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- 1 - ZMODEM PROTOCOL REFERENCE The ZMODEM Inter Application File Transfer Protocol Chuck Forsberg Please distribute as widely as possible. Questions to Chuck Forsberg Omen Technology Inc 17505-V Northwest Sauvie Island Road Portland Oregon 97231 Voice: 503-621-3406 Modem (Telegodzilla): 503-621-3746 Speed 1200,300 Compuserve: 70715,131 UUCP: ...!tektronix!reed!omen!caf - 2 - 1. ACKNOWLEDGMENTS Encouragement and suggestions by Stuart Mathison, Thomas Buck, John Wales, Ward Christensen, and Irv Hoff are gratefully acknowledged. 2. ROSETTA STONE Here are some definitions which reflect the current vernacular in the computer media. The attempt here is identify the file transfer protocol rather than specific programs. Frame A ZMODEM frame consists of a header packet and 0 or more data packets. Response Time This is the maximum expected delay between the receiver sending a packet to the transmitter and receiving the beginning of a response from the transmitter. XMODEM refers to the original 1979 file transfer etiquette introduced by Ward Christensen's 1979 MODEM2 program. It's also called the MODEM or MODEM2 protocol. Some who are unaware of MODEM7's unusual batch file mode call it MODEM7. Other aliases include "CP/M Users's Group" and "TERM II FTP 3". This protocol is supported by every serious communications program because of its universality, simplicity, and reasonable performance. XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte Cyclical Redundancy Check (CRC-16), giving modern error detection protection. YMODEM refers to the XMODEM/CRC protocol with the throughput and/or batch transmission enhancements described in YMODEM.DOC. ZMODEM Zmodem is a second generation streaming protocol for text and binary file transmission between applications running on microcomputers and mainframes. Chapter 3 DRAFT 3-24-86 2 DRAFT ZMODEM Protocol Ref 02-23-86 3 3. YET ANOTHER PROTOCOL, AGAIN? Since its development half a decade ago, the Ward Christensen modem protocol has enabled a wide variety of computer systems to interchange data. There is hardly a communications program that doesn't at least claim to support this protocol now called XMODEM. Advances in computing, modems and networking have spread the XMODEM protocol far beyond the close coupled micro to micro environment for which it was designed. These application have exposed some weaknesses. o+ The short block length causes throughput to suffer when used with timesharing systems, packet switched networks, satellite circuits, and buffered (error correcting) modems. o+ The 8 bit arithmetic checksum and other aspects allows line impairments to interfere with dependable, accurate 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 extraneous bytes. o+ The modification date of the file is lost. A number of other protocols have been developed over the years, but none have displaced XMODEM to date. o+ Lack of public domain documentation and example programs have kept proprietary protocols such as MNP, Blast, and others tightly bound to the fortunes of their suppliers. o+ Hardware and/or software complexity discourages the widespread application of BISYNC, SDLC, HDLC, X.25, and X.PC protocols. o+ Link level protocols such as X.25, X.PC, and MNP modems 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 accomodate non transparent environments limit Kermit's efficiency. Chapter 3 DRAFT 3-24-86 3 DRAFT ZMODEM Protocol Ref 02-23-86 4 Even with completely transparent channels, Kermit control character quoting limits the efficiency of binary file transfers to about 75 per cent. Kermit Sliding Windows ("SuperKermit") improves throughput over networks at the cost of increased complexity. SuperKermit state transitions are encoded in a special language "wart" which requires a C compiler. The SuperKermit C code requires full duplex communications and the ability to check for the presence of characters in the input queue, precluding its implementation on some operating systems. A number of extensions to the XMODEM protocol have been made under the collective name YMODEM. o+ YMODEM-k reduces the overhead from transmission delays by 87 per cent compared to XMODEM, but network delays can still degrade