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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: 70715,131 UUCP: ...!tektronix!reed!omen!caf Chapter 0 rev051486 Printed 5-16-86 1 Chapter 0 rev051486 Printed 5-16-86 2 1. INTENDED AUDIENCE This document is intended for systems programmers and other technically qualified people who choose and implement asynchronous file transfer protocols over dial-up networks and related environments. 2. ACKNOWLEDGMENTS Encouragement and suggestions by Stuart Mathison, Thomas Buck, John Wales, Ward Christensen, and Irv Hoff are gratefully acknowledged. 3. RELATED DOCUMENTS The following files should be available for reference while studying this document: YMODEM.DOC Describes the XMODEM and YMODEM file transfer protocols ZMODEM.H Provides definitions for the manifest constants referenced herein. rz.c, sz.c, rbsb.c Unix source code for operating ZMODEM programs. rz.1, sz.1 Manual pages for rz and sz. zm.c, zmodem.h Operating system independent ZMODEM subroutines, header file. 4. ROSETTA STONE Here are some definitions which reflect the current vernacular in the computer media. The attempt here is identify the file transfer protocol rather than specific programs. Frame A ZMODEM frame consists of a header packet and 0 or more data packets. XMODEM refers to the original 1979 file transfer etiquette introduced by Ward Christensen's 1979 MODEM2 program. It's also called the MODEM or MODEM2 protocol. Some who are unaware of MODEM7's unusual batch file mode call it MODEM7. Other aliases include "CP/M Users's Group" and "TERM II FTP 3". This protocol is supported by every serious communications program because of its universality, simplicity, and reasonable performance. XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte Cyclical Redundancy Check (CRC-16), giving modern error detection protection. Chapter 4 rev051486 Printed 5-16-86 2 Chapter 4 rev051486 Printed 5-16-86 3 YMODEM refers to the XMODEM/CRC protocol with the throughput and/or batch transmission enhancements described in YMODEM.DOC. ZMODEM Zmodem is a second generation streaming protocol for text and binary file transmission between applications running on microcomputers and mainframes. 5. WHY DEVELOP ZMODEM? Since its development half a decade ago, the Ward Christensen MODEM protocol has enabled a wide variety of computer systems to interchange data. There is hardly a communications program that doesn't at least claim to support this protocol, now called XMODEM. Advances in computing, modems and networking have spread the XMODEM protocol far beyond the micro to micro environment for which it was designed. These application have exposed some weaknesses: + The user interface is suitable for computer hobbyists. Three or four commands must be keyboarded to transfer each file. + The short block length causes throughput to suffer when used with timesharing systems, packet switched networks, satellite circuits, and buffered (error correcting) modems. + The 8 bit checksum and unprotected transactions allow undetected errors and disrupted file transfers. + Only one file can be sent per command. The file name has to be given twice, first to the sending program and then again to the receiving program. + The transmitted file accumulates as many as 127 extraneous bytes. + The modification date and other file attributes are lost. + XMODEM requires complete 8 bit transparency, all 256 codes. XMODEM will not operate over some networks that need flow control. A number of other protocols have been developed over the years, but none have displaced XMODEM to date. + Lack of public domain documentation and example programs have kept proprietary protocols such as MNP, Blast, and others tightly bound to the fortunes of their suppliers. + Hardware and/or software complexity discourages the widespread application of BISYNC, SDLC, HDLC, X.25, and X.PC protocols. Chapter 5 rev051486 Printed 5-16-86 3 Chapter 5 rev051486 Printed 5-16-86 4 + Link level protocols such as X.25, X.PC, and MNP do not manage application to application file transfers. + The Kermit protocol was developed to allow file transfers in environments hostile to XMODEM. The performance compromises necessary to accomodate non transparent environments limit Kermit's efficiency. Even with completely transparent channels, Kermit control character quoting limits the efficiency of binary file transfers to about 75 per cent.[1] Kermit Sliding Windows ("SuperKermit") improves throughput over networks at the cost of increased complexity. SuperKermit state transitions are encoded in a special language "wart" which requires a C compiler. The SuperKermit C code requires full duplex communications and the ability to check for the presence of characters in the input queue, precluding its implementation on some operating systems. A number of submodes are used in various Kermit programs, including different methods of transferring binary files. Two Kermit programs will mysteriously fail to operate with each other if these submodes are not matched. A number of extensions to the XMODEM protocol have been made under the collective name YMODEM. + YMODEM-k uses 1024 byte blocks to reduce the overhead from transmission delays by 87 per cent compared to XMODEM, but network delays can still degrade performance. Some networks may not be transmit the 1024 byte packets unmodified. + The handling of files that are not a multiple of 1024 or 128 bytes is awkward, especially if the file length is not known, or changes during transmission. + YMODEM-g provides efficient batch file transfers, preserving the exact file length and file modification date. YMODEM-g is essentially insensitive to network delays. Because it does not support error recovery, YMODEM-g is usually used hardwired or with a reliable link level protocol. IF YMODEM-g detects a CRC error, data transfers are aborted. YMODEM-g is easy to implement because it closely resembles XMODEM-CRC. Another XMODEM "extension" is protocol cheating, referred to as "Turbo Download" and OverThruster. [2] These sometimes improve XMODEM throughput __________ 1. Some Kermit programs support run length encoding. Chapter 5 rev051486 Printed 5-16-86 4 Chapter 5 rev051486 Printed 5-16-86 5 at the expense of error recovery. The ZMODEM Protocol is proposed as a means of addressing the weaknesses described above while maintaining as much of XMODEM's simplicity and prior art as possible. 