit's inefficiency, complexity and debugging time centered around
its ring buffering and window management. There had to be a better way to
get the job done.
A sore point with XMODEM and its progeny is error recovery. More to the
point, how can the receiver determine whether the sender has responded, or
is ready to respond, to a retransmission request? XMODEM attacks the
problem by throwing away characters until a certain period of silence.
Too short a time allows a spurious pause in output (network or timesharing
congestion) to masquerade as error recovery. Too long a timeout
devastates throughput, and allows a noisy line to lock up the protocol.
SuperKermit solves the problem with a distinct start of packet character
(SOH). WXMODEM and ZMODEM use unique character sequences to delineate the
start of frames. SEAlink and MEGAlink do not address this problem.
A further error recovery problem arises in streaming protocols. How does
the receiver know when (or if) the sender has recognized its error signal?
Is the next packet the correct response to the error signal? Is it
something left over "in the queue"? Or is this new subpacket one of many
that will have to be discarded because the sender did not receive the
error signal? How long should this continue before sending another error
signal? How can the protocol prevent this from degenerating into an
argument about mixed signals?
SuperKermit uses selective retransmission, so it can accept any good
packet it receives. Each time the SuperKermit receiver gets a data
packet, it must decide which outstanding packet (if any) it "wants most"
to receive, and asks for that one. In practice, complex software "hacks"
are needed to attain acceptable robustness.[3]
For ZMODEM, we decided to forgo the complexity of SuperKermit's packet
assembly scheme and its associated buffer management logic and memory
requirements.
Another sore point with XMODEM and WXMODEM is the garbage added to files.
This was acceptable with old CP/M files which had no exact length, but not
with modern systems such as DOS and Unix. YMODEM uses file length
information transmitted in the header block to trim the output file, but
this causes data loss when transferring files that grow during a transfer.
In some cases, the file length may be unknown, as when data is obtained
from a process. Variable length data subpackets solve both of these
__________
3. For example, when SuperKermit encounters certain errors, the wndesr
function is called to determine the next block to request. A burst of
errors generates several wasteful requests to retransmit the same
block.
Chapter 4 Rev 10-27-87 Typeset 10-27-87 8
Chapter 4 ZMODEM Protocol 9
problems.
Since some characters had to be escaped anyway, there wasn't any point
wasting bytes to fill out a fixed packet length or to specify a variable
packet length. In ZMODEM, the length of data subpackets is denoted by
ending each subpacket with an escape sequence similar to BISYNC and HDLC.
The end result is a ZMOEM header containing a "frame type", four bytes of
supervisory information, and its own CRC. Data frames consist of a header
followed by 1 or more data subpackets. In the absence of transmission
errors, an entire file can be sent in one data frame.
Since the sending system may be sensitive to numerous control characters
or strip parity in the reverse data path, all of the headers sent by the
receiver are sent in hex. A common lower level routine receives all
headers, allowing the main program logic to deal with headers and data
subpackets as objects.
With equivalent binary (efficient) and hex (application friendly) frames,
the sending program can send an "invitation to receive" sequence to
activate the receiver without crashing the remote application with
unexpected control characters.
Going "back to scratch" in the protocol design presents an opportunity to
steal good ideas from many sources and to add a few new ones.
From Kermit and UUCP comes the concept of an initial dialog to exchange
system parameters.
ZMODEM generalizes Compuserve B Protocol's host controlled transfers to
single command AutoDownload and command downloading. A Security Challenge
discourages password hackers and Trojan Horse authors from abusing
ZMODEM's power.
We were also keen to the pain and $uffering of legions of
telecommunicators whose file transfers have been ruined by communications
and timesharing faults. ZMODEM's file transfer recovery and advanced file
management are dedicated to these kindred comrades.
