1 2 3 4 - 1 - 5 6 7 8 XMODEM/YMODEM PROTOCOL REFERENCE 9 A compendium of documents describing the 10 11 XMODEM and YMODEM 12 13 File Transfer Protocols 14 15 16 17 18 This document was formatted 10-14-88. 19 20 21 22 23 24 25 26 Edited by Chuck Forsberg 27 28 29 30 31 32 33 34 35 36 This file may be redistributed without restriction 37 provided the text is not altered. 38 39 Please distribute as widely as possible. 40 41 Questions to Chuck Forsberg 42 43 44 45 46 47 Omen Technology Inc 48 The High Reliability Software 49 17505-V Sauvie Island Road 50 Portland Oregon 97231 51 VOICE: 503-621-3406 :VOICE 52 TeleGodzilla BBS: 503-621-3746 Speed 19200(Telebit PEP),2400,1200,300 53 CompuServe: 70007,2304 54 GEnie: CAF 55 UUCP: ...!tektronix!reed!omen!caf 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 - 2 - 71 72 73 74 1. TOWER OF BABEL 75 76 A "YMODEM Tower of Babel" has descended on the microcomputing community 77 bringing with it confusion, frustration, bloated phone bills, and wasted 78 man hours. Sadly, I (Chuck Forsberg) am partly to blame for this mess. 79 80 As author of the early 1980s batch and 1k XMODEM extensions, I assumed 81 readers of earlier versions of this document would implement as much of 82 the YMODEM protocol as their programming skills and computing environments 83 would permit. This proved a rather naive assumption as programmers 84 motivated by competitive pressure implemented as little of YMODEM as 85 possible. Some have taken whatever parts of YMODEM that appealed to them, 86 applied them to MODEM7 Batch, Telink, XMODEM or whatever, and called the 87 result YMODEM. 88 89 Jeff Garbers (Crosstalk package development director) said it all: "With 90 protocols in the public domain, anyone who wants to dink around with them 91 can go ahead." [1] 92 93 Documents containing altered examples derived from YMODEM.DOC have added 94 to the confusion. In one instance, some self styled rewriter of history 95 altered the heading in YMODEM.DOC's Figure 1 from "1024 byte Packets" to 96 "YMODEM/CRC File Transfer Protocol". None of the XMODEM and YMODEM 97 examples shown in that document were correct. 98 99 To put an end to this confusion, we must make "perfectly clear" what 100 YMODEM stands for, as Ward Christensen defined it in his 1985 coining of 101 the term. 102 103 To the majority of you who read, understood, and respected Ward's 104 definition of YMODEM, I apologize for the inconvenience. 105 106 1.1 Definitions 107 108 ARC ARC is a program that compresses one or more files into an archive 109 and extracts files from such archives. 110 111 XMODEM refers to the file transfer etiquette introduced by Ward 112 Christensen's 1977 MODEM.ASM program. The name XMODEM comes from 113 Keith Petersen's XMODEM.ASM program, an adaptation of MODEM.ASM 114 for Remote CP/M (RCPM) systems. It's also called the MODEM or 115 MODEM2 protocol. Some who are unaware of MODEM7's unusual batch 116 file mode call it MODEM7. Other aliases include "CP/M Users' 117 Group" and "TERM II FTP 3". The name XMODEM caught on partly 118 because it is distinctive and partly because of media interest in 119 120 121 __________ 122 123 1. Page C/12, PC-WEEK July 12, 1987 124 125 126 127 128 Chapter 1 129 130 131 132 133 134 135 136 X/YMODEM Protocol Reference June 18 1988 3 137 138 139 140 bulletin board and RCPM systems where it was accessed with an 141 "XMODEM" command. This protocol is supported by every serious 142 communications program because of its universality, simplicity, 143 and reasonable performance. 144 145 XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte Cyclical 146 Redundancy Check (CRC-16), giving modern error detection 147 protection. 148 149 XMODEM-1k Refers to the XMODEM/CRC protocol with 1024 byte data blocks. 150 151 YMODEM Refers to the XMODEM/CRC (optional 1k blocks) protocol with batch 152 transmission as described below. In a nutshell, YMODEM means 153 BATCH. 154 155 YMODEM-g Refers to the streaming YMODEM variation described below. 156 157 True YMODEM(TM) In an attempt to sort out the YMODEM Tower of Babel, Omen 158 Technology has trademarked the term True YMODEM(TM) to represent 159 the complete YMODEM protocol described in this document, including 160 pathname, length, and modification date transmitted in block 0. 161 Please contact Omen Technology about certifying programs for True 162 YMODEM(TM) compliance. 163 164 ZMODEM uses familiar XMODEM/CRC and YMODEM technology in a new protocol 165 that provides reliability, throughput, file management, and user 166 amenities appropriate to contemporary data communications. 167 168 ZOO Like ARC, ZOO is a program that compresses one or more files into 169 a "zoo archive". ZOO supports many different operating systems 170 including Unix and VMS. 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 Chapter 1 195 196 197 198 199 200 201 202 X/YMODEM Protocol Reference June 18 1988 4 203 204 205 206 2. YMODEM MINIMUM REQUIREMENTS 207 208 All programs claiming to support YMODEM must meet the following minimum 209 requirements: 210 211 + The sending program shall send the pathname (file name) in block 0. 212 213 + The pathname shall be a null terminated ASCII string as described 214 below. 215 216 For those who are too lazy to read the entire document: 217 218 + Unless specifically requested, only the file name portion is 219 sent. 220 221 + No drive letter is sent. 222 223 + Systems that do not distinguish between upper and lower case 224 letters in filenames shall send the pathname in lower case only. 225 226 227 + The receiving program shall use this pathname for the received file 228 name, unless explicitly overridden. 229 230 + When the receiving program receives this block and successfully 231 opened the output file, it shall acknowledge this block with an ACK 232 character and then proceed with a normal XMODEM file transfer 233 beginning with a "C" or NAK tranmsitted by the receiver. 234 235 + The sending program shall use CRC-16 in response to a "C" pathname 236 nak, otherwise use 8 bit checksum. 237 238 + The receiving program must accept any mixture of 128 and 1024 byte 239 blocks within each file it receives. Sending programs may 240 arbitrarily switch between 1024 and 128 byte blocks. 241 242 + The sending program must not change the length of an unacknowledged 243 block. 244 245 + At the end of each file, the sending program shall send EOT up to ten 246 times until it receives an ACK character. (This is part of the 247 XMODEM spec.) 248 249 + The end of a transfer session shall be signified by a null (empty) 250 pathname, this pathname block shall be acknowledged the same as other 251 pathname blocks. 252 253 Programs not meeting all of these requirements are not YMODEM compatible, 254 and shall not be described as supporting YMODEM. 255 256 Meeting these MINIMUM requirements does not guarantee reliable file 257 258 259 260 Chapter 2 261 262 263 264 265 266 267 268 X/YMODEM Protocol Reference June 18 1988 5 269 270 271 272 transfers under stress. Particular attention is called to XMODEM's single 273 character supervisory messages that are easily corrupted by transmission 274 errors. 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 Chapter 2 327 328 329 330 331 332 333 334 X/YMODEM Protocol Reference June 18 1988 6 335 336 337 338 3. WHY YMODEM? 339 340 Since its development half a decade ago, the Ward Christensen modem 341 protocol has enabled a wide variety of computer systems to interchange 342 data. There is hardly a communications program that doesn't at least 343 claim to support this protocol. 