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THE TRUTH ABOUT KERMIT FILE TRANSFER PERFORMANCE

Frank da Cruz

(From Kermit News #5, June 1993)

In the early 1980s, the first generation of Kermit software used the basic
Kermit protocol: stop-and-wait exchange of short packets. The basic protocol
is easily implemented and highly robust, and led to its rapid proliferation to
hundreds of hardware and software platforms where it proved a success in
transferring files under even the most difficult conditions.

The new generation of Kermit software improves on the original robust
qualities and dramatically boosts performance without sacrificing
compatibility with the earlier generation. Protocol capabilities are
negotiated automatically so the newest, most advanced versions can still
exchange files with the earliest or most minimal versions.

Kermit's performance features include long packets, sliding windows,
control-prefixing selection, locking shifts, and compression. The first three
have the potential for causing problems, and are not used unless you ask for
them. This article describes Kermit's performance features and tests them
against other popular protocols. The results might surprise you.

Long Packets and Sliding Windows

The maximum packet length in the basic Kermit protocol is 94, chosen to
prevent data loss when the receiver has small input buffers or lacks an
adequate method of flow control, and also to reduce vulnerability to noise.
But since 1985, Kermit's long-packet extension has allowed packets up to 9024
bytes in length to be used when conditions permit.

Longer packets reduce the ratio of protocol overhead to actual data,
increasing the potential file transfer efficiency (the ratio of file
characters transferred per second to the actual connection speed) from 86%
(for 94-byte packets) to 95% (with 2500-byte packets). When conditions
deteriorate on the connection, the packet length is automatically adjusted.

Original, basic Kermit was a stop-and-wait protocol because it had to work on
half-duplex as well as full-duplex connections. But connections through
satellites or packet-switched networks can have delays that seriously impede
the efficiency of a stop-and-wait packet protocol. For example, suppose
packets are 90 bytes = 900 bits long, and there is a one-second transmission
delay. For one packet and its response, the round-trip delay is 2 seconds.
At 300 bits per second (bps), the 3 seconds required to transmit the packet
plus the 2-second delay make 5 seconds, so throughput is 180 bps, 60%
efficiency. At 9600 bps, it takes only 1/10 second to transmit the same
packet, but the delay is still 2 seconds. Throughput is only 428 bps, 4.5%
efficiency. When connections have delays, efficiency can be improved by
lengthening the packets, but only if the connection is clean. On a noisy
connection, longer packets are more likely to be damaged in transmission and
take longer to retransmit.

On full-duplex connections, the new generation of Kermit software (IBM
mainframe Kermit excluded, which always has a half-duplex connection) can
transmit packets in a steady stream, processing the acknowledgements later as
they arrive, thus eliminating the effects of transmission delays, and also
eliminating the overhead of the acknowledgements themselves, since they are
"on the wire" at the same time as the data packets and therefore don't take up
any extra transmission time. This technique is called sliding windows,
because multiple packets are kept in a buffer (window) that "slides" forward
whenever the oldest packet in the window is acknowledged.

Using 94-byte packets without sliding windows on a connection that has a
1-second delay results (according to actual measurements) in an efficiency of
about 8%. Raising the packet length to 1500 on the same connection increases
the efficiency to 63%. Using sliding windows on the same connection raises
the efficiency to 82-90%, depending on the packet length.

------------------------------------------------------------------------
Optimum performance can be achieved on any given connection by choosing
the right combination of packet length and window size.
------------------------------------------------------------------------

To see a dramatic speed improvement using MS-DOS Kermit 3.13 and/or C-Kermit
5A, simply give these commands to each Kermit before file transfer:

SET WINDOW 3
SET RECEIVE PACKET-LENGTH 1000

Adjust as necessary. Longer delays require larger windows; noisier
connections (or devices with small input buffers) need shorter packets.
MS-DOS Kermit 3.13 and most versions of C-Kermit 5A allow the theoretical
maximum sizes, 31 and 9024 respectively, sufficient to overcome any reasonable
delay (for example, between the earth and the moon).