6. ZMODEM Protocol Design Criteria The design of a file transfer protocol is an engineering compromise between conflicting requirements: 6.1 Ease of Use + ZMODEM allows either program to initiate file transfers, passing commands and/or modifiers to the other program. + File names need be entered only once, menu selections are possible. + Wild Card names may be used with batch transfers. + Minimum keystrokes required to initiate transfers. + ZRQINIT packet sent by sending program can trigger automatic downloads. + ZMODEM can step down to YMODEM if the other end does not support ZMODEM.[1] 6.2 Throughput ZMODEM is designed for optimum performance with minimum degradation caused by delays introduced by packet switched networks and timesharing systems. ZMODEM is optimized for best throughput when line hits occur infrequently. This assumption markedly reduces code complexity and memory requirements. ZMODEM protocol features enhance rapid error recovery compared to network compatible XMODEM implementations. It is assumed that many transfers will originate from a timesharing system connected to a packet switched network. ZMODEM provides features to allow for simple, efficient implementation on timesharing hosts. __________________________________________________________________________ 2. Omen Technology Trademark 1. Provided the transmission medium accomodates YMODEM. Chapter 6 rev051486 Printed 5-16-86 5 Chapter 6 rev051486 Printed 5-16-86 6 File transfers begin immediately regardless of which program is started first, without the 10 second delay associated with XMODEM. 6.3 Integrity and Robustness All packets are protected with 16 bit CRC. Proprietary alogrithyms[2] are not needed for reliable transfers. A security challenge guards againgst Trojan Horse messages. 6.4 Ease of Implementation ZMODEM accomodates a wide variety of systems: + Microcomputers that cannot overlap disk and serial i/o + Microcomputers that cannot overlap serial send and receive + Computers and/or networks requiring XON/XOFF flow control + Computers that cannot check the serial input queue for the presence of data without having to wait for the data to arrive. Although ZMODEM provides "hooks" for multiple "threads", ZMODEM is not intended to replace link level protocols such as X.25. ZMODEM accomodates network and timesharing system delays by continuously transmitting data unless the receiver interrupts the sender to request retransmission of garbled data. ZMODEM in effect uses the entire file as a window.[3] ZMODEM provides a general purpose application to application file transfer protocol which may be used directly or with with reliable link level protocols such as X.25, MNP, Fastlink, etc. 7. ZMODEM BASICS __________ 2. Unique to Professional-YAM, PowerCom, etc. 3. Streaming strategey is discussed in a coming chapter. Chapter 7 rev051486 Printed 5-16-86 6 Chapter 7 rev051486 Printed 5-16-86 7 7.1 Packetization ZMODEM frames somewhat different from X/YMODEM blocks. X/YMODEM blocks are not used for the following reasons: + Block numbers are limited to 256 + No provision for variable length blocks + Line hits corrupt protocol signals, causing failed file transfers. In particular, modem errors sometimes generate false block numbers, false EOTs and false ACKs. False ACKs are the most troublesome as they cause the sender to lose synchronization with the receiver. State of the art X/YMODEM programs such as Professional-YAM and PowerCom overcome some of these weaknesses with clever proprietary code, but a stronger protocol is desired. + It is difficult to determine the beginning and ends of X/YMODEM blocks when line hits cause a loss of synchronization. This precludes rapid error recovery. 7.2 Link Escape Encoding ZMODEM acheives data transparency by extending the 8 bit character set (256 codes) with escape sequences based on the ZMODEM data link escape character ZDLE.[1] Link Escape coding permits variable length data packets without the overhead of a separate byte count. It allows the beginning of frames to be detected without special timing techniques, facilitating rapid error recovery. Link Escape coding does add some overhead. The worst case, a file consisting entirely of ZDLE characters, would incur a 50% overhead. The ZDLE character is special. ZDLE represents a control sequence of some sort. If a ZDLE character appears in the data sent within a binary packet, it is prefixed with ZDLE, then sent as ZDLEE. The value for ZDLE is octal 030 (ASCII CAN). This particular value was chosen to allow a string of CAN characters to abort a ZMODEM session, compatible with X/YMODEM session abort. __________ 1. This and other constants are defined in the zmodem.h include file. Please note that constants with a leading 0 are octal constants in C. Chapter 7 rev051486 Printed 5-16-86 7 Chapter 7 rev051486 Printed 5-16-86 8 Since CAN is not used for normal terminal operations, communications programs can monitor the data flow for ZDLE. The following characters can be scanned to detect the ZRQINIT packet, the invitation to automatically download commands or files. Two successive CAN characters will abort a ZMODEM session. Experience with YMODEM file transfers suggests that this does not impair the robustness of the protocol. A minimum of 8 CAN are sent, so the ZMODEM subroutines can be modified to require more successive CAN characters to signal an abort. The receiving program will decode any sequence of ZDLE followed by a byte with bit 6 set and bit 5 reset (upper case letter, either parity) to the equivalent control character by inverting bit 6. This allows the transmitter to escape any control character that cannot be sent by the communications medium. The ZMODEM software currently escapes ZDLE, 021, 0221, 023, and 0223. In addition, the receiver recognizes escapes for 0177 and 0377 should these characters need to be escaped. 7.3 Header Packet Information All ZMODEM frames begin with a header packet which may be sent in binary or HEX form. ZMODEM uses a single routine to recognize binary and hex header packets. Either form of the header packet contains the same raw information: + A type byte[2] Future extensions to ZMODEM may use the high order bits of the type byte to indicate thread selection. + Four bytes of data indicating flags and/or numeric quantities depending on the packet type Figure 1. Order of Bytes in Header Packet TYPE: packet Type F0: Flags least significant byte P0: file Position least significant P3: file Position most significant TYPE F3 F2 F1 F0 -------------- TYPE P0 P1 P2 P3 __________ 2. The packet types are cardinal numbers beginning with 0 to minimize state transition table memory requirements. Chapter 7 rev051486 Printed 5-16-86 8 Chapter 7 rev051486 Printed 5-16-86 9 7.4 Binary Header Packet A binary header packet is only sent by the sending program to the receiving program. A binary header packet begins with the sequence ZPAD, ZDLE, ZBIN. The frame type byte is ZDLE encoded. The four position/flags bytes are ZDLE encoded. A two byte CRC of the frame type and position/flag bytes is ZDLE encoded. 