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technical description and docs for Zmodem
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ZMODEM PROTOCOL REFERENCE
File Transfer Protocol
Please distribute as widely as possible.
Questions to Chuck Forsberg
Omen Technology Inc
17505-V Northwest Sauvie Island Road
Portland Oregon 97231
Modem (Telegodzilla): 503-621-3746 Speed 1200,300
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Encouragement and suggestions by Stuart Mathison, Thomas
Buck, John Wales, Ward Christensen, and Irv Hoff are
2. ROSETTA STONE
Here are some definitions which reflect the current
vernacular in the computer media. The attempt here is
identify the file transfer protocol rather than specific
Frame A ZMODEM frame consists of a header packet and 0 or
more data packets.
Response Time This is the maximum expected delay between the
receiver sending a packet to the transmitter and
receiving the beginning of a response from the
XMODEM refers to the original 1979 file transfer etiquette
introduced by Ward Christensen's 1979 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 every serious
communications program because of its universality,
simplicity, and reasonable performance.
XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte
Cyclical Redundancy Check (CRC-16), giving modern
error detection protection.
YMODEM refers to the XMODEM/CRC protocol with the
throughput and/or batch transmission enhancements
described in YMODEM.DOC.
ZMODEM Zmodem is a second generation streaming protocol for
text and binary file transmission between
applications running on microcomputers and
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3. YET ANOTHER PROTOCOL, AGAIN?
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 close coupled micro to micro
environment for which it was designed. These application
have exposed some weaknesses.
o+ The short block length causes throughput to suffer when
used with timesharing systems, packet switched
networks, satellite circuits, and buffered (error
o+ The 8 bit arithmetic checksum and other aspects allows
line impairments to interfere with dependable, accurate
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
o+ The modification date of the file is lost.
A number of other protocols have been developed over the
years, but none have displaced XMODEM to date.
o+ Lack of public domain documentation and example
programs have kept proprietary protocols such as MNP,
Blast, and others tightly bound to the fortunes of
o+ Hardware and/or software complexity discourages the
widespread application of BISYNC, SDLC, HDLC, X.25, and
o+ Link level protocols such as X.25, X.PC, and MNP modems
do not manage application to application file
o+ The Kermit protocol was developed to allow file
transfers in environments hostile to XMODEM. The
performance compromises necessary to accomodate non
transparent environments limit Kermit's efficiency.
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Even with completely transparent channels, Kermit
control character quoting limits the efficiency of
binary file transfers to about 75 per cent.
Kermit Sliding Windows ("SuperKermit") improves
throughput over networks at the cost of increased
complexity. SuperKermit state transitions are encoded
in a special language "wart" which requires a C
compiler. The SuperKermit C code 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.
A number of extensions to the XMODEM protocol have been made
under the collective name YMODEM.
o+ YMODEM-k reduces the overhead from transmission delays by
87 per cent compared to XMODEM, but network delays can
still degrade performance.
o+ The handling of files that are not a multiple of 1024 or
128 bytes is awkward, especially if the file length is
not known, or changes during transmission.
o+ YMODEM-g is essentially insensitive to network delays.
Because it does not support error recovery, YMODEM-g must
be used hardwired or with a link level protocol.
Another XMODEM "extension" is protocol cheating, referred to
as "Turbo Download" and OverThruster. These improve XMODEM
throughput at the expense of error recovery.
The ZMODEM Protocol is proposed as a means of addressing the
weaknesses described above while maintaining as much of
XMODEM's simplicity and prior art as possible.
4. ZMODEM Protocol Design Criteria
The design of a file transfer protocol is an engineering
compromise between conflicting requirements:
ZMODEM is designed for optimum performance with minimum
degradation caused by delays introduced by packet switched
networks and timesharing systems.
ZMODEM is optimized for best throughput where line hits
occur infrequently. This assumption markedly reduces code
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complexity and memory requirements.
It is assumed that many transfers will originate from a
timesharing system connected to a packet switched network.
ZMODEM provides features to allow for simple, efficient
implementation on timesharing hosts.