344 345 Advances in computing, modems and networking have revealed a number of 346 weaknesses in the original protocol: 347 348 + The short block length caused throughput to suffer when used with 349 timesharing systems, packet switched networks, satellite circuits, 350 and buffered (error correcting) modems. 351 352 + The 8 bit arithmetic checksum and other aspects allowed line 353 impairments to interfere with dependable, accurate transfers. 354 355 + Only one file could be sent per command. The file name had to be 356 given twice, first to the sending program and then again to the 357 receiving program. 358 359 + The transmitted file could accumulate as many as 127 extraneous 360 bytes. 361 362 + The modification date of the file was lost. 363 364 A number of other protocols have been developed over the years, but none 365 have displaced XMODEM to date: 366 367 + Lack of public domain documentation and example programs have kept 368 proprietary protocols such as Blast, Relay, and others tightly bound 369 to the fortunes of their suppliers. 370 371 + Complexity discourages the widespread application of BISYNC, SDLC, 372 HDLC, X.25, and X.PC protocols. 373 374 + Performance compromises and complexity have limited the popularity of 375 the Kermit protocol, which was developed to allow file transfers in 376 environments hostile to XMODEM. 377 378 The XMODEM protocol extensions and YMODEM Batch address some of these 379 weaknesses while maintaining most of XMODEM's simplicity. 380 381 YMODEM is supported by the public domain programs YAM (CP/M), 382 YAM(CP/M-86), YAM(CCPM-86), IMP (CP/M), KMD (CP/M), rz/sz (Unix, Xenix, 383 VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus). Commercial 384 implementations include MIRROR, and Professional-YAM.[1] Communications 385 386 387 388 389 390 391 392 Chapter 3 393 394 395 396 397 398 399 400 X/YMODEM Protocol Reference June 18 1988 7 401 402 403 404 programs supporting these extensions have been in use since 1981. 405 406 The 1k block length (XMODEM-1k) described below may be used in conjunction 407 with YMODEM Batch Protocol, or with single file transfers identical to the 408 XMODEM/CRC protocol except for minimal changes to support 1k blocks. 409 410 Another extension is the YMODEM-g protocol. YMODEM-g provides batch 411 transfers with maximum throughput when used with end to end error 412 correcting media, such as X.PC and error correcting modems, including 9600 413 bps units by TeleBit, U.S.Robotics, Hayes, Electronic Vaults, Data Race, 414 and others. 415 416 To complete this tome, edited versions of Ward Christensen's original 417 protocol document and John Byrns's CRC-16 document are included for 418 reference. 419 420 References to the MODEM or MODEM7 protocol have been changed to XMODEM to 421 accommodate the vernacular. In Australia, it is properly called the 422 Christensen Protocol. 423 424 425 3.1 Some Messages from the Pioneer 426 427 #: 130940 S0/Communications 25-Apr-85 18:38:47 428 Sb: my protocol 429 Fm: Ward Christensen 76703,302 [2] 430 To: all 431 432 Be aware the article[3] DID quote me correctly in terms of the phrases 433 like "not robust", etc. 434 435 It was a quick hack I threw together, very unplanned (like everything I 436 do), to satisfy a personal need to communicate with "some other" people. 437 438 ONLY the fact that it was done in 8/77, and that I put it in the public 439 domain immediately, made it become the standard that it is. 440 441 442 443 444 445 446 447 __________________________________________________________________________ 448 449 1. Available for IBM PC,XT,AT, Unix and Xenix 450 451 2. Edited for typesetting appearance 452 453 3. Infoworld April 29 p. 16 454 455 456 457 458 Chapter 3 459 460 461 462 463 464 465 466 X/YMODEM Protocol Reference June 18 1988 8 467 468 469 470 I think its time for me to 471 472 (1) document it; (people call me and say "my product is going to include 473 it - what can I 'reference'", or "I'm writing a paper on it, what do I put 474 in the bibliography") and 475 476 (2) propose an "incremental extension" to it, which might take "exactly" 477 the form of Chuck Forsberg's YAM protocol. He wrote YAM in C for CP/M and 478 put it in the public domain, and wrote a batch protocol for Unix[4] called 479 rb and sb (receive batch, send batch), which was basically XMODEM with 480 (a) a record 0 containing filename date time and size 481 (b) a 1K block size option 482 (c) CRC-16. 483 484 He did some clever programming to detect false ACK or EOT, but basically 485 left them the same. 486 487 People who suggest I make SIGNIFICANT changes to the protocol, such as 488 "full duplex", "multiple outstanding blocks", "multiple destinations", etc 489 etc don't understand that the incredible simplicity of the protocol is one 490 of the reasons it survived to this day in as many machines and programs as 491 it may be found in! 492 493 Consider the PC-NET group back in '77 or so - documenting to beat the band 494 - THEY had a protocol, but it was "extremely complex", because it tried to 495 be "all things to all people" - i.e. send binary files on a 7-bit system, 496 etc. I was not that "benevolent". I (emphasize > I < ) had an 8-bit UART, 497 so "my protocol was an 8-bit protocol", and I would just say "sorry" to 498 people who were held back by 7-bit limitations. ... 499 500 Block size: Chuck Forsberg created an extension of my protocol, called 501 YAM, which is also supported via his public domain programs for UNIX 502 called rb and sb - receive batch and send batch. They cleverly send a 503 "block 0" which contains the filename, date, time, and size. 504 Unfortunately, its UNIX style, and is a bit weird[5] - octal numbers, etc. 505 BUT, it is a nice way to overcome the kludgy "echo the chars of the name" 506 introduced with MODEM7. Further, chuck uses CRC-16 and optional 1K 507 blocks. Thus the record 0, 1K, and CRC, make it a "pretty slick new 508 protocol" which is not significantly different from my own. 509 510 Also, there is a catchy name - YMODEM. That means to some that it is the 511 "next thing after XMODEM", and to others that it is the Y(am)MODEM 512 513 514 __________ 515 516 4. VAX/VMS versions of these programs are also available. 517 518 5. The file length, time, and file mode are optional. The pathname and 519 file length may be sent alone if desired. 520 521 522 523 524 Chapter 3 525 526 527 528 529 530 531 532 X/YMODEM Protocol Reference June 18 1988 9 533 534 535 536 protocol. I don't want to emphasize that too much - out of fear that 537 other mfgrs might think it is a "competitive" protocol, rather than an 538 "unaffiliated" protocol. Chuck is currently selling a much-enhanced 539 version of his CP/M-80 C program YAM, calling it Professional Yam, and its 540 for the PC - I'm using it right now. VERY slick! 32K capture buffer, 541 script, scrolling, previously captured text search, plus built-in commands 542 for just about everything - directory (sorted every which way), XMODEM, 543 YMODEM, KERMIT, and ASCII file upload/download, etc. You can program it 544 to "behave" with most any system - for example when trying a number for 545 CIS it detects the "busy" string back from the modem and substitutes a 546 diff phone # into the dialing string and branches back to try it. 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 Chapter 3 591 592 593 594 595 596 597 598 X/YMODEM Protocol Reference June 18 1988 10 599 600 601 602 4. XMODEM PROTOCOL ENHANCEMENTS 603 604 This chapter discusses the protocol extensions to Ward Christensen's 1982 605 XMODEM protocol description document. 