After ZMODEM had been operational a short time, Earl Hall pointed out the
obvious: ZMODEM's user friendly AutoDownload was almost useless if the
user must assign transfer options to each of the sending and receiving
programs. Now, transfer options may be specified to/by the sending
program, which passes them to the receiving program in the ZFILE header.
Chapter 5 Rev 10-27-87 Typeset 10-27-87 9
Chapter 5 ZMODEM Protocol 10
5. ROSETTA STONE
Here are some definitions which reflect current vernacular in the computer
media. The attempt here is identify the file transfer protocol rather
than specific programs.
FRAME A ZMODEM frame consists of a header and 0 or more data subpackets.
XMODEM refers to the original 1977 file transfer etiquette introduced by
Ward Christensen's MODEM2 program. It's also called the MODEM or
MODEM2 protocol. Some who are unaware of MODEM7's unusual batch
file mode call it MODEM7. Other aliases include "CP/M Users's
Group" and "TERM II FTP 3". This protocol is supported by most
communications programs because it is easy to implement.
XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte Cyclical
Redundancy Check (CRC-16), improving error detection.
XMODEM-1k Refers to XMODEM-CRC with optional 1024 byte blocks.
YMODEM refers to the XMODEM/CRC protocol with batch transmission and
optional 1024 byte blocks as described in YMODEM.DOC.[1]
6. ZMODEM REQUIREMENTS
ZMODEM requires an 8 bit transfer medium.[1] ZMODEM escapes network
control characters to allow operation with packet switched networks. In
general, ZMODEM operates over any path that supports XMODEM, and over many
that don't.
To support full streaming,[2] the transmission path should either assert
flow control or pass full speed transmission without loss of data.
Otherwise the ZMODEM sender must manage the window size.
6.1 File Contents
6.1.1 Binary Files
ZMODEM places no constraints on the information content of binary files,
except that the number of bits in the file must be a multiple of 8.
__________
1. Available on TeleGodzilla as part of YZMODEM.ZOO
1. The ZMODEM design allows encoded packets for less transparent media.
2. With XOFF and XON, or out of band flow control such as X.25 or CTS
Chapter 6 Rev 10-27-87 Typeset 10-27-87 10
Chapter 6 ZMODEM Protocol 11
6.1.2 Text Files
Since ZMODEM is used to transfer files between different types of computer
systems, text files must meet minimum requirements if they are to be
readable on a wide variety of systems and environments.
Text lines consist of printing ASCII characters, spaces, tabs, and
backspaces.
6.1.2.1 ASCII End of Line
The ASCII code definition allows text lines terminated by a CR/LF (015,
012) sequence, or by a NL (012) character. Lines logically terminated by
a lone CR (013) are not ASCII text.
A CR (013) without a linefeed implies overprinting, and is not acceptable
as a logical line separator. Overprinted lines should print all important
characters in the last pass to allow CRT displays to display meaningful
text. Overstruck characters may be generated by backspacing or by
overprinting the line with CR (015) not followed by LF.
Overstruck characters generated with backspaces should be sent with the
most important character last to accommodate CRT displays that cannot
overstrike. The sending program may use the ZCNL bit to force the
receiving program to convert the received end of line to its local end of
line convention.[3]
__________
3. Files that have been translated in such a way as to modify their
length cannot be updated with the ZCRECOV Conversion Option.
Chapter 6 Rev 10-27-87 Typeset 10-27-87 11
Chapter 6 ZMODEM Protocol 12
7. ZMODEM BASICS
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.
7.2 Link Escape Encoding
ZMODEM achieves data transparency by extending the 8 bit character set
(256 codes) with escape sequences based on the ZMODEM data link escape
character ZDLE.[1]
Link Escape coding permits variable length data subpackets without the
overhead of a separate byte count. It allows the beginning of frames to
be detected without special timing techniques, facilitating rapid error
recovery.
Link Escape coding does add some overhead. The worst case, a file
consisting entirely of escaped characters, would incur a 50% overhead.
