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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: 70007,2304 UUCP: ...!tektronix!reed!omen!caf Chapter 0 Rev091186 Typeset 9-11-86 1 Chapter 0 Rev091186 Typeset 9-11-86 2 1. INTENDED AUDIENCE This document is intended for telecommunications managers, systems programmers, and others who choose and implement asynchronous file transfer protocols over dial-up networks and related environments. 2. EVOLUTION OF ZMODEM In the beginning of ZMODEM development, we thought a few modifications to XMODEM might 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 signals used by XMODEM and YMODEM. 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? Pure binary won't do, some combinations won't pass through networks and operating systems. Other operating systems may not be able to recognize comething coming back unless a break signal or particular code is present.[1] Hmmm, this starts to sound like a PACKET, with variable preamble, encoded data, and an ending sequence, a far cry from XMODEM's simple ACK and NAK. 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. This extrapolates to the better part of 20000 characters at 19.2 kbps. Much of the SuperKermit's complexity and debugging time centered around its ring buffering and window management. There had to be an easier way to get the job done. A sore point in XMODEM and YMODEM 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? Y/YMODEM 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 pass 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 that does not appear anywhere else. WXMODEM and ZMODEM use character sequences to delineate the start of frames. A further error recovery problem arises in streaming protocols. How does __________ 1. Without waiting for a response 2. The WINDOW is the data in transit between sender and receiver. Chapter 2 Rev091186 Typeset 9-11-86 2 Chapter 2 Rev091186 Typeset 9-11-86 3 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 bacause the sender did not receive the error signal? How long should the receiver let 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 goes through a routine to decide which outstanding packet (if any) it "wants most" to receive, and asks for that one. In the ZMODEM development, we decided to forgo the complexity of SuperKermit's selective retransmission scheme and its associated buffer management logic and memory requirements. Another sore point with XMODEM (and WXMODEM) is the garbage added at the end of files. This was accpetable with old CP/M files which had no exact length, but not with modern systems such as DOS and Unix. YMODEM and FAST use file length information 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 problems. Since some characters had to be escaped anyway, there wasn't any point wasting two bytes to represent packet length. The length of data subpackets is denoted by ending each subpacket with an escape sequence analagous to BISYNC and HDLC. The end result was a ZMOEM header containing a "frame type", four bytes of supervisory information, and its own CRC-16. 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 encoded 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 (network friendly) frames, the sending program can send an "invitation to receive" sequence to activate the receiver without undue concern of crashing the remote application with unexpected control characters. Going "back to scratch" in the protocol design presents an oppurtunity 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 Chapter 2 Rev091186 Typeset 9-11-86 3 Chapter 2 Rev091186 Typeset 9-11-86 4 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 is incomplete if the user must assign transfer options to both the sending and receiving programs. As a result, transfer options may be specified to the sending program, which passes them to the receiving program in the ZFILE header. 3. ACKNOWLEDGMENTS Encouragement and suggestions by Thomas Buck, Ward Christensen, Earl Hall, Irv Hoff, Stuart Mathison, and John Wales, are gratefully acknowledged. 4. RELATED DOCUMENTS The following files may be useful 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.1111, SZ.1111 Manual pages for rz and sz (Troff sources). ZM.C, ZMODEM.H Operating system independent ZMODEM subroutines, header file. 