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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


This document is intended for telecommunications managers, systems programmers, and others who choose and implement asynchronous file transfer protocols over dial-up networks and related environments.


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

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

 o The awkward user interface is suitable for computer hobbyists.
   Multiple commands must be keyboarded to transfer each file.
 o Since commands must be given to both programs, simple menu selections
   are not possible.
 o The short block length causes throughput to suffer when used with
   timesharing systems, packet switched networks, satellite circuits,
   and buffered (error correcting) modems.
 o The 8 bit checksum and unprotected supervison allow undetected errors
   and disrupted file transfers.
 o Only one file can be sent per command.  The file name has to be given
   twice, first to the sending program and then again to the receiving
 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

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. 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. Chapter 7 Rev 10-27-87 Typeset 10-27-87 13 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. Chapter 7 Rev 10-27-87 Typeset 10-27-87 14 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.

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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.

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Chapter 9 ZMODEM Protocol 22

A ZRPOS header resets the sender's file offset to the correct position. If possible, the sender should purge its output buffers and/or networks of all unprocessed output data, to minimize the amount of unwanted data the receiver must discard before receiving data starting at the correct file offset. The next transmitted data frame should be a ZCRCW frame followed by a wait to guarantee complete flushing of the network's memory.

If the receiver gets a ZACK header with an address that disagrees with the sender address, it is ignored, and the sender waits for another header. A ZFIN, ZABORT, or TIMEOUT terminates the session; a ZSKIP terminates the processing of this file.

The reverse channel is then sampled for the presence of another header from the receiver.[4] if one is detected, the getinsync() function is again called to read another error header. Otherwise, transmission resumes at the (possibly reset) file offset with a ZDATA header followed by data subpackets.

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

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

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Chapter 9 ZMODEM Protocol 24

9.4 Full Streaming over Error Free Channels

File transfer protocols predicated on the existence of an error free end to end communications channel have been proposed from time to time. Such channels have proven to be more readily available in theory than in actuality. The frequency of undetected errors increases when modem scramblers have more bits than the error detecting CRC.

A ZMODEM sender assuming an error free channel with end to end flow control can send the entire file in one frame without any checking of the reverse stream. If this channel is completely transparent, only ZDLE need be escaped. The resulting protocol overhead for average long files is less than one per cent.[7]

9.5 Segmented Streaming

If the receiver cannot overlap serial and disk I/O, it uses the ZRINIT frame to specify a buffer length which the sender will not overflow. The sending program sends a ZCRCW data subpacket and waits for a ZACK header before sending the next segment of the file.

If the sending program supports reverse data stream sampling or interrupt, error recovery will be faster (on average) than a protocol (such as YMODEM) that sends large blocks.

A sufficiently large receiving buffer allows throughput to closely approach that of full streaming. For example, 16kb segmented streaming adds about 3 per cent to full streaming ZMODEM file transfer times when the round trip delay is five seconds.


The receiving program sends the Attn sequence whenever it detects an error and needs to interrupt the sending program.

The default Attn string value is empty (no Attn sequence). The receiving program resets Attn to the empty default before each transfer session.

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

7. One in 256 for escaping ZDLE, about two (four if 32 bit CRC is used) in 1024 for data subpacket CRC's 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 */ #define ESC8 0200 /* Receiver expects 8th bit to be escaped */ ZP0 and ZP1 contain the size of the receiver's buffer in bytes, or 0 if nonstop I/O is allowed. 11.3 ZSINIT The Sender sends flags followed by a binary data subpacket terminated with ZCRCW. /* Bit Masks for ZSINIT flags byte ZF0 */ #define TESCCTL 0100 /* Transmitter expects ctl chars to be escaped */ #define TESC8 0200 /* Transmitter expects 8th bit to be escaped Chapter 11 Rev 10-27-87 Typeset 10-27-87 25 Chapter 11 ZMODEM Protocol 26 */ The data subpacket contains the null terminated Attn sequence, maximum length 32 bytes including the terminating null. 11.4 ZACK Acknowledgment to a ZSINIT frame, ZCHALLENGE header, ZCRCQ or ZCRCW data subpacket. ZP0 to ZP3 contain file offset. The response to ZCHALLENGE contains the same 32 bit number received in the ZCHALLENGE header. 11.5 ZFILE This frame denotes the beginning of a file transmission attempt. ZF0, ZF1, and ZF2 may contain options. A value of 0 in each of these bytes implies no special treatment. Options specified to the receiver override options specified to the sender with the exception of ZCBIN which overrides any other Conversion Option given to the sender or receiver. 11.5.1 ZF0: Conversion Option If the receiver does not recognize the Conversion Option, an application dependent default conversion may apply. ZCBIN "Binary" transfer - inhibit conversion unconditionally ZCNL Convert received end of line to local end of line convention. The supported end of line conventions are CR/LF (most ASCII based operating systems except Unix and Macintosh), and NL (Unix). Either of these two end of line conventions meet the permissible ASCII definitions for Carriage Return and Line Feed/New Line. Neither the ASCII code nor ZMODEM ZCNL encompass lines separated only by carriage returns. Other processing appropriate to ASCII text files and the local operating system may also be applied by the receiver.[1] ZCRECOV Recover/Resume interrupted file transfer. ZCREVOV is also useful for updating a remote copy of a file that grows without resending of old data. If the destination file exists and is no longer than the source, append to the destination file and start transfer at the offset corresponding to the receiver's end of file. This

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

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Chapter 11 ZMODEM Protocol 27

   option does not apply if the source file is shorter.
   Files that have been converted (e.g., ZCNL) or subject
   to a single ended Transport Option cannot have their
   transfers recovered.