performance. o+ The handling of files that are not a multiple of 1024 or 128 bytes is awkward, especially if the file length is not known, or changes during transmission. o+ YMODEM-g is essentially insensitive to network delays. Because it does not support error recovery, YMODEM-g must be used hardwired or with a link level protocol. Another XMODEM "extension" is protocol cheating, referred to as "Turbo Download" and OverThruster. These improve XMODEM throughput at the expense of error recovery. The ZMODEM Protocol is proposed as a means of addressing the weaknesses described above while maintaining as much of XMODEM's simplicity and prior art as possible. 4. ZMODEM Protocol Design Criteria The design of a file transfer protocol is an engineering compromise between conflicting requirements: 4.1 Throughput ZMODEM is designed for optimum performance with minimum degradation caused by delays introduced by packet switched networks and timesharing systems. ZMODEM is optimized for best throughput where line hits occur infrequently. This assumption markedly reduces code Chapter 4 DRAFT 3-24-86 4 DRAFT ZMODEM Protocol Ref 02-23-86 5 complexity and memory requirements. It is assumed that many transfers will originate from a timesharing system connected to a packet switched network. ZMODEM provides features to allow for simple, efficient implementation on timesharing hosts. File transfers begin immediately regardless of which program is started first, no 10 second delay. 4.2 Integrity and Robustness All packets are protected with 16 bit CRC. 4.3 Ease of use File names need be entered only once. Wild Card names may be used with batch transfers. Minimum keystrokes required to initiate transfers. ZMODEM steps down to X/YMODEM if the other end does not support ZMODEM. 4.4 Ease of implementation ZMODEM accomodates a wide variety of systems: o+ Microcomputers that cannot overlap disk and serial i/o o+ Microcomputers that cannot overlap serial send and 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 provides a near optimal general purpose application to application file transfer protocol to be used with link level protocols such as X.25, MNP, Fastlink, etc. 5. PACKETIZATION ZMODEM uses packets somewhat different from those used in X/YMODEM. X/YMODEM type packets are not used for the following reasons: o+ Block numbers are limited to 256 Chapter 5 DRAFT 3-24-86 5 DRAFT ZMODEM Protocol Ref 02-23-86 6 o+ No provision for variable length blocks o+ Line hits corrupt protocol signals, causing failed file transfers. In particular, modem errors sometimes generate false block numbers, false EOTs and false ACKs. False ACKs are the most troublesome as they cause the sender to lose synchronization with the receiver. State of the art X/YMODEM programs such as Professional- YAM overcome some of these weaknesses with clever proprietary code, but a newer protocol is still useful. o+ It is difficult to determine the beginning and ends of X/YMODEM blocks when they are corrupted by line hits. This discourages rapid error recovery. 5.1 Link Escape Encoding ZMODEM acheives data transparency by extending the 8 bit character set (256 codes) with escape sequences based on the ZMODEM data link escape character ZDLE.1 Link Escape coding permits variable length data packets. It allows the beginning of packets to be detected without special timing techniques, facilitating rapid error recovery. Link Escape coding does add some overhead. The worst case, a file consisting entirely of ZDLE characters, would incur a 50% overhead. The particular ZDLE character was chosen after examining the character distributions of many types of files used with personal computers. The ZDLE character is special. ZDLE represents a control sequence of some sort. If a ZDLE character appears in the data sent within a binary packet, it is prefixed with another ZDLE. An escaped ZDLE is counted once in the CRC. The current value for ZDLE is exclamation point (!). Use of a printing character as ZDLE allows application programs to recognize HEX Header Packets. This particular character was chosen because it does not appear often in many types of files likely to be transferred with this protocol. In __________ 1. This and other constants are defined in the _z_m_o_d_e_m._h include file. Please note that constants with a leading 0 are octal constants in C. Chapter 5 DRAFT 3-24-86 6 DRAFT ZMODEM Protocol Ref 02-23-86 7 addition, no known timesharing system uses it for editing. 