YMODEM.docx

1、YMODEM - 1 - XMODEM/YMODEM PROTOCOL REFERENCE A compendium of documents describing the XMODEM and YMODEM File Transfer Protocols This document was formatted 8-4-87. Edited by Chuck Forsberg Please distribute as widely as possible. Questions to Chuck Forsberg Omen Technology Inc The High Reliability

2、Software 17505-V Sauvie Island Road Portland Oregon 97231 VOICE: 503-621-3406 :VOICE Modem (TeleGodzilla): 503-621-3746 Speed 19200,2400,1200,300 CompuServe: 70007,2304 GEnie: CAF UUCP: .!tektronix!reed!omen!caf - 2 - 1. TOWER OF BABEL A YMODEM Tower of Babel has descended on the microcomputing comm

3、unity bringing with it confusion, frustration, bloated phone bills, and wasted man hours. Sadly, I (Chuck Forsberg) am partly to blame for this mess. As author of the early 1980s batch and 1k XMODEM extensions, I assumed readers of earlier versions of this document would implement as much of the YMO

4、DEM protocol as their programming skills and computing environments would permit. This proved a rather naive assumption as programmers motivated by competitive pressure implemented as little of YMODEM as possible. Some have taken whatever parts of YMODEM that appealed to them, applied them to MODEM7

5、 Batch, Telink, XMODEM or whatever, and called the result YMODEM. Jeff Garbers (Crosstalk package development director) said it all: With protocols in the public domain, anyone who wants to dink around with them can go ahead. 1 Documents containing altered examples derived from YMODEM.DOC have added

6、 to the confusion. In one instance, the heading in YMODEM.DOCs Figure 1 has mutated from 1024 byte Packets to YMODEM/CRC File Transfer Protocol. None of the XMODEM and YMODEM examples shown in one document were correct. To put an end to this confusion, we must make perfectly clear what YMODEM stands

7、 for, as Ward Christensen defined it in his 1985 coining of the term. To the majority of you who read, understood, and respected Wards definition of YMODEM, I apologize for the inconvenience. 1.1 Definitions ARC ARC is a program that compresses one or more files into an archive and extracts files fr

8、om such archives. XMODEM refers to the file transfer etiquette introduced by Ward Christensens 1977 MODEM.ASM program. The name XMODEM comes from Keith Petersens XMODEM.ASM program, an adaptation of MODEM.ASM for Remote CP/M (RCPM) systems. Its also called the MODEM or MODEM2 protocol. Some who are

9、unaware of MODEM7s unusual batch file mode call it MODEM7. Other aliases include CP/M Users Group and TERM II FTP 3. The name XMODEM caught on partly because it is distinctive and partly because of media interest in _ 1. Page C/12, PC-WEEK July 12, 1987 Chapter 1 X/YMODEM Protocol Reference 08-03-87

10、 3 bulletin board and RCPM systems where it was accessed with an XMODEM command. This protocol is supported by every serious communications program because of its universality, simplicity, and reasonable performance. XMODEM/CRC replaces XMODEMs 1 byte checksum with a two byte Cyclical Redundancy Che

11、ck (CRC-16), giving modern error detection protection. XMODEM-1k Refers to the XMODEM/CRC protocol with 1024 byte data blocks. YMODEM Refers to the XMODEM/CRC (optional 1k blocks) protocol with batch transmission as described below. True YMODEM(TM) In an attempt to sort out the YMODEM Tower of Babel

12、, Omen Technology has trademarked the term True YMODEM(TM) to represent the complete YMODEM protocol described in this document, including pathname, length, and modification date transmitted in block 0. Please contact Omen Technology about certifying programs for True YMODEM(TM) compliance. ZMODEM u

13、ses familiar XMODEM/CRC and YMODEM technology in a new protocol that provides reliability, throughput, file management, and user amenities appropriate to contemporary data communications. ZOO Like ARC, ZOO is a program that compresses one or more files into a zoo archive. ZOO supports many different

14、 operating systems including Unix and VMS. Chapter 1 X/YMODEM Protocol Reference 08-03-87 4 2. YMODEM MINIMUM REQUIREMENTS All programs claiming to support YMODEM must meet the following minimum requirements: + The sending program shall send the pathname (file name) in block 0. + The pathname shall