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. Chapter 5 Rev091186 Typeset 9-11-86 4 Chapter 5 Rev091186 Typeset 9-11-86 5 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 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. YMODEM refers to the XMODEM/CRC protocol with the throughput and/or batch transmission enhancements described in YMODEM.DOC. 6. WHY DEVELOP ZMODEM???? Since its development half a decade ago, the Ward Christensen MODEM protocol has enabled a wide variety of computer systems to interchange data. There is hardly a communications program that doesn't at least claim to support this protocol, now called XMODEM. Advances in computing, modems and networking have spread the XMODEM protocol far beyond the micro to micro environment for which it was designed. These application have exposed some weaknesses: o+ The 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 transactions allow undetected errors and disrupted file transfers. o+ Only one file can be sent per command. The file name has to be given twice, first to the sending program and then again to the receiving program. o+ The transmitted file accumulates as many as 127 bytes of garbage. o+ The modification date and other file attributes are lost. o+ XMODEM requires complete 8 bit transparency, all 256 codes. XMODEM will not operate over some networks that use ASCII flow control or escape codes. Setting pure transparency often disables important network control functions for the duration of the call. Chapter 6 Rev091186 Typeset 9-11-86 5 Chapter 6 Rev091186 Typeset 9-11-86 6 A number of other protocols have been developed over the years, but none have displaced XMODEM to date. o+ Lack of public domain documentation and example programs have kept proprietary protocols such as MNP, Blast, and others tightly bound to the fortunes of their suppliers. o+ Link level protocols such as X.25, X.PC, and MNP do not manage application to application file transfers. 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] 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 the user has not correctly specified these submodes. A number of extensions to the XMODEM protocol have been made under the collective name YMODEM. o+ 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. o+ The handling of files that are not a multiple of 1024 or 128 bytes is awkward, especially if the file length is not known, or changes during transmission. o+ YMODEM----G provides efficient batch file transfers, preserving exact file length and file modification date. YMODEM----G is essentially insensitive to network delays. Because it does not support error recovery, __________ 1. Some Kermit programs support run length encoding. Chapter 6 Rev091186 Typeset 9-11-86 6 Chapter 6 Rev091186 Typeset 9-11-86 7 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 by disabling error recovery. The ZMODEM Protocol corrects the weaknesses described above while maintaining as much of XMODEM/CRC's simplicity and prior art as possible. 7. ZMODEM PROTOCOL DESIGN CRITERIA The design of a file transfer protocol is an engineering compromise between conflicting requirements: 7.1 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. 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 X/YMODEM if the other end does not support ZMODEM.[1] 7.2 THROUGHPUT ZMODEM is designed for optimum performance with almost no degradation caused by delays introduced by packet switched networks and timesharing systems. ZMODEM is optimized for best throughput over networks where line hits __________ 2. Omen Technology Trademark 1. Provided the transmission medium accommodates X/YMODEM. Chapter 7 Rev091186 Typeset 9-11-86 7 Chapter 7 Rev091186 Typeset 9-11-86 8 occur infrequently. This assumption markedly reduces code complexity and memory requirements. ZMODEM protocol features enhance rapid error recovery compared to network compatible XMODEM implementations. In the absence of network delays, error recovery is rapid, much faster than YMODEM or network compatible versions of XMODEM. Many transfers will originate from a timesharing system connected to a packet switched network. ZMODEM features allow simple, efficient implementation on a wide variety of timesharing hosts. File transfers begin immediately regardless of which program is started first, without the 10 second delay associated with XMODEM. 7.3 INTEGRITY AND ROBUSTNESS Once a ZMODEM session is begun, all transactions are protected with 16 bit CRC.[2] Complex proprietary techniques such as CYBERNETIC DATA RECOVERY((((TM))))[3] are not needed for reliable transfers. A security challenge mechanism guards against "Trojan Horse" messages. 7.4 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] __________ 2. Except for the CAN-CAN-CAN-CAN-CAN abort sequence. 