


     The ZMODEM Asynchronous Inter Application File Transfer Protocol

                              Chuck Forsberg

                           Omen Technology Inc


                              Chuck Forsberg
                           Omen Technology Inc
                   17505-V Northwest Sauvie Island Road
                          Portland Oregon 97231
                           Voice: 503-621-3406
            Modem (Telegodzilla): 503-621-3746 Speed 1200,300
                          Compuserve: 70715,131
                    UUCP: ...!tektronix!reed!omen!caf











































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1.  INTENDED AUDIENCE

This document is intended for systems programmers and other technically
qualified people who choose and implement asynchronous file transfer
protocols over dial-up networks and related environments.


2.  ACKNOWLEDGMENTS

Encouragement and suggestions by Stuart Mathison, Thomas Buck, John Wales,
Ward Christensen, and Irv Hoff are gratefully acknowledged.


3.  RELATED DOCUMENTS

The following files should be available for reference while studying this
document:

YMODEM.DOC Describes the XMODEM and YMODEM file transfer protocols

ZMODEM.H Provides definitions for the manifest constants referenced
        herein.

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

rz.1, sz.1 Manual pages for rz and sz.

zm.c, zmodem.h Operating system independent ZMODEM subroutines, header
        file.


4.  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 programs.

Frame   A ZMODEM frame consists of a header packet and 0 or more data
        packets.

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.



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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 mainframes.


5.  WHY DEVELOP ZMODEM?

Since its development half a decade ago, the Ward Christensen MODEM
protocol has enabled a wide variety of computer systems to interchange
data.  There is hardly a communications program that doesn't at least
claim to support this protocol, now called XMODEM.

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

   + The user interface is suitable for computer hobbyists.  Three or four
     commands must be keyboarded to transfer each file.

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

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

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

   + The transmitted file accumulates as many as 127 extraneous bytes.

   + The modification date and other file attributes are lost.

   + XMODEM requires complete 8 bit transparency, all 256 codes.  XMODEM
     will not operate over some networks that need flow control.

A number of other protocols have been developed over the years, but none
have displaced XMODEM to date.

   + Lack of public domain documentation and example programs have kept
     proprietary protocols such as MNP, Blast, and others tightly bound to
     the fortunes of their suppliers.

   + Hardware and/or software complexity discourages the widespread
     application of BISYNC, SDLC, HDLC, X.25, and X.PC protocols.





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

   + 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.  Even with completely transparent channels, Kermit
     control character quoting limits the efficiency of binary file
     transfers to about 75 per cent.[1]

     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 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 these submodes
     are not matched.

A number of extensions to the XMODEM protocol have been made under the
collective name YMODEM.

 + YMODEM-k uses 1024 byte blocks to reduce the overhead from transmission
   delays by 87 per cent compared to XMODEM, but network delays can still
   degrade performance.  Some networks may not be transmit the 1024 byte
   packets unmodified.

 + 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.

 + YMODEM-g provides efficient batch file transfers, preserving the exact
   file length and file modification date.  YMODEM-g is essentially
   insensitive to network delays.  Because it does not support error
   recovery, YMODEM-g is usually used hardwired or with a reliable link
   level protocol.  IF YMODEM-g detects a CRC error, data transfers are
   aborted.  YMODEM-g is easy to implement because it closely resembles
   XMODEM-CRC.

Another XMODEM "extension" is protocol cheating, referred to as "Turbo
Download" and OverThruster.  [2] These sometimes improve XMODEM throughput


__________

 1. Some Kermit programs support run length encoding.




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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.



6.  ZMODEM Protocol Design Criteria

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

6.1  Ease of Use

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

 + File names need be entered only once, menu selections are possible.

 + Wild Card names may be used with batch transfers.

 + Minimum keystrokes required to initiate transfers.

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

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

6.2  Throughput

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 when line hits occur infrequently.
This assumption markedly reduces code complexity and memory requirements.
ZMODEM protocol features enhance rapid error recovery compared to network
compatible XMODEM implementations.

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.