606 607 The original document recommends the user be asked whether to continue 608 trying or abort after 10 retries. Most programs no longer ask the 609 operator whether he wishes to keep retrying. Virtually all correctable 610 errors are corrected within the first few retransmissions. If the line is 611 so bad that ten attempts are insufficient, there is a significant danger 612 of undetected errors. If the connection is that bad, it's better to 613 redial for a better connection, or mail a floppy disk. 614 615 616 4.1 Graceful Abort 617 618 The YAM and Professional-YAM X/YMODEM routines recognize a sequence of two 619 consecutive CAN (Hex 18) characters without modem errors (overrun, 620 framing, etc.) as a transfer abort command. This sequence is recognized 621 when is waiting for the beginning of a block or for an acknowledgement to 622 a block that has been sent. The check for two consecutive CAN characters 623 reduces the number of transfers aborted by line hits. YAM sends eight CAN 624 characters when it aborts an XMODEM, YMODEM, or ZMODEM protocol file 625 transfer. Pro-YAM then sends eight backspaces to delete the CAN 626 characters from the remote's keyboard input buffer, in case the remote had 627 already aborted the transfer and was awaiting a keyboarded command. 628 629 630 4.2 CRC-16 Option 631 632 The XMODEM protocol uses an optional two character CRC-16 instead of the 633 one character arithmetic checksum used by the original protocol and by 634 most commercial implementations. CRC-16 guarantees detection of all 635 single and double bit errors, all errors with an odd number of error 636 bits, all burst errors of length 16 or less, 99.9969% of all 17-bit error 637 bursts, and 99.9984 per cent of all possible longer error bursts. By 638 contrast, a double bit error, or a burst error of 9 bits or more can sneak 639 past the XMODEM protocol arithmetic checksum. 640 641 The XMODEM/CRC protocol is similar to the XMODEM protocol, except that the 642 receiver specifies CRC-16 by sending C (Hex 43) instead of NAK when 643 requesting the FIRST block. A two byte CRC is sent in place of the one 644 byte arithmetic checksum. 645 646 YAM's c option to the r command enables CRC-16 in single file reception, 647 corresponding to the original implementation in the MODEM7 series 648 programs. This remains the default because many commercial communications 649 programs and bulletin board systems still do not support CRC-16, 650 especially those written in Basic or Pascal. 651 652 XMODEM protocol with CRC is accurate provided both sender and receiver 653 654 655 656 Chapter 4 XMODEM Protocol Enhancements 657 658 659 660 661 662 663 664 X/YMODEM Protocol Reference June 18 1988 11 665 666 667 668 both report a successful transmission. The protocol is robust in the 669 presence of characters lost by buffer overloading on timesharing systems. 670 671 The single character ACK/NAK responses generated by the receiving program 672 adapt well to split speed modems, where the reverse channel is limited to 673 ten per cent or less of the main channel's speed. 674 675 XMODEM and YMODEM are half duplex protocols which do not attempt to 676 transmit information and control signals in both directions at the same 677 time. This avoids buffer overrun problems that have been reported by 678 users attempting to exploit full duplex asynchronous file transfer 679 protocols such as Blast. 680 681 Professional-YAM adds several proprietary logic enhancements to XMODEM's 682 error detection and recovery. These compatible enhancements eliminate 683 most of the bad file transfers other programs make when using the XMODEM 684 protocol under less than ideal conditions. 685 686 687 4.3 XMODEM-1k 1024 Byte Block 688 689 Disappointing throughput downloading from Unix with YMODEM[1] lead to the 690 development of 1024 byte blocks in 1982. 1024 byte blocks reduce the 691 effect of delays from timesharing systems, modems, and packet switched 692 networks on throughput by 87.5 per cent in addition to decreasing XMODEM's 693 3 per cent overhead (block number, CRC, etc.). 694 695 Some environments cannot accept 1024 byte bursts, including some networks 696 and minicomputer ports. The longer block length should be an option. 697 698 The choice to use 1024 byte blocks is expressed to the sending program on 699 its command line or selection menu.[2] 1024 byte blocks improve throughput 700 in many applications. 701 702 An STX (02) replaces the SOH (01) at the beginning of the transmitted 703 block to notify the receiver of the longer block length. The transmitted 704 block contains 1024 bytes of data. The receiver should be able to accept 705 any mixture of 128 and 1024 byte blocks. The block number (in the second 706 and third bytes of the block) is incremented by one for each block 707 regardless of the block length. 708 709 The sender must not change between 128 and 1024 byte block lengths if it 710 has not received a valid ACK for the current block. Failure to observe 711 712 713 __________ 714 715 1. The name hadn't been coined yet, but the protocol was the same. 716 717 2. See "KMD/IMP Exceptions to YMODEM" below. 718 719 720 721 722 Chapter 4 XMODEM Protocol Enhancements 723 724 725 726 727 728 729 730 X/YMODEM Protocol Reference June 18 1988 12 731 732 733 734 this restriction allows transmission errors to pass undetected. 735 736 If 1024 byte blocks are being used, it is possible for a file to "grow" up 737 to the next multiple of 1024 bytes. This does not waste disk space if the 738 allocation granularity is 1k or greater. With YMODEM batch transmission, 739 the optional file length transmitted in the file name block allows the 740 receiver to discard the padding, preserving the exact file length and 741 contents. 742 743 1024 byte blocks may be used with batch file transmission or with single 744 file transmission. CRC-16 should be used with the k option to preserve 745 data integrity over phone lines. If a program wishes to enforce this 746 recommendation, it should cancel the transfer, then issue an informative 747 diagnostic message if the receiver requests checksum instead of CRC-16. 748 749 Under no circumstances may a sending program use CRC-16 unless the 750 receiver commands CRC-16. 751 752 Figure 1. XMODEM-1k Blocks 753 754 SENDER RECEIVER 755 "sx -k foo.bar" 756 "foo.bar open x.x minutes" 757 C 758 STX 01 FE Data[1024] CRC CRC 759 ACK 760 STX 02 FD Data[1024] CRC CRC 761 ACK 762 STX 03 FC Data[1000] CPMEOF[24] CRC CRC 763 ACK 764 EOT 765 ACK 766 767 Figure 2. Mixed 1024 and 128 byte Blocks 768 769 SENDER RECEIVER 770 "sx -k foo.bar" 771 "foo.bar open x.x minutes" 772 C 773 STX 01 FE Data[1024] CRC CRC 774 ACK 775 STX 02 FD Data[1024] CRC CRC 776 ACK 777 SOH 03 FC Data[128] CRC CRC 778 ACK 779 SOH 04 FB Data[100] CPMEOF[28] CRC CRC 780 ACK 781 EOT 782 ACK 783 784 785 786 787 788 Chapter 4 XMODEM Protocol Enhancements 789 790 791 792 793 794 795 796 X/YMODEM Protocol Reference June 18 1988 13 797 798 799 800 5. YMODEM Batch File Transmission 801 802 The YMODEM Batch protocol is an extension to the XMODEM/CRC protocol that 803 allows 0 or more files to be transmitted with a single command. (Zero 804 files may be sent if none of the requested files is accessible.) The 805 design approach of the YMODEM Batch protocol is to use the normal routines 806 for sending and receiving XMODEM blocks in a layered fashion similar to 807 packet switching methods. 808 809 Why was it necessary to design a new batch protocol when one already 810 existed in MODEM7?[1] The batch file mode used by MODEM7 is unsuitable 811 because it does not permit full pathnames, file length, file date, or 812 other attribute information to be transmitted. Such a restrictive design, 813 hastily implemented with only CP/M in mind, would not have permitted 814 extensions to current areas of personal computing such as Unix, DOS, and 815 object oriented systems. In addition, the MODEM7 batch file mode is 816 somewhat susceptible to transmission impairments. 