0 or more binary data packets will follow depending on the frame type. The function zsbhdr transmits a binary header packet. The function zgethdr receives a binary or hex header packet. Figure 2. Binary Header Packet * * ZDLE TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2 7.5 HEX Header Packet The receiver sends responses in hex header packets. The sender also uses hex header packets when they are not followed by binary data packets. Hex encoding is required to support XON/XOFF flow control. The hex header receiving routine ignores flow control characters. Use of Kermit style encoding for control and paritied characters was considered and rejected because of increased possibility of interacting with some timesharing systems's line edit functions. Use of HEX packets from the receiving program allows control characters to be used to interrupt the sender when errors are detected. Except for header packet types that imply data packets to follow, a HEX header packet may be used in place of a binary header packet. A hex header packet begins with the sequence ZPAD, ZPAD, ZDLE, ZHEX. The zgethdr routine synchronizes in the ZPAD-ZDELE sequence. The extra ZPAD allows other parts of the program to detect a ZMODEM packet and then call zgethdr to receive the packet. The type byte, the four position/flag bytes, and the CRC thereof are sent in hex using the character set 01234567890abcdef. Upper case hex digits are not allowed; they false trigger X/YMODEM programs. A carriage return, line feed, and XON are appended to the HEX header packet but are not considered to be part of it. The CR/LF aids debugging from printouts. The XON releases the sender from spurious XOFF flow control characters generated by line noise, a common occurrence. Chapter 7 rev051486 Printed 5-16-86 9 Chapter 7 rev051486 Printed 5-16-86 10 0 or more ASCII Encoded data packets will follow depending on the frame type. The function zshhdr sends a hex header packet. Figure 3. HEX Header Packet * * ZDLE TYPE F3/P0 F2/P1 F1/P2 F0/P3 CRC-1 CRC-2 CR LF XON (TYPE, F3...F0, CRC-1, and CRC-2 are each sent as two hex digits.) 7.6 Binary Data Packets Binary data packets immediately follow the associated binary header packet. A binary data packet contains 0 to 1024 bytes of data. Recommended length values are 256 bytes below 4800 bps, 1024 above 4800 bps or when the data link is known to be relatively error free. No padding is used with binary data packets. The data bytes are ZDLE encoded and transmitted. A ZDLE and frameend are then sent, followed by two ZDLE encoded CRC bytes. The CRC accumulates the data bytes and frameend. The function zsbdata sends a binary data packet. The function zrbdata receives a binary data packet. 7.7 ASCII Encoded Data Packet The format of ASCII Encoded data packets is not currently specified. These would be used for server commands, etc. 8. PROTOCOL TRANSACTION OVERVIEW As with the XMODEM recommendation, ZMODEM timing is receiver driven. The transmitter should not time out at all, except to abort the program if no packets are received for an extended period of time, say one minute.[1] To start a ZMODEM file transfer session, the sending program is called with the names of the desired file(s) and option(s). The sending program sends the string "rz\r" to invoke the receiving program from a possible command mode. The "rz" followed by carriage return activates a ZMODEM receive program or command if it were not already active. __________ 1. Special considerations apply when sending commands. Chapter 8 rev051486 Printed 5-16-86 10 Chapter 8 rev051486 Printed 5-16-86 11 The sender may then display a message intended for human consumption, such as a list of the files requested, etc. Then the sender sends a ZRQINIT packet. The ZRQINIT packet causes a previously started receive program to send its ZRINIT packet without delay. In an interactive or conversational mode, the receiving application may monitor the data stream for ZDLE. The following characters may be scanned for B000000 indicating a ZRQINIT packet, a command to download a command or data. The sending program awaits a command from the receiving program to start file transfers. If a "C", "G", or NAK is received, an XMODEM or YMODEM file transfer is indicated, and file transfer(s) use the X/YMODEM protocol. Note: With ZMODEM and YMODEM Batch, the sending program provides the file name, but not with XMODEM. When the ZMODEM receive program starts, it immediately sends a ZRINIT packet to initiate ZMODEM file transfers, or a ZCHALLENGE packet to verify the sending program. The receive program resends its packet at repsonse time intervals for a suitable period of time (40 seconds typical) before falling back to X/YMODEM protocol. If the receiving program receives a ZRQINIT packet, it resends the ZRINIT packet. If the sending program receives the ZCHALLENGE packet, it places the data in ZP0...ZP3 in an answering ZACK packet. If the receiving program receives a ZRINIT packet, it is an echo indicating that the sending program is not operational. Eventually the sending program correctly receives the ZRINIT packet. The sender may then respond with an optional ZSINIT frame to set the receiving program's Attention string. The receiver sends a ZACK packet in response, containing the serial number of the receiving program, or 0. The sender then sends a ZFILE header with ZMODEM Conversion, Management, and Transport options[2] followed by a ZCRCW data packet containing the file name, file length, modification date, and other information identical to that used by YMODEM Batch. The receiving program should insure the pathname and options are compatible with its operating environment and local security requirements. __________ 2. See below, under ZFILE packet type. Chapter 8 rev051486 Printed 5-16-86 11 Chapter 8 rev051486 Printed 5-16-86 12 If the receiver has a file with the same name and length, it may respond with a ZCRC packet, which requires the sender to permorm a 16 bit CRC on the file and transmit the CRC in ZP0...ZP1 of a ZCRC packet. The receiver uses this information to determine whether to accept the file or skip it. This sequence is triggered by the ZMCRC Management Option. The receiver may then respond with a ZSKIP packet, which causes the sender to process the next file (if any) in the batch. A ZRPOS packet from the receiver initiates transmission of the file data starting at the offset in the file specified in the ZRPOS packet. Normally the receiver specifies the data transfer begin begin at offset 0 in the file. The receiver may start the transfer further down in the file. This allows a file transfer interrupted by a loss or carrier or system crash to be completed on the next connection without requiring the entire file to be retransmitted.