The ZDLE character is special. ZDLE represents a control sequence of some
sort. If a ZDLE character appears in binary data, it is prefixed with
ZDLE, then sent as ZDLEE.
The value for ZDLE is octal 030 (ASCII CAN). This particular value was
chosen to allow a string of 5 consecutive CAN characters to abort a ZMODEM
__________
1. This and other constants are defined in the zmodem.h include file.
Please note that constants with a leading 0 are octal constants in C.
Chapter 7 Rev 10-27-87 Typeset 10-27-87 12
Chapter 7 ZMODEM Protocol 13
session, compatible with YMODEM session abort.
Since CAN is not used in normal terminal operations, interactive
applications and communications programs can monitor the data flow for
ZDLE. The following characters can be scanned to detect the ZRQINIT
header, the invitation to automatically download commands or files.
Receipt of five successive CAN characters will abort a ZMODEM session.
Eight CAN characters are sent.
The receiving program decodes any sequence of ZDLE followed by a byte with
bit 6 set and bit 5 reset (upper case letter, either parity) to the
equivalent control character by inverting bit 6. This allows the
transmitter to escape any control character that cannot be sent by the
communications medium. In addition, the receiver recognizes escapes for
0177 and 0377 should these characters need to be escaped.
ZMODEM software escapes ZDLE, 020, 0220, 021, 0221, 023, and 0223. If
preceded by 0100 or 0300 (@), 015 and 0215 are also escaped to protect the
Telenet command escape CR-@-CR. The receiver ignores 021, 0221, 023, and
0223 characters in the data stream.
The ZMODEM routines in zm.c accept an option to escape all control
characters, to allow operation with less transparent networks. This
option can be given to either the sending or receiving program.
7.3 Header
All ZMODEM frames begin with a header which may be sent in binary or HEX
form. ZMODEM uses a single routine to recognize binary and hex headers.
Either form of the header contains the same raw information:
o A type byte[2] [3]
o Four bytes of data indicating flags and/or numeric quantities depending
on the frame type
__________
2. The frame types are cardinal numbers beginning with 0 to minimize
state transition table memory requirements.
3. Future extensions to ZMODEM may use the high order bits of the type
byte to indicate thread selection.
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Chapter 7 ZMODEM Protocol 14
Figure 1. Order of Bytes in Header
TYPE: frame type
F0: Flags least significant byte
P0: file Position least significant
P3: file Position most significant
TYPE F3 F2 F1 F0
-------------------
TYPE P0 P1 P2 P3
7.3.1 16 Bit CRC Binary Header
A binary header is sent by the sending program to the receiving program.
ZDLE encoding accommodates XON/XOFF flow control.
A binary header begins with the sequence ZPAD, ZDLE, ZBIN.
The frame type byte is ZDLE encoded.
The four position/flags bytes are ZDLE encoded.
A two byte CRC of the frame type and position/flag bytes is ZDLE encoded.
0 or more binary data subpackets with 16 bit CRC will follow depending on
the frame type.
The function zsbhdr transmits a binary header. The function zgethdr
receives a binary or hex header.
Figure 2. 16 Bit CRC Binary Header
* ZDLE A TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2
7.3.2 32 Bit CRC Binary Header
A "32 bit CRC" Binary header is similar to a Binary Header, except the
ZBIN (A) character is replaced by a ZBIN32 (C) character, and four
characters of CRC are sent. 0 or more binary data subpackets with 32 bit
CRC will follow depending on the frame type.
The common variable Txfcs32 may be set TRUE for 32 bit CRC iff the
receiver indicates the capability with the CANFC32 bit. The zgethdr,
zsdata and zrdata functions automatically adjust to the type of Frame
Check Sequence being used.
Figure 3. 32 Bit CRC Binary Header
* ZDLE C TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2 CRC-3 CRC-4
7.3.3 HEX Header
The receiver sends responses in hex headers. The sender also uses hex
headers when they are not followed by binary data subpackets.
