25-Oct-91 14:45:19-GMT,4410;000000000401
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Date: Fri, 25 Oct 91 10:45:06 EDT
From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: "Glenn E. Thobe" <thobe@getunx.info.com>
Subject: Re: Recent Kermit Protocol Extensions
In-Reply-To: Your message of Thu, 24 Oct 91 23:31:06 PDT
Message-Id: <CMM.0.90.0.688401906.fdc@watsun.cc.columbia.edu>

> Frank-
> In comp.protocols.kermit you write:
> 
> >The next major effort in Kermit protocol expansion (no commitment expressed
> >or implied!) is the addition of an effective and portable compression
> >method. ... For maximum transportability, the
> >result of compression should be a sequence of 7-bit ASCII printable
> >characters (32-126 or a subset of these).  
> 
> No, it should be flexible enough to efficiently use whatever character
> space is available: 32-126 or 32-126,160-255 or ...
> 
> Development of a compression scheme should be combined with the
> development of an efficient means of transporting binary data
> uniformly distributed over the codes 0-255.
> 
We have already done this, to an extent.  But because Kermit will always have
to work on 7-bit connections, and connections where any number of C0 or C1
control characters could interfere with transmission, it would save us a lot
of complexity if the compression method always encodes into ASCII -- that
relieves us of the overhead of prefixing, quoting, and shifting
already-compressed text (to get an idea of what I mean, see the lshift.txt
paper that was mentioned in the same message).

> The current method, control-code prefixing, increases the size of such
> a binary file by a good 25% on an 8-bit channel, dissipating about half 
> the gains achieved by precompressing the file.  On a 7-bit channel, the
> combination of 8-th bit and control-code prefixing increases the
> effective file size by some 75%.  Compare these losses the theoretical
> minimums of 6.5% and 21.5%, respectively.
> 
That's why we need a good compression scheme that automatically gives 8-bit
and C0/C1 transparency.

> For 7-bit channels, a 4-to-5 byte scheme such as used in the tarmail 
> utility solves this problem (the basis of this algorithm is to treat 
> each four bytes as a 32-bit unsigned integer and express it as 5 digits 
> in base 95).  For 8-bit channels, a similar scheme could be worked out 
> to utilize both the lower and upper ascii printables efficiently.  
> 
Right, but better still to design an algorithm that optimally compresses
into ASCII graphics from the start -- then there is no need to expand the
compressed data for transmission.

> Such an alternative to control-code (and even 8th-bit) prefixing
> should be added along with compression, but should be selectable
> (negotiable) even without compression.
> 
There has actually been a lot of talk about this, and this is a possibility.
But the gain in efficiency over current methods is minimal compared (I hope)
to what an effective compression algorithm will yield, so I'd much rather
concentrate on compression than on yet-another-way-of-achieving-transparency-
without-compression.

> (Since a protocol extension is envisioned anyway, you might consider 
> extending the character space to optionally include at least the 
> upper ASCII control characters.)
> 
That'll never work.  C1 controls can be misinterpreted as C0 controls
by communication devices or hosts that ignore the 8th bit, and even when they
are correctly seen as C1 controls can trigger unwanted actions like shifting.

> Sorry, I don't have any good alternative compression algorithms.
> It is just possible that the LZW *program* might be incorporated into
> Kermit or left as a separate module without legal problems on the
> grounds of prior art, publication, etc.  Or the patent holders
> might write an exemption for Kermit use only.
> 
It's a possibility.

> Best wishes.
> 
> -Glenn Thobe <getunx!thobe@uunet.uu.net>
> (A long-time Kermit enthusiast.)
> ---
> P.S. Bravo for your pioneering internationalization efforts!
>
Thanks!  (Do you use this stuff yourself?)

> P.P.S. What can be done to get the most popular BBS's to support Kermit?
> 
Beats me.  Maybe as BBS's become popular outside the USA -- where
(inter)national character set conversion is important and 8-bit transparency
cannot be assumed (because of the PTTs) -- we'll see more Kermit support in
BBS software.

26-Oct-91  6:20:53-GMT,1200;000000000011
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Subject: Compression Protocol
To: fdc@watsun.cc.columbia.edu (Frank da Cruz)
Date: Sat, 26 Oct 91 2:33:03 MET
From: Kristoffer Eriksson <ske@pkmab.se>
X-Mailer: ELM (ver 2.2 PL16)
Message-Id: <9110260233.AA13833@pkmab.se>

>The next major effort in Kermit protocol expansion (no commitment expressed
>or implied!) is the addition of an effective and portable compression
>method.

Isn't this a bit over-ambitious? Why not just provide a portable but
separate compression program that you can run before and after transfer?
That way you would avoid bloating the kermit program itself.

----
Kristoffer Eriksson, Peridot Konsult AB, Hagagatan 6, S-703 40 Oerebro, Sweden
Phone: +46 19-13 03 60 !  e-mail: ske@pkmab.se
Fax:   +46 19-11 51 03 !  or ...!{uunet,mcsun}!mail.swip.net!kullmar!pkmab!ske

28-Oct-91 15:18:19-GMT,1076;000000000001
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From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: Kristoffer Eriksson <ske@pkmab.se>
Subject: Re: Compression Protocol
In-Reply-To: Your message of Sat, 26 Oct 91 2:33:03 MET
Message-Id: <CMM.0.90.0.688663089.fdc@watsun.cc.columbia.edu>

> >The next major effort in Kermit protocol expansion (no commitment expressed
> >or implied!) is the addition of an effective and portable compression
> >method.
> 
> Isn't this a bit over-ambitious? Why not just provide a portable but
> separate compression program that you can run before and after transfer?
> That way you would avoid bloating the kermit program itself.
> 
Because the sending Kermit has to do the record format and character-set
conversions before compressing, and the receiving Kermit has to do them after
decompressing.  Thus we would need not only "n" portable compression programs,
but also "n x (n - 1)" character-set and record-format conversion programs.

- Frank

28-Oct-91 18:25:13-GMT,3632;000000000011
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From: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
To: fdc@watsun.cc.columbia.edu (Frank da Cruz)
Date: Mon, 28 Oct 91 13:17:55 CST
In-Reply-To: <CMM.0.90.0.688662617.fdc@watsun.cc.columbia.edu>; from "Frank da Cruz" at Oct 28, 91 10:10 am

[Compression to printable chars...]
> Right, but better still to design an algorithm that optimally compresses
> into ASCII graphics from the start -- then there is no need to expand the
> compressed data for transmission.

I'm not sure I agree here.  I suppose it might depend on the algorithm,
but I don't see how a 1-pass compress can do any optimization into
a specified character set to make it come out any better than compressing
into 256 characters, then expanding through something like btoa.  That
is, the compress pass is always going to generate an essentially random
output and thus can't do any optimization for the output character set.
The extra processing would be irrelevant compared to the compress pass anyway.

> > Such an alternative to control-code (and even 8th-bit) prefixing
> > should be added along with compression, but should be selectable
> > (negotiable) even without compression.

> There has actually been a lot of talk about this, and this is a possibility.
> But the gain in efficiency over current methods is minimal compared (I hope)
> to what an effective compression algorithm will yield, so I'd much rather
> concentrate on compression than on yet-another-way-of-achieving-transparency-
> without-compression.

But any way you look at it, control-code prefixing is going to cost about
25% of your possible bandwidth, and on many (probably most) connections
it isn't necessary.  I think you need both options and they need to
be coordinated so the compression can use the full character set if
the transfer link allows it.  Likewise, if the material is pre-compressed
with unix compress, zip, zoo, arc or the like, the kermit compression
is unlikely to help and quite likely to expand the data unless you are
careful to allow compression to be disabled when it isn't working
(per-packet would be nice here). In this case if the transfer link
needs control/hi-bit escaping you would still be ahead to pass the
data stream through a btoa style encoder instead of using traditional
kermit escaping, so that should be a separate choice with or without
the compression.   Also, this stuff needs to be processed
so that the CRC checking sees the same thing on both ends.  The current
8-bit quoting is done such that it can be set on one end and not on
the other and the error checking still passes.

> > Sorry, I don't have any good alternative compression algorithms.

You might want to check with: brnstnd@KRAMDEN.ACF.NYU.EDU (Dan Bernstein).
He has posted some original code to usenet and made references to some
work in progress.  I think the intent was to have something available
to use without patent restrictions.

Les Mikesell
  les@fb.com

28-Oct-91 18:56:12-GMT,4752;000000000001
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From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
In-Reply-To: Your message of Mon, 28 Oct 91 13:17:55 CST
Message-Id: <CMM.0.90.0.688676163.fdc@watsun.cc.columbia.edu>

> I'm not sure I agree here.  I suppose it might depend on the algorithm,
> but I don't see how a 1-pass compress can do any optimization into
> a specified character set to make it come out any better than compressing
> into 256 characters, then expanding through something like btoa.  That
> is, the compress pass is always going to generate an essentially random
> output and thus can't do any optimization for the output character set.
> The extra processing would be irrelevant compared to the compress pass
> anyway. 
> 
You might be right, but I think it's worth trying to come up with an algorithm
that compresses into printable ASCII before deciding it can't be done.  You
should be able to use codewords of any bit length you like, so (for example)
we could compress into 6-bit codewords and add some offset (like 33) to get
printable ASCII.  Wasteful but simple.  Needs analysis.

> But any way you look at it, control-code prefixing is going to cost about
> 25% of your possible bandwidth, and on many (probably most) connections
> it isn't necessary.
>
But you never know which control characters are going to cause problems.  The
Kermit programs themselves can't know, because they don't know what kinds of
things are sitting between them.  They can't even send a "test pattern"
because that might put some intermediate box out of commission (e.g. Ctrl-P
might put an X.3 PAD into local mode; Series/1-style 3270 protocol converters
normally will not pass *any* control characters through to the host; etc etc).
I don't think we can ever assume that any particular control characters are
safe, even though some subset of them usually are on any particular connection.

> I think you need both options and they need to be coordinated so the
> compression can use the full character set if the transfer link allows it.
>
But then you put an awful burden on the user.  I'd prefer to see compression
work automatically if both Kermit programs support it, without having to make
the user figure whether and to what degree the link is transparent to control
and 8-bit characters.

> Likewise, if the material is pre-compressed with unix compress, zip, zoo,
> arc or the like, the kermit compression is unlikely to help and quite likely
> to expand the data unless you are careful to allow compression to be
> disabled when it isn't working (per-packet would be nice here).
>
Like all Kermit options (except control-character escaping), compression
will be disablable by the user.  If a file (precompressed or not) has a
random distribution of characters, there will indeed be about 25% expansion
for control prefixing.  On a 7-bit connection, there would also be an
additional 50% for 8th-bit escaping, except that now we have a locking shift
mechanism that reduces that to practically nil.

In the interest of minimizing the internal complexity of the Kermit program
and its user interface, I think it might be better, when compressing, to strip 
away the already-complicated control- and 8th-bit escaping methods and
compress directly into a form that is guaranteed to be transparent to even the
most hostile connections.  It's a tradeoff -- everybody pays a price in some
additional overhead, but everybody gets a compression method that works
without requiring just the right combination of manual tweaking.

> In this case if the transfer link needs control/hi-bit escaping you would
> still be ahead to pass the data stream through a btoa style encoder instead
> of using traditional kermit escaping, so that should be a separate choice
> with or without the compression.
>
btoa is a 4-for-3 scheme, right?  For most types of data (all text, many
kinds of binaries), Kermit is already better than that.

> Also, this stuff needs to be processed
> so that the CRC checking sees the same thing on both ends.  The current
> 8-bit quoting is done such that it can be set on one end and not on
> the other and the error checking still passes.
> 
That only happens if there is a bug in one of the Kermit programs.  The
8th-bit prefixing negotiation is supposed to work.

> You might want to check with: brnstnd@KRAMDEN.ACF.NYU.EDU (Dan Bernstein).
> He has posted some original code to usenet and made references to some
> work in progress.  I think the intent was to have something available
> to use without patent restrictions.
> 
Thanks for the pointer!

- Frank

28-Oct-91 20:36:19-GMT,4650;000000000011
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From: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
To: fdc@watsun.cc.columbia.edu (Frank da Cruz)
Date: Mon, 28 Oct 91 15:29:26 CST
In-Reply-To: <CMM.0.90.0.688676163.fdc@watsun.cc.columbia.edu>; from "Frank da Cruz" at Oct 28, 91 1:56 pm

> You might be right, but I think it's worth trying to come up with an algorithm
> that compresses into printable ASCII before deciding it can't be done.

No question that it can be done - I just don't think it can be done much
more efficiently than compression without regard to the output character
set followed by remapping.

> >
> But you never know which control characters are going to cause problems.

