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Chapter 4 Rev 10-27-87 Typeset 10-27-87 8
Chapter 4 ZMODEM Protocol 9
problems.
Since some characters had to be escaped anyway, there wasn't any point
wasting bytes to fill out a fixed packet length or to specify a variable
packet length. In ZMODEM, the length of data subpackets is denoted by
ending each subpacket with an escape sequence similar to BISYNC and HDLC.
The end result is a ZMOEM header containing a "frame type", four bytes of
supervisory information, and its own CRC. Data frames consist of a header
followed by 1 or more data subpackets. In the absence of transmission
errors, an entire file can be sent in one data frame.
Since the sending system may be sensitive to numerous control characters
or strip parity in the reverse data path, all of the headers sent by the
receiver are sent in hex. A common lower level routine receives all
headers, allowing the main program logic to deal with headers and data
subpackets as objects.
With equivalent binary (efficient) and hex (application friendly) frames,
the sending program can send an "invitation to receive" sequence to
activate the receiver without crashing the remote application with
unexpected control characters.
Going "back to scratch" in the protocol design presents an opportunity to
steal good ideas from many sources and to add a few new ones.
From Kermit and UUCP comes the concept of an initial dialog to exchange
system parameters.
ZMODEM generalizes Compuserve B Protocol's host controlled transfers to
single command AutoDownload and command downloading. A Security Challenge
discourages password hackers and Trojan Horse authors from abusing
ZMODEM's power.
We were also keen to the pain and $uffering of legions of
telecommunicators whose file transfers have been ruined by communications
and timesharing faults. ZMODEM's file transfer recovery and advanced file
management are dedicated to these kindred comrades.
After ZMODEM had been operational a short time, Earl Hall pointed out the
obvious: ZMODEM's user friendly AutoDownload was almost useless if the
user must assign transfer options to each of the sending and receiving
programs. Now, transfer options may be specified to/by the sending
program, which passes them to the receiving program in the ZFILE header.
Chapter 5 Rev 10-27-87 Typeset 10-27-87 9
Chapter 5 ZMODEM Protocol 10
5. RRRROOOOSSSSEEEETTTTTTTTAAAA SSSSTTTTOOOONNNNEEEE
Here are some definitions which reflect 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 and 0 or more data subpackets.
XMODEM refers to the original 1977 file transfer etiquette introduced by
Ward Christensen's 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 most
communications programs because it is easy to implement.
XMODEM/CRC replaces XMODEM's 1 byte checksum with a two byte Cyclical
Redundancy Check (CRC-16), improving error detection.
XMODEM-1k Refers to XMODEM-CRC with optional 1024 byte blocks.