3. Unique to Professional-YAM and PowerCom Chapter 7 Rev091186 Typeset 9-11-86 8 Chapter 7 Rev091186 Typeset 9-11-86 9 ZMODEM provides a general purpose application to application file transfer protocol which may be used directly or with with reliable link level protocols such as X.25, MNP, Fastlink, etc. When used with X.25, MNP, Fastlink, etc., ZMODEM detects and corrects errors in the interfaces between error controlled media and the remainder of the communications link. 8. ZMODEM REQUIREMENTS ZMODEM requires an 8 bit transfer medium. ZMODEM escapes network control characters to allow operation with packet switched networks. In general, ZMODEM operates over any path that supports XMODEM, and over some that don't. To support full streaming, the path should either assert flow control[1] or pass full speed transmission without loss of data. 8.1 FILE CONTENTS 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. 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. 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 accomodate CRT displays that cannot overstrike. A text line is terminated by a CR/LF (015, 012) sequence, or by a NL (012) character. 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. 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 __________________________________________________________________________ 4. Streaming strateges are discussed in coming chapters. 1. With XOFF and XON, or out of band flow control such as X.25 or CTS Chapter 8 Rev091186 Typeset 9-11-86 9 Chapter 8 Rev091186 Typeset 9-11-86 10 text. N.B.:::: Files that have been translated in such a way as to modify their length cannot be updated with the ZCRECOV Conversion Option which allows interrupted transfers to be resumed without loss of data. 9. ZMODEM BASICS 9.1 PACKETIZATION ZMODEM frames differ somewhat from X/YMODEM blocks. X/YMODEM 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 X/YMODEM programs such as Professional-YAM and PowerCom 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 X/YMODEM blocks when line hits cause a loss of synchronization. This precludes rapid error recovery. 9.2 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. __________ 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 9 Rev091186 Typeset 9-11-86 10 Chapter 9 Rev091186 Typeset 9-11-86 11 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 CAN characters to abort a ZMODEM session, compatible with X/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. By default, ZMODEM software currently 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 ZMODEM routines in zm.c accept an option to escape all control characters, to allow operation with less transparent networks. The routines also accept an option to escape only the ZDLE character, for highest possible throughput. 9.3 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] __________ 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 9 Rev091186 Typeset 9-11-86 11 Chapter 9 Rev091186 Typeset 9-11-86 12 o+ Four bytes of data indicating flags and/or numeric quantities depending on the frame type FIGURE 1111. 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 9.3.1 BINARY HEADER A binary header is only sent by the sending program to the receiving program. 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 will follow depending on the frame type. The function zsbhdr transmits a binary header. The function zgethdr receives a binary or hex header. FIGURE 2222. Binary Header * ZDLE A TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2 9.3.2 HEX HEADER The receiver sends responses in hex headers. The sender also uses hex headers when they are not followed by binary data subpackets. Hex encoding accomodates 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 headers from the receiving program allows control characters to be used to interrupt the sender when errors are detected. Except for header types that imply data subpackets to follow, a HEX header may be used in place of a binary header wherever convenient. A hex header begins with the sequence ZPAD, ZPAD, ZDLE, ZHEX. The zgethdr Chapter 9 Rev091186 Typeset 9-11-86 12 Chapter 9 Rev091186 Typeset 9-11-86 13 routine synchronizes with the ZPAD-ZDLE sequence. The extra ZPAD allows other parts of the program to detect a ZMODEM header and then call zgethdr to receive the header. 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[4] are appended to the HEX header but are not strictly a part of it. The CR/LF aids debugging from printouts. The XON releases the sender from spurious XOFF flow control characters generated by line noise, a common occurrence. 0 or more ASCII Encoded data subpackets will follow depending on the frame type. The function zshhdr sends a hex header. FIGURE 3333. 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.) 9.4 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 4800 bps, 1024 above 4800 bps or when the data link is known to be relatively error free. Except for the last subpacket in a file,[5] lengths should be a power of two. 