Compression

To reduce transmission overhead, the Kermit protocol uses a simple, but often
surprisingly effective, compression technique: repeated byte values are
represented by a count+byte combination. This technique is easy to program
and inexpensive in memory and CPU cycles, and is therefore implemented in most
Kermit software, including MS-DOS Kermit, C-Kermit, and IBM mainframe Kermit,
and is used automatically when available.

Analysis of large volumes of both textual and binary data shows an average
compression of 15-20%. Dramatic savings are achieved in certain types of
files, including tabular text (or any other type of text with lots of repeated
characters) and executable programs containing large amounts of pre-zeroed
data.

Prefixing

To achieve its ability to push data through even the most restrictive types of
connections--for example, to mainframes that are sensitive to certain control
characters, or on 7-bit connections, or on very noisy ones (one user said
recently, "Kermit will send data over a communication channel that is only
slightly better than a pair of tin cans connected with a wet string")--Kermit
formats its packets as lines of printable text. This is done by encoding each
control character as a sequence of two printable characters and, on 7-bit
connections only, encoding 8-bit characters as a sequence of two printable
7-bit bytes.

On some connections it is safe to transmit certain control characters "bare,"
without prefixing or encoding. "Unprefixing" of control characters can speed
up the transfer of binary files, particularly precompressed files, which tend
to contain a lot of bytes in the control range. MS-DOS Kermit 3.13 and
C-Kermit 5A(189) give you the ability to specify which control characters are
to be prefixed and which are not. In the benchmarks on pages 7 and -SPEEDY,
only three control characters are prefixed:

SET CONTROL UNPREFIXED ALL
SET CONTROL PREFIXED 0 1 129

This technique can be used even if the Kermit program on the other end doesn't
know anything about it, since well-designed Kermit software will, indeed,
accept bare control characters literally. The three exceptions above are NUL
(0), which is used internally by C-Kermit for string termination, and SOH (1)
and SOH+parity (129), Kermit's normal packet-start indicator. It takes some
experimentation to find the maximum safe set. That's why Kermit prefixes all
control characters by default: first make it work, then make it fast.

On 8-bit connections, Kermit transfers 8-bit data with no additional overhead.
On 7-bit connections, which are quite common--these are the connections that
use even, odd, mark, or space parity, often without the user's
knowledge--8-bit data is encoded using a single-shift technique, a prefix
character for each byte whose 8th bit is 1, similar to holding down the

------------------------------------------------------------------------
The Kermit protocol implementations found in many of the popular
commercial and shareware PC communication software packages are minimal
and perfunctory, usually lacking some or all of the performance
features...
------------------------------------------------------------------------

Shift key on your keyboard for each 8-bit character. This allows Kermit to
work where most other protocols fail. The amount of prefixing ranges from 0%
up to 100%, depending on the type of file.

Locking Shifts

To avoid the high overhead of transferring 8-bit text, particulary Cyrillic,
Hebrew, or Kanji, on 7-bit connections, a new "locking-shift" feature works
like the Caps Lock key on your PC: a special shift prefix applies to a entire
run of 8-bit characters, no matter how long, rather than to a single
character. Locking shifts are used in combination with single shifts to
achieve the most compact encoding.

Locking shifts are supported by MS-DOS Kermit 3.13, C-Kermit 5A, and IBM
Mainframe Kermit 4.2.4, and are negotiated automatically when parity is in use
(including when parity is detected automatically). They reduce the 8th-bit
prefixing penalty anywhere from 0% to 100%, depending on the groupings of the
8-bit characters within the file.

So Why the Bad Reputation?

The Kermit protocol implementations found in many of the popular commercial
and shareware PC communication software packages are minimal and perfunctory,
usually lacking some or all of the performance features just described. Many
of these same packages also include XMODEM, YMODEM, or ZMODEM protocol, which
(when they work at all) usually perform better than the basic short-packet,
stop-and-wait, prefix-everything Kermit protocol. Using a limited Kermit
implementation is like filling your bathtub from a dripping faucet instead of
turning the water on full blast. It is easy to see why users of such packages
might conclude that Kermit file transfers are slow. Nothing could be further
from truth; turn the page and see for yourself.