File transfers begin immediately regardless of which program
is started first, no 10 second delay.
4.2 Integrity and Robustness
All packets are protected with 16 bit CRC.
4.3 Ease of use
File names need be entered only once. Wild Card names may
be used with batch transfers. Minimum keystrokes required
to initiate transfers. ZMODEM steps down to X/YMODEM if the
other end does not support ZMODEM.
4.4 Ease of implementation
ZMODEM accomodates a wide variety of systems:
o+ Microcomputers that cannot overlap disk and serial i/o
o+ Microcomputers that cannot overlap serial send and
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
Although ZMODEM provides "hooks" for multiple "threads",
ZMODEM is not intended to replace link level protocols such
as X.25. ZMODEM provides a near optimal general purpose
application to application file transfer protocol to be used
with link level protocols such as X.25, MNP, Fastlink, etc.
ZMODEM uses packets somewhat different from those used in
X/YMODEM. X/YMODEM type packets are not used for the
o+ Block numbers are limited to 256
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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 X/YMODEM programs such as Professional-
YAM overcome some of these weaknesses with clever
proprietary code, but a newer protocol is still useful.
o+ It is difficult to determine the beginning and ends of
X/YMODEM blocks when they are corrupted by line hits.
This discourages rapid error recovery.
5.1 Link Escape Encoding
ZMODEM acheives 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 packets. It
allows the beginning of packets 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 ZDLE characters, would incur a
50% overhead. The particular ZDLE character was chosen
after examining the character distributions of many types of
files used with personal computers.
The ZDLE character is special. ZDLE represents a control
sequence of some sort. If a ZDLE character appears in the
data sent within a binary packet, it is prefixed with
another ZDLE. An escaped ZDLE is counted once in the CRC.
The current value for ZDLE is exclamation point (!). Use of
a printing character as ZDLE allows application programs to
recognize HEX Header Packets. This particular character was
chosen because it does not appear often in many types of
files likely to be transferred with this protocol. In
1. This and other constants are defined in the _z_m_o_d_e_m._h
include file. Please note that constants with a leading
0 are octal constants in C.
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addition, no known timesharing system uses it for editing.
5.2 Header Packet Information
All ZMODEM frames begin with a header packet which may be
sent in binary or HEX form. ZMODEM uses a single routine to
recognize binary and hex header packets. Either form of the
header packet contains the same raw information:
o+ A type byte 2 Future extensions to ZMODEM may use the
high order bits of the type byte to indicate thread
o+ Four bytes of data indicating flags and/or numeric
quantities depending on the packet type
Order of Bytes in Header Packet
T: packet Type
F0: Flags least significant byte
P0: file Position least significant
P3: file Position most significant
T F3 F2 F1 F0
T P0 P1 P2 P3
5.3 Binary Header Packet
A binary header packet is only sent by the sending program
to the receiving program.
A binary header packet begins with the sequence ZPAD, ZDLE,
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
0 or more binary data packets will follow depending on the
2. The packet types are cardinal numbers beginning with 0
to minimize state transition table memory requirements.
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The function _z_s_b_h_d_r transmits a binary header packet. The
function _z_g_e_t_h_d_r receives a binary or hex header packet.
5.4 HEX Header Packet
The receiver sends responses in hex header packets.
Hex encoding is required to support XON/XOFF flow control.
Use of Kermit style encoding for control and paritied
characters was considered and rejected because of increased
possibility of interacting with some timesharing systems's
line edit functions. Use of HEX packets from the receiving
program allows control characters to be used to interrupt
the sender when errors are detected. Except for header
packet types that imply data packets to follow, a HEX header
packet may be used in place of a binary header packet.
A hex header packet begins with the sequence ZPAD, ZPAD,
ZDLE, ZHEX. The _z_g_e_t_h_d_r routine synchronizes in the ZPAD-
ZDELE sequence. The extra ZPAD allows other parts of the
program to detect a ZMODEM packet and then call _z_g_e_t_h_d_r to
receive the packet.