11.5.2 ZF1: Management Option If the receiver does not recognize the Management Option, the file should be transferred normally.

The ZMSKNOLOC bit instructs the receiver to bypass the current file if the receiver does not have a file with the same name.

Five bits (defined by ZMMASK) define the following set of mutually exclusive management options.

ZMNEWL Transfer file if destination file absent. Otherwise,

   transfer file overwriting destination if the source file
   is newer or longer.

ZMCRC Compare the source and destination files. Transfer if

   file lengths or file polynomials differ.

ZMAPND Append source file contents to the end of the existing

   destination file (if any).

ZMCLOB Replace existing destination file (if any).

ZMDIFF Transfer file if destination file absent. Otherwise,

   transfer file overwriting destination if files have
   different lengths or dates.

ZMPROT Protect destination file by transferring file only if

   the destination file is absent.

ZMNEW Transfer file if destination file absent. Otherwise,

   transfer file overwriting destination if the source file
   is newer.

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

ZTCRYPT Encryption. An initial null terminated string

   identifies the key.  Details to be determined.

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Chapter 11 ZMODEM Protocol 28

ZTRLE Run Length encoding, Details to be determined.

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

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

ZTSPARS Special processing for sparse files, or sender managed

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

11.6 ZSKIP

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

11.7 ZNAK

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


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

11.9 ZFIN

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

11.10 ZRPOS

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

2. Or ZCOMPL in case of server mode. Chapter 11 Rev 10-27-87 Typeset 10-27-87 28 Chapter 11 ZMODEM Protocol 29 11.11 ZDATA ZP0…ZP3 contain file offset. One or more data subpackets follow. 11.12 ZEOF Sender reports End of File. ZP0…ZP3 contain the ending file offset. 11.13 ZFERR Error in reading or writing file, protocol equivalent to ZABORT. 11.14 ZCRC Request (receiver) and response (sender) for file polynomial. ZP0…ZP3 contain file polynomial. 11.15 ZCHALLENGE Request sender to echo a random number in ZP0…ZP3 in a ZACK frame. Sent by the receiving program to the sending program to verify that it is connected to an operating program, and was not activated by spurious data or a Trojan Horse message. 11.16 ZCOMPL Request now completed. 11.17 ZCAN This is a pseudo frame type returned by gethdr() in response to a Session Abort sequence. 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 sent in a binary frame. ZF0 contains 0 or ZCACK1 (see below). A ZCRCW data subpacket follows, with the ASCII text command string terminated with a NULL character. If the command is intended to be executed by the operating system hosting the receiving program (e.g., "shell escape"), it must have "!" as the first character. Otherwise the command is meant to be executed by the application program which receives the command. Chapter 11 Rev 10-27-87 Typeset 10-27-87 29 Chapter 11 ZMODEM Protocol 30 If the receiver detects an illegal or badly formed command, the receiver immediately responds with a ZCOMPL header with an error code in ZP0…ZP3. If ZF0 contained ZCACK1, the receiver immediately responds with a ZCOMPL header with 0 status. Otherwise, the receiver responds with a ZCOMPL header when the operation is completed. The exit status of the completed command is stored in ZP0…ZP3. A 0 exit status implies nominal completion of the command. If the command causes a file to be transmitted, the command sender will see a ZRQINIT frame from the other computer attempting to send data. The sender examines ZF0 of the received ZRQINIT header to verify it is not an echo of its own ZRQINIT header. It is illegal for the sending program to command the receiving program to send a command. If the receiver program does not implement command downloading, it may display the command to the standard error output, then return a ZCOMPL header. 12. SESSION TRANSACTION EXAMPLES 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 Chapter 12 Rev 10-27-87 Typeset 10-27-87 30 Chapter 12 ZMODEM Protocol 31 12.2 Challenge and Command Download Sender Receiver "rz\r" ZRQINIT(ZCOMMAND) ZCHALLENGE(random-number) ZACK(same-number) ZRINIT ZCOMMAND, ZDATA (Performs Command) ZCOMPL ZFIN ZFIN OO 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.: The pathname (file name) field is mandatory. Pathname The pathname (conventionally, the file name) is sent as a null terminated ASCII string. This is the filename format used by the handle oriented MSDOS(TM) functions and C library fopen functions. An assembly language example follows: DB '',0 No spaces are included in the pathname. Normally only the file name stem (no directory prefix) is transmitted unless the sender has selected YAM's f option to send the full absolute or relative pathname. The source drive designator (A:, B:, etc.) usually is not sent. Filename Considerations o File names should be translated to lower case unless the sending system supports upper/lower case file names. This is a convenience for users of systems (such as Unix) which store filenames in upper and lower case. o The receiver should accommodate file names in lower and upper case. o When transmitting files between different operating systems, file names must be acceptable to both the sender and receiving operating systems. If not, transformations should be applied to make the file names acceptable. If the transformations are unsuccessful, a new file name may be invented be the receiving program. Chapter 13 Rev 10-27-87 Typeset 10-27-87 31 Chapter 13 ZMODEM Protocol 32 If directories are included, they are delimited by /; i.e., "subdir/foo" is acceptable, "subdir\foo" is not. Length The file length and each of the succeeding fields are optional.[1] The length field is stored as a decimal string counting the number of data bytes in the file. The ZMODEM receiver uses the file length as an estimate only. It may be used to display an estimate of the transmission time, and may be compared with the amount of free disk space. The actual length of the received file is determined by the data transfer. A file may grow after transmission commences, and all the data will be sent. Modification Date A single space separates the modification date from the file length. The mod date is optional, and the filename and length may be sent without requiring the mod date to be sent. The mod date is sent as an octal number giving the time the contents of the file were last changed measured in seconds from Jan 1 1970 Universal Coordinated Time (GMT). A date of 0 implies the modification date is unknown and should be left as the date the file is received. This standard format was chosen to eliminate ambiguities arising from transfers between different time zones. File Mode A single space separates the file mode from the modification date. The file mode is stored as an octal string. Unless the file originated from a Unix system, the file mode is set to 0. rz(1) checks the file mode for the 0x8000 bit which indicates a Unix type regular file. Files with the 0x8000 bit set are assumed to have been sent from another Unix (or similar) system which uses the same file conventions. Such files are not translated in any way. Serial Number A single space separates the serial number from the file mode. The serial number of the transmitting program is stored as an octal string. Programs which do not have a serial number should omit this field, or set it to 0. The receiver's use of this field is optional.