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Chapter 7 ZMODEM Protocol 15
Hex encoding protects the reverse channel from random control characters.
The hex header receiving routine ignores parity.
Use of Kermit style encoding for control and paritied characters was
considered and rejected because of increased possibility of interacting
with some timesharing systems' line edit functions. Use of HEX headers
from the receiving program allows control characters to be used to
interrupt the sender when errors are detected. A HEX header may be used
in place of a binary header wherever convenient. If a data packet follows
a HEX header, it is protected with CRC-16.
A hex header begins with the sequence ZPAD, ZPAD, ZDLE, ZHEX. The zgethdr
routine synchronizes with the ZPAD-ZDLE sequence. The extra ZPAD
character allows the sending program to detect an asynchronous header
(indicating an error condition) and then call zgethdr to receive the
header.
The type byte, the four position/flag bytes, and the 16 bit CRC thereof
are sent in hex using the character set 01234567890abcdef. Upper case hex
digits are not allowed; they false trigger XMODEM and YMODEM programs.
Since this form of hex encoding detects many patterns of errors,
especially missing characters, a hex header with 32 bit CRC has not been
defined.
A carriage return and line feed are sent with HEX headers. The receive
routine expects to see at least one of these characters, two if the first
is CR. The CR/LF aids debugging from printouts, and helps overcome
certain operating system related problems.
An XON character is appended to all HEX packets except ZACK and ZFIN. The
XON releases the sender from spurious XOFF flow control characters
generated by line noise, a common occurrence. XON is not sent after ZACK
headers to protect flow control in streaming situations. XON is not sent
after a ZFIN header to allow clean session cleanup.
0 or more data subpackets will follow depending on the frame type.
The function zshhdr sends a hex header.
Figure 4. HEX Header
* * ZDLE B TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2 CR LF XON
(TYPE, F3...F0, CRC-1, and CRC-2 are each sent as two hex digits.)
Chapter 7 Rev 10-27-87 Typeset 10-27-87 15
Chapter 7 ZMODEM Protocol 16
7.4 Binary Data Subpackets
Binary data subpackets immediately follow the associated binary header
packet. A binary data packet contains 0 to 1024 bytes of data.
Recommended length values are 256 bytes below 2400 bps, 512 at 2400 bps,
and 1024 above 4800 bps or when the data link is known to be relatively
error free.[4]
No padding is used with binary data subpackets. The data bytes are ZDLE
encoded and transmitted. A ZDLE and frameend are then sent, followed by
two or four ZDLE encoded CRC bytes. The CRC accumulates the data bytes
and frameend.
The function zsdata sends a data subpacket. The function zrdata receives
a data subpacket.
7.5 ASCII Encoded Data Subpacket
The format of ASCII Encoded data subpackets is not currently specified.
These could be used for server commands, or main transfers in 7 bit
environments.
8. PROTOCOL TRANSACTION OVERVIEW
As with the XMODEM recommendation, ZMODEM timing is receiver driven. The
transmitter should not time out at all, except to abort the program if no
headers are received for an extended period of time, say one minute.[1]
8.1 Session Startup
To start a ZMODEM file transfer session, the sending program is called
with the names of the desired file(s) and option(s).
The sending program may send the string "rz\r" to invoke the receiving
program from a possible command mode. The "rz" followed by carriage
return activates a ZMODEM receive program or command if it were not
already active.
The sender may then display a message intended for human consumption, such
__________
4. Strategies for adjusting the subpacket length for optimal results
based on real time error rates are still evolving. Shorter subpackets
speed error detection but increase protocol overhead slightly.
1. Special considerations apply when sending commands.
Chapter 8 Rev 10-27-87 Typeset 10-27-87 16
Chapter 8 ZMODEM Protocol 17
as a list of the files requested, etc.
Then the sender may send a ZRQINIT header. The ZRQINIT header causes a
previously started receive program to send its ZRINIT header without
delay.