Right - in my original message I suggested explicit commands for
SET CONTROL-ESCAPING ON/OFF/MINIMAL/LIST  so you could tell it how
to handle a particular link optimally.  Minimal would be
xon/xoff/ctl-p/and the kermit start-of-packet.  Off would probably
still have to include the start-of-packet.  This would need to be
negotiated with the remote, although I suspect that most kermits would
receive correctly anyway if the sender unilaterally only escaped the
minimal set.

> > I think you need both options and they need to be coordinated so the
> > compression can use the full character set if the transfer link allows it.
> >
> But then you put an awful burden on the user.  I'd prefer to see compression
> work automatically if both Kermit programs support it, without having to make
> the user figure whether and to what degree the link is transparent to control
> and 8-bit characters.

It's not a burden if the default is for the worst-case.  Then the user
doesn't need to know, but if he does know about a particular link he can
easily set the option to take advantage of it. 

> In the interest of minimizing the internal complexity of the Kermit program
> and its user interface, I think it might be better, when compressing, to strip 
> away the already-complicated control- and 8th-bit escaping methods and
> compress directly into a form that is guaranteed to be transparent to even the
> most hostile connections.  It's a tradeoff -- everybody pays a price in some
> additional overhead, but everybody gets a compression method that works
> without requiring just the right combination of manual tweaking.

Yes, that is the right thing to do for the default case, but I still think
you should accomodate the people who are using expensive but transparent
links and want the optimum speed.  In this case, the data is very likely
going to be pre-compressed anyway.

> > In this case if the transfer link needs control/hi-bit escaping you would
> > still be ahead to pass the data stream through a btoa style encoder instead
> > of using traditional kermit escaping, so that should be a separate choice
> > with or without the compression.
> >
> btoa is a 4-for-3 scheme, right?  For most types of data (all text, many
> kinds of binaries), Kermit is already better than that.

But the people who care about transfer time are probably compressing the
files already.  They are probably also using some other transfer
protocol if they have transparent links, but kermit could handle it
with a simple option.  The 4-3 coding just makes things fall out on
byte boundaries.  You could do 8-bit to n-bit coding any way you like
as long as it is internal to kermit. The ideal thing would be to make
the compression and encoding options settable per-packet (at least when
you use long packets).  Then the sender could prepare compressed/encoded,
straight encoded, and traditional kermit escaping packets and send whichever
happens to be shorter.  No matter what compression scheme you pick there
will always be data that won't compress.  If you look at tests of MNP5
vs. V.42bis modem transfer rates you will see that the V.42bis handles
pre-compressed data much better because it knows enough not to expand
the stream accidentally for any length of time.   

Les Mikesell
  les@fb.com

28-Oct-91 21:34:03-GMT,5365;000000000001
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From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
In-Reply-To: Your message of Mon, 28 Oct 91 15:29:26 CST
Message-Id: <CMM.0.90.0.688685632.fdc@watsun.cc.columbia.edu>

> > You might be right, but I think it's worth trying to come up with an
> > algorithm that compresses into printable ASCII before deciding it can't 
> > be done.
> 
> No question that it can be done - I just don't think it can be done much
> more efficiently than compression without regard to the output character
> set followed by remapping.
> 
Right, probably somewhat less efficiently -- but more portably (in the same
way that Kermit protocol is more portable than Xmodem, but at some expense
in efficiency -- however, Xmodem's efficiency is zero on a 7-bit link, so it
kind of averages out).

> > But you never know which control characters are going to cause problems.
> 
> Right - in my original message I suggested explicit commands for
> SET CONTROL-ESCAPING ON/OFF/MINIMAL/LIST  so you could tell it how
> to handle a particular link optimally.
>
This is, of course, a possibility and there's nothing in the protocol that
prevents this.  You are correct that most Kermit programs would accept control
characters in data packets "bare" (but there's no good way to take a complete
census).  But it's a cost vs benefit thing -- the cost in complexity and
confusion would be high, few people would actually use it; those that did
would become confused when the "transparency mask" changed from connection to
connection (and it will), etc.  The overall gain for text is minimal, and
even for binary files with uniform distribution of chars, 25% at best.  So
yes, it can be done, but the payoff would not be nearly so dramatic as for a
good compression scheme, so it's lower on the priority list.  Btw, there have
also been some very clever alternate escaping mechanisms proposed for
combinations of control and 8-bit chars, but again, the benefit/cost ratio is
low.

> Minimal would be xon/xoff/ctl-p/and the kermit start-of-packet.
>
...and carriage return, and linefeed, and 0xff (telnet), and Ctrl-C (most
things), and Ctrl-I (often gets expanded to spaces), and Ctrl-N/Ctrl-O
(shift-in/shift-out), and various 8-bit shifts (SS2, SS3, LS2, LS3, LS2R,...),
etc etc.

> Off would probably still have to include the start-of-packet.  This would
> need to be negotiated with the remote,
>
But it can't be.  They don't know, and there is no way (unless I'm being
incredibly dense) that they can figure it out for themselves.  "Hey, here's
a Ctrl-S, did you get it?" -- what happens next (now that the link is Xoff'd).
Similar scenarios for each Ctrl character...

> It's not a burden if the default is for the worst-case.  Then the user
> doesn't need to know, but if he does know about a particular link he can
> easily set the option to take advantage of it. 
> 
Well, yes, that's an option.  The thing can be designed (for example -- truly
off the top of the head) to use, say, 6-bit codewords on a 7-bit link and
7-bit codewords on an 8-bit link (etc, into the future).  But we'd have to be
pretty clever about designing an algorithm that generalizes to n-bit codewords
(and we should be!).

> But the people who care about transfer time are probably compressing the
> files already.
>
Actually, there are two things going on here.  Precompression for quick
transfers is one thing, and hopefully Kermit will make that relatively
unnecessary.  People also put things into archives for EASE of transport --
many unlike files collected into a single flat file, which floats around from
place to place until it finally reaches its final destination where it is
de-archived.  Unfortunately, these schemes are mostly system-specific -- DOS
ZIP archives, UNIX TAR archives, VMS BACKUP archives, etc -- they depend on
the home system's encoding and record format conventions.

> The 4-3 coding just makes things fall out on byte boundaries.  You could do
> 8-bit to n-bit coding any way you like as long as it is internal to kermit.
> The ideal thing would be to make the compression and encoding options
> settable per-packet (at least when you use long packets).  Then the sender
> could prepare compressed/encoded, straight encoded, and traditional kermit
> escaping packets and send whichever happens to be shorter.
>
This is a worthy goal, but phew!  -- Try to picture the programs that
implement it.  My instinct tells me that if we don't keep it pretty simple,
programmers are just not going to tackle it.  Also we have the issue of CPU
speed here -- on your typical 80286 and below, the computation could easily
wind up being the bottleneck.

> No matter what compression scheme you pick there will always be data that
> won't compress.  If you look at tests of MNP5 vs. V.42bis modem transfer
> rates you will see that the V.42bis handles pre-compressed data much better
> because it knows enough not to expand the stream accidentally for any length
> of time.
> 
Just the kind of info I'm looking for.  Have you seen any analyses of V.42bis
published anywhere?  I have (at least part of) the V.42bis specification
(text, no pictures), but no analysis at all.

Thanks!  - Frank

28-Oct-91 23:51:52-GMT,6495;000000000011
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From: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
To: fdc@watsun.cc.columbia.edu (Frank da Cruz)
Date: Mon, 28 Oct 91 18:41:42 CST
In-Reply-To: <CMM.0.90.0.688685632.fdc@watsun.cc.columbia.edu>; from "Frank da Cruz" at Oct 28, 91 4:33 pm

> > No question that it can be done - I just don't think it can be done much
> > more efficiently than compression without regard to the output character
> > set followed by remapping.

> Right, probably somewhat less efficiently -- but more portably

Keeping the phases separate doesn't affect portability - and it lets you
use each for its own purpose.

[reduced escaping...]
> This is, of course, a possibility and there's nothing in the protocol that
> prevents this. 

> The overall gain for text is minimal, and
> even for binary files with uniform distribution of chars, 25% at best.  So
> yes, it can be done, but the payoff would not be nearly so dramatic as for a
> good compression scheme, so it's lower on the priority list.  Btw, there have
> also been some very clever alternate escaping mechanisms proposed for
> combinations of control and 8-bit chars, but again, the benefit/cost ratio is
> low.

What's low?  We spend about $6-10,000/month on phone charges for data
communications and that's probably low for a national organization. 
I'd think 25% of that would be worth thinking about.
Certainly worth figuring out whether you should issue a set command in
your start-up scripts.  Most of it isn't using kermit now, but I'd switch
some things over if it could match the other protocols in speed.

> > Off would probably still have to include the start-of-packet.  This would
> > need to be negotiated with the remote,
> >
> But it can't be.  They don't know, and there is no way (unless I'm being
> incredibly dense) that they can figure it out for themselves. 

Right - we have to assume that the sender knows what he is doing, or at least
is capable of learning from the transfer failures when he does it wrong.  The
negotiation part would just be to be assured that the remote knows how to
receive the unescaped control characters - it's really up to the sender to
do it all.  Basically you are just asking the user to be aware of the same
things they currently need to know when they chose a protocol, if the
list of choices includes xmodem, etc.

> Actually, there are two things going on here.  Precompression for quick
> transfers is one thing, and hopefully Kermit will make that relatively
> unnecessary.  People also put things into archives for EASE of transport --
> many unlike files collected into a single flat file, which floats around from
> place to place until it finally reaches its final destination where it is
> de-archived.  Unfortunately, these schemes are mostly system-specific -- DOS
> ZIP archives, UNIX TAR archives, VMS BACKUP archives, etc -- they depend on
> the home system's encoding and record format conventions.

This is becoming much less of an issue with portable versions of zoo, arc
and zip around, as well as tar and cpio.  I happen to prefer zoo at the
moment, but I may change my mind when I get around to pickup up the zip
source.   The real challenge for portable kermit compression has got to
be the contortions you have to go through when ascii<->ebcdic conversions
are done by something else between the sender and receiver, but I'm not
sure it's worth crippling other transfers just to allow this one to be
a little bit faster.  (I do use a unix <-> ibm mainframe kermit link,
but it's local and completely script driven, so the speed doesn't matter
much to me.)

> > The 4-3 coding just makes things fall out on byte boundaries.  You could do
> > 8-bit to n-bit coding any way you like as long as it is internal to kermit.
> > The ideal thing would be to make the compression and encoding options
> > settable per-packet (at least when you use long packets).  Then the sender
> > could prepare compressed/encoded, straight encoded, and traditional kermit
> > escaping packets and send whichever happens to be shorter.
> >
> This is a worthy goal, but phew!  -- Try to picture the programs that
> implement it.  My instinct tells me that if we don't keep it pretty simple,
> programmers are just not going to tackle it.  Also we have the issue of CPU
> speed here -- on your typical 80286 and below, the computation could easily
> wind up being the bottleneck.

The worst case of this is the one you mostly want to try, though.  That is,
you need to go through the motions of compressing and doing the portable
encoding anyway.  The other 2 choices (encoding only/traditional kermit)
are trivial in computation for the sender, and all you need are 3 bits in
a packet header to allow an on-the-fly choice of which to send (although
you do have to be careful about compression methods that change their
tables dynamically).  If the encoding/decompression steps are separate,
the receiver doesn't have to be much more complicated at all.  You will
already need traditional kermit packet handling, so you just add the 
new alternative routines and process each step according to the header
indication.  I'm not sure it's worth going down to the short packets
with this, though.

> Just the kind of info I'm looking for.  Have you seen any analyses of V.42bis
> published anywhere?  I have (at least part of) the V.42bis specification
> (text, no pictures), but no analysis at all.

No, I've only seen some commentary on usenet.  Toby Nixon (an engineer
at Hayes) has explained some of the theory (like what happens when you
send a worst-case data stream), but unfortunately I didn't save any of
it.  Basically, after a certain interval of expanding data it will dump
its tables and start retraining, somehow syncing this with the remote
end.

Les Mikesell
  les@chinet.chi.il.us

29-Oct-91  0:19:42-GMT,2071;000000000001
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Date: Mon, 28 Oct 91 19:18:01 EST
From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
In-Reply-To: Your message of Mon, 28 Oct 91 18:41:42 CST
Message-Id: <CMM.0.90.0.688695481.fdc@watsun.cc.columbia.edu>

> What's low?  We spend about $6-10,000/month on phone charges for data
> communications and that's probably low for a national organization. 
> I'd think 25% of that would be worth thinking about.
>
Total agreement.  But wouldn't you rather reduce your phone bill by, say,
50%, 100%, or more, instead of 25%?  With only a couple people willing to put
in whatever spare-time hours they can grab, nights and weekends, we have to
put our efforts where the biggest payoff is.  Changing Kermit protocol and
software to transmit & negotiate bare controls would take a lot longer than
you think -- probably a year for the flame wars alone...