The type byte, the four position/flag bytes, and the CRC
thereof are sent in hex using the character set
01234567890abcdef. Upper case hex digits are not allowed;
they would false trigger X/YMODEM programs.
A carriage return, line feed, and XON are appended to the
HEX header packet but are not considered to be part of it.
The CR/LF aids debugging from printouts. The XON releases
the sender from spurious XOFF flow control characters
generated by line noise, a common occurrence.
0 or more HEX data packets will follow depending on the
The function _z_s_h_h_d_r sends a hex header packet.
5.5 Binary Data Packets
Binary data packets 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 4800 bps, 1024 above 4800 bps or when the data link is
known to be relatively error free.
No padding is used with binary data packets. The data bytes
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are ZDLE encoded and transmitted. A ZDLE and frameend are
then sent, followed by two ZDLE encoded CRC bytes. The CRC
accumulates the data bytes and frameend.
The function _z_s_b_d_a_t_a sends a binary data packet. The
function _z_r_b_d_a_t_a receives a binary data packet.
5.6 HEX Data Packet
The format of HEX data packets is not currently specified.
These would be used for server commands, etc.
6. 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 packets are received for an
extended period of time, say one minute.
To start a ZMODEM file transfer session, the sending program
is called with the names of the desired file(s).
The sending program sends the string "rz\r" and a single HEX
ZRQRINIT packet with data = 0. The "rz" followed by
carriage return activates a ZMODEM receive program or
command if it were not already active. If the receiving
program receives the ZRQRINIT packet, it totally ignores it
as the sending program will be responding to the RINIT
packet sent by the receiver. The sending program should
also ignore this packet type, which would be an echo of its
Since the ZRQRINIT sequence contains no exotic control
characters, it can be accepted by the application program as
a command to begin ZMODEM receive. The sequence prints as
"rz^M**!B00000000000000^M^J" where ^M represents CR and ^J
The sending program awaits a command from the receiving
program to start file transfers. If a "C" or NAK is
received, an XMODEM or YMODEM file transfer is indicated,
and file transfer(s) use the X/YMODEM protocol. Note: With
ZMODEM and YMODEM Batch, the sending program provides the
file name, but not with XMODEM.
When the ZMODEM receive program starts, it immediately sends
a ZRINIT packet to initiate ZMODEM file transfers. The
receive program resends the ZRINIT packet at _r_e_p_s_o_n_s_e _t_i_m_e
intervals for a suitable period of time (40 seconds typical)
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before falling back to X/YMODEM protocol. If the receiving
program receives a ZRINIT packet, it is an echo indicating
that the sending program is not operational.
If the receiving program receives a ZRQRINIT packet, it
Eventually the sending program correctly receives the ZRINIT
The sender may respond with an optional
ZCRYPT packet, which the receiver
acknowledges with a suitable frame. (Details
to be worked out later)
The sender may respond with an optional ZSINIT frame to set
the receiving program's Attention string. The receiver
sends a ZACK packet in response.
The sender then sends a ZFILE packet containing the file
name, file length, modification date, and other information
identical to that used by YMODEM Batch.
The receiver may respond to this with a
ZGETCRC packet, which requires the sender to
permorm a CRC on the file and transmit same
with a ZCRC packet. The receiver may use
this information to determine whether to
accept the file or skip it.
The receiver may respond with a ZSKIP packet, which causes
the file to be passed over.
A ZRPOS packet from the receiver initiates transmission of
the file data starting at the offset in the file specified
in the ZRPOS packet. Normally the receiver specifies the
data transfer 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. 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 header packet (with file
position) followed by one or more data packets.
The receiver compares the file position in the ZDATA
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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.
A data packet terminated by ZCRCGO and CRC does not
elicit a response unless an error is detected; more data
packet(s) follow immediately.
ZCRCQ data packets expect a ZACK response
(with the file offset) if no error,
otherwise a ZRPOS response (with the last
good file offset). Another data packet
continues immediately. ZCRCQ packets are
not used if the receiver does not indicate
FDX ability with the CANFDX bit.