In an interactive or conversational mode, the receiving application may
monitor the data stream for ZDLE. The following characters may be scanned
for B00 indicating a ZRQINIT header, a command to download a command or
data.
The sending program awaits a command from the receiving program to start
file transfers. If a "C", "G", or NAK is received, an XMODEM or YMODEM
file transfer is indicated, and file transfer(s) use the YMODEM protocol.
Note: With ZMODEM and YMODEM, the sending program provides the file name,
but not with XMODEM.
In case of garbled data, the sending program can repeat the invitation to
receive a number of times until a session starts.
When the ZMODEM receive program starts, it immediately sends a ZRINIT
header to initiate ZMODEM file transfers, or a ZCHALLENGE header to verify
the sending program. The receive program resends its header at response
time (default 10 second) intervals for a suitable period of time (40
seconds total) before falling back to YMODEM protocol.
If the receiving program receives a ZRQINIT header, it resends the ZRINIT
header. If the sending program receives the ZCHALLENGE header, it places
the data in ZP0...ZP3 in an answering ZACK header.
If the receiving program receives a ZRINIT header, it is an echo
indicating that the sending program is not operational.
Eventually the sending program correctly receives the ZRINIT header.
The sender may then send an optional ZSINIT frame to define the receiving
program's Attn sequence, or to specify complete control character
escaping.[2]
If the ZSINIT header specifies ESCCTL or ESC8, a HEX header is used, and
the receiver activates the specified ESC modes before reading the
following data subpacket.
The receiver sends a ZACK header in response, optionally containing the
__________
2. If the receiver specifies the same or higher level of escaping, the
ZSINIT frame need not be sent unless an Attn sequence is needed.
Chapter 8 Rev 10-27-87 Typeset 10-27-87 17
Chapter 8 ZMODEM Protocol 18
serial number of the receiving program, or 0.
8.2 File Transmission
The sender then sends a ZFILE header with ZMODEM Conversion, Management,
and Transport options[3] followed by a ZCRCW data subpacket containing the
file name, file length, modification date, and other information identical
to that used by YMODEM Batch.
The receiver examines the file name, length, and date information provided
by the sender in the context of the specified transfer options, the
current state of its file system(s), and local security requirements. The
receiving program should insure the pathname and options are compatible
with its operating environment and local security requirements.
The receiver may respond with a ZSKIP header, which makes the sender
proceed to the next file (if any) in the batch.
If the receiver has a file with the same name and length,
it may respond with a ZCRC header, which requires the
sender to perform a 32 bit CRC on the file and transmit the
complement of the CRC in a ZCRC header.[4] The receiver
uses this information to determine whether to accept the
file or skip it. This sequence is triggered by the ZMCRC
Management Option.
A ZRPOS header from the receiver initiates transmission of the file data
starting at the offset in the file specified in the ZRPOS header.
Normally the receiver specifies the data transfer to begin begin at
offset 0 in the file.
The receiver may start the transfer further down in the
file. This allows a file transfer interrupted by a loss
or carrier or system crash to be completed on the next
connection without requiring the entire file to be
retransmitted.[5] If downloading a file from a timesharing
system that becomes sluggish, the transfer can be
interrupted and resumed later with no loss of data.
The sender sends a ZDATA binary header (with file position) followed by
one or more data subpackets.
__________
3. See below, under ZFILE header type.
4. The crc is initialized to 0xFFFFFFFF.
5. This does not apply to files that have been translated.
Chapter 8 Rev 10-27-87 Typeset 10-27-87 18
Chapter 8 ZMODEM Protocol 19
The receiver compares the file position in the ZDATA header with the
number of characters successfully received to the file. If they do not
agree, a ZRPOS error response is generated to force the sender to the
right position within the file.[6]
A data subpacket terminated by ZCRCG and CRC does not elicit a response
unless an error is detected; more data subpacket(s) follow immediately.