[3] If downloading a file from a timesharing system that becomes sluggish, the transfer can be interrupted and resumed later with no loss of data. The sender sends a ZDATA binary header packet (with file position) followed by one or more data packets. The receiver compares the file position in the ZDATA header with the number of characters successfully received to the file. If they do not agree, a ZRPOS error response is generated to force the sender to the right position within the file.[4] A data packet terminated by ZCRCGO and CRC does not elicit a response unless an error is detected; more data packet(s) follow immediately. ZCRCQ data packets expect a ZACK response (with the file offset) if no error, otherwise a ZRPOS response (with the last good file offset). Another data packet continues immediately. ZCRCQ packets are not used if the receiver does not indicate FDX ability with the CANFDX bit. ZCRCW data packets expect a response before the next frame is sent. If the receiver does not indicate overlapped I/O capability with the __________ 3. This does not apply to files that have been translated. 4. If the ZMSPARS option is used, the receiver instead seeks to position in the ZDATA packet. Chapter 8 rev051486 Printed 5-16-86 12 Chapter 8 rev051486 Printed 5-16-86 13 CANOVIO bit, or by setting a buffer size, the sender uses the ZCRCW to allow the receiver to write its buffer before sending more data. A zero length data frame may be used as a sending idle packet to prevent the receiver from timing out in case data is not immediately available to the sender. In the absence of fatal error, the sender eventually encounters end of file. If the end of file is encountered within a frame, the frame is closed with a ZCRCE data packet which does not elicit a response except in case of error. The sender sends a ZEOF packet with the file ending offset equal to the number of characters in the file. The receiver compares this number with the number of characters received. If the receiver has received all of the file, it closes the file. If the file close was satisfactory, the receiver responds with ZRINIT. If the receiver has not received all the bytes of the file, the receiver sends ZRPOS with the current file offset, forcing the sender to resend the missing data. (If the receiver cannot properly close the file, a ZFERR packet is sent.) After all files are processed, any further protocol errors should not prevent the sending program from returning with a success status. The sender closes the session with a ZEXIT header packet. The receiver acknowledges this with its own ZEXIT packet. When the sender receives the acknowledging packet, it sends two characters, "OO" (Over and Out) and exits to the operating system or application that invoked it. The receiver waits briefly for the "O" characters, then exits whether they were received or not. 8.1 Session Cancel Packet The Cancel packet consists of two ZPAD characters, eight CAN characters, and an optional ten backspace characters. First, the Attn sequence is executed if the receiving program has been receiving data in streaming mode. The ZPAD characters allow sending programs that sample the reverse data stream to check for a single character code indicating a packet from the receiver. The trailing backspace characters attempt to erase the effects of the other characters if they are received by a command interpreter. static char canistr[] = { ZPAD,ZPAD,24,24,24,24,24,24,24,24,8,8,8,8,8,8,8,8,8,8,0 }; Chapter 9 rev051486 Printed 5-16-86 13 Chapter 9 rev051486 Printed 5-16-86 14 9. ZMODEM STREAMING TECHNIQUES ZMODEM allows choices of several data streaming methods selected according to the limitations of the sending environment, receiving environment, and transmission channel(s). 9.1 Full Streaming with Sampling If the computers can overlap serial I/O with disk I/O, and if the sender can sample the reverse channel for the presence of data without having to wait, full streaming can be used with no Attn sequence required. The sender begins data transmission with a ZDATA header and continuous ZCRCG data packets. When the receiver detects an error, it executes the Attn sequence and then sends a ZRPOS packet to force the sender back to the correct position within the file. At the end of each transmitted packet, the sender checks for the presence of an error packet from the receiver. To do this, the sender may sample the reverse data stream for the presence of a ZPAD character. Such a program would sample the reverse channel for ZPAD. If seen, an empty ZCRCW data packet is sent (in case the receiver was still reading packets) and then the receiver's response packet is read and acted upon. The code fragment in sz.c beginning at NOTDEF_DOS would perform this function. 9.2 Full Streaming with Interrupt The method above cannot be used if if the reverse data stream cannot be sampled without entering an I/O wait. An alternate method is to instruct the receiver to interrupt the sending program when an error is detected. The receiver can interrupt the sender with a control character, break signal, or combination thereof, as specified in the ZSINIT frame sent by the sending program. When the sending program "catches" this interrupt, it reads a HEX packet (normally ZRPOS) from the receiver and takes appropriate action. The Unix sb.c program uses a setjmp/longjmp call and the getinsync() function to read the receiver's error packet and take appropriate action. Chapter 9 rev051486 Printed 5-16-86 14 Chapter 9 rev051486 Printed 5-16-86 15 9.3 Full Streaming with a Sliding Window If none of the above methods is applicable, hope is not yet lost. If the sender can buffer responses from the receiver, the sender can use ZCRCQ packets to get ACKs from the receiver without interrupting the transmission of data. After a sufficient number of ZCRCQ packets have been sent, the sender can read one of the one or more packets that should have arrived in it's receive interrupt buffer. A problem with this method is the probability of wasting an excessive amount of time responding to the receiver's error packet. 9.4 No Streaming If the receiver cannot overlap serial and disk I/O, it uses the ZRINIT frame to specify a buffer length which the sender will not overflow. The sending program sends a ZCRCW packet and waits for an ZACK packet before sending the next segment of the file. If the sending program supports reverse data stream sampling or interrupt, error recovery will be faster (on average) than a protocol (such as YMODEM) that sends "monolithic" blocks. 