> Right - we have to assume that the sender knows what he is doing, or at least
> is capable of learning from the transfer failures when he does it wrong.
>
Yes, but...  Can you suggest a method that a mere mortal can follow?

> The real challenge for portable kermit compression has got to
> be the contortions you have to go through when ascii<->ebcdic conversions
> are done by something else between the sender and receiver...
>
Not just ASCII/EBCDIC, but Latin-1, JIS Kanji, etc etc.  We've already figured
this one out -- it's how mainframe Kermit works.  We're adding compression at
a level (sort of "high presentation") where this is not an issue (long story).

> The worst case of this is the one you mostly want to try, though.  That is,
> you need to go through the motions of compressing and doing the portable
> encoding anyway.  The other 2 choices (encoding only/traditional kermit)
> are trivial...
>
I must say, this is an interesting idea (mixing compressed and uncompressed
packets) but, no doubt, a lot trickier than it sounds.

- Frank

29-Oct-91 16:06:22-GMT,5542;000000000011
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From: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
To: fdc@watsun.cc.columbia.edu (Frank da Cruz)
Date: Tue, 29 Oct 91 10:52:50 CST
In-Reply-To: <CMM.0.90.0.688695481.fdc@watsun.cc.columbia.edu>; from "Frank da Cruz" at Oct 28, 91 7:18 pm

> Total agreement.  But wouldn't you rather reduce your phone bill by, say,
> 50%, 100%, or more, instead of 25%?  With only a couple people willing to put
> in whatever spare-time hours they can grab, nights and weekends, we have to
> put our efforts where the biggest payoff is.  Changing Kermit protocol and
> software to transmit & negotiate bare controls would take a lot longer than
> you think -- probably a year for the flame wars alone...

I can't argue about the politics, but from a practical point of view, I'd
rather have the ability to pass the control characters. You are unlikely
to beat V.42bis compression, and that is increasingly available in the
modem hardware, and in the cases where it really matters we can always
pre-compress before transmission.  The thing we currently can't do is
move the bytes with any efficiency after they have been compressed.
I'd be happy with a simple non-negotiated command to turn off escaping
on everything but start-of-packet, ^C, ^P, and if flow control is
enabled, ^S and ^Q (I suppose you'd have to look at how most versions
handle received nulls, too...).  Then just start fixing versions to do
the right thing on the receiving side if they don't already.  I don't
see how anyone can flame about something that is an option and not
enabled unless you want it.  But the important thing is to get this
ability in place so an alternate encoding method can follow the setting.
Typical text compression might be in the 60-70% range using the bandwith
provided by an 8-bit data stream.  The nature of compression is going
to make the character distribution essentially random (otherwise the
compression isn't optimal), so restricting the character set for
transmission is going to take a statistical 1/256th of your efficiency
for every character you disallow, plus the overhead of the mapping. 
There is just no magical way to avoid this.

>>Right - we have to assume that the sender knows what he is doing, or at least
>> is capable of learning from the transfer failures when he does it wrong.
>>
> Yes, but...  Can you suggest a method that a mere mortal can follow?

Yes - the way you learn everything else: if it hurts, don't do it!

It's no more complicated than getting your parity set right or any of
the other things you already have to know.  People are already making
this particular decision, except that instead of telling kermit to
work efficiently they just use a different protocol that is more efficient
by design.  But, programs like C-kermit 5A and the MSDOS 3.11 are so good
that they could handle just about everything else anyone would want to do.
Maybe it would be simpler to just graft a zmodem option into those programs.

> Not just ASCII/EBCDIC, but Latin-1, JIS Kanji, etc etc.  We've already figured
> this one out -- it's how mainframe Kermit works.  We're adding compression at
> a level (sort of "high presentation") where this is not an issue (long story).

I hope this means that the decompression will be done before the checksum/
CRC is computed so that any screwups will appear as a transmission error
and not go undetected.  I've had this happen with 8-bit quoting with one
end happily gobbling all my "&" characters and converting the next character
and reporting that the transfer went fine.  C source is not a pretty sight
after this happens.  I know you said that this was supposed to be negotiated,
so it was probably an error in the program, but it still should have failed
the transfer instead of reporting success with different file contents at
the other end.

> I must say, this is an interesting idea (mixing compressed and uncompressed
> packets) but, no doubt, a lot trickier than it sounds.

The only tricky part is that the compressor needs to know about it.  You
couldn't just take the stream from unix compress and do that because
the compression tables are part of the data stream and they adapt to the
data on the fly.  Also, compression code generally builds a bit-stream,
so you might have to fill out to a byte boundary, effectively going through
the motions of ending a file at the compression level and then starting up
from scratch on the next compressable packet.  How complex this is would
depend on the nature of the compression code.  Off the top of my head, though,
I would bet that you could build non-adaptive tables for english text that
would work as well as any adaptive scheme if you simply pass through the
pieces that don't compress.  But, I suppose you need to support other
languages as well so you need some sort of adaptive method.

Les Mikesell
  les@fb.com

29-Oct-91 17:06:01-GMT,2579;000000000001
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Date: Tue, 29 Oct 91 12:04:56 EST
From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
In-Reply-To: Your message of Tue, 29 Oct 91 10:52:50 CST
Message-Id: <CMM.0.90.0.688755896.fdc@watsun.cc.columbia.edu>

> I can't argue about the politics, but from a practical point of view, I'd
> rather have the ability to pass the control characters. You are unlikely
> to beat V.42bis compression, and that is increasingly available in the
> modem hardware, and in the cases where it really matters we can always
> pre-compress before transmission.
>
But pre-compression usually only works between like systems, or a small set
of unlike systems (e.g. where "portable zip" runs).  Even then, it doesn't
handle the character set conversions, and often doesn't record format
conversions either, which is why you really want to slip compression into
Kermit's presentation layer after all that has been done.

> > Yes, but...  Can you suggest a method that a mere mortal can follow?
> Yes - the way you learn everything else: if it hurts, don't do it!
> It's no more complicated than getting your parity set right or any of
> the other things you already have to know.
>
Well, it's actually much more complicated.  Parity is usually an "on" or
"off" decision; only in rare cases does it actually matter which kind of
(non-none) parity to use.  At most we have 5 choices.  For optimal control
character transparency the user has to test over 60 different characters for
each connection.  That could take hours, since any given test could blow the
connection away.  There goes your efficiency.

> I hope this means that the decompression will be done before the checksum/
> CRC is computed so that any screwups will appear as a transmission error
> and not go undetected.
>
Definitely.  The entire packet is not compressed, only the contents of the
data field, and the checksum is computed on the characters that go into the
packet.

> I've had this happen with 8-bit quoting with one
> end happily gobbling all my "&" characters and converting the next character
> and reporting that the transfer went fine.
>
Like I said, that's a bug -- it should not happen.  Before C-Kermit 5A is
released, I'll be sure this is working right.

> I would bet that you could build non-adaptive tables for english text that...
>
We actually did this once in a pilot project.  But it's a no-no for Kermit.
No anglocentrism allowed.

- Frank

29-Oct-91 18:55:14-GMT,4225;000000000005
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From: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
To: fdc@watsun.cc.columbia.edu (Frank da Cruz)
Date: Tue, 29 Oct 91 13:48:32 CST
In-Reply-To: <CMM.0.90.0.688755896.fdc@watsun.cc.columbia.edu>; from "Frank da Cruz" at Oct 29, 91 12:04 pm

> But pre-compression usually only works between like systems, or a small set
> of unlike systems (e.g. where "portable zip" runs).

Then that's a problem in its own right and should be addressed directly,
perhaps by promoting the use of "zoo" which should be available for
the common ascii machines.

> Even then, it doesn't
> handle the character set conversions, and often doesn't record format
> conversions either, which is why you really want to slip compression into
> Kermit's presentation layer after all that has been done.

I hesitate to comment on this because I think this belongs elsewhere as
well.  That is, any machine needing file format preservation should have
a way to encapsulate the file into a byte stream and do the reverse. A
file being sent over a communications line may traverse many machines
before it's actually used again.  I'd prefer for the transfer protocol
to transfer the file contents unchanged so that the proper conversions
can be done only at the end point.  For the terminal emulation portion
you need to do it in real time, and you don't have any choice about the
ascii-ebcdic conversions when going through a mainframe front-end, but
files can always be run through a separate conversion.  Isn't that why
we put things in data files in the first place?  Plus there are many
things that you might want to preserve (like the ownership, permissions,
filenames that may not map reversibly on the current machine) that can
only be done by encapsulating the file and attributes for transmission
because intermediate filesystems may not share the same concepts.

> Well, it's actually much more complicated.  Parity is usually an "on" or
> "off" decision; only in rare cases does it actually matter which kind of
> (non-none) parity to use.  At most we have 5 choices. 

Make that 5 choices at each end, thus 25 possibilities, plus flow control,
plus file type, plus a few other settings that have to be set right just
to get a single file across unchanged.  If you are arguing that kermit
is now simple to use and would be rendered complex by another settable
option, I disagree.  Kermit already has a deserved reputation for being
difficult to use - one settable option more or less isn't going to change
that.

> For optimal control
> character transparency the user has to test over 60 different characters for
> each connection.  That could take hours, since any given test could blow the
> connection away.  There goes your efficiency.

That's an unlikely worst-case scenario.  In practice, someone will know
if a particular link isn't transparent (and why), and tell the people
using it the proper settings to use.  How long would it take you to
find the right mainframe handshake character?  The same argument would
apply there.  And I still say that people already know this and just
use different protocols without being overwhelmed by complexity.  Ideally,
kermits could be set up to send an attribute packet defining any known
problem characters for the link at start-up (escaped or in hex) but
that would only work for point to point links with computers that have
some concept of which device they are using, so the lack of such an
indication shouldn't automatically mean the link is transparent. 

Les Mikesell
  les@fb.com

29-Oct-91 20:21:58-GMT,2050;000000000001
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Date: Tue, 29 Oct 91 15:21:40 EST
From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
In-Reply-To: Your message of Tue, 29 Oct 91 13:48:32 CST
Message-Id: <CMM.0.90.0.688767700.fdc@watsun.cc.columbia.edu>

> > Even then, it doesn't handle the character set conversions, and often
> > doesn't record format conversions either, which is why you really want to
> > slip compression into Kermit's presentation layer after all that has been
> > done.
> 
> I hesitate to comment on this because I think this belongs elsewhere as
> well.
>
Here we're getting into very old arguments.  I'll only say that the notion
of a "common intermediate representation" is essential to all protocols.
Without them, every computer would need knowledge of the formats and codes of
every other kind of computer.

> That is, any machine needing file format preservation should have
> a way to encapsulate the file into a byte stream and do the reverse.
> A file being sent over a communications line may traverse many machines
> before it's actually used again.  I'd prefer for the transfer protocol
> to transfer the file contents unchanged so that the proper conversions
> can be done only at the end point.
>
Done, not only by zip/zoo/tar/backup and friends, but also by Kermit's very
own "set file type binary".  You can even have Kermit do record format
conversion but skip the character-set translations: "set file type text", "set
transfer character-set transparent".  The Kermit protocol also has an
"archiving" notion that has never been implemented (basically, saving the
attribute packet along with the data).

> ...but files can always be run through a separate conversion.
>
Then if we have n kinds of computers, we need n x (n-1) conversion programs.
Now add a new kind of computer: write a new conversion program for it, and
modify the other n x (n-1) programs to know about it.

- Frank

30-Oct-91  3:45:41-GMT,1771;000000000001
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From: les@fb.com (Les Mikesell)
Subject: Re: Info-Kermit Digest V14
To: fdc@watsun.cc.columbia.edu (Frank da Cruz)
Date: Tue, 29 Oct 91 21:40:32 CST
In-Reply-To: <CMM.0.90.0.688767700.fdc@watsun.cc.columbia.edu>; from "Frank da Cruz" at Oct 29, 91 3:21 pm

> > ...but files can always be run through a separate conversion.
> >
> Then if we have n kinds of computers, we need n x (n-1) conversion programs.
> Now add a new kind of computer: write a new conversion program for it, and
> modify the other n x (n-1) programs to know about it.

This has nothing to do with the concept of including such conversions
in a file transfer protocol - it could still be done by with a single
program for each machine to handle the local <-> portable conversions.
I don't really want to argue against having those options available
in kermit, but the one thing that can't be done anywhere but in
the communication code is efficiently moving the bytes.  I just think
it is a mistake to let functions that could be done elsewhere interfere
with the thing that can't.