1. Fields may not be skipped.

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Chapter 13 ZMODEM Protocol 33

Number of Files Remaining Iff the number of files remaining is sent,

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

Number of Bytes Remaining Iff the number of bytes remaining is sent,

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

The file information is terminated by a null. If only the pathname is sent, the pathname is terminated with two nulls. The length of the file information subpacket, including the trailing null, must not exceed 1024 bytes; a typical length is less than 64 bytes.


14.1 Compatibility

Extensive testing has demonstrated ZMODEM to be compatible with satellite links, packet switched networks, microcomputers, minicomputers, regular and error correcting buffered modems at 75 to 19200 bps. ZMODEM's economy of reverse channel bandwidth allows modems that dynamically partition bandwidth between the two directions to operate at optimal speeds.

14.2 Throughput

Between two single task PC-XT computers sending a program image on an in house Telenet link, SuperKermit provided 72 ch/sec throughput at 1200 baud. YMODEM-k yielded 85 chars/sec, and ZMODEM provided 113 chars/sec. XMODEM was not measured, but would have been much slower based on observed network propagation delays.

Recent tests downloading large binary files to an IBM PC (4.7 mHz V20) running YAMK 16.30 with table driven 32 bit CRC calculation yielded a throughput of 1870 cps on a 19200 bps direct connection.

Tests with TELEBIT TrailBlazer modems have shown transfer rates approaching 1400 characters per second for long files. When files are compressed, effective transfer rates of 2000 characters per second are possible.

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Chapter 14 ZMODEM Protocol 34

14.3 Error Recovery

Some tests of ZMODEM protocol error recovery performance have been made. A PC-AT with SCO SYS V Xenix or DOS 3.1 was connected to a PC with DOS 2.1 either directly at 9600 bps or with unbuffered dial-up 1200 bps modems. The ZMODEM software was configured to use 1024 byte data subpacket lengths above 2400 bps, 256 otherwise.

Because no time delays are necessary in normal file transfers, per file negotiations are much faster than with YMODEM, the only observed delay being the time required by the program(s) to update logging files.

During a file transfer, a short line hit seen by the receiver usually induces a CRC error. The interrupt sequence is usually seen by the sender before the next data subpacket is completely sent, and the resultant loss of data throughput averages about half a data subpacket per line hit. At 1200 bps this is would be about .75 second lost per hit. At 10-5 error rate, this would degrade throughput by about 9 per cent.

The throughput degradation increases with increasing channel delay, as more data subpackets in transit through the channel are discarded when an error is detected.

A longer noise burst that affects both the receiver and the sender's reception of the interrupt sequence usually causes the sender to remain silent until the receiver times out in 10 seconds. If the round trip channel delay exceeds the receiver's 10 second timeout, recovery from this type of error may become difficult.