5.2 Header Packet Information All ZMODEM frames begin with a header packet which may be sent in binary or HEX form. ZMODEM uses a single routine to recognize binary and hex header packets. Either form of the header packet contains the same raw information: o+ A type byte 2 Future extensions to ZMODEM may use the high order bits of the type byte to indicate thread selection. o+ Four bytes of data indicating flags and/or numeric quantities depending on the packet type Order of Bytes in Header Packet T: packet Type F0: Flags least significant byte P0: file Position least significant P3: file Position most significant T F3 F2 F1 F0 -------------- T P0 P1 P2 P3 5.3 Binary Header Packet A binary header packet is only sent by the sending program to the receiving program. A binary header packet begins with the sequence ZPAD, ZDLE, ZBIN. The frame type byte is ZDLE encoded. The four position/flags bytes are ZDLE encoded. A two byte CRC of the frame type and position/flag bytes is ZDLE encoded. 0 or more binary data packets will follow depending on the __________ 2. The packet types are cardinal numbers beginning with 0 to minimize state transition table memory requirements. Chapter 5 DRAFT 3-24-86 7 DRAFT ZMODEM Protocol Ref 02-23-86 8 frame type. The function _z_s_b_h_d_r transmits a binary header packet. The function _z_g_e_t_h_d_r receives a binary or hex header packet. 5.4 HEX Header Packet The receiver sends responses in hex header packets. Hex encoding is required to support XON/XOFF flow control. Use of Kermit style encoding for control and paritied characters was considered and rejected because of increased possibility of interacting with some timesharing systems's line edit functions. Use of HEX packets from the receiving program allows control characters to be used to interrupt the sender when errors are detected. Except for header packet types that imply data packets to follow, a HEX header packet may be used in place of a binary header packet. A hex header packet begins with the sequence ZPAD, ZPAD, ZDLE, ZHEX. The _z_g_e_t_h_d_r routine synchronizes in the ZPAD- ZDELE sequence. The extra ZPAD allows other parts of the program to detect a ZMODEM packet and then call _z_g_e_t_h_d_r to receive the packet. The type byte, the four position/flag bytes, and the CRC thereof are sent in hex using the character set 01234567890abcdef. Upper case hex digits are not allowed; they would false trigger X/YMODEM programs. A carriage return, line feed, and XON are appended to the HEX header packet but are not considered to be part of it. The CR/LF aids debugging from printouts. The XON releases the sender from spurious XOFF flow control characters generated by line noise, a common occurrence. 0 or more HEX data packets will follow depending on the frame type. The function _z_s_h_h_d_r sends a hex header packet. 5.5 Binary Data Packets Binary data packets immediately follow the associated binary header packet. A binary data packet contains 0 to 1024 bytes of data. Recommended length values are 256 bytes below 4800 bps, 1024 above 4800 bps or when the data link is known to be relatively error free. No padding is used with binary data packets. The data bytes Chapter 5 DRAFT 3-24-86 8 DRAFT ZMODEM Protocol Ref 02-23-86 9 are ZDLE encoded and transmitted. A ZDLE and frameend are then sent, followed by two ZDLE encoded CRC bytes. The CRC accumulates the data bytes and frameend. The function _z_s_b_d_a_t_a sends a binary data packet. The function _z_r_b_d_a_t_a receives a binary data packet. 