Les Mikesell
  les@fb.com

30-Oct-91 10:39:12-GMT,1072;000000000005
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From: tankus@hsi86.hsi.com (Ed Tankus)
Date: Wed, 30 Oct 1991 05:34:33 EST
X-Mailer: Mail User's Shell (7.1.2 7/11/90)
To: fdc@watsun.cc.columbia.edu
Subject: Compresssion for Kermit

Frank,

You may wish to speak with the author of ARJ, Robert K. Jung.  The ARJ program
is the fastest and best-compression scheme in the DOS world.  Robert has a
C source de-archiver which is very portable and plans to release a C source
simple archiver during 1Q92 when he releases a UNIX flavor of his commercial
product.

Robert K. Jung can be reached via email at robjung@world.std.com.  The ARJ
software is available for FTP from simtel20, wuarchive, or me.

Hope this helps.


"For every word, there is a song upon which inspiration lies ...". 
Ed Tankus.   {uunet,yale}!hsi!tankus -- OR -- tankus@hsi.com

30-Oct-91 10:54:57-GMT,1332;000000000005
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From: Kai Uwe Rommel <rommel@informatik.tu-muenchen.de>
Reply-To: <rommel@dszenger9.informatik.tu-muenchen.de>
Message-Id: <9110300957.AA25284@dszenger9.informatik.tu-muenchen.de>
Subject: Re: [Bo Kullmar <bk@kullmar.se>: C-Kermit for OS/2]
To: fdc@watsun.cc.columbia.edu
Date: 	Wed, 30 Oct 91 10:57:29 +0100
In-Reply-To: <CMM.0.90.0.680906442.fdc@watsun.cc.columbia.edu>; from "Frank da Cruz" at Jul 30, 91 4:40 pm
X-Mailer: ELM [version 2.3 PL11]

I forgot to mention, I have the compression part of the programs already
working on 16-bit Microsoft C, only the decompressor fails currently.
I needed the algorithms for my thesis which I am currently preparing.

Kai Uwe Rommel

-- 
/* Kai Uwe Rommel, Munich ----- rommel@informatik.tu-muenchen.de */

DOS ... is still a real mode only non-reentrant interrupt
handler, and always will be.                -Russell Williams

30-Oct-91 11:21:29-GMT,1713;000000000005
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From: Kai Uwe Rommel <rommel@informatik.tu-muenchen.de>
Reply-To: <rommel@dszenger9.informatik.tu-muenchen.de>
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Subject: Re: [Bo Kullmar <bk@kullmar.se>: C-Kermit for OS/2]
To: fdc@watsun.cc.columbia.edu
Date: 	Wed, 30 Oct 91 10:55:54 +0100
In-Reply-To: <CMM.0.90.0.680906442.fdc@watsun.cc.columbia.edu>; from "Frank da Cruz" at Jul 30, 91 4:40 pm
X-Mailer: ELM [version 2.3 PL11]

I don't have news regarding the OS/2 port of C-Kermit. Probably I will
only have time during christmas.

On another topic - you are looking for a compression algorithm. You
could ask Ross Williams for his LZRW-[123][a] programs. They are
portable C versions, running at 200k/sec to 100k/sec on my 24 MHz 386
and work blocked, so you can back up in case of transmission errors.
They currently work on 32-bit machines only, but that can be fixed, I
think. His address is ross@spam.ua.oz.au.

Kai Uwe Rommel

-- 
/* Kai Uwe Rommel, Munich ----- rommel@informatik.tu-muenchen.de */

DOS ... is still a real mode only non-reentrant interrupt
handler, and always will be.                -Russell Williams

30-Oct-91 19:38:23-GMT,4639;000000000015
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To: Frank da Cruz <fdc@watsun.cc.columbia.edu>
Cc: INFO-ZIP@CS.UCLA.EDU, Jean-loup Gailly <jloup@chorus.fr>
Subject: Re: Recent Kermit Protocol Extensions
Date: Wed, 30 Oct 91 20:21:56 +0100
From: Jean-loup Gailly <jloup@chorus.fr>

Frank,

> The next major effort in Kermit protocol expansion (no commitment
> expressed or implied!) is the addition of an effective and portable
> compression method.  We're in the information-gathering phase now.

I suggest that you use an algorithm in the LZ77 class [Ziv77].
They are often slower than algorithms of the LZ78 class [Ziv78], but
generally produce a much better compression ratio. They also require
less memory, and it is quite easy to set any desired tradeoff
between speed and compression ratio. In short, the main difference
between LZ77 and LZ78 is that duplicated strings are designated with
a (pointer,length) pair in LZ77 and a pointer only in LZ78.

Algorithms using Markov modeling can get an excellent compression
ratio but they are slow and eat a lot of memory.

> The method that is finally chosen must be single-pass,

zip 1.0 uses two passes, but future versions will have an option
to use one pass only.

> in the clear legally (unlike LZW, which is tied up in patent
> infringement litigation,

Given the rate at which software patents on data compression are
currently granted, it will be very soon impossible to write an
efficient non-patented algorithm. LZ78 has been patented twice and
several patents on LZ77 have appeared recently. The fastest LZ77
algorithm, published in particular by Ross Williams in his LZRW
series, is patented by Gibson and Graybill (U.S. Patent 5,049,881).
As mentioned by Rich Wales, Phil Katz also obtained a patent on LZ77,
but surprisingly his algorithm does not seem practical and does
not cover the simpler technique used in zip, arj and freeze.

The algorithms used in lha and zoo 2.1 are probably covered by the
Fiala and Greene patent, but I have not read the patent claims. yabba
written by Dan Bernstein is free of patents, but requires a lot of
memory and does not compress as well as the LZ77 compressors.
whap, also written by Dan Bernstein, is patented by Storer.
zip, arj, freeze and lharc seem to be free of patents for the moment,
but Robert Jung (author of arj) has applied for a patent in this area.

(One correction about LZW: this algorithm is patented by IBM and
Unisys, but both companies are not actually suing people for using
Unix 'compress'.)

> effective for all types of data (7-bit text or 8-bit text
> in any language, binary executables, numeric data, raster images,
> etc),

LZ77 is much better than LZ78 for binary data.

> For maximum transportability, the result of
> compression should be a sequence of 7-bit ASCII printable characters
> (32-126 or a subset of these).

I think that Kermit should take care of transforming the compressed
binary data to printable characters. It would be easier to plug in
existing compression algorithms.

> Suggestions, pointers to specifications for various methods and
> analyses of them, etc, would be most appreciated.

For more details on the compression programs mentioned above, you
can contact:

Rahul Dhesi     <dhesi@cirrus.com>            about zoo
Leonid Broukhis <leo@s514.ipmce.su>           about freeze
Ross Williams   <ross@spam.ua.oz.au>          about lzrw*
Dan Bernstein   <brnstnd@kramden.acf.nyu.edu> about yabba
Y.Tagawa        ???                           about Unix lharc
myself          <jloup@chorus.fr>             about zip

zip is currently much faster than its portable LZ77 competitors (zoo,
freeze, lharc), but zoo compresses slightly better. The non patented
variants of lzrw should be fast as well, but I have not tested them.
I don't give references for lha and arj since they only run under
Msdos and the source code of arj is not available anyway.
lharc, zip, zoo, and freeze have been posted to comp.sources.misc.

Jean-loup Gailly

References:

[Ziv77] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data
Compression", IEEE Transactions on Information Theory", Vol. 23, No. 3,
pp. 337-343.

[Ziv78] Ziv J., Lempel A., "Compression of Individual Sequences via
Variable-Rate Coding", IEEE Transactions on Information Theory,
Vol. 24, No. 5, pp. 530-536.

30-Oct-91 20:05:31-GMT,3110;000000000001
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To: Frank da Cruz <fdc@watsun.cc.columbia.edu>
Cc: Jean-loup Gailly <jloup@chorus.fr>
Subject: Re: Recent Kermit Protocol Extensions 
In-Reply-To: Your message of Wed, 30 Oct 91 14:42:51 -0500.
             <CMM.0.90.0.688851771.fdc@watsun.cc.columbia.edu> 
Date: Wed, 30 Oct 91 21:01:04 +0100
From: Jean-loup Gailly <jloup@chorus.fr>

> It is also a little bit discouraging (as to the patents, etc)

Yes, I am quite upset with these software patents. The US patent
office is absolutely incompetent to determine obviousness and
the existence of prior art. Most software patents are invalid
on both grounds. But the simple threat of a lawsuit is often
too much for individuals like us, developers of zip. We learned
about the Phil Katz patent a few days before zip became public and we
had to study it in detail to convince ourselves that it did not
cover our implementation.

> What do you know about V.42bis?  Is it a derivative of one of the
> patented methods?

I am afraid so. I think it is LZW. But it is very unlikely that
Unisys or IBM will do anything about this, because LZW is used
about everywhere now, and moreover they probably do not want to
fight each other because both patents apply to the same algorithm.
Of course the patent office did not see that.

> Have you seen analyses or benchmarks published anywhere? 

Benchmarks are regularly published in comp.compression. Ask
Peter Gutman <pgut1@cs.aukuni.ac.nz> for his latest results,
because the last posting did not include zip 1.0 which was
not yet public. I am giving below an extract of a posting I made
in comp.compression on Oct 7.

Jean-loup

-----------------------------------------------------------------------
Here are some results on the standard text compression corpus
(the 14 files bib book1 book2 geo news obj1 obj2 paper1 paper2 pic
progc progl progp trans) available at fsa.cpsc.ucalgary.ca in
/pub/text.compression.corpus.
                                           user     system     archive size

$ time zip    ../corp *                   37.630u  5.750s     1,111,595

$ time zoo ah ../corp *                   88.070u 12.280s     1,097,258

$ time lharc a   ../corp *                99.180u  9.720s     1,201,148

$ tar cf - * | time freeze > ../corp.F   110.590u  9.140s     1,137,586

So, on large files, zip is more than twice faster than all other unix
compressors with comparable compression ratio (yabba and compress are
faster but have much worse compression ratio). zoo has however slightly
better compression because its archive format is better. (zip cannot
change its archive format because of the compatibility constraint
with PKZIP.) The versions used where zip 1.0, zoo 2.1, lharc 1.02
and freeze 2.2.1, run under SunOS 4.1 on a Sparc.

30-Oct-91 20:53:44-GMT,5208;000000000011
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To: Frank da Cruz <fdc@watsun.cc.columbia.edu>
Cc: info-zip@cs.ucla.edu, Jean-loup Gailly <jloup@chorus.fr>
Subject: zip compression algorithm (was: Info-Zip and compression for Kermit)
Date: Wed, 30 Oct 91 21:48:48 +0100
From: Jean-loup Gailly <jloup@chorus.fr>

Frank da Cruz asks:

> is there a single document, or at least a small number of files
> (or books, or papers) where Zip's compression algorithm is
> presented and analyzed?

Let me try a short summary. Zip's implosion algorithm finds duplicated
strings in the input data. The second occurrence of a string is
replaced by a pointer to the previous string, in the form of a pair
(distance, length).  Distances are limited to 8K bytes, and lengths
are limited to 320 bytes. When a string does not occur anywhere in the
previous 8K bytes, it is emitted as a sequence of literal bytes.
(In this description, 'string' must be taken as an arbitrary sequence
of bytes, and is not restricted to printable characters.)

In the compressed output, literal bytes are distinguished from pointers
with one leading bit. (This is one of the unfortunate features of the
zip format, since better formats use less than one bit.)

Distances and lengths are themselves compressed using Shannon-Fano
encoding, which is somewhat similar to Huffman coding: frequent values
are encoded on fewer bits than unlikely values. Literal bytes can
also be optionally encoded using a Shannon-Fano tree. The Shannon-Fano
trees are stored in a compacted (but non optimal) form at the head
of compressed output. zip 1.0 uses a two pass algorithm to determine
optimal trees, but a single pass algorithm is possible using static
trees.

Duplicated strings are found using a hash table. All input strings of
length 2 (for binary data) or 3 (for ascii data) are inserted
in the hash table and removed when they fall off the sliding window.
A hash index is computed for the next 2 or 3 bytes. If the hash
chain for this index is not empty, all strings in the chain are
compared with the current input string, and the longest match
is selected.

The hash chains are searched starting with the most recent strings,
to favor small distances and thus take advantage of the Shannon-Fano
encoding. The hash chains are doubly linked to get fast deletion.

To avoid a worst case situation, very long hash chains are arbitrarily
truncated at a certain length, determined by a runtime option (zip -0
to -9). So zip does not always find the longest possible match but
generally finds a match which is long enough.

zip also defers the selection of matches with a lazy evaluation
mechanism. After a match of length N has been found, zip searches for
a longer match at the next input byte. If a longer match is found, the
previous match is truncated to a length of one (thus producing a
single literal byte) and the longer match is emitted afterwards.
Otherwise, the original match is kept, and the next match search is
attempted only N steps later.