817 818 As in the case of single a file transfer, the receiver initiates batch 819 file transmission by sending a "C" character (for CRC-16). 820 821 The sender opens the first file and sends block number 0 with the 822 following information.[2] 823 824 Only the pathname (file name) part is required for batch transfers. 825 826 To maintain upwards compatibility, all unused bytes in block 0 must be set 827 to null. 828 829 Pathname The pathname (conventionally, the file name) is sent as a null 830 terminated ASCII string. This is the filename format used by the 831 handle oriented MSDOS(TM) functions and C library fopen functions. 832 An assembly language example follows: 833 DB 'foo.bar',0 834 No spaces are included in the pathname. Normally only the file name 835 stem (no directory prefix) is transmitted unless the sender has 836 selected YAM's f option to send the full pathname. The source drive 837 (A:, B:, etc.) is not sent. 838 839 Filename Considerations: 840 841 842 843 __________ 844 845 1. The MODEM7 batch protocol transmitted CP/M FCB bytes f1...f8 and 846 t1...t3 one character at a time. The receiver echoed these bytes as 847 received, one at a time. 848 849 2. Only the data part of the block is described here. 850 851 852 853 854 Chapter 5 XMODEM Protocol Enhancements 855 856 857 858 859 860 861 862 X/YMODEM Protocol Reference June 18 1988 14 863 864 865 866 + File names are forced to lower case unless the sending system 867 supports upper/lower case file names. This is a convenience for 868 users of systems (such as Unix) which store filenames in upper 869 and lower case. 870 871 + The receiver should accommodate file names in lower and upper 872 case. 873 874 + When transmitting files between different operating systems, 875 file names must be acceptable to both the sender and receiving 876 operating systems. 877 878 If directories are included, they are delimited by /; i.e., 879 "subdir/foo" is acceptable, "subdir\foo" is not. 880 881 Length The file length and each of the succeeding fields are optional.[3] 882 The length field is stored in the block as a decimal string counting 883 the number of data bytes in the file. The file length does not 884 include any CPMEOF (^Z) or other garbage characters used to pad the 885 last block. 886 887 If the file being transmitted is growing during transmission, the 888 length field should be set to at least the final expected file 889 length, or not sent. 890 891 The receiver stores the specified number of characters, discarding 892 any padding added by the sender to fill up the last block. 893 894 Modification Date The mod date is optional, and the filename and length 895 may be sent without requiring the mod date to be sent. 896 897 Iff the modification date is sent, a single space separates the 898 modification date from the file length. 899 900 The mod date is sent as an octal number giving the time the contents 901 of the file were last changed, measured in seconds from Jan 1 1970 902 Universal Coordinated Time (GMT). A date of 0 implies the 903 modification date is unknown and should be left as the date the file 904 is received. 905 906 This standard format was chosen to eliminate ambiguities arising from 907 transfers between different time zones. 908 909 910 911 912 913 __________ 914 915 3. Fields may not be skipped. 916 917 918 919 920 Chapter 5 XMODEM Protocol Enhancements 921 922 923 924 925 926 927 928 X/YMODEM Protocol Reference June 18 1988 15 929 930 931 932 Mode Iff the file mode is sent, a single space separates the file mode 933 from the modification date. The file mode is stored as an octal 934 string. Unless the file originated from a Unix system, the file mode 935 is set to 0. rb(1) checks the file mode for the 0x8000 bit which 936 indicates a Unix type regular file. Files with the 0x8000 bit set 937 are assumed to have been sent from another Unix (or similar) system 938 which uses the same file conventions. Such files are not translated 939 in any way. 940 941 942 Serial Number Iff the serial number is sent, a single space separates the 943 serial number from the file mode. The serial number of the 944 transmitting program is stored as an octal string. Programs which do 945 not have a serial number should omit this field, or set it to 0. The 946 receiver's use of this field is optional. 947 948 949 Other Fields YMODEM was designed to allow additional header fields to be 950 added as above without creating compatibility problems with older 951 YMODEM programs. Please contact Omen Technology if other fields are 952 needed for special application requirements. 953 954 The rest of the block is set to nulls. This is essential to preserve 955 upward compatibility.[4] 956 957 If the filename block is received with a CRC or other error, a 958 retransmission is requested. After the filename block has been received, 959 it is ACK'ed if the write open is successful. If the file cannot be 960 opened for writing, the receiver cancels the transfer with CAN characters 961 as described above. 962 963 The receiver then initiates transfer of the file contents with a "C" 964 character, according to the standard XMODEM/CRC protocol. 965 966 After the file contents and XMODEM EOT have been transmitted and 967 acknowledged, the receiver again asks for the next pathname. 968 969 Transmission of a null pathname terminates batch file transmission. 970 971 Note that transmission of no files is not necessarily an error. This is 972 possible if none of the files requested of the sender could be opened for 973 reading. 974 975 976 977 __________ 978 979 4. If, perchance, this information extends beyond 128 bytes (possible 980 with Unix 4.2 BSD extended file names), the block should be sent as a 981 1k block as described above. 982 983 984 985 986 Chapter 5 XMODEM Protocol Enhancements 987 988 989 990 991 992 993 994 X/YMODEM Protocol Reference June 18 1988 16 995 996 997 998 Most YMODEM receivers request CRC-16 by default. 999 1000 The Unix programs sz(1) and rz(1) included in the source code file 1001 RZSZ.ZOO should answer other questions about YMODEM batch protocol. 1002 1003 Figure 3. YMODEM Batch Transmission Session (1 file) 1004 1005 SENDER RECEIVER 1006 "sb foo.*<CR>" 1007 "sending in batch mode etc." 1008 C (command:rb) 1009 SOH 00 FF foo.c NUL[123] CRC CRC 1010 ACK 1011 C 1012 SOH 01 FE Data[128] CRC CRC 1013 ACK 1014 SOH 02 FC Data[128] CRC CRC 1015 ACK 1016 SOH 03 FB Data[100] CPMEOF[28] CRC CRC 1017 ACK 1018 EOT 1019 NAK 1020 EOT 1021 ACK 1022 C 1023 SOH 00 FF NUL[128] CRC CRC 1024 ACK 1025 1026 Figure 7. YMODEM Header Information and Features 1027 1028 _____________________________________________________________ 1029 | Program | Length | Date | Mode | S/N | 1k-Blk | YMODEM-g | 1030 |___________|________|______|______|_____|________|__________| 1031 |Unix rz/sz | yes | yes | yes | no | yes | sb only | 1032 |___________|________|______|______|_____|________|__________| 1033 |VMS rb/sb | yes | no | no | no | yes | no | 1034 |___________|________|______|______|_____|________|__________| 1035 |Pro-YAM | yes | yes | no | yes | yes | yes | 1036 |___________|________|______|______|_____|________|__________| 1037 |CP/M YAM | no | no | no | no | yes | no | 1038 |___________|________|______|______|_____|________|__________| 1039 |KMD/IMP | ? | no | no | no | yes | no | 1040 |___________|________|______|______|_____|________|__________| 1041 1042 5.1 KMD/IMP Exceptions to YMODEM 1043 1044 KMD and IMP use a "CK" character sequence emitted by the receiver to 1045 trigger the use of 1024 byte blocks as an alternative to specifying this 1046 option to the sending program. This two character sequence generally 1047 works well on single process micros in direct communication, provided the 1048 programs rigorously adhere to all the XMODEM recommendations included 1049 1050 1051 1052 Chapter 5 XMODEM Protocol Enhancements 1053 1054 1055 1056 1057 1058 1059 1060 X/YMODEM Protocol Reference June 18 1988 17 1061 1062 1063 1064 Figure 4. YMODEM Batch Transmission Session (2 files) 1065 1066 SENDER RECEIVER 1067 "sb foo.c baz.c<CR>" 1068 "sending in batch mode etc." 1069 C (command:rb) 1070 SOH 00 FF foo.c NUL[123] CRC CRC 1071 ACK 1072 C 1073 SOH 01 FE Data[128] CRC CRC 1074 ACK 1075 SOH 02 FC Data[128] CRC CRC 1076 ACK 1077 SOH 03 FB Data[100] CPMEOF[28] CRC CRC 1078 ACK 1079 EOT 1080 NAK 1081 EOT 1082 ACK 1083 C 1084 SOH 00 FF baz.c NUL[123] CRC CRC 1085 ACK 1086 C 1087 SOH 01 FB Data[100] CPMEOF[28] CRC CRC 1088 ACK 1089 EOT 1090 NAK 1091 EOT 1092 ACK 1093 C 1094 SOH 00 FF NUL[128] CRC CRC 1095 ACK 1096 1097 Figure 5. YMODEM Batch Transmission Session-1k Blocks 1098 1099 SENDER RECEIVER 1100 "sb -k foo.