__________________________________________________________________________

 2. Omen Technology Trademark

 1. Provided the transmission medium accomodates YMODEM.




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


6.3  Integrity and Robustness

All packets are protected with 16 bit CRC.  Proprietary alogrithyms[2] are
not needed for reliable transfers.

A security challenge guards againgst Trojan Horse messages.

6.4  Ease of Implementation

ZMODEM accomodates a wide variety of systems:

 + Microcomputers that cannot overlap disk and serial i/o

 + Microcomputers that cannot overlap serial send and receive

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

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

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

ZMODEM accomodates 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.[3]

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.


7.  ZMODEM BASICS







__________

 2. Unique to Professional-YAM, PowerCom, etc.

 3. Streaming strategey is discussed in a coming chapter.




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7.1  Packetization

ZMODEM frames somewhat different from X/YMODEM blocks.  X/YMODEM blocks
are not used for the following reasons:

 + Block numbers are limited to 256

 + No provision for variable length blocks

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

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

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

7.2  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 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 ZDLE 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 the data sent within a binary
packet, 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 CAN characters to abort a ZMODEM session,
compatible with X/YMODEM session abort.



__________

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




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

Two successive CAN characters will abort a ZMODEM session.  Experience
with YMODEM file transfers suggests that this does not impair the
robustness of the protocol.  A minimum of 8 CAN are sent, so the ZMODEM
subroutines can be modified to require more successive CAN characters to
signal an abort.

The receiving program will decode 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.  The ZMODEM software currently escapes ZDLE, 021,
0221, 023, and 0223.  In addition, the receiver recognizes escapes for
0177 and 0377 should these characters need to be escaped.

7.3  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:

 + A type byte[2] Future extensions to ZMODEM may use the high order bits
   of the type byte to indicate thread selection.

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

                Figure 1.  Order of Bytes in Header Packet


                   TYPE:  packet 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



__________

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




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7.4  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, 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 packets will follow depending on the frame type.

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

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


7.5  HEX Header Packet

The receiver sends responses in hex header packets.  The sender also uses
hex header packets when they are not followed by binary data packets.

Hex encoding is required to support XON/XOFF flow control.  The hex header
receiving routine ignores flow control characters.

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
zgethdr routine synchronizes in the ZPAD-ZDELE sequence.  The extra ZPAD
allows other parts of the program to detect a ZMODEM packet and then call
zgethdr 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 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.



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0 or more ASCII Encoded data packets will follow depending on the frame
type.

The function zshhdr sends a hex header packet.

                       Figure 3.  HEX Header Packet
       * * ZDLE 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.)


7.6  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 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 zsbdata sends a binary data packet.  The function zrbdata
receives a binary data packet.

7.7  ASCII Encoded Data Packet

The format of ASCII Encoded data packets is not currently specified.
These would be used for server commands, etc.


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
packets are received for an extended period of time, say one minute.[1]

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 sends 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.


__________

 1. Special considerations apply when sending commands.




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The sender may then display a message intended for human consumption, such
as a list of the files requested, etc.

Then the sender sends a ZRQINIT packet.  The ZRQINIT packet causes a
previously started receive program to send its ZRINIT packet 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 B000000 indicating a ZRQINIT packet, 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 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, or a ZCHALLENGE packet to verify
the sending program.  The receive program resends its packet at repsonse
time intervals for a suitable period of time (40 seconds typical) before
falling back to X/YMODEM protocol.  If the receiving program receives a
ZRQINIT packet, it resends the ZRINIT packet.  If the sending program
receives the ZCHALLENGE packet, it places the data in ZP0...ZP3 in an
answering ZACK packet.

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

Eventually the sending program correctly receives the ZRINIT packet.

The sender may then respond with an optional ZSINIT frame to set the
receiving program's Attention string.  The receiver sends a ZACK packet in
response, containing the serial number of the receiving program, or 0.

The sender then sends a ZFILE header with ZMODEM Conversion, Management,
and Transport options[2] followed by a ZCRCW data packet containing the
file name, file length, modification date, and other information identical
to that used by YMODEM Batch.