ZCRCW data packets expect a response before the next
frame is sent. If the receiver does not indicate
overlapped I/O capability with the CANOVIO bit, the
sender uses the ZCRCW to allow the receiver to write its
buffer before sending more data.
A zero length data packet may also be
used as a sending idle packet to prevent
the receiver from timing out in case data
is not immediately available to the
In the absence of fatal error, the sender encounters
end of file. If the end of file is encountered within
a frame, the frame is closed with a ZCRCE data packet
which does not elicit a response except in case of
The sender sends a ZEOF packet 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 and responds with
ZRINIT. Otherwise the receiver sends ZRPOS with the
current file offset, forcing the sender to send the
After all files are processed, any further protocol
errors should not prevent the sending program from
returning with a success status.
The sender closes the session with a ZEXIT header
packet. The receiver acknowledges this with its own
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When the sender receives the acknowledging packet, 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.
7. STREAMING TECHNIQUE
ZMODEM allows a choice of data streaming methodmethods
selected according to the limitations of the sending
program operating environment, receiving program
operating environment, and the transmission channel(s).
If the computers can overlap serial I/O with disk I/O,
the sender begins data transmission with a ZDATA header
and continuous ZCRCG data packets. When the receiver
detects an error, it sends the Attn sequence and a
ZRPOS packet to force the sender back to the correct
position within the file.
At the end of each transmitted packet, the sender
checks for the presence of a error packet from the
receiver. To do this, the sender may sample the
reverse data stream for the presence of a ZPAD
If the reverse data stream cannot be sampled without
entering an I/O wait, the receiver can interrupt the
sender with a control character, break, or combinations
thereof, as specified in the ZSINIT frame sent by the
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 overfill before sending a ZCRCW
8. ATTENTION SEQUENCE
The receiving program sends the Attn sequence whenever
it detects an error and needs to interrupt the sending
The default Attn string value is empty (no Attn
sequence). The characters in the Attn string are
terminated with a null. Two characters perform special
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o+ \335 Sends a break signal
o+ \336 Pauses one second
9. PACKET/FRAME TYPES
The numeric values for the values shown in boldface are
given in _z_m_o_d_e_m._h.
Sent by the sending program to call up the receiving
program. P0...P3 are zero.
Sent by the receiving program. ZF0 and ZF1 contain
receiver capability flags:
#define CANFDX 1 /* Rx can send and receive FDX */
#define CANOVIO 2 /* Rx can receive during disk I/O */
#define CANBRK 4 /* Rx can send a break signal */
#define CANCRY 8 /* Receiver can decrypt */
ZP0 and ZP1 contain the size of the receiver's buffer
in bytes, or 0 if overlapped I/O is allowed.
Sender sends capability flags (none currently defined)
followed by a binary data packet terminated with ZCRCW.
Data contains the Attn string, maximum length 32 bytes.
The ZSINIT is only sent to programs that indicate the
ability to overlap serial data and disk I/O (CANOVIO).
Acknowedgement to ZSINIT header packet or ZCRCW data
packet. ZP0 to ZP3 contain file offset.
This packet indicates the beginning of a file
transmission attempt. ZF0 and ZF1 contain special file
o+ ZBIN This is a binary file
A ZCRCW data packet follows with file name, file
length, modification date, and other information
described in a later chapter.
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Sent by the receiver in response to ZFILE, makes the
sender skip to the next file.
Indicates last packet header was garbled. (See also
Sent by receiver to terminate batch file transfers when
requested by the user. Sender initiates a ZFIN
Sent by sending program to terminate ZMODEM. Receiver
responds with ZFIN.
Sent by receiver to force file transfer to resume at
file offset given in ZP0...ZP3.
ZP0...ZP3 contain file offset. One or more data
ZP0...ZP3 contain file offset. Sender reports End of
Error in reading or writing file, equivalent to ZABORT.
1. Or ZCOMPL in case of server mode.
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Request (receiver) and response (sender) for file CRC.
ZP0 and ZP1 contain 16 bit file CRC.
Negotiation for encryption.
Server request now completed.
A simple transaction, one file, no errors, overlapped
ZDATA data ...