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 ZDLE encoded CRC bytes. The CRC accumulates the data bytes and frameend. The function zsdata sends a data subpacket. The function zrdata receives a data subpacket. __________ 4. XON is not sent after a ZFIN header, to allow clean session cleanup. 5. Or file segment if a sparse file is being processed Chapter 9 Rev091186 Typeset 9-11-86 13 Chapter 9 Rev091186 Typeset 9-11-86 14 9.5 ASCII ENCODED DATA SUBPACKET The format of ASCII Encoded data subpackets is not currently specified. These would be used for server commands, or main transfers in 7 bit environments. 10. 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] 10.1 SESSION STARTUP To start a ZMODEM file transfer session, the sending program is called with the names of the desired file(s) and option(s). The sending program may send the string "rz\r" to invoke the receiving program from a possible command mode. The "rz" followed by carriage return activates a ZMODEM receive program or command if it were not already active. The sender may then display a message intended for human consumption, such as a list of the files requested, etc. Then the sender may send a ZRQINIT header. The ZRQINIT header causes a previously started receive program to send its ZRINIT header without delay. In an interactive or conversational mode, the receiving application may monitor the data stream for ZDLE. The following characters may be scanned for B00000000 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 X/YMODEM protocol. Note: With ZMODEM and YMODEM Batch, 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. __________ 1. Special considerations apply when sending commands. Chapter 10 Rev091186 Typeset 9-11-86 14 Chapter 10 Rev091186 Typeset 9-11-86 15 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 X/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. The receiver sends a ZACK header in response, optionally containing the serial number of the receiving program, or 0. 10.2 FILE TRANSMISSION The sender then sends a ZFILE header with ZMODEM Conversion, Management, and Transport options[2] 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 CRC on the file and transmit the CRC in a ZCRC header. The receiver uses this information to determine whether to accept the file or skip it. This sequence is triggered by the ZMCRC Management Option.[3] __________ 2. See below, under ZFILE header type. 3. The type of CRC has not been determined yet. The obvious choice of the CRC-16 used to protect packets may not be optimum for detecting differences between long files. The fact that the file lengths are identical may give some guidance to the selection of CRC. Chapter 10 Rev091186 Typeset 9-11-86 15 Chapter 10 Rev091186 Typeset 9-11-86 16 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 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.[4] If downloading a file from a timesharing system that becomes sluggish, the transfer can be interrupted and resumed later with no loss of data. The sender sends a ZDATA binary header (with file position) followed by one or more data subpackets. The receiver compares the file position in the ZDATA header with the number of characters successfully received to the file. If they do not agree, a ZRPOS error response is generated to force the sender to the right position within the file.[5] A data subpacket terminated by ZCRCGO 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 __________ 4. This does not apply to files that have been translated. 5. If the ZMSPARS option is used, the receiver instead seeks to the position given in the ZDATA header. Chapter 10 Rev091186 Typeset 9-11-86 16 Chapter 10 Rev091186 Typeset 9-11-86 17 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 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 header is sent.) After all files are processed, any further protocol errors should not prevent the sending program from returning with a success status. 10.3 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. 10.4 SESSION CANCEL SEQUENCE If the receiving program has been receiving data in streaming mode, the ATTN sequence is executed to interrupt data transmission. The Cancel sequence of eight CAN characters[6] and ten backspace characters is sent. static char canistr[] = { 24,24,24,24,24,24,24,24,8,8,8,8,8,8,8,8,8,8,0 }; __________ 6. The trailing backspace characters attempt to erase the effects of the CAN characters if they are received by a command interpreter. Chapter 10 Rev091186 Typeset 9-11-86 17 Chapter 10 Rev091186 Typeset 9-11-86 18 11. 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). 11.1 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. Any other character is ignored.[1] 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 ZCRCW data subpacket is sent to force the receiver to send a ZACK header in case the ZPAD or CAN was spurious and the receiver was still reading data subpackets. Then the receiver's response header is read and acted upon.