The receiving program should insure the pathname and options are
compatible with its operating environment and local security requirements.




__________

 2. See below, under ZFILE packet type.




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       If the receiver has a file with the same name and length,
       it may respond with a ZCRC packet, which requires the
       sender to permorm a 16 bit CRC on the file and transmit the
       CRC in ZP0...ZP1 of a ZCRC packet.  The receiver uses this
       information to determine whether to accept the file or skip
       it.  This sequence is triggered by the ZMCRC Management
       Option.

The receiver may then respond with a ZSKIP packet, which causes the
sender to process the next file (if any) in the batch.

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.[3] 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 packet (with file position)
followed by one or more data packets.

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.[4]

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


__________

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

 4. If the ZMSPARS option is used, the receiver instead seeks to position
    in the ZDATA packet.




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CANOVIO bit, or by setting 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 a sending idle
       packet 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 packet which does not elicit a response
except in case of error.

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.  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 sends ZRPOS with
the current file offset, forcing the sender to resend the missing
data.  (If the receiver cannot properly close the file, a ZFERR packet
is sent.)

       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 ZEXIT packet.

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.

8.1  Session Cancel Packet

The Cancel packet consists of two ZPAD characters, eight CAN
characters, and an optional ten backspace characters.  First, the
Attn sequence is executed if the receiving program has been receiving
data in streaming mode.  The ZPAD characters allow sending programs
that sample the reverse data stream to check for a single character
code indicating a packet from the receiver.  The trailing backspace
characters attempt to erase the effects of the other characters if
they are received by a command interpreter.

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







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9.  ZMODEM STREAMING TECHNIQUES

ZMODEM allows choices of 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 computers 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 packets.  When the receiver detects
an error, it executes the Attn sequence and then sends 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 an error packet from the receiver.  To do this, the
sender may sample the reverse data stream for the presence of a ZPAD
character.

Such a program would sample the reverse channel for ZPAD.  If seen,
an empty ZCRCW data packet is sent (in case the receiver was still
reading packets) and then the receiver's response packet is read and
acted upon.  The code fragment in sz.c beginning at NOTDEF_DOS would
perform this function.


9.2  Full Streaming with Interrupt

The method above cannot be used if 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 ZSINIT frame sent
by the sending program.

When the sending program "catches" this interrupt, it reads a HEX
packet (normally ZRPOS) from the receiver and takes appropriate
action.  The Unix sb.c program uses a setjmp/longjmp call and the
getinsync() function to read the receiver's error packet and take
appropriate action.










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9.3  Full Streaming with a 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 packets to get ACKs from the receiver without interrupting the
transmission of data.  After a sufficient number of ZCRCQ packets
have been sent, the sender can read one of the one or more packets
that should have arrived in it's receive interrupt buffer.

A problem with this method is the probability of wasting an excessive
amount of time responding to the receiver's error packet.

9.4  No 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 packet and waits for an
ZACK packet 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 "monolithic" blocks.


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

Two meta-characters perform special functions:

   + \335 (octal) Sends a break signal

   + \336 (octal) Pauses one second


11.  PACKET/FRAME TYPES

The numeric values for the values shown in boldface are given in
zmodem.h.







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11.1  ZRQINIT

Sent once by the sending program, to trigger the receiving program to
send its ZRINIT packet.  This aviods the aggravatimg startup delay
associated with XMODEM and Kermit transfers.

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 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 nonstop I/O is allowed.

11.3  ZSINIT

Sender sends capability flags (currently all 0) (none currently
defined) followed by a binary data packet terminated with ZCRCW.  The
data packet contains the null terminated Attn sequence, maximum
length 32 bytes including the terminating null.

11.4  ZACK

Acknowedgement to ZSINIT header packet, ZCHALLENGE header packet, or
ZCRCW data packet.  ZP0 to ZP3 contain file offset.  Response to
ZCHALLENGE contains the same 32 bits as received.