5.6 HEX Data Packet The format of HEX data packets is not currently specified. These would be used for server commands, etc. 6. PROTOCOL TRANSACTION OVERVIEW As with the XMODEM recommendation, ZMODEM timing is receiver driven. The transmitter should not time out at all, except to abort the program if no packets are received for an extended period of time, say one minute. To start a ZMODEM file transfer session, the sending program is called with the names of the desired file(s). The sending program sends the string "rz\r" and a single HEX ZRQRINIT packet with data = 0. The "rz" followed by carriage return activates a ZMODEM receive program or command if it were not already active. If the receiving program receives the ZRQRINIT packet, it totally ignores it as the sending program will be responding to the RINIT packet sent by the receiver. The sending program should also ignore this packet type, which would be an echo of its own packet. Since the ZRQRINIT sequence contains no exotic control characters, it can be accepted by the application program as a command to begin ZMODEM receive. The sequence prints as "rz^M**!B00000000000000^M^J" where ^M represents CR and ^J represents LF. The sending program awaits a command from the receiving program to start file transfers. If a "C" or NAK is received, an XMODEM or YMODEM file transfer is indicated, and file transfer(s) use the X/YMODEM protocol. Note: With ZMODEM and YMODEM Batch, the sending program provides the file name, but not with XMODEM. When the ZMODEM receive program starts, it immediately sends a ZRINIT packet to initiate ZMODEM file transfers. The receive program resends the ZRINIT packet at _r_e_p_s_o_n_s_e _t_i_m_e intervals for a suitable period of time (40 seconds typical) Chapter 6 DRAFT 3-24-86 9 DRAFT ZMODEM Protocol Ref 02-23-86 10 before falling back to X/YMODEM protocol. If the receiving program receives a ZRINIT packet, it is an echo indicating that the sending program is not operational. If the receiving program receives a ZRQRINIT packet, it ignores it. Eventually the sending program correctly receives the ZRINIT packet. The sender may respond with an optional ZCRYPT packet, which the receiver acknowledges with a suitable frame. (Details to be worked out later) The sender may respond with an optional ZSINIT frame to set the receiving program's Attention string. The receiver sends a ZACK packet in response. The sender then sends a ZFILE packet containing the file name, file length, modification date, and other information identical to that used by YMODEM Batch. The receiver may respond to this with a ZGETCRC packet, which requires the sender to permorm a CRC on the file and transmit same with a ZCRC packet. The receiver may use this information to determine whether to accept the file or skip it. The receiver may respond with a ZSKIP packet, which causes the file to be passed over. A ZRPOS packet from the receiver initiates transmission of the file data starting at the offset in the file specified in the ZRPOS packet. Normally the receiver specifies the data transfer begin begin at offset 0 in the file. The receiver may start the transfer further down in the file. This allows a file transfer interrupted by a loss or carrier or system crash to be completed on the next connection without requiring the entire file to be retransmitted. If downloading a file from a timesharing system that becomes sluggish, the transfer can be interrupted and resumed later with no loss of data. The sender sends a ZDATA header packet (with file position) followed by one or more data packets. The receiver compares the file position in the ZDATA Chapter 6 DRAFT 3-24-86 10 DRAFT ZMODEM Protocol Ref 02-23-86 11 header with the number of characters successfully received to the file. If they do not agree, a ZRPOS error response is generated to force the sender to the right position within the file. A data packet terminated by ZCRCGO and CRC does not elicit a response unless an error is detected; more data packet(s) follow immediately. ZCRCQ data packets expect a ZACK response (with the file offset) if no error, otherwise a ZRPOS response (with the last good file offset). Another data packet continues immediately. ZCRCQ packets are not used if the receiver does not indicate FDX ability with the CANFDX bit. ZCRCW data packets expect a response before the next frame is sent. If the receiver does not indicate overlapped I/O capability with the CANOVIO bit, the sender uses the ZCRCW to allow the receiver to write its buffer before sending more data. A zero length data packet may also be used as a sending idle packet to prevent the receiver from timing out in case data is not immediately available to the sender. In the absence of fatal