10. ATTENTION SEQUENCE The receiving program sends the Attn sequence whenever it detects an error and needs to interrupt the sending program. The default Attn string value is empty (no Attn sequence). The receiving program resets Attn to the empty default before each transfer session. The sender speficies the Attn sequence in its optional ZSINIT frame. The Attn string is terminated with a null. Two meta-characters perform special functions: + \335 (octal) Sends a break signal + \336 (octal) Pauses one second 11. PACKET/FRAME TYPES The numeric values for the values shown in boldface are given in zmodem.h. Chapter 11 rev051486 Printed 5-16-86 15 Chapter 11 rev051486 Printed 5-16-86 16 11.1 ZRQINIT Sent once by the sending program, to trigger the receiving program to send its ZRINIT packet. This aviods the aggravatimg startup delay associated with XMODEM and Kermit transfers. ZF0 contains ZCOMMAND if the program is attempting to send a command, 0 otherwise. 11.2 ZRINIT Sent by the receiving program. ZF0 and ZF1 contain the bitwise or of the receiver capability flags: #define CANFDX 1 /* Rx can send and receive FDX */ #define CANOVIO 2 /* Rx can receive during disk I/O */ #define CANBRK 4 /* Rx can send a break signal */ #define CANCRY 8 /* Receiver can decrypt */ ZP0 and ZP1 contain the size of the receiver's buffer in bytes, or 0 if nonstop I/O is allowed. 11.3 ZSINIT Sender sends capability flags (currently all 0) (none currently defined) followed by a binary data packet terminated with ZCRCW. The data packet contains the null terminated Attn sequence, maximum length 32 bytes including the terminating null. 11.4 ZACK Acknowedgement to ZSINIT header packet, ZCHALLENGE header packet, or ZCRCW data packet. ZP0 to ZP3 contain file offset. Response to ZCHALLENGE contains the same 32 bits as received. 11.5 ZFILE This packet denotes the beginning of a file transmission attempt. ZF0, ZF1, and ZF2 may contain options. A value of 0 in each of these bytes implies no special treatment. If options are specified to the reciever, they override options specified to the sender with the exception of the ZCBIN option, which overrides any other Conversion Option. 11.5.1 ZF0: Conversion Option If the receiver does not recognize the Conversion Option, an application dependent default conversion may apply. ZCBIN "Binary" transfer - inhibit conversion unconditionally Chapter 11 rev051486 Printed 5-16-86 16 Chapter 11 rev051486 Printed 5-16-86 17 ZCNL Convert received end of line to local end of line convention. The suported end line conventions are CR/LF (most ASCII based operating systems except Unix and Macintosh), and NL (Unix). Neither of these two end of line conventions violate the permissible ASCII definitions for Carriage Return and Line Feed/New Line. ZCRECOV Recover interrupted file transfer; start transfer at location corresponding to the receiver's end of file. This option does not apply if the source file is shorter. Files that have been converted (e.g., ZCNL) or subject to a single ended Transport Option cannot have their transfers recovered. 11.5.2 ZF1: Management Option If the receiver does not recognize the Management Option, the file should be transferred normally. ZMNEW Compare the source and destination files. Transfer file if source newer or different length ZMCRC Compare the source and destination files. Transfer if different file length or CRC ZMAPND Append source file contents to existing destination file (if any) ZMCLOB Replace existing destination file (if any) ZTSPARS Special processing for sparse file; each file segment is transmitted as a separate frame, where the frames are not necessarily contiguous. 11.5.3 ZF2: Transport Option If the receiver does not implement the particular transport option, the file is copied without conversion for later processing. ZTLZW Lempel-Ziv compression. Transmitted data will be identical to that produced by compress 4.0 operating on a computer with VAX byte ordering, using 12 bit encoding. ZTCRYPT Encryption. An initial null terminated string identifies the key. Details to be determined. ZTRLE Run Length encoding Details to be determined. A ZCRCW data packet follows with file name, file length, modification date, and other information described in a later chapter. Chapter 11 rev051486 Printed 5-16-86 17 Chapter 11 rev051486 Printed 5-16-86 18 11.6 ZSKIP Sent by the receiver in response to ZFILE, makes the sender skip to the next file. 11.7 ZNAK Indicates last packet header was garbled. (See also ZRPOS). 11.8 ZABORT Sent by receiver to terminate batch file transfers when requested by the user. Sender initiates a ZFIN sequence.[1] 11.9 ZFIN Sent by sending program to terminate a ZMODEM session. Receiver responds with ZFIN. 11.10 ZRPOS Sent by receiver to force file transfer to resume at file offset given in ZP0...ZP3. 11.11 ZDATA ZP0...ZP3 contain file offset. One or more data packets follow. 11.12 ZEOF Sender reports End of File. ZP0...ZP3 contain the ending file offset. 11.13 ZFERR Error in reading or writing file, protocol equivalent to ZABORT. 11.14 ZCRC Request (receiver) and response (sender) for file CRC. ZP0 and ZP1 contain 16 bit file CRC. __________ 1. Or ZCOMPL in case of server mode. Chapter 11 rev051486 Printed 5-16-86 18 Chapter 11 rev051486 Printed 5-16-86 19 11.15 ZCHALLENGE Request to echo a random number in ZP0...ZP3 in a ZACK frame. Sent by the receiving program to the sending program to verify that it is connected to an operating program, and was not activated by spurious data or a Trojan Horse message. 11.16 ZCOMPL Request now completed. 11.17 ZCAN This is a pseudo frame type returned by gethdr() in response to a Cancel sequence. 11.18 ZFREECNT Sending program requests a ZACK frame with ZP0...ZP3 containing the number of free bytes on the current file system. A value of 0 represents an indefinite amount of free space. 