*<CR>" 1101 "sending in batch mode etc." 1102 C (command:rb) 1103 SOH 00 FF foo.c NUL[123] CRC CRC 1104 ACK 1105 C 1106 STX 01 FD Data[1024] CRC CRC 1107 ACK 1108 SOH 02 FC Data[128] CRC CRC 1109 ACK 1110 SOH 03 FB Data[100] CPMEOF[28] CRC CRC 1111 ACK 1112 EOT 1113 NAK 1114 EOT 1115 1116 1117 1118 Chapter 5 XMODEM Protocol Enhancements 1119 1120 1121 1122 1123 1124 1125 1126 X/YMODEM Protocol Reference June 18 1988 18 1127 1128 1129 1130 ACK 1131 C 1132 SOH 00 FF NUL[128] CRC CRC 1133 ACK 1134 1135 Figure 6. YMODEM Filename block transmitted by sz 1136 1137 -rw-r--r-- 6347 Jun 17 1984 20:34 bbcsched.txt 1138 1139 00 0100FF62 62637363 6865642E 74787400 |...bbcsched.txt.| 1140 10 36333437 20333331 34373432 35313320 |6347 3314742513 | 1141 20 31303036 34340000 00000000 00000000 |100644..........| 1142 30 00000000 00000000 00000000 00000000 1143 40 00000000 00000000 00000000 00000000 1144 50 00000000 00000000 00000000 00000000 1145 60 00000000 00000000 00000000 00000000 1146 70 00000000 00000000 00000000 00000000 1147 80 000000CA 56 1148 1149 herein. Programs with marginal XMODEM implementations do not fare so 1150 well. Timesharing systems and packet switched networks can separate the 1151 successive characters, rendering this method unreliable. 1152 1153 Sending programs may detect the CK sequence if the operating enviornment 1154 does not preclude reliable implementation. 1155 1156 Instead of the standard YMODEM file length in decimal, KMD and IMP 1157 transmit the CP/M record count in the last two bytes of the header block. 1158 1159 1160 6. YMODEM-g File Transmission 1161 1162 Developing technology is providing phone line data transmission at ever 1163 higher speeds using very specialized techniques. These high speed modems, 1164 as well as session protocols such as X.PC, provide high speed, nearly 1165 error free communications at the expense of considerably increased delay 1166 time. 1167 1168 This delay time is moderate compared to human interactions, but it 1169 cripples the throughput of most error correcting protocols. 1170 1171 The g option to YMODEM has proven effective under these circumstances. 1172 The g option is driven by the receiver, which initiates the batch transfer 1173 by transmitting a G instead of C. When the sender recognizes the G, it 1174 bypasses the usual wait for an ACK to each transmitted block, sending 1175 succeeding blocks at full speed, subject to XOFF/XON or other flow control 1176 exerted by the medium. 1177 1178 The sender expects an inital G to initiate the transmission of a 1179 particular file, and also expects an ACK on the EOT sent at the end of 1180 each file. This synchronization allows the receiver time to open and 1181 1182 1183 1184 Chapter 6 XMODEM Protocol Enhancements 1185 1186 1187 1188 1189 1190 1191 1192 X/YMODEM Protocol Reference June 18 1988 19 1193 1194 1195 1196 close files as necessary. 1197 1198 If an error is detected in a YMODEM-g transfer, the receiver aborts the 1199 transfer with the multiple CAN abort sequence. The ZMODEM protocol should 1200 be used in applications that require both streaming throughput and error 1201 recovery. 1202 1203 Figure 8. YMODEM-g Transmission Session 1204 1205 SENDER RECEIVER 1206 "sb foo.*<CR>" 1207 "sending in batch mode etc..." 1208 G (command:rb -g) 1209 SOH 00 FF foo.c NUL[123] CRC CRC 1210 G 1211 SOH 01 FE Data[128] CRC CRC 1212 STX 02 FD Data[1024] CRC CRC 1213 SOH 03 FC Data[128] CRC CRC 1214 SOH 04 FB Data[100] CPMEOF[28] CRC CRC 1215 EOT 1216 ACK 1217 G 1218 SOH 00 FF NUL[128] CRC CRC 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 Chapter 6 XMODEM Protocol Enhancements 1251 1252 1253 1254 1255 1256 1257 1258 X/YMODEM Protocol Reference June 18 1988 20 1259 1260 1261 1262 7. XMODEM PROTOCOL OVERVIEW 1263 1264 8/9/82 by Ward Christensen. 1265 1266 I will maintain a master copy of this. Please pass on changes or 1267 suggestions via CBBS/Chicago at (312) 545-8086, CBBS/CPMUG (312) 849-1132 1268 or by voice at (312) 849-6279. 1269 1270 7.1 Definitions 1271 1272 <soh> 01H 1273 <eot> 04H 1274 <ack> 06H 1275 <nak> 15H 1276 <can> 18H 1277 <C> 43H 1278 1279 1280 7.2 Transmission Medium Level Protocol 1281 1282 Asynchronous, 8 data bits, no parity, one stop bit. 1283 1284 The protocol imposes no restrictions on the contents of the data being 1285 transmitted. No control characters are looked for in the 128-byte data 1286 messages. Absolutely any kind of data may be sent - binary, ASCII, etc. 1287 The protocol has not formally been adopted to a 7-bit environment for the 1288 transmission of ASCII-only (or unpacked-hex) data , although it could be 1289 simply by having both ends agree to AND the protocol-dependent data with 1290 7F hex before validating it. I specifically am referring to the checksum, 1291 and the block numbers and their ones- complement. 1292 1293 Those wishing to maintain compatibility of the CP/M file structure, i.e. 1294 to allow modemming ASCII files to or from CP/M systems should follow this 1295 data format: 1296 1297 + ASCII tabs used (09H); tabs set every 8. 1298 1299 + Lines terminated by CR/LF (0DH 0AH) 1300 1301 + End-of-file indicated by ^Z, 1AH. (one or more) 1302 1303 + Data is variable length, i.e. should be considered a continuous 1304 stream of data bytes, broken into 128-byte chunks purely for the 1305 purpose of transmission. 1306 1307 + A CP/M "peculiarity": If the data ends exactly on a 128-byte 1308 boundary, i.e. CR in 127, and LF in 128, a subsequent sector 1309 containing the ^Z EOF character(s) is optional, but is preferred. 1310 Some utilities or user programs still do not handle EOF without ^Zs. 1311 1312 1313 1314 1315 1316 Chapter 7 Xmodem Protocol Overview 1317 1318 1319 1320 1321 1322 1323 1324 X/YMODEM Protocol Reference June 18 1988 21 1325 1326 1327 1328 + The last block sent is no different from others, i.e. there is no 1329 "short block". 1330 Figure 9. XMODEM Message Block Level Protocol 1331 1332 Each block of the transfer looks like: 1333 <SOH><blk #><255-blk #><--128 data bytes--><cksum> 1334 in which: 1335 <SOH> = 01 hex 1336 <blk #> = binary number, starts at 01 increments by 1, and 1337 wraps 0FFH to 00H (not to 01) 1338 <255-blk #> = blk # after going thru 8080 "CMA" instr, i.e. 1339 each bit complemented in the 8-bit block number. 1340 Formally, this is the "ones complement". 1341 <cksum> = the sum of the data bytes only. Toss any carry. 1342 1343 7.3 File Level Protocol 1344 1345 7.3.1 Common_to_Both_Sender_and_Receiver 1346 All errors are retried 10 times. For versions running with an operator 1347 (i.e. NOT with XMODEM), a message is typed after 10 errors asking the 1348 operator whether to "retry or quit". 