The lazy match evaluation is also subject to a runtime parameter.
If the current match is long enough, zip does not search for
a longer match, thus speeding up the whole process. If compression
ratio is more important than speed, zip attempts a second search
even the first one is already quite long.


For small files, zip also tries a completely different algorithm
called shrink, which is an improvement on the LZW algorithm used in
the well known 'compress' program. The improvement is to re-use
leaves of the string tree, instead of erasing the whole tree when
the compression ratio degrades. As in implosion, the shrinking
algorithm finds duplicated strings, but it can only finds a subset
of them, because new strings are inserted in the dictionary only
at the end of the previous string match. (Implosion inserts a new
string at every input byte.)

The decompressor reconstructs the same dictionary as the compressor,
and is thus able to determine string lengths given a string pointer
only. So the shrink output is only a sequence of pointers and does not
include lengths. The pointers are encoded on the minimal number of
bits required to represent them, but they are not further compressed
with Huffman or Shannon-Fano encoding.

Shrinking is not attempted by default on large files because implosion
is generally better, once the initial cost of storing the Shanon-Fano
trees is amortized.

I hope that this little introduction has been helpful. For more
details about data compression algorithms, I recommend the book

   Text Compression
   Timothy C. Bell, John G. Cleary, Ian  H. Witten
   ISBN 0-13-911991-4
   Price: approx. US$40, much higher outside the US
   Publisher: Prentice Hall

or the review paper

   Modeling for Text Compression
   Timothy C. Bell, Ian  H. Witten, John G. Cleary
   ACM computing surveys, Vol 21, No 4, Dec 1989, p557-591

Jean-loup

31-Oct-91  3:05:17-GMT,3931;000000000411
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Date: Wed, 30 Oct 91 16:26:32 PST
From: "Glenn E. Thobe" <thobe@getunx.info.com>
Message-Id: <9110310026.AA04026@getunx.info.com>
To: fdc@watsun.cc.columbia.edu
Subject: Re: Recent Kermit Protocol Extensions

Frank-

Thank you for your detailed reply of 25 Oct 91 to my letter
regarding proposed Kermit extensions, namely compression.

On the issue of whether the compressed data should be able to 
use the upper ASCII printables when available, a quick calculation 
reveals that using the extra codes adds about 15% to the channel 
capacity, i.e. not using them increases the message size by 15%.
(log 192 / log 96 = 1.15)  That's 15% extra cost no matter how
good the compression.

>> P.S. Bravo for your pioneering internationalization efforts!

>Thanks!  (Do you use this stuff yourself?)

I do a lot of Cyrillic text processing on my 3b1/UnixPC.  I have even
built a Russian TeX.  As for Cyrillic data communications, I'm pretty
much limited to network e-mail just because there is no one to exhange
texts with via Kermit.  I am hoping that when 8-bit mailers become
universal, ISO 8895 (-5 for Cyrillic) will become the standard of 
exchange as you have made it for Kermit.  For now, it seems impossible 
to get two people to agree on the same Cyrillic transliteration.  I found
the articles in issue 4 of your Kermit mailout regarding Kermit in Russia 
and internationalization to be quite informative.  (I haven't gotten #5 
yet, the announcement said if you got #4, they would send #5 automatically.)

>> P.P.S. What can be done to get the most popular BBS's to support Kermit?

>Beats me.  Maybe as BBS's become popular outside the USA -- where
>(inter)national character set conversion is important and 8-bit transparency
>cannot be assumed (because of the PTTs) -- we'll see more Kermit support in
>BBS software.

Chuck Forsberg has had pretty much success getting BBS's to use his Z-modem.
And he doesn't have a world-wide army of volunteers like Kermit does.
He even charges money for his product.  I think part of the reason is that
Kermit has gotten a rep for being slow partly because of this prefixing
issue, and for other reasons which have been fixed, but people don't
know it yet.

A stripped MS-Kermit for remote use on BBS's, if such doesn't already 
exist, would help, along with specific information on how to install it 
on the major BBS systems (FIDO, Waffle, etc.).  By stripped I mean no 
terminal emulation facilities (e.g. rz/sz), do everything from the command 
line.  Most BBS's give you a choice of protocols, and it shouldn't be hard 
to make one of them Kermit.  The designers and purveyors of BBS systems 
could be approached directly.  I don't see a lot of technical obstacles here.  
Maybe a note in info-kermit would be sufficient to mobilize the forces.

When I encounter a BBS that doesn't support Kermit, I first proselytize
the sysop.  Often they are interested, but I don't know exactly what to
say to them other than to start explaining about the WATSUN archive,
the Kermit books, the universality of Kermit implementations, the
importance of getting a current release, etc.

Then I escape to my local Kermit and "!rz" to download files.  (It's 
easier to swim around a rock than through it.)

-Glenn <getunx!thobe@uunet.uu.net>

31-Oct-91 16:19:02-GMT,728;000000000001
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From: Peter Mauzey <mtune>
Message-Id: <9110311619.AA01558@watsun.cc.columbia.edu>
To: fdc

Frank -  (from Peter Mauzey)   October 31, 1991

Regarding V.42bis being "in the clear," there is no doubt that Microcom,
which claims to represent UNISYS, has asserted that the technology
represented by V.42bis is covered by U.S. Patent 4,558,302.

The contact at Microcom (VP Technology Planning) is Gregory Pearson,
818-986-4212, last I heard (Sherman Oaks, CA).

My Department's representatives to the CCITT are now overseas but I
will ask them when they return "what's new" with V42bis.

 4-Nov-91  6:19:26-GMT,1485;000000000001
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To: Frank da Cruz <fdc@watsun.cc.columbia.edu>
Cc: brnstnd@nyu.edu
Subject: Re: Compression 
In-Reply-To: Your message of Mon, 28 Oct 91 14:21:04 EST.
             <CMM.0.90.0.688677664.fdc@watsun.cc.columbia.edu> 
Date: Mon, 04 Nov 91 01:19:21 +0000
From: brnstnd@KRAMDEN.ACF.NYU.EDU

In message <CMM.0.90.0.688677664.fdc@watsun.cc.columbia.edu> you write:
> Pointers to specifications, analyses, etc, would be much appreciated.
> Thanks!

You should pick up my yabbawhap package, comp.sources.unix volume 24.
The yabba part uses Y coding, which is described in a paper included
with the package and which is free of patents. Y is an LZ variant which
generally outperforms compress by 10-20%; on English text it usually
beats even zip, lharc, freeze, and other LZ-Huffman hybrids. I'm working
on dabba, which applies a statistical coder to the output of Y coding
and which so far appears to be competitive with PPMC, the most effective
known compression method. (PPMC's problem is that it's awfully complex
to implement and rather slow.) In the meantime, yabba is public-domain.
Feel free to use it. Last I heard the GNU project was switching to yabba
from compress.

---Dan

 4-Nov-91 14:48:12-GMT,897;000000000001
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To: Frank da Cruz <fdc@watsun.cc.columbia.edu>
Cc: Jean-loup Gailly <jloup@chorus.fr>
Subject: v42bis
In-Reply-To: Your message of Wed, 30 Oct 91 14:42:51 -0500.
             <CMM.0.90.0.688851771.fdc@watsun.cc.columbia.edu> 
Date: Mon, 04 Nov 91 15:43:12 +0100
From: Jean-loup Gailly <jloup@chorus.fr>

> What doyou know about V.42bis?

The specification is available by anonymous ftp at bruno.cs.colorado.edu
in pub/standards/ccitt/1992/v/v42bis.asc. It is indeed the shrink
algorithm used in zip, and derived from LZW.

Jean-loup

14-Nov-91 17:57:26-GMT,10517;000000000005
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From: PK6811S@ACAD.DRAKE.EDU
Subject: Kermit Compression Experiments
To: fdc@watsun.cc.columbia.edu
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One more weekend volunteer reporting in.
Let's keep it simple, and let's try it out to see what happens.  My experiment
uses run-length and 'Huffman' coding.  I'm no expert on the differences 
between Huffman and Shannon-Fano coding, but I understand the principle
of using short codes for frequently-occuring characters.

I had a go at writing a program on the Mac which reduce the number
of transmitted characters significantly.  I used four test files:
        DuTermSource    - tokenized zbasic source code (terminal emulator)
                        - has lots of text, with some 8-bit
        Kingdon200      - 200 dpi 2x2 scan of 8.5 by 14 document
        Excel.mb        - Microsoft Excel program converted by Macbinary
                        - program to one data fork
        Crashbang       - sound file

    Following are my results, given in 1024 bytes. Transmit sizes include packet
headers and blockcheck, with packetlength=1000.  Repeat prefixing was allowed in
all cases.

                      WITHOUT COMPRESSION         ! WITH COMPRESSION
                 ! ----------------------------------------------------
                 !  Original    8bit      7bit    !   8bit      7bit
                 !    File    Transmit  Transmit  ! Transmit  Transmit
    File         !    Size      Size      Size    !   Size      Size
    ----         !  --------  --------  --------  ! --------  --------
    DuTermSource !     255       273       299    !    210       245
    Kingdon200   !     262       439       521    !    216       252
    Excel.mb     !     729      1010      1202    !    777       907
    Crashbang    !      46        68        90    !     46        53


    I'll summarize my algorithm, then give the details and reasoning.
But first, we must understand the concept of a compression window.  This
is a fixed number of input characters considered together for compression.
The window size can be smaller, equal to, or greater than the packet length;
for my tests, the window size was 256 with the packet length 1000

    Summary of Algorithm:
        1.  Apply run-length encoding to enough input characters to fill
            a window.  Somewhat like kermit's repeat-prefixing.
        2.  Convert window to 'Huffman' codes
        3.  Update Huffman code tables
        4.  Convert window to printable characters 
        5.  Accumulate enough windows to fill a packet.


RUN-LENGTH ENCODING:

Any character 'c' repeating from 5 to 255 times is replaced with c:r:n,
where c is the character, r is the repeat-flag character, and
n is the number of characters replaced.  When the repeat-flag character is
encountered, it is replaced with r:0.  When decoding, when the repeat-flag is
found, first check the next character for count.  If count is zero, then 
output the repeat-flag character, else output 'count' number of the previously
decoded character.
    
Usually this pass will compress the input data somewhat, and can be very
effective against certain kinds of files, or portions of files.  It is possible
in cases where there are few or no repeating characters, and many occurances
of the repeat-flag character that this will actually add to the input size.
It might be possible to choose either the encoded or the unencoded version
of the window, appropriately flagged, and pass that to step 2, to prevent
undue expansion of the file, but I did not do that in my tests.

HUFFMAN CODING

Huffman coding allows the most frequently occuring characters to be replaced
by short (< 8 bits) codes, and less frequently occuring ones to be
replaced by long (= or > 8 bits) codes.  For example, if hex 00 might be
the most frequently occuring character, then every hex 00 should be replaced
by the shortest possible code.  We first construct a table of codes, from
shortest to longest, my table is as follows:

    binary    1xxxx     - the 16 most frequent characters
    binary   01xxxxx    - the next 32 most frequent characters
    binary  001xxxxxx   - the next 64 most frequent characters
    binary  000xxxxxxxx - the rest

To decode, check the next bit, bit1.
    If bit1 = 1
        decode the first group from the next 4 bits
    else
        check the next bit, bit2
        if bit2 = 1
            decode the second group from the next 5 bits
        else
            check the next bit, bit3
            if bit3 = 1
                decode the third group from the next 6 bits
            else
                decode the fourth group from the next 8 bits.

This produces a stream of bits which should be much shorter than the original
input data where every character is represented by 8 bits.



UPDATE HUFFMAN CODE TABLES

It is important to identify the most frequently occuring characters.  Therefore
we count the occurances of each character in the window, and sort them from
highest-count to lowest.  This becomes the table used in encoding the next
window.  Here is an important concept.  The receiver can't decode a window
without having the same table that the sender used, therefore the table must
be based on the previous window.  Hopefully, the distribution of characters
doesn't change much from one window to the next.  There will be instances
in the data where changes occur, but there should be long stretches where
they don't.

As a side note, I found that the efficiency of the sort algorithm was a
limiting factor in speed of processing.  A simple bubble sort slowed the
compression program terribly, and required changing to a shell-sort which
with some tuning gave satisfactory results.


CONVERT WINDOW TO PRINTABLE CHARACTERS

Huffman coding leaves us with a stream of bits stored in 8-bit bytes.  We
now must make them kermit-legal to avoid eight-bit and control prefixing.
Alas, now is where we give up much of our hard-won compression.