11.19 ZCOMMAND ZCOMMAND is only sent as a binary header packet. ZP0...ZP2 contain a unique cardinal number to differentiate this command from other commands[2]. ZF0 contains 0 or ZCACK1. A ZCRCW data packet follows, with the ASCII text command string terminated with a NULL character. If the command is intended to be executed by the operating system hosting the receiving program (e.g., "shell escape"), it must have "!" as the first character. Otherwise the command is meant to be executed by the application program which received the command. If ZF0 contained ZCACK1, the receiver immediately responds with a ZCOMPL header. Otherwise, the receiver responds with a ZCOMPL header when the operation is completed. The exit status of the completed command is stored in ZP0...ZP3 (0 if ZCACK1). A 0 exit status implies nominal completion of the command. If the command caused a file to be transmitted, the command sender will see a ZRQINIT frame from the other computer attempting to send __________ 2. Currently unused. Chapter 11 rev051486 Printed 5-16-86 19 Chapter 11 rev051486 Printed 5-16-86 20 data. The sender examines ZF0 of the received ZRQINIT packet to determine it is not an echo of its own ZRQINIT packet. It is illegal for the sending program to command the receiving program to send a command. 12. TRANSACTION EXAMPLE 12.1 A simple file transfer A simple transaction, one file, no errors, no CHALLENGE, overlapped I/O: Sender Receiver "rz\r" ZRQINIT(0) ZRINIT ZFILE ZRPOS ZDATA data ... ZEOF ZRINIT ZFIN ZFIN OO 12.2 Challenge and Command Download Sender Receiver "rz\r" ZRQINIT(ZCOMMAND) ZCHALLENGE(rnd) ZACK(same-rnd) ZRINIT ZCOMMAND (Perform Command) ZCOMPL ZFIN ZFIN OO Chapter 13 rev051486 Printed 5-16-86 20 Chapter 13 rev051486 Printed 5-16-86 21 13. ZFILE FRAME FILE INFORMATION ZMODEM sends the same file information with the ZFILE frame data that YMODEM Batch sends in its block 0. N.B.: Only the pathname (file name) part is required for batch transfers. Pathname The pathname (conventionally, the file name) is sent as a null terminated ASCII string. This is the filename format used by the handle oriented MSDOS(TM) functions and C library fopen functions. An assembly language example follows: DB 'foo.bar',0 No spaces are included in the pathname. Normally only the file name stem (no directory prefix) is transmitted unless the sender has selected YAM's f option to send the full relative pathname. The source drive designator (A:, B:, etc.) is not sent. Filename Considerations: + File names should be translated to lower case unless the sending system supports upper/lower case file names. This is a convenience for users of systems (such as Unix) which store filenames in upper and lower case. + The receiver should accommodate file names in lower and upper case. + The rb(1) program on Unix systems normally translates the filename to lower case unless one or more letters in the filename are already in lower case. + When transmitting files between different operating systems, file names must be acceptable to both the sender and receiving operating systems. If directories are included, they are delimited by /; i.e., "subdir/foo" is acceptable, "subdir\foo" is not. Length The file length and each of the succeeding fields are optional.[1] The length field is stored as a decimal string counting the number of data bytes in the file. With ZMODEM, the receiver uses the file length only for display (progress reporting) purposes; the actual length is determined __________ 1. Fields may not be skipped. Chapter 13 rev051486 Printed 5-16-86 21 Chapter 13 rev051486 Printed 5-16-86 22 by the data transfer. Modification Date A single space separates the modification date from the file length. The mod date is optional, and the filename and length may be sent without requiring the mod date to be sent. The mod date is sent as an octal number giving the time the contents of the file were last changed measured in seconds from Jan 1 1970 Universal Coordinated Time (GMT). A date of 0 implies the modification date is unknown and should be left as the date the file is received. This standard format was chosen to eliminate ambiguities arising from transfers between different time zones. Two Microsoft blunders complicate the use of modification dates in file transfers with MSDOS(TM) systems. The first is the lack of timezone standardization in MS-DOS. A file's creation time can not be known unless the timezone of the system that wrote the file[2] is known. Unix solved this problem (for planet Earth, anyway) by stamping files with Universal Time (GMT). Microsoft would have to include the timezone of origin in the directory entries, but does not. Professional-YAM gets around this problem by using the z parameter which is set to the number of minutes local time lags GMT. For files known to originate from a different timezone, the -zT option may be used to specify T as the timezone for an individual file transfer. The second problem is the lack of a separate file creation date in DOS. Since some backup schemes used with DOS rely on the file creation date to select files to be copied to the archive, back-dating the file modification date could interfere with the safety of the transferred files. For this reason, Professional-YAM does not modify the date of received files with the header information unless the d parameter is non zero. Mode A single space separates the file mode from the modification date. The file mode is stored as an octal string. Unless the file originated from a Unix system, the file mode is set to 0. rb(1) checks the file mode for the 0x8000 bit which indicates a Unix type regular file. Files with the 0x8000 bit set are assumed to have been sent from another Unix (or similar) system __________ 2. Not necessarily that of the transmitting system! Chapter 13 rev051486 Printed 5-16-86 22 Chapter 13 rev051486 Printed 5-16-86 23 which uses the same file conventions. Such files are not translated in any way. Serial Number A single space separates the serial number from the file mode. The serial number of the transmitting program is stored as an octal string. Programs which do not have a serial number should omit this field, or set it to 0. The receiver's use of this field is optional. The file information is terminated by a null. If only the pathname is sent, the pathname will be terminated by two nulls. The length of the file information packet, including the trailing null, must not exceed 1024 bytes; a typical length is less than 64 bytes. 