1349 1350 Some versions of the protocol use <can>, ASCII ^X, to cancel transmission. 1351 This was never adopted as a standard, as having a single "abort" character 1352 makes the transmission susceptible to false termination due to an <ack> 1353 <nak> or <soh> being corrupted into a <can> and aborting transmission. 1354 1355 The protocol may be considered "receiver driven", that is, the sender need 1356 not automatically re-transmit, although it does in the current 1357 implementations. 1358 1359 1360 7.3.2 Receive_Program_Considerations 1361 The receiver has a 10-second timeout. It sends a <nak> every time it 1362 times out. The receiver's first timeout, which sends a <nak>, signals the 1363 transmitter to start. Optionally, the receiver could send a <nak> 1364 immediately, in case the sender was ready. This would save the initial 10 1365 second timeout. However, the receiver MUST continue to timeout every 10 1366 seconds in case the sender wasn't ready. 1367 1368 Once into a receiving a block, the receiver goes into a one-second timeout 1369 for each character and the checksum. If the receiver wishes to <nak> a 1370 block for any reason (invalid header, timeout receiving data), it must 1371 wait for the line to clear. See "programming tips" for ideas 1372 1373 Synchronizing: If a valid block number is received, it will be: 1) the 1374 expected one, in which case everything is fine; or 2) a repeat of the 1375 previously received block. This should be considered OK, and only 1376 indicates that the receivers <ack> got glitched, and the sender re- 1377 transmitted; 3) any other block number indicates a fatal loss of 1378 synchronization, such as the rare case of the sender getting a line-glitch 1379 1380 1381 1382 Chapter 7 Xmodem Protocol Overview 1383 1384 1385 1386 1387 1388 1389 1390 X/YMODEM Protocol Reference June 18 1988 22 1391 1392 1393 1394 that looked like an <ack>. Abort the transmission, sending a <can> 1395 1396 1397 7.3.3 Sending_program_considerations 1398 While waiting for transmission to begin, the sender has only a single very 1399 long timeout, say one minute. In the current protocol, the sender has a 1400 10 second timeout before retrying. I suggest NOT doing this, and letting 1401 the protocol be completely receiver-driven. This will be compatible with 1402 existing programs. 1403 1404 When the sender has no more data, it sends an <eot>, and awaits an <ack>, 1405 resending the <eot> if it doesn't get one. Again, the protocol could be 1406 receiver-driven, with the sender only having the high-level 1-minute 1407 timeout to abort. 1408 1409 1410 Here is a sample of the data flow, sending a 3-block message. It includes 1411 the two most common line hits - a garbaged block, and an <ack> reply 1412 getting garbaged. <xx> represents the checksum byte. 1413 1414 Figure 10. Data flow including Error Recovery 1415 1416 SENDER RECEIVER 1417 times out after 10 seconds, 1418 <--- <nak> 1419 <soh> 01 FE -data- <xx> ---> 1420 <--- <ack> 1421 <soh> 02 FD -data- xx ---> (data gets line hit) 1422 <--- <nak> 1423 <soh> 02 FD -data- xx ---> 1424 <--- <ack> 1425 <soh> 03 FC -data- xx ---> 1426 (ack gets garbaged) <--- <ack> 1427 <soh> 03 FC -data- xx ---> <ack> 1428 <eot> ---> 1429 <--- <anything except ack> 1430 <eot> ---> 1431 <--- <ack> 1432 (finished) 1433 1434 7.4 Programming Tips 1435 1436 + The character-receive subroutine should be called with a parameter 1437 specifying the number of seconds to wait. The receiver should first 1438 call it with a time of 10, then <nak> and try again, 10 times. 1439 1440 After receiving the <soh>, the receiver should call the character 1441 receive subroutine with a 1-second timeout, for the remainder of the 1442 message and the <cksum>. Since they are sent as a continuous stream, 1443 timing out of this implies a serious like glitch that caused, say, 1444 127 characters to be seen instead of 128. 1445 1446 1447 1448 Chapter 7 Xmodem Protocol Overview 1449 1450 1451 1452 1453 1454 1455 1456 X/YMODEM Protocol Reference June 18 1988 23 1457 1458 1459 1460 + When the receiver wishes to <nak>, it should call a "PURGE" 1461 subroutine, to wait for the line to clear. Recall the sender tosses 1462 any characters in its UART buffer immediately upon completing sending 1463 a block, to ensure no glitches were mis- interpreted. 1464 1465 The most common technique is for "PURGE" to call the character 1466 receive subroutine, specifying a 1-second timeout,[1] and looping 1467 back to PURGE until a timeout occurs. The <nak> is then sent, 1468 ensuring the other end will see it. 1469 1470 + You may wish to add code recommended by John Mahr to your character 1471 receive routine - to set an error flag if the UART shows framing 1472 error, or overrun. This will help catch a few more glitches - the 1473 most common of which is a hit in the high bits of the byte in two 1474 consecutive bytes. The <cksum> comes out OK since counting in 1-byte 1475 produces the same result of adding 80H + 80H as with adding 00H + 1476 00H. 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 __________ 1508 1509 1. These times should be adjusted for use with timesharing systems. 1510 1511 1512 1513 1514 Chapter 7 Xmodem Protocol Overview 1515 1516 1517 1518 1519 1520 1521 1522 X/YMODEM Protocol Reference June 18 1988 24 1523 1524 1525 1526 8. XMODEM/CRC Overview 1527 1528 Original 1/13/85 by John Byrns -- CRC option. 1529 1530 Please pass on any reports of errors in this document or suggestions for 1531 improvement to me via Ward's/CBBS at (312) 849-1132, or by voice at (312) 1532 885-1105. 1533 1534 The CRC used in the Modem Protocol is an alternate form of block check 1535 which provides more robust error detection than the original checksum. 1536 Andrew S. Tanenbaum says in his book, Computer Networks, that the CRC- 1537 CCITT used by the Modem Protocol will detect all single and double bit 1538 errors, all errors with an odd number of bits, all burst errors of length 1539 16 or less, 99.997% of 17-bit error bursts, and 99.998% of 18-bit and 1540 longer bursts.[1] 1541 1542 The changes to the Modem Protocol to replace the checksum with the CRC are 1543 straight forward. If that were all that we did we would not be able to 1544 communicate between a program using the old checksum protocol and one 1545 using the new CRC protocol. An initial handshake was added to solve this 1546 problem. The handshake allows a receiving program with CRC capability to 1547 determine whether the sending program supports the CRC option, and to 1548 switch it to CRC mode if it does. This handshake is designed so that it 1549 will work properly with programs which implement only the original 1550 protocol. A description of this handshake is presented in section 10. 1551 1552 Figure 11. Message Block Level Protocol, CRC mode 1553 1554 Each block of the transfer in CRC mode looks like: 1555 <SOH><blk #><255-blk #><--128 data bytes--><CRC hi><CRC lo> 1556 in which: 1557 <SOH> = 01 hex 1558 <blk #> = binary number, starts at 01 increments by 1, and 1559 wraps 0FFH to 00H (not to 01) 1560 <255-blk #> = ones complement of blk #. 