15、be a null terminated ASCII string as described below. + The receiving program shall use this pathname for the received file name, unless explicitly overridden. + The sending program shall use CRC-16 in response to a C pathname nak, otherwise use 8 bit checksum. + The receiving program must accept an

16、y mixture of 128 and 1024 byte blocks it receives. + The sending program must not change the length of an unacknowledged block. + At the end of each file, the sending program shall send EOT up to ten times until it receives an ACK character. (This is part of the XMODEM spec.) + The end of a transfer

17、 session shall be signified by a null (empty) pathname. Programs not meeting all of these requirements are not YMODEM compatible. Meeting the minimum requirements does not guarantee reliable file transfers under stress. Chapter 2 X/YMODEM Protocol Reference 08-03-87 5 3. WHY YMODEM? Since its develo

18、pment 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 doesnt at least claim to support this protocol. Recent advances in computing, modems and networking have revealed a number of

19、 weaknesses in the original protocol: + The short block length caused throughput to suffer when used with timesharing systems, packet switched networks, satellite circuits, and buffered (error correcting) modems. + The 8 bit arithmetic checksum and other aspects allowed line impairments to interfere

20、 with dependable, accurate transfers. + Only one file could be sent per command. The file name had to be given twice, first to the sending program and then again to the receiving program. + The transmitted file could accumulate as many as 127 extraneous bytes. + The modification date of the file was

21、 lost. 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 Blast, Relay, and others tightly bound to the fortunes of their suppliers. + Complexity di

22、scourages the widespread application of BISYNC, SDLC, HDLC, X.25, and X.PC protocols. + Performance compromises and complexity have limited the popularity of the Kermit protocol, which was developed to allow file transfers in environments hostile to XMODEM. The XMODEM protocol extensions and YMODEM

23、Batch address some of these weaknesses while maintaining most of XMODEMs simplicity. YMODEM is supported by the public domain programs YAM (CP/M), YAM(CP/M-86), YAM(CCPM-86), IMP (CP/M), KMD (CP/M), rz/sz (Unix, Xenix, VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus). Commercial implement

24、ations include MIRROR, and Professional-YAM.1 Communications Chapter 3 X/YMODEM Protocol Reference 08-03-87 6 programs supporting these extensions have been in use since 1981. The 1k block length (XMODEM-1k) described below may be used in conjunction with YMODEM Batch Protocol, or with single file t

25、ransfers identical to the XMODEM/CRC protocol except for minimal changes to support 1k blocks. Another extension is the YMODEM-g protocol. YMODEM-g provides maximum throughput when used with end to end error correcting media, such as X.PC and error correcting modems, including 9600 bps units by Tele

26、Bit, U.S.Robotics, Hayes, Electronic Vaults, Data Race, and others. To complete this tome, edited versions of Ward Christensens original protocol document and John Byrnss CRC-16 document are included for reference. References to the MODEM or MODEM7 protocol have been changed to XMODEM to accommodate

27、 the vernacular. In Australia, it is properly called the Christensen Protocol. 3.1 Some Messages from the Pioneer #: 130940 S0/Communications 25-Apr-85 18:38:47 Sb: my protocol Fm: Ward Christensen 76703,302 2 To: all Be aware the article3 DID quote me correctly in terms of the phrases like not robu

28、st, etc. It was a quick hack I threw together, very unplanned (like everything I do), to satisfy a personal need to communicate with some other people. ONLY the fact that it was done in 8/77, and that I put it in the public domain immediately, made it become the standard that it is. I think its time

29、 for me to (1) document it; (people call me and say my product is going to include it - what can I reference, or Im writing a paper on it, what do I put in the bibliography) and _ 1. Available for IBM PC,XT,AT, Unix and Xenix 2. Edited for typesetting appearance 3. Infoworld April 29 p. 16 Chapter 3

30、 X/YMODEM Protocol Reference 08-03-87 7 (2) propose an incremental extension to it, which might take exactly the form of Chuck Forsbergs YAM protocol. He wrote YAM in C for CP/M and put it in the public domain, and wrote a batch protocol for Unix4 called rb and sb (receive batch, send batch), which

31、was basically XMODEM with (a) a record 0 containing filename date time and size (b) a 1K block size option (c) CRC-16. He did some clever programming to detect false ACK or EOT, but basically left them the same. People who suggest I make SIGNIFICANT changes to the protocol, such as full duplex, multiple outstanding blocks, multiple destinations, etc etc dont understand that the incred