14. PERFORMANCE RESULTS 14.1 Throughput Between two single task PC-XT computrers, on a Telenet link through the local Telenet, SuperKermit gave 72 ch/sec throughput at 1200 baud. YMODEM-k yielded 85 chars/sec, and ZMODEM provided 113 chat sec. ZMODEM was not measured, but would have given much less. 14.2 Error Recovery Some tests of ZMODEM protocol performance have been made. A PC-AT with SCO SYS V Xenix or DOS 3.1 was connected to a PC with DOS 2.1 either directly at 9600 bps or with unbuffered dial-up 1200 bps modems. The ZMODEM software was configured to use 1024 byte packet lengths above 2400 bps, 256 otherwise. Because no time delays are necessary in normal file transfers, per file negotiations are much faster than with YMODEM, the only observed impidiment being the time required by the program(s) to update logging files. During a file transfer, a short line hit seen by the receiver usually induces a CRC error. The interrupt packet is usually seen by the sender before the next packet is sent, and the resultant loss of data throughput averages about half a packet. At 1200 bps this is would be about .75 second lost per hit. At 10-5 error rate, this would degrade throughput by about 9 per cent. The throughput degradation increases with the channel delay, as the packets in transit through the channel are discarded on error. A longer noise burst that affects both the receiver and the sender's reception of the interrupt packet usually causes the sender to remain silent until the receiver times out in 10 seconds. If the round trip channel delay exceeds the receiver's 10 second timeout, recovery from Chapter 14 rev051486 Printed 5-16-86 23 Chapter 14 rev051486 Printed 5-16-86 24 this type of error may become difficult. Noise affecting only the sender is usually ignored, with one common exception. Spurious XOFF characters generated by noise stop the sender until the receiver times out and sends an interrupt packet which concludes with an XON. In summation, ZMODEM performance in the presence of errors resembles that of X.PC and SuperKermit. Short bursts cause minimuml data loss. Long bursts (such as pulse dialing noises) often require a timeout error to restore the flow of data. 15. PACKET SWITCHED NETWORK CONSIDERATIONS Flow control is necessary for printing messages and directories, and for streaming file transfer protocols including Kermit Sliding Windows and ZMODEM. A non transparent flow control is incompatible with XMODEM and YMODEM transfers. XMODEM and YMODEM protocols require complete transparency of all 256 8 bit codes to operate properly. The most desireable flow control (when X.25 or hardware CTS is unavailable) does not "eat" any characters at all. When the PAD's buffer almost fills up, an XOFF should be emitted. When the buffer is no longer nearly full, send an XON. Otherwise, the network should neither generate nor eat XON or XOFF control characters. On Telenet, this can be met by setting CCIT X3 5:1 and 12:0 at both ends of the network. For best throughput, parameter 64 (advance ACK) should be set to something like 4. Packets should be sent when the packet is a full 128 bytes, or after a moderate delay (3:0,4:10,6:0). For YMODEM, PAD buffering should guarantee that a minimum of 1040 characters can be sent in a burst without loss of data or generation of flow control characters. Failure to provide this buffering will generate excessive retries with YMODEM. Figure 4. Flow Control Compatibility Connectivity Interactive XMODEM KERMIT ZMODEM Direct Connection YES YES YES YES Network, no flow control NO YES (1) (1) Network, transparent f.c. YES YES YES YES Network, semi-transparent f.c. YES NO YES YES Network, 7 bit YES NO YES(2) NO(3) (1) Cannot operate in streaming mode. Kermit is very slow because of 96 byte max packet size. ZMODEM can adjust burst length to maximum for faster transfers. Chapter 15 rev051486 Printed 5-16-86 24 Chapter 15 rev051486 Printed 5-16-86 25 (2) Parity bits must be encoded, slowing binary transfers. (3) Extension possible for encoding data to 7 bits. 16. PERFORMANCE COMPARISION TABLES "Round Trip Delay Time" includes the time for the last byte in a packet to propagate through the operating systems and network to the receiver, plus the time for the receiver's response to that packet to propogate back to the sender. The figures shown below are calculated for round trip delay times of 40 milliseconds and 5 seconds. Shift registers in the two computers and a pair of 212 modems generate a round trip delay time on the order of 40 milliseconds. Operation with busy timesharing computers and networks can easily generate round trip delays of five seconds. Because the round trip delays cause visible interruptions of data transfer when using XMODEM protocol, the subjective effect of these delays is greatly exaggerated, especially when the user is paying for connect time. A 102400 byte binary file with randomly distributed codes is sent at 1200 bps 8 data bits, 1 stop bit. The calculations assume no transmission errors. For each of the protocols, only the per file functions are considered. Processor and I/O overhead are not included. YM-k refers to YMODEM with 1024 byte packets. YM-g refers to the YMODEM "g" option. ZMODEM uses 256 byte packets for this example. SuperKermit uses maximum packet size, 8 bit transparent transmission, no run length compression. For comparison, a straight "dump" of the file contents with no file management or error checking takes 853 seconds. Figure 5. Protocol Overhead Information Protocol XMODEM YM-k YM-g ZMODEM S-Kermit Protocol Round Trips 803 103 5 5 5 Trip Time at 40ms 32s 4s 0 0 0 Trip Time at 5s 4015s 515s 25s 25s 25 Overhead Characters 4803 603 503 3600 38280 Transfer Time at 0s 893s 858s 857s 883s 1172s Transfer Time at 40ms 925s 862s 857s 883s 1172s Transfer Time at 5s 5761s 1373s 882s 918s 1197s Chapter 16 rev051486 Printed 5-16-86 25 Chapter 16 rev051486 Printed 5-16-86 26 Figure 6. Transmission Time Comparision (5 Second Round Trip) ************************************************** XMODEM ************ YMODEM-K ********** SuperKermit (Sliding Windows) ******* YMODEM-G ******* ZMODEM Figure 7. Y/ZMODEM Header Information usage Program Batch Length Date Mode S/N YMODEM-g ZMODEM Unix rb/sb yes yes yes yes no sb only no Unix rz/sz yes yes yes yes no sz only yes VMS rb/sb yes yes no no no no no Pro-YAM yes yes yes no yes yes yes CP/M YAM yes no no no no no no KMD/IMP yes yes- no no no no no MEX no no no no no no no 17. MORE INFORMATION More information may be obtained by calling Telegodzilla at 503-621-3746. UUCP sites can obtain the nroff/troff source to this file with uucp omen!