11.5  ZFILE

This packet 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.  If options are specified to the
reciever, they override options specified to the sender with the
exception of the ZCBIN option, which overrides any other Conversion
Option.


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





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ZCNL Convert received end of line to local end of line convention.
     The suported end line conventions are CR/LF (most ASCII based
     operating systems except Unix and Macintosh), and NL (Unix).
     Neither of these two end of line conventions violate the
     permissible ASCII definitions for Carriage Return and Line
     Feed/New Line.

ZCRECOV Recover interrupted file transfer; start transfer at location
     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.

ZMNEW Compare the source and destination files.  Transfer file if
     source newer or different length

ZMCRC Compare the source and destination files.  Transfer if
     different file length or CRC

ZMAPND Append source file contents to existing destination file (if
     any)

ZMCLOB Replace existing destination file (if any)

ZTSPARS Special processing for sparse file; each file segment is
     transmitted as a separate frame, where the frames are not
     necessarily contiguous.

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 packet follows with file name, file length, modification
date, and other information described in a later chapter.







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11.6  ZSKIP

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

11.7  ZNAK

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

11.8  ZABORT

Sent by receiver to terminate batch file transfers when requested by
the user.  Sender initiates a ZFIN sequence.[1]

11.9  ZFIN

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

11.10  ZRPOS

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

11.11  ZDATA

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

11.12  ZEOF

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

11.13  ZFERR

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

11.14  ZCRC

Request (receiver) and response (sender) for file CRC.  ZP0 and ZP1
contain 16 bit file CRC.






__________

 1. Or ZCOMPL in case of server mode.




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11.15  ZCHALLENGE

Request 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
Cancel sequence.

11.18  ZFREECNT

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

11.19  ZCOMMAND

ZCOMMAND is only sent as a binary header packet.  ZP0...ZP2 contain a
unique cardinal number to differentiate this command from other
commands[2].  ZF0 contains 0 or ZCACK1.


A ZCRCW data packet 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
received the command.

If ZF0 contained ZCACK1, the receiver immediately responds with a
ZCOMPL header.  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 (0 if
ZCACK1).  A 0 exit status implies nominal completion of the command.

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


__________

 2. Currently unused.




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data.

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



12.  TRANSACTION EXAMPLE

12.1  A simple file transfer

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

Sender          Receiver

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


12.2  Challenge and Command Download


Sender          Receiver

"rz\r"
ZRQINIT(ZCOMMAND)
                ZCHALLENGE(rnd)
ZACK(same-rnd)
                ZRINIT
ZCOMMAND
                (Perform Command)
                ZCOMPL
ZFIN
                ZFIN
OO








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13.  ZFILE FRAME FILE INFORMATION

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

N.B.: Only the pathname (file name) part is
s required for batch
transfers.

Pathname The pathname (conventionally, the file name) is sent as a
     null terminated ASCII string.  This is the filename format used
     by the handle oriented MSDOS(TM) functions and C library fopen
     functions.  An assembly language example follows:
                           DB      'foo.bar',0
     No spaces are included in the pathname.  Normally only the file
     name stem (no directory prefix) is transmitted unless the sender
     has selected YAM's f option to send the full relative pathname.
     The source drive designator (A:, B:, etc.) is not sent.

     Filename Considerations:

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

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

        + 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.

        + When transmitting files between different operating
          systems, file names must be acceptable to both the sender
          and receiving operating systems.

     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.

     With ZMODEM, the receiver uses the file length only for display
     (progress reporting) purposes; the actual length is determined


__________

 1. Fields may not be skipped.




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     by the data transfer.

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

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

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

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

     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 file[2] 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


__________

 2. Not necessarily that of the transmitting system!




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     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.

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.  PERFORMANCE RESULTS

14.1  Throughput

Between two single task PC-XT computrers, on a Telenet link through
the local Telenet, SuperKermit gave 72 ch/sec throughput at 1200
baud.  YMODEM-k yielded 85 chars/sec, and ZMODEM provided 113 chat
sec.  ZMODEM was not measured, but would have given much less.