[2] A ZRPOS header resets the sender's file offset to the correct position. If a ZACK header is recieved, the reverse channel is sampled for the possible presence of another header from the receiver. If a ZRPOS header is received, it is processed as above. A ZFIN, ZABORT, or TIMEOUT terminates the session; a ZSKIP terminates the processing of this file. Otherwise, transmission resumes at the (possibly reset) file offset with a ZDATA header followed by data subpackets. __________ 1. The call to rdchk() in SZ.C performs this function. 2. The call to getinsync() in SZ.C performs this function. Chapter 11 Rev091186 Typeset 9-11-86 18 Chapter 11 Rev091186 Typeset 9-11-86 19 11.2 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. 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. 11.3 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 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 reciever's ATTN sequence to flush the sender's interrupt buffer before sending the ZRPOS header. 11.4 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. 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.[3] Chapter 11 Rev091186 Typeset 9-11-86 19 Chapter 11 Rev091186 Typeset 9-11-86 20 11.5 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. 12. ATTENTION SEQUENCE The receiving program sends the ATTN sequence whenever it detects an error and needs to interrupt the sending program. The default ATTN string value is empty (no Attn sequence). The receiving program resets Attn to the empty default before each transfer session. The sender specifies the Attn sequence in its optional ZSINIT frame. The ATTN string is terminated with a null. Two meta-characters perform special functions: o+ \335 (octal) Send a break signal o+ \336 (octal) Pause one second 13. 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. __________________________________________________________________________ 3. One in 256 for escaping ZDLE, about two in 1024 for data subpacket CRC's Chapter 13 Rev091186 Typeset 9-11-86 20 Chapter 13 Rev091186 Typeset 9-11-86 21 13.1 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. 13.2 ZRINIT Sent by the receiving program. ZF0 and ZF1 contain the bitwise or of the receiver capability flags: #define CANFDX 1 /* Receiver can send and receive simultaneously */ #define CANOVIO 2 /* Receiver 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. 13.3 ZSINIT Sender sends capability flags (currently all 0) (none currently defined) followed by a binary data subpacket terminated with ZCRCW. The data subpacket contains the null terminated ATTN sequence, maximum length 32 bytes including the terminating null. 13.4 ZACK Acknowedgement to a ZSINIT frame, ZCHALLENGE header, 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. 13.5 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. 13.5.1 ZF0000:::: CONVERSION OPTION If the receiver does not recognize the Conversion Option, an application dependent default conversion may apply. Chapter 13 Rev091186 Typeset 9-11-86 21 Chapter 13 Rev091186 Typeset 9-11-86 22 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. ZCRECOV Recover/Resume interrupted file transfer. ZCREVOV is also useful for updating a remote copy of a file that grows without resending of old data. If the destination file exists and is no longer than the source, append to the destination file and start transfer at the offset corresponding to the receiver's end of file. This option does not apply if the source file is shorter. Files that have been converted (e.g., ZCNL) or subject to a single ended Transport Option cannot have their transfers recovered. 13.5.2 ZF1111:::: MANAGEMENT OPTION If the receiver does not recognize the Management Option, the file should be transferred normally. ZMNEW Transfer file if destination file absent. Otherwise, transfer file overwriting destination if source file 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). ZTSPARS Special processing for sparse file; 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 (or possibly ZCRCQ) data subpacket and process the expected ZACK to insure no data is lost. 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. Chapter 13 Rev091186 Typeset 9-11-86 22 Chapter 13 Rev091186 Typeset 9-11-86 23 13.5.3 ZF2222:::: 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 4444.0000 operating on a computer with VAX byte ordering, using 12 bit encoding. ZTCRYPT Encryption. An initial null terminated string identifies the key. Details to be determined. ZTRLE Run Length encoding, Details to be determined. A ZCRCW data subpacket follows with file name, file length, modification date, and other information described in a later chapter. 