Noise affecting only the sender is usually ignored, with one common exception. Spurious XOFF characters generated by noise stop the sender until the receiver times out and sends an interrupt sequence which concludes with an XON.

In summation, ZMODEM performance in the presence of errors resembles that of X.PC and SuperKermit. Short bursts cause minimal data retransmission. Long bursts (such as pulse dialing noises) often require a timeout error to restore the flow of data.


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

The "best" flow control (when X.25 or hardware CTS is unavailable)

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Chapter 15 ZMODEM Protocol 35

would not "eat" any characters at all. When the PAD's buffer almost fills up, an XOFF should be emitted. When the buffer is no longer nearly full, send an XON. Otherwise, the network should neither generate nor eat XON or XOFF control characters.

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

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

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

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

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

     TABLE 1.  Network and Flow Control Compatibility

| Connectivity | Interactive| XMODEM| WXMODEM| SUPERKERMIT| ZMODEM | |_|_||_|_|_|

_|_||_|_|_| |Direct Connect | YES | YES | YES | YES | YES | |_____
Network, no FC no YES (4) (6) YES (1)
_|_||_|_|_| |Net, transparent FC| YES | YES | YES | YES | YES | |_____
Net, non-trans. FC YES no no (5) YES YES
_|_||_|_|_| |Network, 7 bit | YES | no | no | YES (2) | YES (3)| |_____

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

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Chapter 15 ZMODEM Protocol 36


"Round Trip Delay Time" includes the time for the last byte in a packet to propagate through the operating systems and network to the receiver, plus the time for the receiver's response to that packet to propagate back to the sender.

The figures shown below are calculated for round trip delay times of 40 milliseconds and 5 seconds. Shift registers in the two computers and a pair of 212 modems generate a round trip delay time on the order of 40 milliseconds. Operation with busy timesharing computers and networks can easily generate round trip delays of five seconds. Because the round trip delays cause visible interruptions of data transfer when using XMODEM protocol, the subjective effect of these delays is greatly exaggerated, especially when the user is paying for connect time.

A 102400 byte binary file with randomly distributed codes is sent at 1200 bps 8 data bits, 1 stop bit. The calculations assume no transmission errors. For each of the protocols, only the per file functions are considered. Processor and I/O overhead are not included. YM-k refers to YMODEM with 1024 byte data packets. YM-g refers to the YMODEM "g" option. ZMODEM uses 256 byte data subpackets for this example. SuperKermit uses maximum standard packet size, 8 bit transparent transmission, no run length compression. The 4 block WXMODEM window is too small to span the 5 second delay in this example; the resulting thoughput degradation is ignored.

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

	 TABLE 2.  Protocol Overhead Information
   (102400 byte binary file, 5 Second Round Trip)

     Protocol		 XMODEM   YM-k	  YM-g	 ZMODEM   SKermit   WXMODEM

Protocol Round Trips 804 104 5 5 5 4 Trip Time at 40ms 32s 4s 0 0 0 0 Trip Time at 5s 4020s 520s 25s 25s 25 20 Overhead Characters 4803 603 503 3600 38280 8000 Line Turnarounds 1602 204 5 5 2560 1602

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Chapter 16 ZMODEM Protocol 37

Transfer Time at 0s 893s 858s 857s 883s 1172s 916s
Transfer Time at 40ms 925s 862s 857s 883s 1172s 916s
Transfer Time at 5s 5766s 1378s 882s 918s 1197s 936s
	 Figure 5.  Transmission Time Comparison
   (102400 byte binary file, 5 Second Round Trip)