TRUE-LIFE BENCHMARKS

Table 1 illustrates the performance of the Kermit protocol implementations
found in different PC software packages. These measurements were made on a
direct 19200-bps serial connection, downloading a typical ASCII text file (the
VM/CMS Kermit-370 manual), 135087 bytes in length, from a Sun SPARCserver-10
with C-Kermit 5A(189) to the hard disk of an IBM PS/2 Model 70.

Table 1: Kermit Implementations Compared
------------------------------------------------------------------------------
Window Packet Time Speed
PC Software Size Length secs cps Effic. Remarks
------------------------------------------------------------------------------
Telix 1 94 131 1052 55% Long packets and s/w not avail
MTEZ 1 94 119 1158 60% Long packets and s/w not avail
Smartcom III 1 94 113 1220 64% Long packets and s/w not avail
PROCOMM PLUS 14 1000 77 1790 93% Window size not selectable
Zstem 340 2 1000 74 1863 97% Maximum window size 2
MS-DOS Kermit 3 1000 72 1915 99% Full control-character prefixing
MS-DOS Kermit 3 1000 69 1999 104% Only 0, 1, and 129 prefixed
------------------------------------------------------------------------------

The results speak for themselves.

If you thought Kermit file transfer was slow, you were probably not
using real Kermit software!

The UNIX-resident copy of the file, like all UNIX text files, uses only
linefeed (LF) for line termination. During text-mode transfer, each LF
becomes carriage return and linefeed (CRLF). There are 2814 lines in the
file, so the actual data size during (and after) transfer is 137901. Since
the connection runs at 1920 characters per second (10 bits per character), a
100%-efficiency transfer should take 137901 / 1920 = 71.8 seconds. The
following PC communications software was used:

MS-DOS Kermit 3.13 Columbia University, New York, NY, USA
MTEZ 1.16 MagicSoft, Inc., Lombard, IL, USA
PROCOMM PLUS 2.0 Datastorm Technologies, Inc., Columbia, MO, USA
Smartcom III 1.0A Hayes Microcomputer Products, Inc, Norcross, GA, US
Telix 3.21 deltaComm Development, Cary, NC, USA
Zstem 340 1.0.4 KEA Systems Ltd., Burnaby, BC, Canada

Kermit and X-Y-ZMODEM

XMODEM, YMODEM, and ZMODEM are the file tranfer protocols most commonly
compared with Kermit, and which are found in numerous shareware and commercial
communication software packages. XMODEM and YMODEM are stop-and-wait
protocols; XMODEM uses short blocks (128 data bytes), YMODEM uses longer ones
(1024 data bytes). ZMODEM is a streaming protocol.

The tables on page 8 compare XMODEM, YMODEM, ZMODEM, and Kermit transfers
between the PC and UNIX. The file transfer software on the UNIX system is sx
(XMODEM) / sb (YMODEM) / sz (ZMODEM) 3.24 (June 1993) and C-Kermit 5A(189).
On the PC, X-, Y- and ZMODEM transfers were done with Telix and PROCOMM PLUS
(which gave exactly the same results). For fairness, four types of files are
transferred:

ASCII Text: IKCKER.DOC 137901 bytes Our original ASCII text file
UNIX Binary: uuencode 24576 bytes A Sun SPARC binary executable program
PC Binary: KERMIT.EXE 197928 bytes An MS-DOS binary executable program
Precompressed: KERMIT.ZIP 238584 bytes A compressed ZIP archive

Tests were performed on four types of connections and in each trial, Kermit
transfers used a window size of 5 and a packet length of 5000, and control
prefixing was disabled except for NUL (0), Ctrl-A (1), and 129. As the tables
show, Kermit outperforms the competition every time.