ZCRCQ data subpackets expect a ZACK response with the
receiver's file offset if no error, otherwise a ZRPOS
response with the last good file offset. Another data
subpacket continues immediately. ZCRCQ subpackets are
not used if the receiver does not indicate FDX ability
with the CANFDX bit.
ZCRCW data subpackets expect a response before the next frame is sent.
If the receiver does not indicate overlapped I/O capability with the
CANOVIO bit, or sets a buffer size, the sender uses the ZCRCW to allow
the receiver to write its buffer before sending more data.
A zero length data frame may be used as an idle
subpacket to prevent the receiver from timing out in
case data is not immediately available to the sender.
In the absence of fatal error, the sender eventually encounters end of
file. If the end of file is encountered within a frame, the frame is
closed with a ZCRCE data subpacket which does not elicit a response
except in case of error.
The sender sends a ZEOF header with the file ending offset equal to
the number of characters in the file. The receiver compares this
number with the number of characters received. If the receiver has
received all of the file, it closes the file. If the file close was
satisfactory, the receiver responds with ZRINIT. If the receiver has
not received all the bytes of the file, the receiver ignores the ZEOF
because a new ZDATA is coming. If the receiver cannot properly close
the file, a ZFERR header is sent.
After all files are processed, any further protocol
errors should not prevent the sending program from
returning with a success status.
__________
6. If the ZMSPARS option is used, the receiver instead seeks to the
position given in the ZDATA header.
Chapter 8 Rev 10-27-87 Typeset 10-27-87 19
Chapter 8 ZMODEM Protocol 20
8.3 Session Cleanup
The sender closes the session with a ZFIN header. The receiver
acknowledges this with its own ZFIN header.
When the sender receives the acknowledging header, it sends two
characters, "OO" (Over and Out) and exits to the operating system or
application that invoked it. The receiver waits briefly for the "O"
characters, then exits whether they were received or not.
8.4 Session Abort Sequence
If the receiver is receiving data in streaming mode, the Attn
sequence is executed to interrupt data transmission before the Cancel
sequence is sent. The Cancel sequence consists of eight CAN
characters and ten backspace characters. ZMODEM only requires five
Cancel characters, the other three are "insurance".
The trailing backspace characters attempt to erase the effects of the
CAN characters if they are received by a command interpreter.
static char canistr[] = {
24,24,24,24,24,24,24,24,8,8,8,8,8,8,8,8,8,8,0
};
Chapter 8 Rev 10-27-87 Typeset 10-27-87 20
Chapter 8 ZMODEM Protocol 21
9. STREAMING TECHNIQUES / ERROR RECOVERY
It is a fact of life that no single method of streaming is applicable
to a majority of today's computing and telecommunications
environments. ZMODEM provides several data streaming methods
selected according to the limitations of the sending environment,
receiving environment, and transmission channel(s).
9.1 Full Streaming with Sampling
If the receiver can overlap serial I/O with disk I/O, and if the
sender can sample the reverse channel for the presence of data
without having to wait, full streaming can be used with no Attn
sequence required. The sender begins data transmission with a ZDATA
header and continuous ZCRCG data subpackets. When the receiver
detects an error, it executes the Attn sequence and then sends a
ZRPOS header with the correct position within the file.
At the end of each transmitted data subpacket, the sender checks for
the presence of an error header from the receiver. To do this, the
sender samples the reverse data stream for the presence of either a
ZPAD or CAN character.[1] Flow control characters (if present) are
acted upon.
Other characters (indicating line noise) increment a counter which is
reset whenever the sender waits for a header from the receiver. If
the counter overflows, the sender sends the next data subpacket as
ZCRCW, and waits for a response.