XMODEM YMODEM-K SuperKermit (Sliding Windows) * ZMODEM 16kb Segmented Streaming * ZMODEM Full Streaming * YMODEM-G TABLE 3. Local Timesharing Computer Download Performance | Command | Protocol| Time/HD| Time/FD| Throughput| Efficiency| |_||_|_||| |_||_|_||| |kermit -x | Kermit | 1:49 | 2:03 | 327 | 34% | |_||_|_||| |sz -Xa phones.t| XMODEM | 1:20 | 1:44 | 343 | 36% | |_||_|_||| |sz -a phones.t | ZMODEM | :39 | :48 | 915 | 95% | |_||_|_||| Times were measured downloading a 35721 character text file at 9600 bps, from Santa Cruz SysV 2.1.2 Xenix on a 9 mHz IBM PC-AT to DOS 2.1 on an IBM PC. Xenix was in multiuser mode but otherwise idle. Transfer times to PC hard disk and floppy disk destinations are shown. C-Kermit 4.2(030) used server mode and file compression, sending to Pro-YAM 15.52 using 0 delay and a "get phones.t" command. Crosstalk XVI 3.6 used XMODEM 8 bit checksum (CRC not available) and an "ESC rx phones.t" command. The Crosstalk time does not include the time needed to enter the extra commands not needed by Kermit and ZMODEM. Professional-YAM used ZMODEM AutoDownload. ZMODEM times included a security challenge to the sending program. Chapter 16 Rev 10-27-87 Typeset 10-27-87 37 Chapter 16 ZMODEM Protocol 38 TABLE 4. File Transfer Speeds | Prot file bytes bps ch/sec Notes | || |X jancol.c 18237 2400 53 Tymnet PTL 5/3/87 | |X 6143 2400 56 Source/Telenet PTL 5/29/87| |X jancol.c 18237 2400 64 Tymnet PTL | |B jancol.c 18237 1200 87 DataPac (604-687-7144) | |XN tsrmaker.arc 25088 1200 94 GEnie PTL | |B/ovth emaibm.arc 51200 1200 101 CIS PTL MNP | |UUCP 74 files, each >7000 1200 102 Average, Various callers | |ZM jancol.c 18237 1200 112 DataPac (604-687-7144) | |X/ovth emaibm.arc 51200 1200 114 CIS PTL MNP | |ZM emaibm.arc 51200 1200 114 CIS PTL MNP | |B jancol.c 18237 2400 124 Tymnet PTL | |B YI0515.87 9081 2400 157 CIS PTL node 5/29/87 | |SK 6143 2400 170 Source/Telenet PTL 5/29/87| |ZM jancol.c 18237 2400 221 Tymnet PTL upl/dl | |B/ovth destro.gif 33613 2400 223 CIS/PTL LEVEL 5 9-12-87 | |ZM jancol.c 18237 2400 224 Tymnet PTL | |ZM jancol.c 18237 2400 226/218 TeleGodzilla upl | |ZM jancol.c 18237 2400 226 Tymnet PTL 5/3/87 | |ZM zmodem.ov 35855 2400 227 CIS PTL node | |C jancol.c 18237 2400 229 Tymnet PTL 5/3/87 | |ZM jancol.c 18237 2400 229/221 TeleGodzilla | |ZM zmodem.ov 35855 2400 229 CIS PTL node upl | |ZM jancol.c 18237 2400 232 CIS PTL node | |ZM mbox 473104 9600 948/942 TeleGodzilla upl | |ZM zmodem.arc 318826 14k 1357/1345 TeleGodzilla | |ZM mbox 473104 14k 1367/1356 TeleGodzilla upl | |ZM c2.doc 218823 38k 3473 Xenix 386 Toolkit upl | |ZM mbox 511893 38k 3860 386 Xenix 2.2 Beta # | |ZM c.doc 218823 57k 5611 |

Times are for downloads unless noted. Where two speeds are noted, the faster speed is reported by the receiver because its transfer time calculation excludes the security check and transaction log file processing. The TeleGodzilla computer is a 4.77 mHz IBM PC with a 10 MB hard disk. The 386 computer uses an Intel motherboard at 18 mHz 1ws. The AT Clone (QIC) runs at 8 mHz 0ws. Abbreviations: B Compuserve B Protocol B/ovth CIS B with Omen Technology OverThruster(TM) C Capture DC2/DC4 (no protocol) K Kermit MNP Microcom MNP error correcting SX/1200 modem PTL Portland Oregon network node SK Sliding Window Kermit (SuperKermit) w=15 X XMODEM

Chapter 16 Rev 10-27-87 Typeset 10-27-87 38

Chapter 16 ZMODEM Protocol 39

XN XMODEM protocol implemented in network modes X/ovth XMODEM, Omen Technology OverThruster(TM) ZM ZMODEM Tk Xenix 386 Toolkit, rz compiled -M3, dumb serial port ** AT Clone ramdisk to 386 ramdisk, or either ramdisk to nul # On the fly format translation NL to CR/LF

	       TABLE 5.  Protocol Checklist


_||||__|| |IN SERVICE | 1977 | 1986 | 1982 | 1985 | 1986 | 1985 | |__|_
User Friendly I/F - - - - YES -
Commands/batch 2*N 2*N 2 2 1 1(1)
Commands/file 2 2 0 0 0 0
Command Download - - - - YES YES(6)
Menu Compatible - - - - YES -
Transfer Recovery - - - - YES -
File Management - - - - YES -
Security Check - - - - YES -
_||||__|| |COMPATIBILITY | | | | | | | |Dynamic Files | YES | YES | FAIL | FAIL | YES | YES | |Packet SW NETS | - | YES | - | - | YES | YES | |7 bit PS NETS | - | - | - | - | (8) | YES | |Old Mainframes | - | - | - | - | (8) | YES | |CP/M-80 | YES | YES | YES | - | YES(9)| - | |__|_
Reliability(5) fair poor fair(5) none BEST HIGH
Streaming - YES - YES YES YES
Overhead(2) 7% 7% 1% 1% 1% 30%
Faithful Xfers - - YES YES YES YES
Preserve Date - - YES YES YES -
_||||__|| |COMPLEXITY | | | | | | | |No-Wait Sample | - | REQD | - | - | opt | REQD | |Ring Buffers | - | REQD | - | - | opt | REQD | |XMODEM Similar | YES | LOW | HIGH | HIGH | LOW | NONE | |Complexity | LOW(5)| MED | LOW(5) | LOW | MED | HIGH | |__|_
Server Operation - - - - YES(4) YES
Multiple Threads - - - - future -

