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Text File | 1987-08-01 | 9.5 KB | 197 lines

KERMIT File Transfer Protocol


by Larry Jordan


 KERMIT is a file transfer protocol similar to XMODEM. Like
XMODEM, KERMIT transfers files between computers in blocks or
packets and each packet is checked for communication errors. The
transmitting computer calculates a checksum based on the contents
of each packet and sends the checksum along with the packet. Like
XMODEM, the receiving computer calculates its own checksum based
on its contents of each received packet and compares the value to
the received checksum. The receiving computer requests a
retransfer of a packet when the received checksum does not match
the calculated checksum. Like XMODEM, KERMIT repeats this process
packet-by-packet until an entire file is transferred or until the
file transfer is abnormally aborted.

 For XMODEM users, this KERMIT block mode transfer sounds all too
familiar. The users of PC-TALK.III and CROSSTALK XVI have watched
block counts methodically increment as utility and other assorted
software moved from a distant TCOMM or RBBS-PC to a local
floppy disk. These communications users have confidence in files
transferred using XMODEM because they know Ward Christensen
designed the protocol to ensure better than 99.6% error-free
transmission. Many XMODEM enthusiasts will question the need for
yet another public domain protocol, but there is a legitimate
need for KERMIT.

 The XMODEM protocol requires a full 8 data bits to properly
transfer a file. The communications software and hardware on both
ends of a communications link must be set for no parity error-
checking and 8 data bits. When these parameters are properly set,
XMODEM simply divides a file into 128 byte (each byte contains 8
bits) blocks and moves the file one block at a time.  There is no
conversion or translation of the data.    The file may contain
text, data or binary program execution code. WordStar files and
compiled source code are transferred "as is" and can be used in
the receiving computer just as they were used in the transmitting
computer, assuming the computers are compatible.

 Unfortunately, the ubiquitous XMODEM cannot perform its majestic
work with some computer systems. Some computer front-end
processors can only operate using 7 data bits. Other computers
that can operate at 8 data bits cough and choke when they receive
bit patterns that match control characters (characters with ASCII
values between 0 and 31 and the DEL character--ASCII 127). And
finally, some modems lock up when they receive bit patterns that
appear to be command mode signals. To accommodate these cranky
but useful devices (that are already paid for) it is necessary to
use an asynchronous file transfer protocol that ensures "error-
free" transfer while overcoming the equipment limitations.
Columbia University has developed and released a public domain
protocol called KERMIT that accomplishes all these objectives.

 KERMIT can handle any kind of file and can be used with just
about every kind of computer. The protocol is capable of
converting all data into the equivalent of printable 7-bit ASCII
characters and numbers when necessary. When KERMIT is forced to
communicate 8-bit binary files, it simply sends the extra bit by
itself. This "prefixing" or "8-bit quoting" process overcomes the
7-bit limitation of some old mainframe front-end processors.
When Kermit is asked to transmit a control character, it encodes
the character in a process which produces a printable character
that can pass through control character sensitive devices without
a hitch.  In both of these cases, the KERMIT on the receiving end
of the file transfer has to work hard to decode each block of
data before it is transferred from the KERMIT program to disk.

  Because of the relatively slow speed of microprocessors (yes
the IBM PC is slower than the VAX 11/780) most microcomputer
KERMIT implementations are written in assembly language.
Implementations on faster machines are often written in PL/1 or
some other high-level language with good string handling
capabilities.  The original KERMIT was written in C to operate
under the UNIX operating system, but other more recent
implementations are generally written in the most powerful and
productive language available on a specific computer.