error, the sender encounters end of file. If the end of file is encountered within a frame, the frame is closed with a ZCRCE data packet which does not elicit a response except in case of error. The sender sends a ZEOF packet with the file ending offset equal to the number of characters in the file. The receiver compares this number with the number of characters received. If the receiver has received all of the file, it closes the file and responds with ZRINIT. Otherwise the receiver sends ZRPOS with the current file offset, forcing the sender to send the missing data. After all files are processed, any further protocol errors should not prevent the sending program from returning with a success status. The sender closes the session with a ZEXIT header packet. The receiver acknowledges this with its own ZEXIT packet. Chapter 6 DRAFT 3-24-86 11 DRAFT ZMODEM Protocol Ref 02-23-86 12 When the sender receives the acknowledging packet, it sends two characters, "OO" (Over and Out) and exits to the operating system or application that invoked it. The receiver waits briefly for the "O" characters, then exits whether they were received or not. 7. STREAMING TECHNIQUE ZMODEM allows a choice of data streaming methodmethods selected according to the limitations of the sending program operating environment, receiving program operating environment, and the transmission channel(s). If the computers can overlap serial I/O with disk I/O, the sender begins data transmission with a ZDATA header and continuous ZCRCG data packets. When the receiver detects an error, it sends the Attn sequence and a ZRPOS packet to force the sender back to the correct position within the file. At the end of each transmitted packet, the sender checks for the presence of a error packet from the receiver. To do this, the sender may sample the reverse data stream for the presence of a ZPAD character. If the reverse data stream cannot be sampled without entering an I/O wait, the receiver can interrupt the sender with a control character, break, or combinations thereof, as specified in the ZSINIT frame sent by the sending program. If the receiver cannot overlap serial and disk I/O, it uses the ZRINIT frame to specify a buffer length which the sender will not overfill before sending a ZCRCW packet. 8. 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 characters in the Attn string are terminated with a null. Two characters perform special functions: Chapter 8 DRAFT 3-24-86 12 DRAFT ZMODEM Protocol Ref 02-23-86 13 o+ \335 Sends a break signal o+ \336 Pauses one second 9. PACKET/FRAME TYPES The numeric values for the values shown in boldface are given in _z_m_o_d_e_m._h. 9.1 ZRQRINIT Sent by the sending program to call up the receiving program. P0...P3 are zero. 9.2 ZRINIT Sent by the receiving program. ZF0 and ZF1 contain receiver capability flags: #define CANFDX 1 /* Rx can send and receive FDX */ #define CANOVIO 2 /* Rx can receive during disk I/O */ #define CANBRK 4 /* Rx can send a break signal */ #define CANCRY 8 /* Receiver can decrypt */ ZP0 and ZP1 contain the size of the receiver's buffer in bytes, or 0 if overlapped I/O is allowed. 9.3 ZSINIT Sender sends capability flags (none currently defined) followed by a binary data packet terminated with ZCRCW. Data contains the Attn string, maximum length 32 bytes. The ZSINIT is only sent to programs that indicate the ability to overlap serial data and disk I/O (CANOVIO). 9.4 ZACK Acknowedgement to ZSINIT header packet or ZCRCW data packet. ZP0 to ZP3 contain file offset. 9.5 ZFILE This packet indicates the beginning of a file transmission attempt. ZF0 and ZF1 contain special file processing flags: o+ ZBIN This is a binary file A ZCRCW data packet follows with file name, file length, modification date, and other information described in a later chapter. Chapter 9 DRAFT 3-24-86 13 DRAFT ZMODEM Protocol Ref 02-23-86 14 9.6 ZSKIP Sent by the receiver in response to ZFILE, makes the sender skip to the next file. 9.7 ZNAK Indicates last packet header was garbled. (See also ZRPOS). 9.8 ZABORT Sent by receiver to terminate batch file transfers when requested by the user. Sender initiates a ZFIN sequence.1 9.9 ZFIN Sent by sending program to terminate ZMODEM. Receiver responds with ZFIN. 9.10 ZRPOS Sent by receiver to force file transfer to resume at file offset given in ZP0...ZP3. 