NOTES: (1) Server mode or Omen Technology Kermit AutoDownload

Chapter 16 Rev 10-27-87 Typeset 10-27-87 39

Chapter 16 ZMODEM Protocol 40

(2) Character count, binary file, transparent channel (3) 32 bit math needed for accurate transfer (no garbage added) (4) AutoDownload operation (5) Cybernetic Data Recovery(TM) improves XMODEM and YMODEM reliability with complex proprietary logic. (6) Server commands only (7) No provision for transfers across time zones (8) Future enhancement provided for (9) With Segmented Streaming WXMODEM: XMODEM derivative protocol with data encoding and windowing

Chapter 16 Rev 10-27-87 Typeset 10-27-87 40

Chapter 16 ZMODEM Protocol 41


Future extensions include:

 o Compatibility with 7 bit networks
 o Server/Link Level operation: An END-TO-END error corrected
   program to program session is required for financial and other
   sensitive applications.
 o Multiple independent threads
 o Encryption
 o Compression
 o File Comparison
 o Selective transfer within a file (e.g., modified segments of a
   database file)
 o Selective Retransmission for error correction


10-27-87 Optional fields added for number of files remaining to be sent and total number of bytes remaining to be sent. 07-31-1987 The receiver should ignore a ZEOF with an offset that does not match the current file length. The previous action of responding with ZRPOS caused transfers to fail if a CRC error occurred immediately before end of file, because two retransmission requests were being sent for each error. This has been observed under exceptional conditions, such as data transmission at speeds greater than the receiving computer's interrupt response capabilitiy or gross misapplication of flow control.

Discussion of the Tx backchannel garbage count and ZCRCW after error ZRPOS was added. Many revisions for clarity. 07-09-87 Corrected XMODEM's development date, incorrectly stated as 1979 instead of the actual August 1977. More performance data was added. 05-30-87 Added ZMNEW and ZMSKNOLOC 05-14-87 Window management, ZACK zshhdr XON removed, control character escaping, ZMSPARS changed to ZXPARS, editorial changes. 04-13-87 The ZMODEM file transfer protocol's public domain status is emphasized. 04-04-87: minor editorial changes, added conditionals for overview version. 03-15-87: 32 bit CRC added. 12-19-86: 0 Length ZCRCW data subpacket sent in response to ZPAD or

Chapter 18 Rev 10-27-87 Typeset 10-27-87 41

Chapter 18 ZMODEM Protocol 42

ZDELE detected on reverse channel has been changed to ZCRCE. The reverse channel is now checked for activity before sending each ZDATA header. 11-08-86: Minor changes for clarity. 10-2-86: ZCNL definition expanded. 9-11-86: ZMPROT file management option added. 8-20-86: More performance data included. 8-4-86: ASCII DLE (Ctrl-P, 020) now escaped; compatible with previous versions. More document revisions for clarity. 7-15-86: This document was extensively edited to improve clarity and correct small errors. The definition of the ZMNEW management option was modified, and the ZMDIFF management option was added. The cancel sequence was changed from two to five CAN characters after spurious two character cancel sequences were detected.


Please contact Omen Technology for troff source files and typeset copies of this document.

19.1 TeleGodzilla Bulletin Board

More information may be obtained by calling the TeleGodzilla bulletin board at 503-621-3746. TeleGodzilla supports 19200 (Telebit PEP), 2400 and 1200 bps callers with automatic speed recognition.


Useful commands for TeleGodzilla include "menu", "dir", "sx file (XMODEM)", "kermit sb file …", and "sz file …".

19.2 Unix UUCP Access

UUCP sites can obtain the current version of this file with

      uucp omen!/u/caf/public/zmodem.doc /tmp

A continually updated list of available files is stored in /usr/spool/uucppublic/FILES.

   uucp  omen!~uucp/FILES   /usr/spool/uucppublic

The following L.sys line allows UUCP to call site "omen" via Omen's bulletin board system "TeleGodzilla". TeleGodzilla is an instance of Omen Technology's Professional-YAM in host operation, acting as a bulletin board and front ending a Xenix system.

In response to TeleGodzilla's "Name Please:" (e:–e:), uucico gives the Pro-YAM "link" command as a user name. Telegodzilla then asks for a link password (d:). The password (Giznoid) controls access to

Chapter 19 Rev 10-27-87 Typeset 10-27-87 42

Chapter 19 ZMODEM Protocol 43

the Xenix system connected to the IBM PC's other serial port. Communications between Pro-YAM and Xenix use 9600 bps; YAM converts this to the caller's speed.

Finally, the calling uucico sees the Xenix "Login:" message (n:– n:), and logs in as "uucp". No password is used for the uucp account.