 KERMIT has four other advantages over XMODEM.    First, it allows
wildcard file transfers.  An asterisk can be used in place of
several file name letters in order to sequentially and
automatically transmit many files with common filenames or
extensions.  XMODEM has to be manually restarted at the end of
each file transfer.  Second, minicomputer and mainframe versions
of KERMIT often include a SERVER mode.    After the host KERMIT is
placed in the SERVER mode, the local KERMIT performs all
handshaking necessary to both send and receive files--the remote
host KERMIT is taken over by the local KERMIT and is slave to the
local KERMIT's wishes.  Both the KERMIT wildcard and SERVER
feature are time savers when several files have to be
transferred. Third, Kermit saves time during ASCII and executable
EXE types of file transfers by supporting data compression.
Characters that are repeated in a file are only sent once for
each repeat sequence.  This often results in a 10 to 30 percent
faster file transfer for Kermit compared to Xmodem for the same
file.  Finally, Kermit avoids the "deadly embrace" that can
abnormally abort XMODEM transfers when handshake character
synchronization are thrown off by bad telephone connections.

 Will KERMIT displace XMODEM for error-checked file transfers?
Not in the near future, but perhaps it will eventually win out as
the public domain protocol of choice. It is already in use in
many universities, and several major federal government
organizations have standardized on the protocol. The National
Institute of Health in Bethesda has standardized on KERMIT for
file transfers. THE SOURCE has also implemented KERMIT and
supports the protocol for their subscribers. Both the BLAST
and the Microcom Networking Protocol file transfer techniques
that are being widely touted as the future national standard for
asynchronous communications have characteristics that prevent
their use with certain computers without special add-on
equipment. Kermit does not suffer this limitation.

  It is often vitally important to move data from one computer
to another with assurance that errors are not introduced in the
data as a result of the transfer process.  When such a transfer
has to be done using the asynchronous method of communications,
KERMIT is a good candidate for the job.  It is available at
reasonable cost, and it is a proven protocol.  If it has not
already been implemented for a specific hardware configuration,
it can be implemented quickly because the protocol is well
documented. The protocol documentation manual comes complete with
a full source code listing of a UNIX KERMIT written in C.


Ordering Information

The KERMIT software is free and is distributed by Columbia
University. The university is not, however, set up to distribute
free software on the scale required for KERMIT. Therefore, to
defray costs for media, printing, postage, labor, and
computing resources, the university requests a moderate donation
from sites that request KERMIT directly. The schedule is as
follows:

       Complete KERMIT Distribution   $100.00
     (Tape, Users Guide, and Protocol Manual)

       Printed Documents      $5.00 each
     (Users Guide, Protocol Manual, or Any Source Listing)

Other sites are free to redistribute KERMIT on their own terms,
and are encouraged to do so, with the following stipulations:
KERMIT should not be sold for profit; credit should be given
where it is due; and new material should be sent back to Columbia
University at the address below so that we can maintain a
definitive and comprehensive set of KERMIT implementations for
further distribution.

To order KERMIT from Columbia University, send a letter
requesting either:

   (a) The manuals or source listings you desire
       (specify each one), or
   (b) A 9-track magnetic tape in one of the
       following formats:

      System    Tape Format         Densities
      TOPS-10    BACKUP/Interchange,     Unlabeled 800, 1600
      TOPS-20    DUMPER, Unlabeled     800, 1600
      IBM VM/CMS  EBCDIC, CMS Format     1600, 6250
       or EBCDIC, OS Standard Label  1600, 6250
      Other    ASCII, ANSI Label, Format ``D'' 800, 1600

      Specify system, format, and density. One copy of each
      manual is included with the tape. The University supplies
      the tape, packaging, and postage.

Columbia University can only make tapes in the formats listed
above. They cannot produce floppy disks; bootstrapping procedures
are provided to allow the microcomputer versions to be downloaded
from the mainframe for which the tape is produced. The tape
includes all source programs, documentation, and, when practical,
binaries or hex.

Send your letter to:

       KERMIT Distribution
       Columbia University Center for Computing Activities
       7th Floor, Watson Laboratory
       612 West 115th Street
       New York, N.Y. 10025

Please list the machines and operating systems you expect to run
KERMIT on. You should also specify the tape format or the
listings desired. Make checks payable to Columbia University
Center for Computing Activities.


---------------------------------------------------------------
Larry Jordan coauthored the book
COMMUNICATIONS and NETWORKING for the IBM PC,
published by the Robert J. Brady Company.