9.11 ZDATA ZP0...ZP3 contain file offset. One or more data packets follow. 9.12 ZEOF ZP0...ZP3 contain file offset. Sender reports End of File. 9.13 ZFERR Error in reading or writing file, equivalent to ZABORT. __________ 1. Or ZCOMPL in case of server mode. Chapter 9 DRAFT 3-24-86 14 DRAFT ZMODEM Protocol Ref 02-23-86 15 9.14 ZCRC Request (receiver) and response (sender) for file CRC. ZP0 and ZP1 contain 16 bit file CRC. 9.15 ZCRYPT Negotiation for encryption. 9.16 ZCOMPL Server request now completed. 10. Transaction A simple transaction, one file, no errors, overlapped I/O: Sender Receiver ZRINIT ZFILE ZRPOS ZDATA data ... ZEOF ZRINIT ZFIN ZFIN OO 11. PERFORMANCE Some tests of ZMODEM protocol performance have been made. A PC-AT with SCO SYS V Xenix or DOS 3.1 was connected to a PC with DOS 2.1 either directly at 9600 bps or with dial-up 1200 bps modems. The ZMODEM software was configured to use 1024 byte packet lengths above 2400 bps, 256 otherwise. Because no time delays are necessary in normal file transfers, per file negotiations are much faster than Chapter 11 DRAFT 3-24-86 15 DRAFT ZMODEM Protocol Ref 02-23-86 16 with YMODEM, the only observed impidiment being the time required by the program(s) to update logging files. During a file transfer, a short line hit seen by the receiver usually induces a CRC error. The interrupt packet is usually seen by the sender before the next packet is sent, and the resultant loss of data throughput averages about half a packet. At 1200 bps this is would be about .75 second lost per hit. At 10-5 error rate, this would degrade throughput by about 9 per cent. The throughput degradation increases with the channel delay, as the packets in transit through the channel are discarded on error. A longer noise burst that affects both the receiver and the sender's reception of the interrupt packet usually causes the sender to remain silent until the receiver times out in 10 seconds. If the round trip channel delay exceeds the receiver's 10 second timeout, recovery from this type of error may become difficult. Noise affecting only the sender is usually ignored, with one common exception. Spurious XOFF characters generated by noise stop the sender until the receiver times out and sends an interrupt packet which concludes with an XON. In summation, ZMODEM performance in the presence of errors resembles that of X.PC and SuperKermit. Short bursts cause minimuml data loss. Long bursts (such as pulse dialing noises) often require a timeout error to restore the flow of data. 12. TABLES Figure 1. Protocol Overhead Information Figures are calculated for round trip delay times of 40 milliseconds and 5 seconds. A 102400 byte randomly distributed binary file is sent at 1200 bps 8 data bits, 1 stop bit. The calculations assume no transmission errors. For each of the protocols, only the per file functions are considered. Processor and I/O overhead are not included. YM-k refers to YMODEM with 1024 byte packets. YM-g refers to the YMODEM "g" option. ZMODEM uses 256 byte packets for this example. SuperKermit uses amximum packet size, 8 bit transparent Chapter 12 DRAFT 3-24-86 16 DRAFT ZMODEM Protocol Ref 02-23-86 17 transmission, no run length compression. Protocol dump XMODEM YM-k YM-G ZMODEM S-Kermit Round-Trips - 803 103 5 5 5? Time@40ms - 32s 4s - - - Time@5s - 4015s 515s 25s 25s 25 Chars-Ovhd - 4803 603 503 2000 7460 Time@0s 853s 893s 858s 857s 870s 1135s Time@40ms 853s 925s 862s 857s 870s 1135s Time@5s 853s 5761s 1373s 882s 905s 1160s Figure 2. Transmission Time Comparision (5 second delay) ********************************************************** XMODEM ************** YMODEM-K ************ Kermit Sliding Windows ********* YMODEM-G ********* ZMODEM Figure 3. Y/ZMODEM Header Information usage Program Batch Length Date Mode S/N YMODEM-g ZMODEM Unix rb/sb yes yes yes yes no sb only no Unix rz/sz yes yes yes yes no sb only yes VMS rb/sb yes yes no no no no no Pro-YAM yes yes yes no yes yes yes CP/M YAM yes no no no no no no KMD/IMP yes no|- no no no no no MEX no no no no no no no 13. ZFILE FRAME FILE INFORMATION Only the pathname (file name) part is required for batch