1561 <CRC hi> = byte containing the 8 hi order coefficients of the CRC. 1562 <CRC lo> = byte containing the 8 lo order coefficients of the CRC. 1563 1564 8.1 CRC Calculation 1565 1566 8.1.1 Formal_Definition 1567 To calculate the 16 bit CRC the message bits are considered to be the 1568 coefficients of a polynomial. This message polynomial is first multiplied 1569 by X^16 and then divided by the generator polynomial (X^16 + X^12 + X^5 + 1570 1571 1572 __________ 1573 1574 1. This reliability figure is misleading because XMODEM's critical 1575 supervisory functions are not protected by this CRC. 1576 1577 1578 1579 1580 Chapter 8 Xmodem Protocol Overview 1581 1582 1583 1584 1585 1586 1587 1588 X/YMODEM Protocol Reference June 18 1988 25 1589 1590 1591 1592 1) using modulo two arithmetic. The remainder left after the division is 1593 the desired CRC. Since a message block in the Modem Protocol is 128 bytes 1594 or 1024 bits, the message polynomial will be of order X^1023. The hi order 1595 bit of the first byte of the message block is the coefficient of X^1023 in 1596 the message polynomial. The lo order bit of the last byte of the message 1597 block is the coefficient of X^0 in the message polynomial. 1598 1599 Figure 12. Example of CRC Calculation written in C 1600 1601 The following XMODEM crc routine is taken from "rbsb.c". Please refer to 1602 the source code for these programs (contained in RZSZ.ZOO) for usage. A 1603 fast table driven version is also included in this file. 1604 1605 /* update CRC */ 1606 unsigned short 1607 updcrc(c, crc) 1608 register c; 1609 register unsigned crc; 1610 { 1611 register count; 1612 1613 for (count=8; --count>=0;) { 1614 if (crc & 0x8000) { 1615 crc <<= 1; 1616 crc += (((c<<=1) & 0400) != 0); 1617 crc ^= 0x1021; 1618 } 1619 else { 1620 crc <<= 1; 1621 crc += (((c<<=1) & 0400) != 0); 1622 } 1623 } 1624 return crc; 1625 } 1626 1627 8.2 CRC File Level Protocol Changes 1628 1629 8.2.1 Common_to_Both_Sender_and_Receiver 1630 The only change to the File Level Protocol for the CRC option is the 1631 initial handshake which is used to determine if both the sending and the 1632 receiving programs support the CRC mode. All Modem Programs should support 1633 the checksum mode for compatibility with older versions. A receiving 1634 program that wishes to receive in CRC mode implements the mode setting 1635 handshake by sending a <C> in place of the initial <nak>. If the sending 1636 program supports CRC mode it will recognize the <C> and will set itself 1637 into CRC mode, and respond by sending the first block as if a <nak> had 1638 been received. If the sending program does not support CRC mode it will 1639 not respond to the <C> at all. After the receiver has sent the <C> it will 1640 wait up to 3 seconds for the <soh> that starts the first block. If it 1641 receives a <soh> within 3 seconds it will assume the sender supports CRC 1642 mode and will proceed with the file exchange in CRC mode. If no <soh> is 1643 1644 1645 1646 Chapter 8 Xmodem Protocol Overview 1647 1648 1649 1650 1651 1652 1653 1654 X/YMODEM Protocol Reference June 18 1988 26 1655 1656 1657 1658 received within 3 seconds the receiver will switch to checksum mode, send 1659 a <nak>, and proceed in checksum mode. If the receiver wishes to use 1660 checksum mode it should send an initial <nak> and the sending program 1661 should respond to the <nak> as defined in the original Modem Protocol. 1662 After the mode has been set by the initial <C> or <nak> the protocol 1663 follows the original Modem Protocol and is identical whether the checksum 1664 or CRC is being used. 1665 1666 1667 8.2.2 Receive_Program_Considerations 1668 There are at least 4 things that can go wrong with the mode setting 1669 handshake. 1670 1671 1. the initial <C> can be garbled or lost. 1672 1673 2. the initial <soh> can be garbled. 1674 1675 3. the initial <C> can be changed to a <nak>. 1676 1677 4. the initial <nak> from a receiver which wants to receive in checksum 1678 can be changed to a <C>. 1679 1680 The first problem can be solved if the receiver sends a second <C> after 1681 it times out the first time. This process can be repeated several times. 1682 It must not be repeated too many times before sending a <nak> and 1683 switching to checksum mode or a sending program without CRC support may 1684 time out and abort. Repeating the <C> will also fix the second problem if 1685 the sending program cooperates by responding as if a <nak> were received 1686 instead of ignoring the extra <C>. 1687 1688 It is possible to fix problems 3 and 4 but probably not worth the trouble 1689 since they will occur very infrequently. They could be fixed by switching 1690 modes in either the sending or the receiving program after a large number 1691 of successive <nak>s. This solution would risk other problems however. 1692 1693 1694 8.2.3 Sending_Program_Considerations 1695 The sending program should start in the checksum mode. This will insure 1696 compatibility with checksum only receiving programs. Anytime a <C> is 1697 received before the first <nak> or <ack> the sending program should set 1698 itself into CRC mode and respond as if a <nak> were received. The sender 1699 should respond to additional <C>s as if they were <nak>s until the first 1700 <ack> is received. This will assist the receiving program in determining 1701 the correct mode when the <soh> is lost or garbled. After the first <ack> 1702 is received the sending program should ignore <C>s. 1703 1704 1705 1706 1707 1708 1709 1710 1711 1712 Chapter 8 Xmodem Protocol Overview 1713 1714 1715 1716 1717 1718 1719 1720 X/YMODEM Protocol Reference June 18 1988 27 1721 1722 1723 1724 8.3 Data Flow Examples with CRC Option 1725 1726 Here is a data flow example for the case where the receiver requests 1727 transmission in the CRC mode but the sender does not support the CRC 1728 option. This example also includes various transmission errors. <xx> 1729 represents the checksum byte. 1730 1731 Figure 13. Data Flow: Receiver has CRC Option, Sender Doesn't 1732 1733 SENDER RECEIVER 1734 <--- <C> 1735 times out after 3 seconds, 1736 <--- <C> 1737 times out after 3 seconds, 1738 <--- <C> 1739 times out after 3 seconds, 1740 <--- <C> 1741 times out after 3 seconds, 1742 <--- <nak> 1743 <soh> 01 FE -data- <xx> ---> 1744 <--- <ack> 1745 <soh> 02 FD -data- <xx> ---> (data gets line hit) 1746 <--- <nak> 1747 <soh> 02 FD -data- <xx> ---> 1748 <--- <ack> 1749 <soh> 03 FC -data- <xx> ---> 1750 (ack gets garbaged) <--- <ack> 1751 times out after 10 seconds, 1752 <--- <nak> 1753 <soh> 03 FC -data- <xx> ---> 1754 <--- <ack> 1755 <eot> ---> 1756 <--- <ack> 1757 1758 Here is a data flow example for the case where the receiver requests 1759 transmission in the CRC mode and the sender supports the CRC option. This 1760 example also includes various transmission errors. <xxxx> represents the 1761 2 CRC bytes. 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 Chapter 8 Xmodem Protocol Overview 1779 1780 1781 1782 1783 1784 1785 1786 X/YMODEM Protocol Reference June 18 1988 28 1787 1788 1789 1790 Figure 14. Receiver and Sender Both have CRC Option 1791 1792 SENDER RECEIVER 1793 <--- <C> 1794 <soh> 01 FE -data- <xxxx> ---> 1795 <--- <ack> 1796 <soh> 02 FD -data- <xxxx> ---> (data gets line hit) 1797 <--- <nak> 1798 <soh> 02 FD -data- <xxxx> ---> 1799 <--- <ack> 1800 <soh> 03 FC -data- <xxxx> ---> 1801 (ack gets garbaged) <--- <ack> 1802 times out after 10 seconds, 1803 <--- <nak> 1804 <soh> 03 FC -data- <xxxx> ---> 1805 <--- <ack> 1806 <eot> ---> 1807 <--- <ack> 1808 1809 1810 1811 1812 1813 1814 1815 1816 1817 1818 1819 1820 1821 1822 1823 1824 1825 1826 1827 1828 1829 1830 1831 1832 1833 1834 1835 1836 1837 1838 1839 1840 1841 1842 1843 1844 Chapter 8 Xmodem Protocol Overview 1845 1846 1847 1848 1849 1850 1851 1852 X/YMODEM Protocol Reference June 18 1988 29 1853 1854 1855 1856 9. MORE INFORMATION 1857 1858 Please contact Omen Technology for troff source files and typeset copies 1859 of this document. 1860 1861 1862 9.1 TeleGodzilla Bulletin Board 1863 1864 More information may be obtained by calling TeleGodzilla at 503-621-3746. 