Some tests of ZMODEM protocol 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 dial-up 1200 bps modems. The ZMODEM
software was configured to use 1024 byte packet lengths
above 2400 bps, 256 otherwise.
Because no time delays are necessary in normal file
transfers, per file negotiations are much faster than
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with YMODEM, the only observed impidiment being the
time required by the program(s) to update logging
During a file transfer, a short line hit seen by the
receiver usually induces a CRC error. The interrupt
packet is usually seen by the sender before the next
packet is sent, and the resultant loss of data
throughput averages about half a packet. 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
the channel delay, as the packets in transit through
the channel are discarded on error.
A longer noise burst that affects both the receiver and
the sender's reception of the interrupt packet 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 packet which concludes
with an XON.
In summation, ZMODEM performance in the presence of
errors resembles that of X.PC and SuperKermit. Short
bursts cause minimuml data loss. Long bursts (such as
pulse dialing noises) often require a timeout error to
restore the flow of data.
Figure 1. Protocol Overhead Information
Figures are calculated for round trip delay times of 40
milliseconds and 5 seconds. A 102400 byte randomly
distributed binary file 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 packets. YM-g refers to the YMODEM "g"
option. ZMODEM uses 256 byte packets for this example.
SuperKermit uses amximum packet size, 8 bit transparent
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transmission, no run length compression.
Protocol dump XMODEM YM-k YM-G ZMODEM S-Kermit
Round-Trips - 803 103 5 5 5?
Time@40ms - 32s 4s - - -
Time@5s - 4015s 515s 25s 25s 25
Chars-Ovhd - 4803 603 503 2000 7460
Time@0s 853s 893s 858s 857s 870s 1135s
Time@40ms 853s 925s 862s 857s 870s 1135s
Time@5s 853s 5761s 1373s 882s 905s 1160s
Figure 2. Transmission Time Comparision
(5 second delay)
************ Kermit Sliding Windows
Figure 3. Y/ZMODEM Header Information usage
Program Batch Length Date Mode S/N YMODEM-g ZMODEM
Unix rb/sb yes yes yes yes no sb only no
Unix rz/sz yes yes yes yes no sb only yes
VMS rb/sb yes yes no no no no no
Pro-YAM yes yes yes no yes yes yes
CP/M YAM yes no no no no no no
KMD/IMP yes no|- no no no no no
MEX no no no no no no no
13. ZFILE FRAME FILE INFORMATION
Only the pathname (file name) part is required for
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:
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 pathname. The source
drive (A:, B:, etc.) is never sent.
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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
o+ The receiver should accommodate file names in
lower and upper case.
o+ The rb(1) program on Unix systems normally
translates the filename to lower case unless
one or more letters in the filename are
already in lower case.
o+ When transmitting files between different
operating systems, file names must be
acceptable to both the sender and receiving
If directories are included, they are delimited by
/; i.e., "subdir/foo" is acceptable, "subdir\foo"
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.
With ZMODEM, the receiver uses the file length
only for display (progress reporting) purposes;
the actual length is determined by the data
Modification Date A single space separates the
modification date from the file length.
The mod date is optional, and the filename and
length may be sent without requiring the mod date
to be sent.
The mod date is sent as an octal number giving the
time the contents of the file were last changed
measured in seconds from Jan 1 1970 Universal
1. Fields may not be skipped.
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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.
Two Microsoft blunders complicate the use of
modification dates in file transfers with
MSDOS(TM) systems. The first is the lack of
timezone standardization in MS-DOS. A file's
creation time can not be known unless the timezone
of the system that wrote the file2 is known. Unix
solved this problem (for planet Earth, anyway) by
stamping files with Universal Time (GMT).
Microsoft would have to include the timezone of
origin in the directory entries, but does not.
Professional-YAM gets around this problem by using
the z parameter which is set to the number of
minutes local time lags GMT. For files known to
originate from a different timezone, the -zT
option may be used to specify T as the timezone
for an individual file transfer.