ZPAD indicates some sort of error header from the receiver. A CAN
suggests the user is attempting to "stop the bubble machine" by
keyboarding CAN characters. If one of these characters is seen, an
empty ZCRCE data subpacket is sent. Normally, the receiver will have
sent an ZRPOS or other error header, which will force the sender to
resume transmission at a different address, or take other action. In
the unlikely event the ZPAD or CAN character was spurious, the
receiver will time out and send a ZRPOS header.[2]
Then the receiver's response header is read and acted upon.[3]
__________
1. The call to rdchk() in sz.c performs this function.
2. The obvious choice of ZCRCW packet, which would trigger an ZACK from
the receiver, is not used because multiple in transit frames could
result if the channel has a long propagation delay.
3. The call to getinsync() in sz.c performs this function.
Chapter 9 Rev 10-27-87 Typeset 10-27-87 21
Chapter 9 ZMODEM Protocol 22
A ZRPOS header resets the sender's file offset to the correct
position. If possible, the sender should purge its output buffers
and/or networks of all unprocessed output data, to minimize the
amount of unwanted data the receiver must discard before receiving
data starting at the correct file offset. The next transmitted data
frame should be a ZCRCW frame followed by a wait to guarantee
complete flushing of the network's memory.
If the receiver gets a ZACK header with an address that disagrees
with the sender address, it is ignored, and the sender waits for
another header. A ZFIN, ZABORT, or TIMEOUT terminates the session; a
ZSKIP terminates the processing of this file.
The reverse channel is then sampled for the presence of another
header from the receiver.[4] if one is detected, the getinsync()
function is again called to read another error header. Otherwise,
transmission resumes at the (possibly reset) file offset with a ZDATA
header followed by data subpackets.
9.1.1 Window Management
When sending data through a network, some nodes of the network store
data while it is transferred to the receiver. 7000 bytes and more of
transient storage have been observed. Such a large amount of storage
causes the transmitter to "get ahead" of the reciever. This can be
fatal with MEGAlink and other protocols that depend on timely
notification of errors from the receiver. This condition is not
fatal with ZMODEM, but it does slow error recovery.
To manage the window size, the sending program uses ZCRCQ data
subpackets to trigger ZACK headers from the receiver. The returning
ZACK headers inform the sender of the receiver's progress. When the
window size (current transmitter file offset - last reported receiver
file offset) exceeds a specified value, the sender waits for a
ZACK[5] packet with a receiver file offset that reduces the window
size.
Unix sz versions beginning with May 9 1987 control the window size
with the "-w N" option, where N is the maximum window size. Pro-YAM,
ZCOMM and DSZ versions beginning with May 9 1987 control the window
size with "zmodem pwN". This is compatible with previous versions of
these programs.[6]
__________
4. If sampling is possible.
5. ZRPOS and other error packets are handled normally.
6. When used with modems or networks that simultaneously assert flow
Chapter 9 Rev 10-27-87 Typeset 10-27-87 22
Chapter 9 ZMODEM Protocol 23
9.2 Full Streaming with Reverse Interrupt
The above method cannot be used if the reverse data stream cannot be
sampled without entering an I/O wait. An alternate method is to
instruct the receiver to interrupt the sending program when an error
is detected.
The receiver can interrupt the sender with a control character, break
signal, or combination thereof, as specified in the Attn sequence.
After executing the Attn sequence, the receiver sends a hex ZRPOS
header to force the sender to resend the lost data.
When the sending program responds to this interrupt, it reads a HEX
header (normally ZRPOS) from the receiver and takes the action
described in the previous section. The Unix sz.c program uses a
setjmp/longjmp call to catch the interrupt generated by the Attn
sequence. Catching the interrupt activates the getinsync() function
to read the receiver's error header and take appropriate action.
When compiled for standard SYSTEM III/V Unix, sz.c uses an Attn
sequence of Ctrl-C followed by a 1 second pause to interrupt the
sender, then give the sender (Unix) time to prepare for the
receiver's error header.