Table 2 shows the figures for transferring all four files with each of the
four protocols on same direct connection used for Table 1. In this and the
following tables, the secs column shows the elapsed time of transfer in
seconds, the cps column shows actual file characters transferred per second,
and the eff column shows the percent efficiency (file characters per second
divided by the connection speed).

Table 2: X- Y- and ZMODEM vs Kermit on a 19200-bps Direct Connection
------------------------------------------------------------------------------
XMODEM YMODEM ZMODEM KERMIT
File Type secs cps eff secs cps eff secs cps eff secs cps eff
------------------------------------------------------------------------------
ASCII Text 89 1549 81% 76 1814 95% 73 1889 98% 69 1999 104%
UNIX Binary 15 1638 85% 13 1890 98% 13 1890 98% 9 2648 138%
PC Binary 127 1558 81% 109 1816 95% 107 1850 96% 100 1979 103%
Precompressed 153 1559 81% 133 1794 93% 130 1835 96% 129 1849 96%
-------------------------------------------------------------------------------

Table 3 shows the results for a local-call dialup connection using Telebit
T3000 modems, V.32bis modulation (14400 bps), V.42 error correction, V.42bis
compression, RTS/CTS hardware flow control, and an interface speed of 57600
bps. The efficiencies in this table are based on the modem's 14400-bps
connection speed, and therefore also reflect the modem's compression methods.

Table 3: X- Y- and ZMODEM vs Kermit with High-Speed Modems
------------------------------------------------------------------------------
XMODEM YMODEM ZMODEM KERMIT
File Type secs cps eff secs cps eff secs cps eff secs cps eff
------------------------------------------------------------------------------
ASCII Text 221 624 43% 79 1746 121% 42 3283 228% 39 3535 246%
UNIX Binary 32 768 53% 13 1890 131% 15 1638 114% 3 8192 569%
PC Binary 346 572 40% 129 1534 106% 83 2385 166% 80 2474 172%
Precompressed 500 477 33% 208 1147 79% 149 1601 111% 148 1612 112%
------------------------------------------------------------------------------

So far we've looked only at connections with no delays. Table 4 (also see
cover, left group) shows the results for a V.32 9600-bps cross-country dialup
connection from the same PC to a PC/486-50 running UNIX System V R4, with the
same C-Kermit, sx, sb, and sz software as on the Sun. The round-trip delay is
a fraction of a second. No error correction or compression is done by the
modems, but the connection is clean and no errors occurred.

Table 4: X- Y- and ZMODEM vs Kermit with Delays at 9600 bps
------------------------------------------------------------------------------
XMODEM YMODEM ZMODEM KERMIT
File Type secs cps eff secs cps eff secs cps eff secs cps eff
------------------------------------------------------------------------------
ASCII Text 422 327 33% 253 545 57% 217 635 66% 151 913 95%
UNIX Binary 73 337 35% 41 599 62% 32 768 80% 8 3072 320%
PC Binary 536 369 38% 319 620 65% 271 730 76% 207 956 99%
Precompressed 710 336 37% 363 657 68% 314 759 79% 284 840 87%
------------------------------------------------------------------------------

But if we always had clean connections, why would we need error-correcting
file-transfer protocols? Table 5 (and middle group, cover) shows the results
for the same cross-country connection, same settings, but with short bursts of
noise injected every two seconds, which cause errors and retransmissions in
all four protocols.

Table 5: X- Y- and ZMODEM vs Kermit with Delays and Noise at 9600 bps
------------------------------------------------------------------------------
XMODEM YMODEM ZMODEM KERMIT
File Type secs cps eff secs cps eff secs cps eff secs cps eff
------------------------------------------------------------------------------
ASCII Text 3346 41 4% fail 0 0% 438 315 33% 206 669 70%
UNIX Binary 573 43 4% 58 424 44% 144 171 18% 9 2736 284%
PC Binary 5154 42 4% fail 0 0% 566 350 36% 281 706 74%
Precompressed 5917 40 4% fail 0 0% 694 344 36% 385 621 65%
------------------------------------------------------------------------------

What about 7-Bit Connections? No Contest!