The second problem is the lack of a separate file
creation date in DOS. Since some backup schemes
used with DOS rely on the file creation date to
select files to be copied to the archive, back-
dating the file modification date could interfere
with the safety of the transferred files. For
this reason, Professional-YAM does not modify the
date of received files with the header information
unless the d parameter is non zero.
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. rb(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
2. Not necessarily that of the transmitting system!
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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.
The file information is terminated by a null. If only
the pathname is sent, the pathname will be terminated
by two nulls. The length of the file information
packet, including the trailing null, must not exceed
1024 bytes; a typical length is less than 64 bytes.
14. MORE INFORMATION
More information may be obtained by calling
Telegodzilla at 503-621-3746.
UUCP sites can obtain the nroff/troff source to this
uucp omen!/usr/caf/public/zmodem.mm /tmp
A continually updated list of available files is stored
The following L.sys line calls Telegodzilla (Pro-YAM in
host operation). Telegodzilla waits for carriage
returns to determine the incoming speed. If none is
detected, 1200 bps is assumed and a greeting is
In response to "Name Please:" uucico gives the Pro-YAM
"link" command as a user name. 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 logs in as uucp.
omen Any ACU 1200 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp
Chapter 15 DRAFT 3-24-86 20
DRAFT ZMODEM Protocol Ref 02-23-86 21
15. ZMODEM Programs
A demonstration version of Professional-YAM is
available as YAMDEMO.LQR (A SQueezed Novosielski
library). This may be used to test ZMODEM and YMODEM
Unix programs supporting the YMODEM protocol are
available on Telegodzilla in the "upgrade" subdirectory
as RBSB.SHQ (a SQueezed shell archive). 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
A CP/M assembly version is available as MODEM76.AQM and
Irv Hoff has added YMODEM 1k packets and basic YMODEM
batch 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.
MEX and MEX-PC also support some of the YMODEM
Questions about YMODEM, the Professional-YAM
communications program, and requests for evaluation
copies may be directed to:
Omen Technology Inc
17505-V Sauvie Island Road
Portland Oregon 97231
Modem: 503-621-3746 Speed: 1200,300
Chapter 15 DRAFT 3-24-86 21
1. ACKNOWLEDGMENTS.................................... 2
2. ROSETTA STONE...................................... 2
3. YET ANOTHER PROTOCOL, AGAIN?....................... 3
4. ZMODEM Protocol Design Criteria.................... 4
4.1 Throughput................................... 4
4.2 Integrity and Robustness..................... 5
4.3 Ease of use.................................. 5
4.4 Ease of implementation....................... 5
5. PACKETIZATION...................................... 5
5.1 Link Escape Encoding......................... 6
5.2 Header Packet Information.................... 7
5.3 Binary Header Packet......................... 7
5.4 HEX Header Packet............................ 8
5.5 Binary Data Packets.......................... 8
5.6 HEX Data Packet.............................. 9
6. PROTOCOL TRANSACTION OVERVIEW...................... 9
7. STREAMING TECHNIQUE................................ 12
8. ATTENTION SEQUENCE................................. 12
9. PACKET/FRAME TYPES................................. 13
9.1 ZRQRINIT..................................... 13
9.2 ZRINIT....................................... 13
9.3 ZSINIT....................................... 13
9.4 ZACK......................................... 13
9.5 ZFILE........................................ 13
9.6 ZSKIP........................................ 14
9.7 ZNAK......................................... 14
9.8 ZABORT....................................... 14
9.9 ZFIN......................................... 14
9.10 ZRPOS........................................ 14
9.11 ZDATA........................................ 14
9.12 ZEOF......................................... 14
9.13 ZFERR........................................ 14
9.14 ZCRC......................................... 15
9.15 ZCRYPT....................................... 15
9.16 ZCOMPL....................................... 15
10. Transaction........................................ 15
11. PERFORMANCE........................................ 15
- i -
12. TABLES............................................. 16
13. ZFILE FRAME FILE INFORMATION....................... 17
14. MORE INFORMATION................................... 20
15. ZMODEM Programs.................................... 21
- ii -
LIST OF FIGURES
Figure 1. Protocol Overhead Information................ 16
Figure 2. Transmis
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