9.3 Full Streaming with Sliding Window
If none of the above methods is applicable, hope is not yet lost. If
the sender can buffer responses from the receiver, the sender can use
ZCRCQ data subpackets to get ACKs from the receiver without
interrupting the transmission of data. After a sufficient number of
ZCRCQ data subpackets have been sent, the sender can read one of the
headers that should have arrived in its receive interrupt buffer.
A problem with this method is the possibility of wasting an excessive
amount of time responding to the receiver's error header. It may be
possible to program the receiver's Attn sequence to flush the
sender's interrupt buffer before sending the ZRPOS header.
__________________________________________________________________________
control with XON and XOFF characters and pass XON characters that
violate flow control, the receiving program should have a revision
date of May 9 or later.
Chapter 9 Rev 10-27-87 Typeset 10-27-87 23
Chapter 9 ZMODEM Protocol 24
9.4 Full Streaming over Error Free Channels
File transfer protocols predicated on the existence of an error free
end to end communications channel have been proposed from time to
time. Such channels have proven to be more readily available in
theory than in actuality. The frequency of undetected errors
increases when modem scramblers have more bits than the error
detecting CRC.
A ZMODEM sender assuming an error free channel with end to end flow
control can send the entire file in one frame without any checking of
the reverse stream. If this channel is completely transparent, only
ZDLE need be escaped. The resulting protocol overhead for average
long files is less than one per cent.[7]
9.5 Segmented Streaming
If the receiver cannot overlap serial and disk I/O, it uses the
ZRINIT frame to specify a buffer length which the sender will not
overflow. The sending program sends a ZCRCW data subpacket and waits
for a ZACK header before sending the next segment of the file.
If the sending program supports reverse data stream sampling or
interrupt, error recovery will be faster (on average) than a protocol
(such as YMODEM) that sends large blocks.
A sufficiently large receiving buffer allows throughput to closely
approach that of full streaming. For example, 16kb segmented
streaming adds about 3 per cent to full streaming ZMODEM file
transfer times when the round trip delay is five seconds.
10. ATTENTION SEQUENCE
The receiving program sends the Attn sequence whenever it detects an
error and needs to interrupt the sending program.
The default Attn string value is empty (no Attn sequence). The
receiving program resets Attn to the empty default before each
transfer session.
The sender specifies the Attn sequence in its optional ZSINIT frame.
The Attn string is terminated with a null.
__________
7. One in 256 for escaping ZDLE, about two (four if 32 bit CRC is used)
in 1024 for data subpacket CRC's
Chapter 10 Rev 10-27-87 Typeset 10-27-87 24
Chapter 10 ZMODEM Protocol 25
Two meta-characters perform special functions:
o \335 (octal) Send a break signal
o \336 (octal) Pause one second
11. FRAME TYPES
The numeric values for the values shown in boldface are given in
zmodem.h. Unused bits and unused bytes in the header (ZP0...ZP3) are
set to 0.
11.1 ZRQINIT
Sent by the sending program, to trigger the receiving program to send
its ZRINIT header. This avoids the aggravating startup delay
associated with XMODEM and Kermit transfers. The sending program may
repeat the receive invitation (including ZRQINIT) if a response is
not obtained at first.
ZF0 contains ZCOMMAND if the program is attempting to send a command,
0 otherwise.
11.2 ZRINIT
Sent by the receiving program. ZF0 and ZF1 contain the bitwise or
of the receiver capability flags:
#define CANCRY 8 /* Receiver can decrypt */
#define CANFDX 01 /* Rx can send and receive true FDX */
#define CANOVIO 02 /* Rx can receive data during disk I/O */
#define CANBRK 04 /* Rx can send a break signal */
#define CANCRY 010 /* Receiver can decrypt */
#define CANLZW 020 /* Receiver can uncompress */
#define CANFC32 040 /* Receiver can use 32 bit Frame Check */
#define ESCCTL 0100 /* Receiver expects ctl chars to be escaped