14.2  Error Recovery

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 unbuffered 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 with YMODEM, the only observed
impidiment 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 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



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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.


15.  PACKET SWITCHED NETWORK CONSIDERATIONS

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

The most desireable flow control (when X.25 or hardware CTS is
unavailable) does 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 sent when the
packet is a full 128 bytes, or after a moderate delay (3:0,4:10,6:0).

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.

                  Figure 4.  Flow Control Compatibility

         Connectivity            Interactive   XMODEM   KERMIT   ZMODEM

Direct Connection                YES           YES      YES      YES
Network, no flow control         NO            YES      (1)      (1)
Network, transparent f.c.        YES           YES      YES      YES
Network, semi-transparent f.c.   YES           NO       YES      YES
Network, 7 bit                   YES           NO       YES(2)   NO(3)

(1) Cannot operate in streaming mode.  Kermit is very slow because of
96 byte max packet size.  ZMODEM can adjust burst length to maximum
for faster transfers.



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(2) Parity bits must be encoded, slowing binary transfers.

(3) Extension possible for encoding data to 7 bits.



16.  PERFORMANCE COMPARISION TABLES


"Round Trip Delay Time" includes the time for the last byte in a
packet to propagate through the operating systems and network to the
receiver, plus the time for the receiver's response to that packet to
propogate 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 packets.  YM-g refers
to the YMODEM "g" option.  ZMODEM uses 256 byte packets for this
example.  SuperKermit uses maximum packet size, 8 bit transparent
transmission, no run length compression.

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

                 Figure 5.  Protocol Overhead Information

      Protocol          XMODEM   YM-k    YM-g   ZMODEM   S-Kermit

Protocol Round Trips    803      103     5      5        5
Trip Time at 40ms       32s      4s      0      0        0
Trip Time at 5s         4015s    515s    25s    25s      25

Overhead Characters     4803     603     503    3600     38280

Transfer Time at 0s     893s     858s    857s   883s     1172s
Transfer Time at 40ms   925s     862s    857s   883s     1172s
Transfer Time at 5s     5761s    1373s   882s   918s     1197s





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                 Figure 6.  Transmission Time Comparision
                        (5 Second Round Trip)

************************************************** XMODEM
************ YMODEM-K
********** SuperKermit (Sliding Windows)
******* YMODEM-G
******* ZMODEM

               Figure 7.  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    sz 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     yes-     no     no     no    no         no
 MEX          no      no       no     no     no    no         no


17.  MORE INFORMATION

More information may be obtained by calling Telegodzilla at
503-621-3746.

UUCP sites can obtain the nroff/troff source to this file with
              uucp omen!/usr/caf/public/zmodem.mm /tmp
A continually updated list of available files is stored in
/usr/spool/uucppublic/FILES.

The following L.sys line calls site "omen" yia UUCP.  Telegodzilla
uses Pro-YAM in host operation.

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 e:--e: link d: Giznoid n:--n: uucp










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18.  ZMODEM PROGRAMS

A demonstration version of Professional-YAM is available as
YAMDEMO.ARC on TeleGodzilla..  This file must be unpacked with the
"ARC" program, version 5 or later.  A copy of ARC is available as
"ARC.EXE" or "ARC510.COM" on TeleGodzilla.


This may be used to test ZMODEM and YMODEM implementations.  A
flash-up tree structured help file and processor are provided in
YAMHELP.LQR.



19.  YMODEM PROGRAMS

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 same
directory.

Irv Hoff has added YMODEM 1k packets and 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.

Many other programs, including MEX and MEX-PC also support some of
the YMODEM extensions.