/usr/caf/public/zmodem.mm /tmp A continually updated list of available files is stored in /usr/spool/uucppublic/FILES. The following L.sys line calls site "omen" yia UUCP. Telegodzilla uses Pro-YAM in host operation. In response to "Name Please:" uucico gives the Pro-YAM "link" command as a user name. The password (Giznoid) controls access to the Xenix system connected to the IBM PC's other serial port. Communications between Pro-YAM and Xenix use 9600 bps; YAM converts this to the caller's speed. Finally, the calling uucico logs in as uucp. omen Any ACU 1200 1-503-621-3746 e:--e: link d: Giznoid n:--n: uucp Chapter 18 rev051486 Printed 5-16-86 26 Chapter 18 rev051486 Printed 5-16-86 27 18. ZMODEM PROGRAMS A demonstration version of Professional-YAM is available as YAMDEMO.ARC on TeleGodzilla.. This file must be unpacked with the "ARC" program, version 5 or later. A copy of ARC is available as "ARC.EXE" or "ARC510.COM" on TeleGodzilla. This may be used to test ZMODEM and YMODEM implementations. A flash-up tree structured help file and processor are provided in YAMHELP.LQR. 19. YMODEM PROGRAMS Unix programs supporting the YMODEM protocol are available on Telegodzilla in the "upgrade" subdirectory as RBSB.SHQ (a SQueezed shell archive). Most Unix like systems are supported, including V7, Sys III, 4.2 BSD, SYS V, Idris, Coherent, and Regulus. A version for VAX-VMS is available in VRBSB.SHQ, in the same directory. Irv Hoff has added YMODEM 1k packets and YMODEM batch transfers to the KMD and IMP series programs, which replace the XMODEM and MODEM7/MDM7xx series respectively. Overlays are available for a wide variety of CP/M systems. Many other programs, including MEX and MEX-PC also support some of the YMODEM extensions. Questions about YMODEM, the Professional-YAM communications program, and requests for evaluation copies may be directed to: Chuck Forsberg Omen Technology Inc 17505-V Sauvie Island Road Portland Oregon 97231 Voice: 503-621-3406 Modem (Telegodzilla): 503-621-3746 Usenet: ...!tektronix!reed!omen!caf Compuserve: 70715,131 Source: TCE022 Yours very truly, Chapter 19 rev051486 Printed 5-16-86 27 CONTENTS 1. INTENDED AUDIENCE................................................ 2 2. ACKNOWLEDGMENTS.................................................. 2 3. RELATED DOCUMENTS................................................ 2 4. ROSETTA STONE.................................................... 2 5. WHY DEVELOP ZMODEM?.............................................. 3 6. ZMODEM Protocol Design Criteria.................................. 5 6.1 Ease of Use............................................... 5 6.2 Throughput................................................ 5 6.3 Integrity and Robustness.................................. 6 6.4 Ease of Implementation.................................... 6 7. ZMODEM BASICS.................................................... 6 7.1 Packetization............................................. 7 7.2 Link Escape Encoding...................................... 7 7.3 Header Packet Information................................. 8 7.4 Binary Header Packet...................................... 9 7.5 HEX Header Packet......................................... 9 7.6 Binary Data Packets....................................... 10 7.7 ASCII Encoded Data Packet................................. 10 8. PROTOCOL TRANSACTION OVERVIEW.................................... 10 8.1 Session Cancel Packet..................................... 13 9. ZMODEM STREAMING TECHNIQUES...................................... 14 9.1 Full Streaming with Sampling.............................. 14 9.2 Full Streaming with Interrupt............................. 14 9.3 Full Streaming with a Sliding Window...................... 15 9.4 No Streaming.............................................. 15 10. ATTENTION SEQUENCE............................................... 15 11. PACKET/FRAME TYPES............................................... 15 11.1 ZRQINIT................................................... 16 11.2 ZRINIT.................................................... 16 11.3 ZSINIT.................................................... 16 11.4 ZACK...................................................... 16 11.5 ZFILE..................................................... 16 11.6 ZSKIP..................................................... 18 11.7 ZNAK...................................................... 18 11.8 ZABORT.................................................... 18 11.9 ZFIN...................................................... 18 11.10 ZRPOS..................................................... 18 11.11 ZDATA..................................................... 18 - i - 11.12 ZEOF...................................................... 18 11.13 ZFERR..................................................... 18 11.14 ZCRC...................................................... 18 11.15 ZCHALLENGE................................................ 19 11.16 ZCOMPL.................................................... 19 11.17 ZCAN...................................................... 19 11.18 ZFREECNT.................................................. 19 11.19 ZCOMMAND.................................................. 19 12. TRANSACTION EXAMPLE.............................................. 20 12.1 A simple file transfer.................................... 20 12.2 Challenge and Command Download............................ 20 13. ZFILE FRAME FILE INFORMATION..................................... 21 14. PERFORMANCE RESULTS.............................................. 23 14.1 Throughput................................................ 23 14.2 Error Recovery............................................ 23 15. PACKET SWITCHED NETWORK CONSIDERATIONS........................... 24 16. PERFORMANCE COMPARISION TABLES................................... 25 17. MORE INFORMATION................................................. 26 18. ZMODEM PROGRAMS.................................................. 27 19. YMODEM PROGRAMS.................................................. 27 - ii - LIST OF FIGURES Figure 1. Order of Bytes in Header Packet............................ 8 Figure 2. Binary Header Packet....................................... 9 Figure 3. HEX Header Packet.......................................... 10 Figure 4. Flow Control Compatibility................................. 24 Figure 5. Protocol Overhead Information.............................. 25 Figure 6. Transmission Time Comparision.............................. 26 Figure 7. Y/ZMODEM Header Information usage.......................... 26 - iii - The ZMODEM Asynchronous Inter Application File Transfer Protocol Chuck Forsberg Omen Technology Inc ABSTRACT The ZMODEM file transfer protocol greatly simplifies file transfers compared to XMODEM. In addition to supporting a transparent user interface, ZMODEM provides Personal Computer and other users an efficient, accurate, robust file transfer method. ZMODEM provides especially efficient file transfers with timesharing systems, satellite relays, and wide area packet switched networks. A choice of buffering and windowing modes allow ZMODEM to operate efficiently on systems that cannot support some other streaming protocols. ZMODEM provides advanced file management features including AutoDownload, remote file compare, aborted transfer recovery, selective file transfers, and security verified command downloading.