13.6 ZSKIP Sent by the receiver in response to ZFILE, makes the sender skip to the next file. 13.7 ZNAK Indicates last header was garbled. (See also ZRPOS). 13.8 ZABORT Sent by receiver to terminate batch file transfers when requested by the user. Sender responds with a ZFIN sequence.[1] 13.9 ZFIN Sent by sending program to terminate a ZMODEM session. Receiver responds with its own ZFIN. 13.10 ZRPOS Sent by receiver to force file transfer to resume at file offset given in ZP0...ZP3. __________ 1. Or ZCOMPL in case of server mode. Chapter 13 Rev091186 Typeset 9-11-86 23 Chapter 13 Rev091186 Typeset 9-11-86 24 13.11 ZDATA ZP0...ZP3 contain file offset. One or more data subpackets follow. 13.12 ZEOF Sender reports End of File. ZP0...ZP3 contain the ending file offset. 13.13 ZFERR Error in reading or writing file, protocol equivalent to ZABORT. 13.14 ZCRC Request (receiver) and response (sender) for file polynomial. ZP0...ZP3 contain file polynomial. 13.15 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. 13.16 ZCOMPL Request now completed. 13.17 ZCAN This is a pseudo frame type returned by gethdr() in response to a Session Abort sequence. 13.18 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. 13.19 ZCOMMAND ZCOMMAND is sent in a binary frame. ZF0000 contains 0000 or ZCACK1111 (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 received the command. Chapter 13 Rev091186 Typeset 9-11-86 24 Chapter 13 Rev091186 Typeset 9-11-86 25 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 ZCACK1111, 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 should display the command to the standard error output, then return a ZCOMPL header. 14. SESSION TRANSACTION EXAMPLES 14.1 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 14 Rev091186 Typeset 9-11-86 25 Chapter 14 Rev091186 Typeset 9-11-86 26 14.2 CHALLENGE AND COMMAND DOWNLOAD Sender Receiver "rz\r" ZRQINIT(ZCOMMAND) ZCHALLENGE(rnd) ZACK(rnd) ZRINIT ZCOMMAND (Performs Command) ZCOMPL ZFIN ZFIN OO 15. 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 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 '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: 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 Chapter 15 Rev091186 Typeset 9-11-86 26 Chapter 15 Rev091186 Typeset 9-11-86 27 the transformations are unsuccessful, an file name should be invented be the receiving program. If directories are included, they are delimited by /; i.e., "subdir/foo" is acceptable, "subdir\foo" is not. LENGTH The file length and each of the succeeding fields are optional.[1] The length field is stored as a decimal string counting the number of data bytes in the file. With ZMODEM, the 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. 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 __________ 1. Fields may not be skipped. 2. Not necessarily that of the transmitting system! Chapter 15 Rev091186 Typeset 9-11-86 27 Chapter 15 Rev091186 Typeset 9-11-86 28 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. 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. 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. 16. PERFORMANCE RESULTS 16.1 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 marked economy of reverse channel bandwith allows modems that dynamically partition bandwidth between the two directions to operate at optimal speeds. 16.2 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. Chapter 16 Rev091186 Typeset 9-11-86 28 Chapter 16 Rev091186 Typeset 9-11-86 29 Recent tests downloading program images to an IBM PC (4.7 mHz V20) running YAMK 15.68 with table driven CRC calculation yielded a throughput of about 17kb on a 19.2 kb direct connection. 16.3 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 channel delay, as 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 loss. Long bursts (such as pulse dialing noises) often require a timeout error to restore the flow of data. Chapter 17 Rev091186 Typeset 9-11-86 29 Chapter 17 Rev091186 Typeset 9-11-86 30 17. PACKET SWITCHED NETWORK CONSIDERATIONS 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) 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 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. 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 1111. 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 optimize burst length for fastest transfers. (2) Parity bits must be encoded, slowing binary transfers. (3) Protocol extension possible for encoding data to 7 bits. Chapter 18 Rev091186 Typeset 9-11-86 30 Chapter 18 Rev091186 Typeset 9-11-86 31 18. PERFORMANCE COMPARISON 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 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 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 2222. 