The foregoing benchmarks were conducted in environments where XMODEM, YMODEM,
and ZMODEM can work, namely 8-bit transparent connections that are not
sensitive to any control characters. Now let's look a different, but very
common, situation. Table 6 (and right group, cover) shows the results of
downloading the same files from an IBM Mainframe running VM/CMS and Kermit-370
4.2.5 to the PS/2 over a 19200-bps serial connection through an IBM 7171
protocol converter, which uses even parity and Xon/Xoff flow control.
Kermit's window size is 1 because the mainframe can operate only in half
duplex, and the packet length is 1920, the largest allowed by the 7171. All
control characters are prefixed.

Table 6: File Transfer on a 7-Bit Connection
------------------------------------------------------------------------------
XMODEM YMODEM ZMODEM KERMIT
File Type secs cps eff secs cps eff secs cps eff secs cps eff
------------------------------------------------------------------------------
ASCII Text - 0 0% - 0 0% - 0 0% 81 1702 88%
UNIX Binary - 0 0% - 0 0% - 0 0% 9 2730 142%
PC Binary - 0 0% - 0 0% - 0 0% 162 1221 63%
Precompressed - 0 0% - 0 0% - 0 0% 243 981 51%
------------------------------------------------------------------------------

The table shows Kermit file transfer to be infinitely more efficient than
X-Y-Z-MODEM transfer with IBM mainframes, because X-Y-Z-MODEM implementations
do not work with IBM mainframe operating systems such as VM/CMS, MVS/TSO, or
CICS, whereas Kermit works with all of them. Of course, 7-bit connections are
not peculiar to IBM mainframes. They are also used by other types of
mainframes and front ends as well as many types of networks and devices,
including some X.25-based public data networks and certain terminal servers.
You can use Kermit to transfer files on these connections, but not X-Y-Z-MODEM
protocols.

Locking Shifts

Although Kermit, unlike X-Y-Z-MODEM, can transfer 8-bit data over 7-bit
connections, there is often a performance penalty. This penalty is
particularly unfair to people whose written languages are encoded primarily in
8-bit characters, as are Russian, Hebrew, and Japanese. Russian text encoded
in any of the commonly used 8-bit Cyrillic character sets typically consists
of about 80% 8-bit characters and Japanese Kanji text often consists of
nearly 100% 8-bit characters.

Table 7 shows the results of attempting to upload typical Russian and Japanese
8-bit text files over a 19200-bps 7-bit serial connection to an IBM mainframe
using X-Y-Z-MODEM (it can't be done), Kermit with only single shifts (SS), and
Kermit with locking shifts (LS). The Kermit window size is 1 and the packet
length is 1920. In these cases, locking shifts improve the speed of transfer
30-40%.

Table 7: Effect of Locking Shifts
-------------------------------------------------------------------------------
X-Y-Z-MODEM KERMIT (SS) KERMIT (LS)
File Type Size secs cps eff secs cps eff secs cps eff
-------------------------------------------------------------------------------
Russian Text 52046 - 0 0% 55 946 49% 39 1334 69%
Japanese Text 29706 - 0 0% 34 873 45% 20 1485 77%
-------------------------------------------------------------------------------

Conclusion

Kermit protocol works in practically every communication and computing
environment. You don't have to be a data communications expert to transfer a
file with Kermit software. Its first priority is getting your data through
safe and sound, and its second is efficiency. Kermit's conservative protocol
defaults reflect these priorities: First make it work, then make it fast.

But as the tests show, today's Kermit software, when given several simple
commands to enable its efficiency features, outperforms X-, Y-, and ZMODEM
protocol transfers every time. And real Kermit software also outperforms the
Kermit protocol implementations found in commercial and shareware
communications programs. Skeptical? Run your own tests!