If we are using a 7-bit channel, I simply take 6 bits at a time - giving
values hex 00 to hex 3f, and add hex 20 to them to make an 8-bit character
whose values lie between hex 20 and hex 5f, inclusively, corresponding to 
printable characters, space through underscore.

If we are using an 8-bit channel, I take 7 bits at a time - giving values
hex 00 to hex 7f.  For values hex 00 to hex 3f, I add hex 20 as above.  
For values hex 40 to hex 7f, I add hex a0 to place them into the 8-bit 
versions of the same characters, space through underscore.

These conversions give characters which require no prefixing for transmission.


MAKE UP A PACKET

Now we simply accumulate windows until we have a full packet, append the
appropriate packet header and block-check, and write the packets to our
output file.  Now we encounter a problem of granularity.  If our window
size is not a factor of our packet size, we will not be able to fill each
packet to the maximum allowable size.  Consider this example:  our packet
size is 1000 and our window size is 256.  After Huffman coding and printable-
conversion, we could produce a series of windows sized 200, 190, 210, 220, 230.
The first four windows add up to 820, adding the fifth gives 1050.  Our
actual average packet size will then be somewhat smaller than that allowable,
creating additional overhead.

I did not follow up on this problem, but some method which allows windows
to span packets may be applied.


CONCLUSION AND SUGGESTIONS

My experiments show that a reduction of transmitted characters is possible,
up to about 50%.  The coding needed is on the order of that required for
kermit-windows (which makes me think I need to find another name for these
compression 'windows').

There is a way to make a further reduction in characters.  You will notice
that this program doesn't output printable characters in the range hex 60 to
hex 7e.  Now whenever we have unused character values we should consider how
to use them to pass information that aids our compression.  Let's say we
output a hex 40 (space).  We could just as well output a hex 60 "`" - if
the receiver understood that it really was a hex 40, cleverly encoded to
flag some kind of information.  So a space is either hex 40 or hex 60, the
flag is either up or down, on or off, zero or one.  If we have a series of
such flags we can construct a stream of bits out of them;  and when we have
accumulated enough bits to make a character, we insert that character in 
the decoded stream.  Over a 7-bit channel we are taking our huffman stream
in 6-bit chunks, so that we need 6 flags to insert a character.  Over an
8-bit channel we need 7 flags.  Since half of our input characters,
hex 40 to hex 5e, can serve as flags we can insert a character for every
12 or 14 transmitted characters on the average - reducing our transmittal
by one-twelfth or one-fourteenth

There is also another approach to Huffman tables.  Rather than sort characters
by their frequency in the window, we can take advantage of the natural
assocation of one character to another.  For example, in English text, the
letter 'd' may be the most frequent character to follow the letter 'e', and
the letter 'u' is certainly the most frequent character to follow
the letter 'q'.  If 'd' follows 'e' in the input, it should be replaced by
the shortest possible code, regardless of how many 'd's appear in the window.
And 'u' following 'q' should be replaced by the same shortest possible code.
To do this, we keep a table of counts of characters occuring after one 
another, and sort each character's 'follower table' separately.  The receiver,
having constructed the same tables, knows that the same code following an 'e',
means 'd', but after a 'q', means 'u'.  Unfortunately this implies a large
table, at least 256 by 256 bytes, plus lots of sort time.  I did not attempt
this experiment, but someone may be able to find an efficient implementation.

Hope this is helpful.  A lot of us would like to see shorter file transmission
times.  

Paul D. Kline
Associate Director of Adminstrative Computing
Drake University
Des Moines, IA  50311
(515)271-2166
internet pk6811s@acad.drake.edu

15-Nov-91 20:46:42-GMT,714;000000000005
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From: mtdcc!ptm@mtune.att.com
Date: Fri, 15 Nov 91 15:42 EST
To: fdc@watsun.cc.columbia.edu

Frank -  (from Peter Mauzey)   November 15, 1991

A little more about V.42bis.  In addition to UNISYS claiming a
relevant patent, British Telecom claims the algorithm was the
subject of a UK patent application GB 8815978 filed 5th July 1988.

Our representative to the standards organizations says that
companies that use V.42bis will have to pay an amount that is
separately negotiated with each company.

Bye for now.

16-Nov-91 20:56:15-GMT,3212;000000000001
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Date: Sat, 16 Nov 91 15:56:05 EST
From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: PK6811S@ACAD.DRAKE.EDU
Subject: Re: Kermit Compression Experiments
In-Reply-To: Your message of Thu, 14 Nov 1991 11:58 CST
Message-Id: <CMM.0.90.0.690324965.fdc@watsun.cc.columbia.edu>

Paul, many thanks for your message!  It's a good idea, too.  I had always
thought of Huffman encoding as a two-pass process if we were to develop an
ideal key for a particular file, but your idea (let me see if I understand it
right) is to read the first chunk ("window"), get the byte frequencies and
compute the key, send the first chunk uncompressed, then use the first chunk's
key to encode and send with the second chunk, etc.

A refinement on this might be: read a chunk, get the frequencies, construct
the key, send the key with its own chunk.  This assumes we can keep a whole
chunk in memory.  But in most cases we do that anyway -- most Kermit programs
read data from the input file into a buffer, a fairly long piece at a time --
so now we just make a pass over the input buffer to compute the frequencies
before compressing the data putting it in the packet.

How do you encode and transmit the key?  I assume the key would be 256 bytes
long, before Kermit encoding.  Later, in an example, you mention a window size
of 256.  This means the key for a window is as long as the window itself.
Obviously we need either a compact way of transmitting the key, or a much
bigger window size, or both.  Tricky business, because we have memory and
addressing limitations to cope with on small machines -- so we'd need to have
two compression-capable Kermit programs negotiation the compression chunk
size.  If it is too small, compression won't pay off.

I agree there is no reason why compression chunks and packet boundaries need
to have anything to do with each other.  We just need a way of distinguishing
the key from the compressed data in the packet -- easy to do, using either
length fields or flagging via unused characters as you suggested.

Another refinement would be to make better use of the GL and GR character code
space by "tighter packing", using, say, base 94 (or 188) rather than 64 (or
128).

I'm not sure that second-or-higher-order analysis of the data (for pairs,
triplets, etc) would be worthwhile -- as you point out, we'd need at least a
64K byte (or word!) table, lots of sorting, etc.  Maybe in 10 years, when CPU
speed and memory are no longer the bottlenecks.  More detailed analysis and
testing will tell (higher-order analysis is comparable to the string-matching
approach used by LZW, V.24bis, and friends -- all of which are patented).

Anyway, the final results of your tests are definitely better than today's
Kermit, achieving up to a 100% savings in transmission time -- nothing to
sneeze at.  Your method is simple, easy to understand and program, relatively
compact in memory requirements, and best of all it is clear of the kind of
legal hassles impinging on all the fancier LZW-class (string-based)
algorithms.  It's very much worth considering, and much appreciated.  Thanks!

- Frank

21-Nov-91 21:48:30-GMT,900;000000000001
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To: fdc@watsun.cc.columbia.edu

Frank -  (from Peter Mauzey)   November 21, 1991

Regarding file compression, here's a quote: "Using the 'implode' public
domain compression algorithm that is optimized for speed rather than
compression, we were able to achieve speed-ups ranging from 18% to 85%
on the transfer of a variety of files, varying from 70 kbytes to 1.1
Mbytes."

That's from "Software Technology for Wireless Mobile Computing" by
Daniel Duchamp, Steven K. Feiner, and Gerald Q. Maguire, Jr., in the
November 1991 IEEE Network magazine (ISSN 0890-8044).  Page 17.

All three authors are at Columbia, Computer Science Department.

22-Nov-91  1:34:03-GMT,7604;000000000001
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Date: Thu, 21 Nov 1991 18:40 CST
From: PK6811S@ACAD.DRAKE.EDU
Subject: Re: Kermit Compression Experiments
To: fdc@watsun.cc.columbia.edu
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Frank, you're welcome.  With regard to transmitting the key - I don't.  I start
with a default key, then each chunk is encoded using the key calculated from
the previous chunk.  Here's an attempt at  diagramming:

For the sender:

    chunk1  +  key0 -> encode1
    chunk1  -> key1
    chunk2  +  key1 -> encode2
    chunk2  -> key2
    chunk3  +  key2 -> encode3
    chunk3  -> key3
      . . .
      . . .
      . . .
    encode1 thru encodeX are blocked into packets and transmitted.

For the receiver:

    encode1  +  key0 -> chunk1
    chunk1   -> key1
    encode2  +  key1 -> chunk2
    chunk2   -> key2
    encode3  +  key2 -> chunk3
    chunk3   -> key3
      . . .
      . . .
      . . .

It's obvious that each key is non-optimal for any given chunk of data,
because it is derived from the previous chunk, however, my Huffman codes
are not so tight as to make much difference.  The first 16 characters are
all coded in 5 bits, the next 32 in 7 bits, etc., so if any character is 
one or two positions off - there won't be much loss of efficiency.  This
works well when there are long stretches of homogeneous data.  When there
is a change in type, like in the resource fork of a Macintosh program with
pics, sounds, text strings, and executable code - there will be 
inefficient coding for the first chunk of each different stretch, then
the key will adapt and be correct for subsequent chunks.

Now, if you were to compute an optimal key for a chunk, by counting the
frequencies therein, you would need to send the key.  You wouldn't need
to send the last group of characters (144 count), so you would only
be sending the first 112 characters.  Or you might only send the
changes in the key... Char(i)+Newposition(i).

-----------------------------------------------------------------------
After sending you those results, I made two tweaks to the program.
First, I changed the key calculation routine slightly, to allow it to
ride over rough parts in the data without disrupting the keys completely
for the next chunk.  Instead of zeroing out the count for each
character, I simply divide the previous count by 2 (shift right 1).  This
allows some 'memory' of previous chunks and prevents a frequently-
recurring character from going to the end of the table just because it
was left out for a short stretch.

Second, I divided the fourth group of 144 characters into two groups, so
that my Huffman codes now look as follows:

   binary     1xxxx     -  1st 16 characters
   binary    01xxxxx    - next 32 characters
   binary   001xxxxxx   - next 64 characters
   binary  0001xxxxxx   - next 64 characters
   binary  0000xxxxxxx  - next 80 characters

The fourth group is now encoded in 10 bits instead of 11 as before.

With these two changes, I ran the compression against some of the previous
files:  (all are 8bit)

               before          after
File           tweak           tweak
----           ------          ------
DuTermSource    210             207
Kingdon200      216             214
Crashbang        46              45

I tried the program against the text compression corpus available at
fsa.cpsc.ucalgary.ca in /pub/text.compression.corpus.  This time I
monitored the total size of chunks after Huffman encoding as well as
the total size after kermit-legal expansion:

        Huffman-    Kermit-    Original
        Encoded    Transmit      File
File     Size        Size        Size
----    -------     ------     --------
bib       80375      92806      111261
book1    517426     601001      768771
bbok2    416858     483608      610856
geo       86897     100230      102400
news     265430     308191      377109
obj1      15486      16861       21504
obj2     195992     226658      246814
paper1    36699      41581       53161
paper2    55451      63852       82199
pic       79312      91365      513216
progc     28243      31960       39611
progl     46402      53255       71646
progp     32737      37227       49379
trans     67333      77783       93695
-----   -------    -------     -------
total   1924641    2226378     3141622

Some comments:
- First, no file transmit size was larger than it's
original size, though 'geo' was pretty close.  This bodes well for applying
the algorithm in practice.
- Second, the expansion from Huffman to transmit size is right at 16%
for all files, which is due to the data-independent algorithm.
- Third, the amazing result for the file 'pic' can be explained by the
existence of long strings of repeated characters which were reduced through
run-length encoding.
- Fourth, these results can be compared with those given by Jean-loup Gailly
for zip, zoo, lharc, and tar - whose archive totals ranged from 1097258 to
1201148.  They are much smaller, but he did not give the transmit sizes for
those archives, which is the real comparison.

----------------------------------------------------------------------
As to the question of long input buffers and such, I tried chunks as large
as 4k, but the compression varied from slightly worse to slightly better,
on the order of 1% difference from the 256-byte chunks.  Reason this way:
either a character occurs frequently or it doesn't.  If it does, then
most chunks, whatever size, will show that.  And, as before, my Huffman
codes are not so tight that a character's frequency can't move around
some without affecting efficiency.  

There is also the characteristic of 'locality', which means that different
stretches within the data will have somewhat different profiles.  A good
compressor will adapt as it proceeds.  In my case adaptivity is determined
by chunk size.  The smaller the chunk, the more adaptive.  This is an
advantage above some 'noise' level, below which, the counts become too
low to be meaningful - they no longer predict the actual frequency of 
the characters.  Above that level, increasing the chunk size doesn't
improve the efficiency, and at some higher level the efficiency decreases
as we lose adaptivity.