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 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 Chapter 18 Rev091186 Typeset 9-11-86 31 Chapter 18 Rev091186 Typeset 9-11-86 32 FIGURE 4444. 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 3333. 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 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 of the sending program. Chapter 18 Rev091186 Typeset 9-11-86 32 Chapter 18 Rev091186 Typeset 9-11-86 33 TABLE 4444. Protocol Checklist Item FAST XMODEM WXM YMDM-k YMDM-g ZMODEM SKermit IN SERVICE NO 1979 NO 1982 1985 1986 1985 USER FEATURES User Friendly I/F - - - - - YES - Commands/batch 2 2*N 2*N 2 2 1 1(1) Commands/file 0 2 2 0 0 0 0 Command Download - - - - - YES YES(6) Menu Compatible - - - - - YES - Transfer Recovery - - - - - YES - File Management - - - - - YES - Security Check - - - - - YES - X/YMODEM Fallback - YES YES - - YES - COMPATIBILITY Dynamic Files FAIL 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) - ATTRIBUTES Reliability(5) FAIL fair ??? fair FAIL HIGH HIGH Streaming YES - YES - YES YES YES Overhead(2) 0% 7% 7% 1% 1% 1% 30% Faithful Xfers YES - - YES(3) YES(3) YES YES Preserve Date NO(7) - - YES YES YES - COMPLEXITY CRC-16 REQD - REQD REQD REQD REQD opt 32 bit math REQD - - (3) (3) REQD - No-Wait Sample - - REQD - - opt REQD Ring Buffers - - REQD - - opt REQD XMODEM Similar NONE YES LOW HIGH HIGH LOW NONE Complexity MED LOW(5) MED LOW(5) LOW MED HIGH EXTENSIONS Server Operation - - - - - YES(4) YES Multiple Threads - - - - - future - NOTES: (1) Server mode only (2) Character count, binary file, transparent channel (3) 32 bit math needed for accurate transfer (no garbage added) (4) AutoDownload operation (5) X/YMODEM Reliability enhancemnets greatly add to complexity (6) Server commands only (7) No provision for transfers across time zones Chapter 18 Rev091186 Typeset 9-11-86 33 Chapter 18 Rev091186 Typeset 9-11-86 34 (8) Future enhancement provided for (9) With Segmented Streaming WXM: XMODEM derivative with data encoding and Windowing FAST: File transfer protocol requiring end-to-end 8-bit transparent, error free communications. 19. FUTURE EXTENSIONS Future extensions include: o+ Compatibility with 7 bit networks o+ Server/Link Level operation: An END-TO-END error corrected program to program session is required for financial and other sensitive applications. o+ 32 bit CRC: for sensitive applications o+ Multiple independent threads o+ Encryption o+ Compression o+ File Comparision o+ Selective transfer within a file (e.g., modified segments of a database file). 20. REVISIONS 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. Chapter 21 Rev091186 Typeset 9-11-86 34 Chapter 21 Rev091186 Typeset 9-11-86 35 21. 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/zmodem.mm /tmp A continually updated list of available files is stored in /usr/spool/uucppublic/FILES. The following L.sys line allows UUCP to call site "omen" via Omen's bulletin board system "TeleGodzilla". TeleGodzilla uses Pro-YAM in host operation. In response to TeleGodzilla's "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 22. ZMODEM PROGRAMS A copy of this document, a demonstration version of Professional-YAM, a flash-up tree structured help file and processor, are available in ZMODEM.ARC on TeleGodzilla. This file must be unpacked with ARC-E.COM compatible with ARC5x files. ARC- E.COM is also available on TeleGodzilla. Source code and manual pages for UNIX programs are available on TeleGodzilla in RZSZ1.SHQ and RZSZ2.SHQ, squeezed "shell archives". To use these files, unsqueeze them with YAMDEMO's "usq" command, upload them to Unix, and then execute them as shell scripts to break them into the program and documentation source files. More detailed instructions may be found in Chapter 8 of the Professional-YAM User's manual. Most Unix like systems are supported, including V7, Sys III, 4.x BSD, SYS V, Idris, Coherent, and Regulus. 22.1 ADDING ZMODEM TO DOS PROGRAMS DOS programs such as bulletin boards may call YAMDEMO.EXE with the DOS EXEC function to support fast, reliable ZMODEM file transfers. This method allows program developers to add ZMODEM support with a minimum of software development at the expense of higher memory Chapter 22 Rev091186 Typeset 9-11-86 35 Chapter 22 Rev091186 Typeset 9-11-86 36 utilization than built-in routines. YAMDEMO.EXE beginning with Version 15.61 include an xmodem command which performs the following functions: o+ Sets restricted opertaion, restricting pathnames and disallowing modification of existing files o+ Causes error messages to be sent to the modem o+ Forces an exit after the command is finished When YAMDEMO.EXE is used in this way, the default setup file DEMOPHON.T should be overriden with the DOS environment variable PHONES: set PHONES=C:/newphone.t where newphone.t contains setup return (other commands may be added as necessary). If a comm port other than COM1 is used, the DPORT environment variable should be set: set DPORT=2 The Online help processor included in ZMODEM.ARC and the Professional-YAM User's Manual contain other useful information that applies to YAMDEMO.EXE. YAMDEMO.EXE unmodified may be copied and used without licensing or other liability. Omen Technology requests that YAMDEMO.EXE be distributed only in conjunction with all the files included in ZMODEM.ARC, as distributed by Omen, with no files deleted. 