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

     Chuck Forsberg
     Omen Technology Inc
     17505-V Sauvie Island Road
     Portland Oregon 97231
     Voice: 503-621-3406
     Modem (Telegodzilla): 503-621-3746
     Usenet: ...!tektronix!reed!omen!caf
     Compuserve: 70715,131
     Source: TCE022

                                  Yours very truly,








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                              CONTENTS


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

 2.  ACKNOWLEDGMENTS..................................................   2

 3.  RELATED DOCUMENTS................................................   2

 4.  ROSETTA STONE....................................................   2

 5.  WHY DEVELOP ZMODEM?..............................................   3

 6.  ZMODEM Protocol Design Criteria..................................   5
     6.1    Ease of Use...............................................   5
     6.2    Throughput................................................   5
     6.3    Integrity and Robustness..................................   6
     6.4    Ease of Implementation....................................   6

 7.  ZMODEM BASICS....................................................   6
     7.1    Packetization.............................................   7
     7.2    Link Escape Encoding......................................   7
     7.3    Header Packet Information.................................   8
     7.4    Binary Header Packet......................................   9
     7.5    HEX Header Packet.........................................   9
     7.6    Binary Data Packets.......................................  10
     7.7    ASCII Encoded Data Packet.................................  10

 8.  PROTOCOL TRANSACTION OVERVIEW....................................  10
     8.1    Session Cancel Packet.....................................  13

 9.  ZMODEM STREAMING TECHNIQUES......................................  14
     9.1    Full Streaming with Sampling..............................  14
     9.2    Full Streaming with Interrupt.............................  14
     9.3    Full Streaming with a Sliding Window......................  15
     9.4    No Streaming..............................................  15

10.  ATTENTION SEQUENCE...............................................  15

11.  PACKET/FRAME TYPES...............................................  15
     11.1   ZRQINIT...................................................  16
     11.2   ZRINIT....................................................  16
     11.3   ZSINIT....................................................  16
     11.4   ZACK......................................................  16
     11.5   ZFILE.....................................................  16
     11.6   ZSKIP.....................................................  18
     11.7   ZNAK......................................................  18
     11.8   ZABORT....................................................  18
     11.9   ZFIN......................................................  18
     11.10  ZRPOS.....................................................  18
     11.11  ZDATA.....................................................  18



                                  - i -











     11.12  ZEOF......................................................  18
     11.13  ZFERR.....................................................  18
     11.14  ZCRC......................................................  18
     11.15  ZCHALLENGE................................................  19
     11.16  ZCOMPL....................................................  19
     11.17  ZCAN......................................................  19
     11.18  ZFREECNT..................................................  19
     11.19  ZCOMMAND..................................................  19

12.  TRANSACTION EXAMPLE..............................................  20
     12.1   A simple file transfer....................................  20
     12.2   Challenge and Command Download............................  20

13.  ZFILE FRAME FILE INFORMATION.....................................  21

14.  PERFORMANCE RESULTS..............................................  23
     14.1   Throughput................................................  23
     14.2   Error Recovery............................................  23

15.  PACKET SWITCHED NETWORK CONSIDERATIONS...........................  24

16.  PERFORMANCE COMPARISION TABLES...................................  25

17.  MORE INFORMATION.................................................  26

18.  ZMODEM PROGRAMS..................................................  27

19.  YMODEM PROGRAMS..................................................  27


























                                  - ii -














                             LIST OF FIGURES


Figure 1.  Order of Bytes in Header Packet............................   8

Figure 2.  Binary Header Packet.......................................   9

Figure 3.  HEX Header Packet..........................................  10

Figure 4.  Flow Control Compatibility.................................  24

Figure 5.  Protocol Overhead Information..............................  25

Figure 6.  Transmission Time Comparision..............................  26

Figure 7.  Y/ZMODEM Header Information usage..........................  26



































                                 - iii -









     The ZMODEM Asynchronous Inter Application File Transfer Protocol

                              Chuck Forsberg

                           Omen Technology Inc


                                 ABSTRACT



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

ZMODEM provides especially efficient file transfers with timesharing
systems, satellite relays, and wide area packet switched networks.  A
choice of buffering and windowing modes allow ZMODEM to operate
efficiently on systems that cannot support some other streaming protocols.

ZMODEM provides advanced file management features including AutoDownload,
remote file compare, aborted transfer recovery, selective file transfers,
and security verified command downloading.




