transfers. Pathname The pathname (conventionally, the file name) is sent as a null terminated ASCII string. This is the filename format used by the handle oriented MSDOS(TM) functions and C library fopen functions. An assembly language example follows: DB 'foo.bar',0 No spaces are included in the pathname. Normally only the file name stem (no directory prefix) is transmitted unless the sender has selected YAM's f option to send the full pathname. The source drive (A:, B:, etc.) is never sent. Chapter 13 DRAFT 3-24-86 17 DRAFT ZMODEM Protocol Ref 02-23-86 18 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+ The rb(1) program on Unix systems normally translates the filename to lower case unless one or more letters in the filename are already in lower case. o+ When transmitting files between different operating systems, file names must be acceptable to both the sender and receiving operating systems. If directories are included, they are delimited by /; i.e., "subdir/foo" is acceptable, "subdir\foo" is not. Length The file length and each of the succeeding fields are optional.1 The length field is stored as a decimal string counting the number of data bytes in the file. With ZMODEM, the receiver uses the file length only for display (progress reporting) purposes; the actual length is determined by the data transfer. Modification Date A single space separates the modification date from the file length. The mod date is optional, and the filename and length may be sent without requiring the mod date to be sent. The mod date is sent as an octal number giving the time the contents of the file were last changed measured in seconds from Jan 1 1970 Universal __________ 1. Fields may not be skipped. Chapter 13 DRAFT 3-24-86 18 DRAFT ZMODEM Protocol Ref 02-23-86 19 Coordinated Time (GMT). A date of 0 implies the modification date is unknown and should be left as the date the file is received. This standard format was chosen to eliminate ambiguities arising from transfers between different time zones. Two Microsoft blunders complicate the use of modification dates in file transfers with MSDOS(TM) systems. The first is the lack of timezone standardization in MS-DOS. A file's creation time can not be known unless the timezone of the system that wrote the file2 is known. Unix solved this problem (for planet Earth, anyway) by stamping files with Universal Time (GMT). Microsoft would have to include the timezone of origin in the directory entries, but does not. Professional-YAM gets around this problem by using the z parameter which is set to the number of minutes local time lags GMT. For files known to originate from a different timezone, the -zT option may be used to specify T as the timezone for an individual file transfer. The second problem is the lack of a separate file creation date in DOS. Since some backup schemes used with DOS rely on the file creation date to select files to be copied to the archive, back- dating the file modification date could interfere with the safety of the transferred files. For this reason, Professional-YAM does not modify the date of received files with the header information unless the d parameter is non zero. Mode A single space separates the file mode from the modification date. The file mode is stored as an octal string. Unless the file originated from a Unix system, the file mode is set to 0. rb(1) checks the file mode for the 0x8000 bit which indicates a Unix type regular file. Files with the 0x8000 bit set are assumed to have been sent from another Unix (or similar) system which uses the same file conventions. Such files are not __________ 2. Not necessarily that of the transmitting system! Chapter 13 DRAFT 3-24-86 19 DRAFT ZMODEM Protocol Ref 02-23-86 20 translated in any way. Serial Number A single space separates the serial number from the file mode. The serial number of the transmitting program is stored as an octal string. Programs which do not have a serial number should omit this field, or set it to 0. The receiver's use of this field is optional. The file information is terminated by a null. If only the pathname is sent, the pathname will be terminated by two nulls. The length of the file information packet, including the trailing null, must not exceed 1024 bytes; a typical length is less than 64 bytes. 