23. 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 Chapter 23 Rev091186 Typeset 9-11-86 36 Chapter 23 Rev091186 Typeset 9-11-86 37 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: 70007,2304 Source: TCE022 Yours very truly, Chapter 23 Rev091186 Typeset 9-11-86 37 CONTENTS 1. INTENDED AUDIENCE............ 2 2. EVOLUTION OF ZMODEM............. 2 3. ACKNOWLEDGMENTS.............. 4 4. RELATED DOCUMENTS............ 4 5. ROSETTA STONE............. 4 6. WHY DEVELOP ZMODEM?............. 5 7. ZMODEM Protocol Design Criteria.......... 7 7.1 Ease of Use.............. 7 7.2 Throughput............ 7 7.3 Integrity and Robustness.......... 8 7.4 Ease of Implementation......... 8 8. ZMODEM REQUIREMENTS............. 9 8.1 File Contents............ 9 9. ZMODEM BASICS............. 10 9.1 Packetization............ 10 9.2 Link Escape Encoding........... 10 9.3 Header............. 11 9.4 Binary Data Subpackets......... 13 9.5 ASCII Encoded Data Subpacket......... 14 10. PROTOCOL TRANSACTION OVERVIEW......... 14 10.1 Session Startup............. 14 10.2 File Transmission........... 15 10.3 Session Cleanup............. 17 10.4 Session Cancel Sequence........... 17 11. STREAMING TECHNIQUES / ERROR RECOVERY....... 18 11.1 Full Streaming with Sampling......... 18 11.2 Full Streaming with Reverse Interrupt...... 19 11.3 Full Streaming with a Sliding Window....... 19 11.4 Full Streaming over Error Free Channels....... 19 11.5 Segmented Streaming............ 20 12. ATTENTION SEQUENCE.............. 20 13. FRAME TYPES............... 20 13.1 ZRQINIT............... 21 13.2 ZRINIT............. 21 13.3 ZSINIT............. 21 13.4 ZACK............... 21 - i - 13.5 ZFILE.............. 21 13.6 ZSKIP.............. 23 13.7 ZNAK............... 23 13.8 ZABORT............. 23 13.9 ZFIN............... 23 13.10 ZRPOS.............. 23 13.11 ZDATA.............. 24 13.12 ZEOF............... 24 13.13 ZFERR.............. 24 13.14 ZCRC............... 24 13.15 ZCHALLENGE............ 24 13.16 ZCOMPL............. 24 13.17 ZCAN............... 24 13.18 ZFREECNT.............. 24 13.19 ZCOMMAND.............. 24 14. SESSION TRANSACTION EXAMPLES.......... 25 14.1 A simple file transfer......... 25 14.2 Challenge and Command Download....... 26 15. ZFILE FRAME FILE INFORMATION.......... 26 16. PERFORMANCE RESULTS............. 28 16.1 Compatibility............ 28 16.2 Throughput............ 28 16.3 Error Recovery........... 29 17. PACKET SWITCHED NETWORK CONSIDERATIONS......... 30 18. PERFORMANCE COMPARISON TABLES......... 31 19. FUTURE EXTENSIONS............ 34 20. REVISIONS.............. 34 21. MORE INFORMATION............. 35 22. ZMODEM PROGRAMS.............. 35 22.1 Adding ZMODEM to DOS Programs........ 35 23. YMODEM PROGRAMS.............. 36 - ii - LIST OF FIGURES Figure 1. Order of Bytes in Header........... 12 Figure 2. Binary Header............. 12 Figure 3. HEX Header............. 13 Figure 4. Transmission Time Comparison.......... 32 - iii - LIST OF TABLES TABLE 1. Flow Control Compatibility.......... 30 TABLE 2. Protocol Overhead Information.......... 31 TABLE 3. Local Timesharing Computer Download Performance.... 32 TABLE 4. Protocol Checklist............ 33 - iv - 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 allowing a friendly user interface, ZMODEM provides Personal Computer and other users an efficient, accurate, robust file transfer method. ZMODEM provides efficient file transfers with timesharing systems, satellite relays, and wide area packet switched networks. ZMODEM provides advanced file management features including AutoDownload (Automatic file Download initiated without user intervention), aborted transfer recovery, selective file transfers, and security verified command downloading. ZMODEM is carefully designed to provide these benefits using a minimum of new technology beyond XMODEM/CRC. 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 primitive XMODEM routines, actual C source code for working ZMODEM programs allows developers to provide solid, reliable ZMODEM implementations with minimum effort.