We also need to conceptually separate our input buffers, compression
chunks, and packets.  They can be optimized independently.  The input
routine reads large buffers to reduce i/o delays.  The compression
routine reads small chunks out of the input buffer for processing, and
puts them somewhere.  The packetizing routine accumulates enough
chunks to make an efficient packet.  The windowing routine manages
packets within it's window space.  The leg bone is connected to the
thigh bone.

------------------------------------------------------------------------
I like the idea of selecting the unencoded chunk or the encoded one,
whichever is shorter.  I use a terminator character, hex 7e, to separate
the chunks within packets.  If we used two terminators, 7e and 7d, they
could indicate whether the chunk was encoded or not.  This would be 
useful for data that was extremely 'noisy' across all 256 8bit codes.

------------------------------------------------------------------------
Sorry about this late response, I was attending an IA-FRS User Conference
in Orlando this week.

 4-Dec-91 21:15:59-GMT,1516;000000000001
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Date: Wed, 4 Dec 91 16:14:38 EST
From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: Dick Elnicki <DICKE%NERVM@cuvmb.cc.columbia.edu>
Cc: Network Mailer <MAILER@cuvmb.cc.columbia.edu>,
        Joe Doupnik <JRD@cc.usu.edu>
Subject: Re: File Compression
In-Reply-To: Your message of Tue, 03 Dec 91 15:59:32 EST
Message-Id: <CMM.0.90.0.691881278.fdc@watsun.cc.columbia.edu>

>   The results clearly suggest anyone serious about using compression
> to save file Xmit times should pre-compress rather than do it on the
> fly during the Xmit.  The Kermit compression procedures would have
> to be at least as good as PKZIP's result to make the inclusion worth
> the trouble.  Or, am I missing something...
> 
Yup -- Kermit runs on hundreds of different kinds of computers.  You can't
rely on any two of them to have the same compression methods.  Even when they
do, they might have different file formats, or use different character
sets.  So the sending Kermit has to convert data to canonical form first,
then compress.  So you can't separate compression from Kermit.  I'm beginning
to think we can't really have a super-effective method built into Kermit,
however, because of complexity, size, execution overhead, and patent/legal
issues.  It might well turn out that we'll settle for something less effective
than PKZIP and friends, but something simpler.  It'll be much better than what
we have now.

- Frank

10-Feb-92 19:09:26-GMT,7097;000000000015
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Date: Mon, 10 Feb 1992 09:06:13 -0800
From: "Glenn E. Thobe" <thobe@getunx.info.com>
Message-Id: <199202101706.AA00280@getunx.info.com>
To: fdc@watsun.cc.columbia.edu
Subject: Encoding speeds transfer of compressed files


Frank-

I have some experimental results in support of my theory that Kermit's
performance in transferring compressed files can be greatly improved
by pre- and post-processing of the files.  I have designed a special
encoding scheme and programmed encoders and decoders to implement it.


------------------------------------------------------------------

	    Preprocessors Speed Kermit File Transfer

		    by Glenn E. Thobe 

		    February 10, 1992


INTRODUCTION


I have written a set of four preprocessors for Kermit which greatly
improve it's efficiency when transferring compressed files.  Kermit's
control prefixing algorithm, although efficient for text files, is
expensive for compressed files.  

Compressed files are fundamentally different from other binary files 
in the way in which the data are statistically distributed:  roughly 
speaking the bytes of a compressed file are uncorrelated and uniformly 
distributed over the 256 8-bit codes are uniformly distributed over.  

The new routines eliminate Kermit's control prefixing overhead while 
utilizing the maximum available code space of 190 or 95 characters.  
This minimizes the number of characters which get transferred, thereby 
shortening the time and reducing the cost.  

They filters are:

    k7enc - encode for 7-bit channels
    k7dec - decode for 7-bit channels
    k8enc - encode for 8-bit channels
    k8dec - decode for 8-bit channels

For testing purposes, additional filters were written which emulate Kermit's 
prefixing algorithms:

    getpkt7 - Kermit encoding (control + 8th-bit prefixing)
    getpkt8 - Kermit encoding (control prefixing)

In practice, one would apply the following processes to transfer
a compressed file:

    k8enc (encode file on sending computer)
    kermit (transfer file)
    k8dec (decode file on receiving computer)

Here we simulate the transfer and predict the performance of the above
process by performing the following steps on a single computer:

    k8enc (encode file)
    getpkt8 (apply control prefixing of non-printable charaters)
    k8dec (decode file)

We can view k8enc and Kermit's encoding as alternative ways of dealing 
with the limited 190 (or 95) letter code space, and compare the 
performance of the two.  We can also compare the performance of k7enc, 
k8enc, and other popular binary-to-printable filters.


RESULTS


Following are simulation results for a typical file using k8enc.

    file=DNLNK/dnlnk.tar.Z
    raw file transfer:
    in = 57725, out = 75436, expansion = 30.68%, efficiency = 76.52%
    encoding alone:
    in = 57725, out = 62118, expansion = 7.61%, efficiency = 92.92%
    encoded file transfer alone:
    in = 62118, out = 63072, expansion = 1.53%, efficiency = 98.48%
    combined coding and transfer:
    in = 57725, out = 63072, expansion = 9.26%, efficiency = 91.52%

Note that k8enc encoding alone significantly outperforms Kermit's
getpkt8 encoding.  Also, that k8enc preprocessed files enjoy greatly
improved transfer performance compared the raw compressed file.

Here, for the same file, are the results with different preprocessors:

    ============================================================
    Combined Efficiencies for Kermit used with various preprocessors
    ============================================================
    file name = DNLNK/dnlnk.tar.Z, file size = 57725 bytes
    7-bit channel (eighth bit prefixing):
    cat             56.77%
    uuencode x      69.2%
    btoa            75.96%
    k7enc           77.36%
    8-bit channel:
    cat             76.52%
    k8enc           91.52%
    ============================================================

Note that k7enc used with a 7-bit channel actually outperforms raw 
file transfer (cat) over an 8-bit channel.  It also outperforms both
uuencode (where each three bytes are spread over four) and atob 
(where each four bytes are spread over five).  K8enc is obviously
the big winner here.


PROTOCOL ENHANCEMENT PROPOSAL


While the improvements are impressive, the user is still left with 
the clumsiness of manually running the pre- and postprocessors, and
keeping track of the file names.  It would be much more convenient 
to incorporate the encoding and decoding processes into the Kermit 
protocol.  

A compressed-file mode should be introduced.  The user would select 
this mode when transferring LZ-compressed, zipped, Gif, and other 
compressed binary data types.  The implementation would be especially 
easy, since in this mode most of the intricate prefixing, RLE, and
locking shift preprocessing involved in packetizing the data can be 
switched off and built-in k7enc or k8enc processing switched on in
its place.

This protocol enhancement should be viewed as separate from the proposed 
built-in file compression, since the latter is of no great benefit when 
the file is already compressed.  Built-in compression should be used
for text files, executables, etc., but not for already compressed files.

A slightly surprising result of this study, is that the cost of Kermit's
design decision to use only a 188 character subset (190 extended ascii 
printables minus two control prefix characters) of the 256 data characters, 
can have so little adverse impact on efficiency.  Typically the cost of 
mapping white noise data over a 256 character alphabet into a 190 character 
subset (8-bit channel) is between 7 and 8 percent as shown experimentally 
by this study.  That is, the files are expanded by only 7 to 8 percent.  
This would seem to be a fair trade off for the ability to operate over 
channels where full 8-bit transparency cannot be guaranteed.


CONCLUSION


The traditional Kermit prefixing algorithm for mapping all file characters
into the ASCII and extended ASCII printable codes is optimized for text
files but performs poorly for compressed files.  File transfer performance 
with compressed files can be improved greatly by complementary pre- and
post processing of the files using the filter programs presented here.
These filters significantly outperform the traditional uuencode and btoa,
optimally exploiting the available channel capacity.

These filters should be introduced into the Kermit protocol in support of 
a special mode for optimizing the transfer of compressed files.

-----------------------------------------------------------------------

-Glenn E. Thobe
<thobe@getunx.info.com>

29-Jan-92  0:24:32-GMT,4031;000000000015
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Date: Tue, 28 Jan 92 19:24:28 EST
From: James Sturdevant <sturdeva@watsun.cc.columbia.edu>
To: fdc
Cc: jrd
Subject: Kermit Data Compression
Message-Id: <CMM.0.90.0.696644668.sturdeva@watsun.cc.columbia.edu>

Greetings,

In my home directory, you will find my compression routines in both C
and 8086 assembler.  Below, you will find my attempt to explain them. 
(The routines are better than the explaination.)

The compression code challenge:  I have added my code to
xx??co.c and added the module ckccmp.c.  Changes in the XX
code have [jrs] comments to flag it.  I have done my best
to speed it up.  Any other code jockies are welcome to
prove themselves better.

Unliketechniques, the compression does not change the
bit width of any non-compressed characters.  (Is that
enough difference to patent it?)  It flags compressed data
with the grave character (`-0x60).  The compression takes
place after the conversion of the character (or run of like
characters) for the packet.  For ease of description, I
will call these tokens.  The basis of this algorithm came
from an article in Computer Language (V8#5, May 91) on a
technique called Sliding Windows.  It's supposed to be a
variation on LZW.  My variations are the position encoding
and fixed sizes for window and data.

The compression works like this:

Initialize a compression buffer and window dictionary.

Step:
Create a token.  Add the token to the end of the existing
buffer.

Search for a match on the buffer in a "window" of bytes
processed previously.  If found, get another token.

When not found,  check the previous match.  If it is
greater than 4 characters (the length of a compression
token), output the string as a compressed token, start with
the new token in the buffer and search again.

If the buffer without the current token is less than 5
characters, output the first token normally and try to find
a match starting at the next token.

In both cases, the text of the output (whether token or
compressed string) is added to the window.  This is done
only when data is actually output so the search doesn't
find data which is pending.

The compressed data consists of the flag character a two
character (base 94) position, and a one character (base 94
offset 4) length.

When the file is complete, flush the remaining characters
from the compression buffer.

To speed the search of the "window", I maintain a linked
list for each character with a pointer to the beginning and
end.  When a character is removed (to add a new character
in its place) the starting link is moved forward.  The
ending link is moved with each character.  Searches are
done by starting at the first character, comparing the
string starting in that location with the buffer.  On
success, the search stops (why continue?).  On failure, the
search follows the link to the next matching character, and
compares again.

On the receiving side, the decompression routine read
characters and processes them until it sees the ` character
at the beginning of a token (i.e. it is not quoted by #, or
a length field for a repeat).  As the characters are
processed, they are added to the window.  It is not
necessary for the receiving side to maintain a list of
characters, but it doesn't hurt.  When the ` character is
seen, the next three characters are used to compute the
location and length of the real string.  The current data
buffer is saved and data is consumed from the calculated
string.  When the string is exausted, processing continues
on the original data.

The initial design criteria included low memory usage
(24K+slop of data), consistancy with the current Kermit
packet style, and no increase on data which cannot
compress.  There is an exception for ` which must be quoted
if compression is turned on.  I chose ` because it is in
the allowable range for flags and is not likely to appear
in much text, c programs or assembler programs (that I am
aware of).

13-Feb-92 12:11:52-GMT,27174;000000000005
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  (5.65c/IDA-1.4.4 for fdc@watsun.cc.columbia.edu); Thu, 13 Feb 1992 01:22:25 -0800
Date: Thu, 13 Feb 1992 01:22:25 -0800
From: "Glenn E. Thobe" <thobe@getunx.info.com>
Message-Id: <199202130922.AA01901@getunx.info.com>
To: fdc@watsun.cc.columbia.edu
Subject: Re: Encoding speeds transfer of compressed files

You wrote:

>Thanks, Glenn!  It'll take me a while to digest this.  As you probably know,
>yours is only one entry of many in the Great Compression Contest (you saw
>kermit/e/compress.txt, right?).  Anyway, we really do want to do something
>like what you have suggested, but we're all pretty swamped right now and will
>have to come back to this later.  Meanwhile, do you want to send in your
>filters?  Thanks again!  - Frank

Frank-

Here is the whole k7- and k8-encoding package.  I revised the document 
I sent you the other day (it should be clearer) and included it.  You 
should be able to build and test everything in minutes on a Unix system.  
Please let me know if any problems show up.  

The technique is not a compression scheme as such, it is more of an 
anti-expansion scheme for already compressed files.  Such a mode should 
be incorporated whether or not a built in compression is adopted.  
Nearly all the files I transfer are compressed already, so I feel the 
need personally.

I can send you my filters which do Kermit-style prefixing and the 
comparison shell scripts if you want to repeat my efficiency tests, 
but I presume you already have a way of doing such things.