1865 Speed detection is automatic for 1200, 2400 and 19200(Telebit PEP) bps. 1866 TrailBlazer modem users may issue the TeleGodzilla trailblazer command to 1867 swith to 19200 bps once they have logged in. 1868 1869 Interesting files include RZSZ.ZOO (C source code), YZMODEM.ZOO (Official 1870 XMODEM, YMODEM, and ZMODEM protocol descriptions), ZCOMMEXE.ARC, 1871 ZCOMMDOC.ARC, and ZCOMMHLP.ARC (PC-DOS shareware comm program with XMODEM, 1872 True YMODEM(TM), ZMODEM, Kermit Sliding Windows, Telink, MODEM7 Batch, 1873 script language, etc.). 1874 1875 1876 9.2 Unix UUCP Access 1877 1878 UUCP sites can obtain the current version of this file with 1879 uucp omen!/u/caf/public/ymodem.doc /tmp 1880 A continually updated list of available files is stored in 1881 /usr/spool/uucppublic/FILES. When retrieving these files with uucp, 1882 remember that the destination directory on your system must be writeable 1883 by anyone, or the UUCP transfer will fail. 1884 1885 The following L.sys line calls TeleGodzilla (Pro-YAM in host operation). 1886 TeleGodzilla determines the incoming speed automatically. 1887 1888 In response to "Name Please:" uucico gives the Pro-YAM "link" command as a 1889 user name. The password (Giznoid) controls access to the Xenix system 1890 connected to the IBM PC's other serial port. Communications between 1891 Pro-YAM and Xenix use 9600 bps; YAM converts this to the caller's speed. 1892 1893 Finally, the calling uucico logs in as uucp. 1894 1895 omen Any ACU 2400 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp 1896 1897 1898 1899 10. REVISIONS 1900 1901 6-18-88 Further revised for clarity. Corrected block numbering in two 1902 examples. 1903 10-27-87 Optional fields added for number of files remaining to be sent 1904 and total number of bytes remaining to be sent. 1905 10-18-87 Flow control discussion added to 1024 byte block descritpion, 1906 minor revisions for clarity per user comments. 1907 1908 1909 1910 Chapter 10 Xmodem Protocol Overview 1911 1912 1913 1914 1915 1916 1917 1918 X/YMODEM Protocol Reference June 18 1988 30 1919 1920 1921 1922 8-03-87 Revised for clarity. 1923 5-31-1987 emphasizes minimum requirements for YMODEM, and updates 1924 information on accessing files. 1925 9-11-1986 clarifies nomenclature and some minor points. 1926 The April 15 1986 edition clarifies some points concerning CRC 1927 calculations and spaces in the header. 1928 1929 1930 11. YMODEM Programs 1931 1932 ZCOMM, A shareware little brother to Professional-YAM, is available as 1933 ZCOMMEXE.ARC on TeleGodzilla and other bulletin board systems. ZCOMM may 1934 be used to test YMODEM amd ZMODEM implementations. 1935 1936 Unix programs supporting YMODEM are available on TeleGodzilla in RZSZ.ZOO. 1937 This ZOO archive includes a ZCOMM/Pro-YAM/PowerCom script ZUPL.T to upload 1938 a bootstrap program MINIRB.C, compile it, and then upload the rest of the 1939 files using the compiled MINIRB. Most Unix like systems are supported, 1940 including V7, Xenix, Sys III, 4.2 BSD, SYS V, Idris, Coherent, and 1941 Regulus. 1942 1943 A version for VAX-VMS is available in VRBSB.SHQ. 1944 1945 Irv Hoff has added 1k blocks and basic YMODEM batch transfers to the KMD 1946 and IMP series programs, which replace the XMODEM and MODEM7/MDM7xx series 1947 respectively. Overlays are available for a wide variety of CP/M systems. 1948 1949 Questions about Professional-YAM communications software may be directed 1950 to: 1951 Chuck Forsberg 1952 Omen Technology Inc 1953 17505-V Sauvie Island Road 1954 Portland Oregon 97231 1955 VOICE: 503-621-3406 :VOICE 1956 Modem: 503-621-3746 Speed: 19200(Telebit PEP),2400,1200,300 1957 Usenet: ...!tektronix!reed!omen!caf 1958 CompuServe: 70007,2304 1959 GEnie: CAF 1960 1961 Unlike ZMODEM and Kermit, XMODEM and YMODEM place obstacles in the path of 1962 a reliable high performance implementation, evidenced by poor reliability 1963 under stress of the industry leaders' XMODEM and YMODEM programs. Omen 1964 Technology provides consulting and other services to those wishing to 1965 implement XMODEM, YMODEM, and ZMODEM with state of the art features and 1966 reliability. 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 Chapter 11 Xmodem Protocol Overview 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 CONTENTS 1989 1990 1991 1. TOWER OF BABEL................................................... 2 1992 1.1 Definitions................................................. 2 1993 1994 2. YMODEM MINIMUM REQUIREMENTS...................................... 4 1995 1996 3. WHY YMODEM?...................................................... 6 1997 3.1 Some Messages from the Pioneer.............................. 7 1998 1999 4. XMODEM PROTOCOL ENHANCEMENTS..................................... 10 2000 4.1 Graceful Abort.............................................. 10 2001 4.2 CRC-16 Option............................................... 10 2002 4.3 XMODEM-1k 1024 Byte Block................................... 11 2003 2004 5. YMODEM Batch File Transmission................................... 13 2005 5.1 KMD/IMP Exceptions to YMODEM................................ 16 2006 2007 6. YMODEM-g File Transmission....................................... 18 2008 2009 7. XMODEM PROTOCOL OVERVIEW......................................... 20 2010 7.1 Definitions................................................. 20 2011 7.2 Transmission Medium Level Protocol.......................... 20 2012 7.3 File Level Protocol......................................... 21 2013 7.4 Programming Tips............................................ 22 2014 2015 8. XMODEM/CRC Overview.............................................. 24 2016 8.1 CRC Calculation............................................. 24 2017 8.2 CRC File Level Protocol Changes............................. 25 2018 8.3 Data Flow Examples with CRC Option.......................... 27 2019 2020 9. MORE INFORMATION................................................. 29 2021 9.1 TeleGodzilla Bulletin Board................................. 29 2022 9.2 Unix UUCP Access............................................ 29 2023 2024 10. REVISIONS........................................................ 29 2025 2026 11. YMODEM Programs.................................................. 30 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 - i - 2043 2044 2045 2046 2047 2048 2049 2050 2051 2052 2053 2054 2055 2056 2057 LIST OF FIGURES 2058 2059 2060 Figure 1. XMODEM-1k Blocks.......................................... 12 2061 2062 Figure 2. Mixed 1024 and 128 byte Blocks............................ 12 2063 2064 Figure 3. YMODEM Batch Transmission Session (1 file)................ 16 2065 2066 Figure 4. YMODEM Batch Transmission Session (2 files)............... 16 2067 2068 Figure 5. YMODEM Batch Transmission Session-1k Blocks............... 16 2069 2070 Figure 6. YMODEM Filename block transmitted by sz................... 16 2071 2072 Figure 7. YMODEM Header Information and Features.................... 16 2073 2074 Figure 8. YMODEM-g Transmission Session............................. 19 2075 2076 Figure 9. XMODEM Message Block Level Protocol....................... 21 2077 2078 Figure 10. Data flow including Error Recovery........................ 22 2079 2080 Figure 11. Message Block Level Protocol, CRC mode.................... 24 2081 2082 Figure 12. Example of CRC Calculation written in C................... 25 2083 2084 Figure 13. Data Flow: Receiver has CRC Option, Sender Doesn't........ 27 2085 2086 Figure 14. Receiver and Sender Both have CRC Option.................. 28 2087 2088 2089 2090 2091 2092 2093 2094 2095 2096 2097 2098 2099 2100 2101 2102 2103 2104 2105 2106 2107 2108 - ii - 2109