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


A copy of this document, a demonstration version of Professional-YAM, a flash-up tree structured help file and processor, are available in YZMODEM.ZOO on TeleGodzilla and other bulletin boards. This file must be unpacked with LOOZ.EXE, also available on TeleGodzilla. YZMODEM.ZOO may be distributed provided none of the files are deleted or modified without the written consent of Omen Technology.

TeleGodzilla and other bulletin boards also feature ZCOMM, a shareware communications program. ZCOMM includes Omen Technology's TurboLearn(TM) Script Writer, ZMODEM, Omen's highly acclaimed XMODEM and YMODEM protocol support, Sliding Windows Kermit, several traditional protocols, a powerful script language, and the most accurate VT100/102 emulation available in a usr supported program. The ZCOMM files include:

o ZCOMMEXE.ARC Executable files and beginner's telephone directory
o ZCOMMDOC.ARC "Universal Line Printer Edition" Manual
o ZCOMMHLP.ARC Tree structured Flash-UP help processor and

Source code and manual pages for the Unix/Xenix rz and sz programs are available on TeleGodzilla in RZSZ.ZOO. This ZOO archive may be unpacked with LOOZ.EXE, also available on TeleGodzilla. Most Unix like systems are supported, including V7, Sys III, 4.x BSD, SYS V, Idris, Coherent, and Regulus.

RZSZ.ZOO includes a ZCOMM/Pro-YAM/PowerCom script ZUPL.T to upload the small (178 lines) YMODEM bootstrap program MINIRB.C without a file transfer protocol. MINIRB uses the Unix stty(1) program to set the required raw tty modes, and compiles without special flags on virtually all Unix and Xenix systems. ZUPL.T directs the Unix system to compile MINIRB, then uses it as a bootstrap to upload the rz/sz source and manual files.

Chapter 20 Rev 10-27-87 Typeset 10-27-87 43

Chapter 20 ZMODEM Protocol 44

The PC-DOS Opus and Nochange bulletin boards support ZMODEM. Integrated ZMODEM support for the Collie bulletin board program is planned.

A number of other bulletin board programs support ZMODEM with external modules (DSZ, etc.).

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

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

Other PC-DOS communications programs with ZMODEM support modules include QMODEM and BOYAN. Many programs are adding direct ZMODEM support, including Crosstalk Mark IV, Telix 3.0, and (expected) Procomm and Qmodem.

The ZMDM communications program by Jwahar Bammi runs on Atari ST machines.

The Online! program for the Amiga supports ZMODEM.

The Compuserve Information Service has ported the Unix rz/sz ZMODEM programs to DECSYSTEM 20 assembler.

20.1 Adding ZMODEM to DOS Programs

DSZ is a small program that supports XMODEM, YMODEM, and ZMODEM file transfers. DSZ is designed to be called from a bulletin board program or another communications program. It may be called as

	     dsz port 2 sz file1 file2

to send files, or as

		   dsz port 2 rz

to receive zero or more file(s), or as

	     dsz port 2 rz filea fileb

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

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

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

Chapter 21 Rev 10-27-87 Typeset 10-27-87 44

Chapter 21 ZMODEM Protocol 45


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

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

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

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

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

   Chuck Forsberg
   Omen Technology Inc
   17505-V Sauvie Island Road
   Portland Oregon 97231
   VOICE: 503-621-3406 :VOICE
   Modem (TeleGodzilla): 503-621-3746
   Usenet: ...!tektronix!reed!omen!caf
   Compuserve: 70007,2304
   Source: TCE022


ZMODEM was developed for the public domain under a Telenet contract. The ZMODEM protocol descriptions and the Unix rz/sz program source code are public domain. No licensing, trademark, or copyright restrictions apply to the use of the protocol, the Unix rz/sz source code and the ZMODEM name.

Encouragement and suggestions by Thomas Buck, Ward Christensen, Earl Hall, Irv Hoff, Stuart Mathison, and John Wales, are gratefully acknowledged. 32 bit CRC code courtesy Gary S. Brown.


The following files may be useful while studying this document:

YMODEM.DOC Describes the XMODEM, XMODEM-1k, and YMODEM batch file

transfer protocols.  This file is available on TeleGodzilla

Chapter 23 Rev 10-27-87 Typeset 10-27-87 45

Chapter 23 ZMODEM Protocol 46

zmodem.h Definitions for ZMODEM manifest constants

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

rz.1, sz.1 Manual pages for rz and sz (Troff sources).

zm.c Operating system independent low level ZMODEM subroutines.

minirb.c A YMODEM bootstrap program, 178 lines.

RZSZ.ZOO,rzsz.arc Contain the C source code and manual pages listed

above, plus a ZCOMM script to upload minirb.c to a Unix or
Xenix system, compile it, and use the program to upload the
ZMODEM source files with error checking.

DSZ.ZOO,dsz.arc Contains DSZ.COM, a shareware X/Y/ZMODEM subprogram,

DESQview "pif" files for background operation in minimum
memory, and DSZ.DOC.

ZCOMM*.ARC Archive files for ZCOMM, a powerful shareware

communications program.