14. MORE INFORMATION More information may be obtained by calling Telegodzilla at 503-621-3746. UUCP sites can obtain the nroff/troff source to this file with uucp omen!/usr/caf/public/zmodem.mm /tmp A continually updated list of available files is stored in /usr/spool/uucppublic/FILES. The following L.sys line calls Telegodzilla (Pro-YAM in host operation). Telegodzilla waits for carriage returns to determine the incoming speed. If none is detected, 1200 bps is assumed and a greeting is displayed. In response to "Name Please:" uucico gives the Pro-YAM "link" command as a user name. The password (Giznoid) controls access to the Xenix system connected to the IBM PC's other serial port. Communications between Pro-YAM and Xenix use 9600 bps; YAM converts this to the caller's speed. Finally, the calling uucico logs in as uucp. omen Any ACU 1200 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp Chapter 15 DRAFT 3-24-86 20 DRAFT ZMODEM Protocol Ref 02-23-86 21 15. ZMODEM Programs A demonstration version of Professional-YAM is available as YAMDEMO.LQR (A SQueezed Novosielski library). This may be used to test ZMODEM and YMODEM implementations. 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. A CP/M assembly version is available as MODEM76.AQM and MODEM7.LIB. Irv Hoff has added YMODEM 1k packets and basic YMODEM batch transfers to the KMD and IMP series programs, which replace the XMODEM and MODEM7/MDM7xx series respectively. Overlays are available for a wide variety of CP/M systems. MEX and MEX-PC also support some of the YMODEM 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: 503-621-3746 Speed: 1200,300 Usenet: ...!tektronix!reed!omen!caf Compuserve: 70715,131 Source: TCE022 Chapter 15 DRAFT 3-24-86 21 CONTENTS 1. ACKNOWLEDGMENTS.................................... 2 2. ROSETTA STONE...................................... 2 3. YET ANOTHER PROTOCOL, AGAIN?....................... 3 4. ZMODEM Protocol Design Criteria.................... 4 4.1 Throughput................................... 4 4.2 Integrity and Robustness..................... 5 4.3 Ease of use.................................. 5 4.4 Ease of implementation....................... 5 5. PACKETIZATION...................................... 5 5.1 Link Escape Encoding......................... 6 5.2 Header Packet Information.................... 7 5.3 Binary Header Packet......................... 7 5.4 HEX Header Packet............................ 8 5.5 Binary Data Packets.......................... 8 5.6 HEX Data Packet.............................. 9 6. PROTOCOL TRANSACTION OVERVIEW...................... 9 7. STREAMING TECHNIQUE................................ 12 8. ATTENTION SEQUENCE................................. 12 9. PACKET/FRAME TYPES................................. 13 9.1 ZRQRINIT..................................... 13 9.2 ZRINIT....................................... 13 9.3 ZSINIT....................................... 13 9.4 ZACK......................................... 13 9.5 ZFILE........................................ 13 9.6 ZSKIP........................................ 14 9.7 ZNAK......................................... 14 9.8 ZABORT....................................... 14 9.9 ZFIN......................................... 14 9.10 ZRPOS........................................ 14 9.11 ZDATA........................................ 14 9.12 ZEOF......................................... 14 9.13 ZFERR........................................ 14 9.14 ZCRC......................................... 15 9.15 ZCRYPT....................................... 15 9.16 ZCOMPL....................................... 15 10. Transaction........................................ 15 11. PERFORMANCE........................................ 15 - i - 12. TABLES............................................. 16 13. ZFILE FRAME FILE INFORMATION....................... 17 14. MORE INFORMATION................................... 20 15. ZMODEM Programs.................................... 21 - ii - LIST OF FIGURES Figure 1. Protocol Overhead Information................ 16 Figure 2. Transmission Time Comparision................ 17 Figure 3. Y/ZMODEM Header Information usage............ 17 - iii -