Xmodem/Ymodem协议说明（完整版）_YMODEM协议说明

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XMODEM/YMODEM PROTOCOL REFERENCE

A compendium of documents describing the

XMODEM and YMODEM

File Transfer Protocols

This document was formatted 10-14-88.

Edited by Chuck Forsberg

This file may be redistributed without restriction

provided the text is not altered.

Please distribute as widely as possible.

Questions to Chuck Forsberg

Omen Technology Inc

The High Reliability Software

17505-V Sauvie Island Road

Portland Oregon 97231

VOICE: 503-621-3406 :VOICE

TeleGodzilla BBS: 503-621-3746 Speed 19200(Telebit PEP),2400,1200,300

CompuServe: 70007,2304

GEnie: CAF

UUCP: ...!tektronix!reed!omen!caf

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1. TOWER OF BABEL

A "YMODEM Tower of Babel" has descended on the microcomputing community

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 YMODEM 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 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

to the confusion. In one instance, some self styled rewriter of history

altered the heading in YMODEM.DOC's Figure 1 from "1024 byte Packets" to

"YMODEM/CRC File Transfer Protocol". None of the XMODEM and YMODEM

examples shown in that document were correct.

To put an end to this confusion, we must make "perfectly clear" what

YMODEM stands for, as Ward Christensen defined it in his 1985 coining of

the term.

To the majority of you who read, understood, and respected Ward's

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 from such archives.

XMODEM refers to the file transfer etiquette introduced by Ward

Christensen's 1977 MODEM.ASM program. The name XMODEM comes from

Keith Petersen's XMODEM.ASM program, an adaptation of MODEM.ASM

for Remote CP/M (RCPM) systems. 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'

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 June 18 1988 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 be a null terminated ASCII string as described

below.

For those who are too lazy to read the entire document:

+ Unless specifically requested, only the file name portion is

sent.

+ No drive letter is sent.

+ Systems that do not distinguish between upper and lower case

letters in filenames shall send the pathname in lower case only.

+ The receiving program shall use this pathname for the received file

name, unless explicitly overridden.

+ When the receiving program receives this block and successfully

opened the output file, it shall acknowledge this block with an ACK

character and then proceed with a normal XMODEM file transfer

beginning with a "C" or NAK tranmsitted by the receiver.

+ The sending program shall use CRC-16 in response to a "C" pathname

nak, otherwise use 8 bit checksum.

+ The receiving program must accept any mixture of 128 and 1024 byte

blocks within each file it receives. Sending programs may

arbitrarily switch between 1024 and 128 byte blocks.

+ 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 session shall be signified by a null (empty)

pathname, this pathname block shall be acknowledged the same as other

pathname blocks.

Programs not meeting all of these requirements are not YMODEM compatible,

and shall not be described as supporting YMODEM.

Meeting these MINIMUM requirements does not guarantee reliable file

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