Chapter 23 Rev 10-27-87 Typeset 10-27-87 46


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

2. WHY DEVELOP ZMODEM?………………………………………. 2

3. ZMODEM Protocol Design Criteria……………………………. 4

   3.1    Ease of Use...............................................	 4
   3.2    Throughput................................................	 5
   3.3    Integrity and Robustness..................................	 6
   3.4    Ease of Implementation....................................	 6

4. EVOLUTION OF ZMODEM………………………………………. 7

5. ROSETTA STONE……………………………………………. 10

6. ZMODEM REQUIREMENTS………………………………………. 10

   6.1    File Contents.............................................  10

7. ZMODEM BASICS……………………………………………. 12

   7.1    Packetization.............................................  12
   7.2    Link Escape Encoding......................................  12
   7.3    Header....................................................  13
   7.4    Binary Data Subpackets....................................  16
   7.5    ASCII Encoded Data Subpacket..............................  16


   8.1    Session Startup...........................................  16
   8.2    File Transmission.........................................  18
   8.3    Session Cleanup...........................................  20
   8.4    Session Abort Sequence....................................  20


   9.1    Full Streaming with Sampling..............................  21
   9.2    Full Streaming with Reverse Interrupt.....................  23
   9.3    Full Streaming with Sliding Window........................  23
   9.4    Full Streaming over Error Free Channels...................  24
   9.5    Segmented Streaming.......................................  24

10. ATTENTION SEQUENCE……………………………………….. 24

11. FRAME TYPES……………………………………………… 25

   11.1   ZRQINIT...................................................  25
   11.2   ZRINIT....................................................  25
   11.3   ZSINIT....................................................  25
   11.4   ZACK......................................................  26
   11.5   ZFILE.....................................................  26
   11.6   ZSKIP.....................................................  28
   11.7   ZNAK......................................................  28
   11.8   ZABORT....................................................  28
			  - i -
   11.9   ZFIN......................................................  28
   11.10  ZRPOS.....................................................  28
   11.11  ZDATA.....................................................  29
   11.12  ZEOF......................................................  29
   11.13  ZFERR.....................................................  29
   11.14  ZCRC......................................................  29
   11.15  ZCHALLENGE................................................  29
   11.16  ZCOMPL....................................................  29
   11.17  ZCAN......................................................  29
   11.18  ZFREECNT..................................................  29
   11.19  ZCOMMAND..................................................  29


   12.1   A simple file transfer....................................  30
   12.2   Challenge and Command Download............................  31


14. PERFORMANCE RESULTS………………………………………. 33

   14.1   Compatibility.............................................  33
   14.2   Throughput................................................  33
   14.3   Error Recovery............................................  34



17. FUTURE EXTENSIONS………………………………………… 41

18. REVISIONS……………………………………………….. 41

19. MORE INFORMATION…………………………………………. 42

   19.1   TeleGodzilla Bulletin Board...............................  42
   19.2   Unix UUCP Access..........................................  42

20. ZMODEM PROGRAMS………………………………………….. 43

   20.1   Adding ZMODEM to DOS Programs.............................  44

21. YMODEM PROGRAMS………………………………………….. 45

22. ACKNOWLEDGMENTS………………………………………….. 45

23. RELATED FILES……………………………………………. 45


Figure 1. Order of Bytes in Header…………………………….. 14

Figure 2. 16 Bit CRC Binary Header…………………………….. 14

			  - ii -

Figure 3. 32 Bit CRC Binary Header…………………………….. 14

Figure 4. HEX Header…………………………………………. 15

Figure 5. Transmission Time Comparison…………………………. 37


TABLE 1. Network and Flow Control Compatibility…………………. 35

TABLE 2. Protocol Overhead Information…………………………. 36

TABLE 3. Local Timesharing Computer Download Performance…………. 37

TABLE 4. File Transfer Speeds…………………………………. 38

TABLE 5. Protocol Checklist…………………………………… 39

			 - iii -
   The ZMODEM Inter Application File Transfer Protocol
		      Chuck Forsberg
		   Omen Technology Inc

The ZMODEM file transfer protocol provides reliable file and command transfers with complete END-TO-END data integrity between application programs. ZMODEM's 32 bit CRC catches errors that continue to sneak into even the most advanced networks.

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

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

ZMODEM provides advanced file management features including AutoDownload (Automatic file Download initiated without user intervention), Crash Recovery, selective file transfers, and security verified command downloading.

ZMODEM protocol features allow implementation on a wide variety of systems operating in a wide variety of environments. A choice of buffering and windowing modes allows ZMODEM to operate on systems that cannot support other streaming protocols. Finely tuned control character escaping allows operation with real world networks without Kermit's high overhead.

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

ZMODEM is carefully designed to provide these benefits using a minimum of new software technology. ZMODEM can be implemented on all but the most brain damaged computers.

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.

/data/webs/external/dokuwiki/data/pages/archive/programming/zmodem6.txt · Last modified: 2002/06/19 03:56 (external edit)