I just ordered bin/e/compress.txt.  Thanks for pointing it out.

-Glenn <thobe@getunx.info.com>

#! /bin/sh
# This is a shell archive.  Remove anything before this line, then unpack
# it by saving it into a file and typing "sh file".  To overwrite existing
# files, type "sh file -c".  You can also feed this as standard input via
# unshar, or by typing "sh <file", e.g..  If this archive is complete, you
# will see the following message at the end:
#		"End of shell archive."
# Contents:  READ.ME Makefile filters.doc k7enc.c k7dec.c k8enc.c
#   k8dec.c k7verify.ksh k8verify.ksh
# Wrapped by thobe@getunx on Thu Feb 13 00:56:42 1992
PATH=/bin:/usr/bin:/usr/ucb ; export PATH
if test -f 'READ.ME' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'READ.ME'\"
else
echo shar: Extracting \"'READ.ME'\" \(2058 characters\)
sed "s/^X//" >'READ.ME' <<'END_OF_FILE'
X/* READ ME */
XThis is the first release of two pairs of complementary filters for
Xfor pre- and post-processing of COMPRESSED FILES in order to speed
Xup the transfer of those files via Kermit.  One pair is for 7 bit 
Xdata paths and the other is for 8.
X
XThe reason these filters are needed is that Kermit allows only the
XASCII or 8-bit extended ASCII codes to be to be transferred unchanged.  
XKermit prefixes control codes and replaces them with printable codes,
Xwhich increases the amount of data to be transferred.  This is efficient 
Xfor text files where control codes are infrequent but it is inefficient 
Xfor compressed files.
X
XThe code has been compiled and tested on an AT&T 7300/UnixPC/3b1
Xcomputer with both gcc and cc.  A make file and a document describing
Xthe rationale are included.  After you build them, test by running
Xk7verify.ksh and k8verify.ksh under either the Bourne or Korn shell.
X
X    /bin/sh k7verify.ksh *
X    /bin/sh k8verify.ksh *
X
XInstructions on use are found in the
Xheaders of the four C source files.  Sorry, no man page.
X
XIt is hoped that these routines will be found helpful in their own
Xright, but most of all it is hoped that they will find their way
Xinto the Kermit protocol as a special mode for efficiently transferring
Xcompressed files.  They may also be combined with future built-in file
Xcompression.  This mode should be adopted whether or not built in file 
Xcompression is added.
X
XThe design and coding of these filters is a completely original
Xcreation of the author.  Note the copyright notice.  It is essentially
XColumbia University's copyright, which should in no way restrict the
XKermit project from making full use of the code or algorithm.
X
X    Copyright (C) 1992, Glenn E. Thobe, Canoga Park, California, USA.
X    All rights reserved.  Permission is granted to any individual 
X    or institution to use, copy, or redistribute this software so 
X    long as it is not sold for profit, provided this copyright 
X    notice is retained.
X
XEnjoy.
X
X-Glenn Thobe
X<thobe@getunx.info.com>
XWed Feb 12 23:27:51 PST 1992
END_OF_FILE
if test 2058 -ne `wc -c <'READ.ME'`; then
    echo shar: \"'READ.ME'\" unpacked with wrong size!
fi
# end of 'READ.ME'
fi
if test -f 'Makefile' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'Makefile'\"
else
echo shar: Extracting \"'Makefile'\" \(721 characters\)
sed "s/^X//" >'Makefile' <<'END_OF_FILE'
X# Makefile for Kermit pre- and postfilters
X# Author: Glenn E. Thobe <thobe@getunx.info.com>
X# Wed Feb 12 00:23:44 PST 1992
X
XCC = gcc 
XCFLAGS = -O
X#LDFLAGS = -s 
XLDFLAGS = -s -shlib	#shared library option for gcc on 3b1/UnixPC
X
XOBJS= k7enc.o k7dec.o k8enc.o k8dec.o
XPROGS= k7enc k7dec k8enc k8dec
XSOURCES= k7enc.c k7dec.c k8enc.c k8dec.c
X
Xall: $(PROGS)
X
Xk7enc: k7enc.o
X	$(CC) -o $@ k7enc.o $(LDFLAGS)
X
Xk7dec: k7dec.o
X	$(CC) -o $@ k7dec.o $(LDFLAGS)
X
Xk8enc: k8enc.o
X	$(CC) -o $@ k8enc.o $(LDFLAGS)
X
Xk8dec: k8dec.o
X	$(CC) -o $@ k8dec.o $(LDFLAGS)
X
Xclean: 
X	rm core $(PROGS) $(OBJS)
X
Xshar:	READ.ME Makefile filters.doc $(SOURCES) k[78]verify.ksh
X	shar READ.ME Makefile filters.doc $(SOURCES) k[78]verify.ksh \
X	>filters.shar
END_OF_FILE
if test 721 -ne `wc -c <'Makefile'`; then
    echo shar: \"'Makefile'\" unpacked with wrong size!
fi
# end of 'Makefile'
fi
if test -f 'filters.doc' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'filters.doc'\"
else
echo shar: Extracting \"'filters.doc'\" \(6957 characters\)
sed "s/^X//" >'filters.doc' <<'END_OF_FILE'
X
X	    Preprocessors Speed Kermit File Transfer
X
X		    by Glenn E. Thobe 
X
X		    rev. February 12, 1992
X
X
XINTRODUCTION
X
X
XI have written a set of four pre- and post-processors which greatly 
Ximprove Kermit's efficiency when transferring compressed files.  
XKermit's control prefixing algorithm, although efficient for text 
Xfiles, is expensive for compressed files.  
X
XCompressed files are fundamentally different from other binary files 
Xin the way in which the data are statistically distributed.  Roughly 
Xspeaking, the bytes of a compressed file are uncorrelated and uniformly 
Xdistributed over the 256 8-bit codes.  Text files, for which Kermit's
Xcontrol prefixing was originally designed, have few characters which
Xare not in the printable range.  For them, control prefixing adds 
Xlittle extra cost.  By contrast, compressed files contain many
Xunprintable characters for which prefixing is inefficient.  Refinements, 
Xsuch as Kermit's run length and shift lock encoding, are ineffective 
Xfor compressed files.
X
XThe preprocessors work by preempting Kermit's control prefixing overhead 
Xwhich was designed to optimize the transmission of text files.  The entire 
Xavailable code space of 190 or 95 (7-bits) printable characters is used in a 
Xstatistically uniform way, and none of the control characters are used at 
Xall.  All forms of encoding will expand compressed files.  Our 
Xpreprocessors expand them as little as possible, certainly much less than 
XKermit's control prefixing strategy.  The test measurements bear out this 
Xcontention.
X
XThe filters are:
X
X    k7enc - encode for 7-bit channels
X    k7dec - decode for 7-bit channels
X    k8enc - encode for 8-bit channels
X    k8dec - decode for 8-bit channels
X
XIn actual use these filters are employed in the following sequence to 
Xtransfer a compressed file in the least amount of time:
X
X    * Using k8enc, encode the file on sending computer.
X    * Transfer the encoded file using Kermit.
X    * Using k8dec, decode file on receiving computer.
X
XFor testing purposes, additional filters were written which implement
XKermit's prefixing algorithms: 
X
X    getpkt7 - Kermit encoding (control + 8th-bit prefixing) 
X    getpkt8 - Kermit encoding (control prefixing) 
X
XIn order to experimentally evaluate our strategy, we simulated Kermit 
Xtransfers using the filters k7enc, k8enc, getpkt7, and getpkt8 in 
Xvarious combinations.  We compared the efficiencies of transferring 
Xfiles with or without the pre- and post-processing.  Efficiency is
Xdefined as the ratio of the number of bytes in the original file to
Xthe number of bytes transmitted over the line (i.e. fully encoded).  
XThe wc utility gives the number of bytes emerging from the preceding 
Xfilters in the pipeline:
X
X    getpkt8 <file.Z | wc                #raw file transmitted by kermit
X    k8enc <file.Z | wc                  #k8encoded file
X    k8enc <file.Z | getpkt8 | wc        #k8encoded file transmitted by Kermit
X
XWe can view k8enc and Kermit's encoding as alternative ways of dealing 
Xwith the limited 190 (or 95) letter code space, and compare the 
Xperformance of the two.  We can also compare the performance of k7enc
Xand k8enc with other popular binary-to-printable filters.
X
X
XRESULTS
X
X
XFollowing are simulation results for a typical file using k8enc.
X
X    file=DNLNK/dnlnk.tar.Z
X    raw file transfer:
X    in = 57725, out = 75436, expansion = 30.68%, efficiency = 76.52%
X    encoding alone:
X    in = 57725, out = 62118, expansion = 7.61%, efficiency = 92.92%
X    encoded file transfer alone:
X    in = 62118, out = 63072, expansion = 1.53%, efficiency = 98.48%
X    combined coding and transfer:
X    in = 57725, out = 63072, expansion = 9.26%, efficiency = 91.52%
X
XNote that k8enc encoding alone significantly outperforms Kermit's
Xgetpkt8 encoding.  Also, that k8enc preprocessed files enjoy greatly
Ximproved transfer performance compared to the raw compressed file.
X
XHere, for the same file, are the results with different preprocessors:
X
X    ============================================================
X    Combined Efficiencies for Kermit used with various preprocessors
X    ============================================================
X    file name = DNLNK/dnlnk.tar.Z, file size = 57725 bytes
X    7-bit channel (eighth bit prefixing):
X    no encoding     56.77%
X    uuencode        69.2%
X    btoa            75.96%
X    k7enc           77.36%
X    8-bit channel:
X    no encoding     76.52%
X    k8enc           91.52%
X    ============================================================
X
XNote that k7enc used with a 7-bit channel actually outperforms raw 
Xfile transfer over an 8-bit channel.  It also outperforms both
Xuuencode (where each three bytes are spread over four) and atob 
X(where each four bytes are spread over five).  K8enc is obviously
Xthe big winner here.
X
X
XPROTOCOL ENHANCEMENT PROPOSAL
X
X
XWhile the improvements are impressive, the user is still left with 
Xthe clumsiness of manually running the pre- and postprocessors, and
Xkeeping track of the file names.  It would be much more convenient 
Xto incorporate the encoding and decoding processes into the Kermit 
Xprotocol.  
X
XA compressed-file mode should be introduced.  The user would select 
Xthis mode when transferring LZ-compressed, zipped, Gif, and other 
Xcompressed binary data types.  The implementation would be especially 
Xeasy, since in this mode most of the intricate prefixing, RLE, and
Xlocking shift preprocessing involved in packetizing the data can be 
Xswitched off and built-in k7enc or k8enc processing switched on in
Xits place.
X
XThis protocol enhancement should be viewed as separate from the proposed 
Xbuilt-in file compression, since the latter is of no great benefit when 
Xthe file is already compressed.  Built-in compression should be used
Xfor text files, executables, etc., but not for already compressed files.
X
XThe good news here is that restricting the code space to the 190 extended
Xascii printables can have so little adverse impact on efficiency.  When
Xthe data are encoded using our k8enc filter, a compressed file is expanded
Xtypically by only 7 to 8 percent, as shown experimentally by our study.
X
X
XCONCLUSION
X
X
XThe traditional Kermit prefixing algorithm for mapping all file characters
Xinto the ASCII and extended ASCII printable codes is optimized for text
Xfiles but performs poorly for compressed files.  File transfer performance 
Xwith compressed files can be improved greatly by complementary pre- and
Xpost processing of the files using the filter programs presented here.
XThese filters significantly outperform not only Kermit's built-in
Xcontrol prefixing, but also the traditional uuencode and btoa.  This
Xis accomplished by optimally exploiting the available channel capacity.
X
XThese filters should be introduced into the Kermit protocol in support of 
Xa special mode for optimizing the transfer of compressed files.
X
X-----------------------------------------------------------------------
X
X-Glenn E. Thobe
X<thobe@getunx.info.com>
END_OF_FILE
if test 6957 -ne `wc -c <'filters.doc'`; then
    echo shar: \"'filters.doc'\" unpacked with wrong size!
fi
# end of 'filters.doc'
fi
if test -f 'k7enc.c' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'k7enc.c'\"
else
echo shar: Extracting \"'k7enc.c'\" \(2881 characters\)
sed "s/^X//" >'k7enc.c' <<'END_OF_FILE'
X/* k7enc.c - encode for Kermit's 7-bit window (0x20-0x7e).
XSpeeds Kermit transfer of compressed files for 7-bit channels, use 
Xwith k7dec.  For 8-bit channels with no parity use k8enc and k8dec.
X
XAuthor:	Glenn E. Thobe	<thobe@getunx.info.com>
X
XCopyright (C) 1992, Glenn E. Thobe, Canoga Park, California, USA.
XAll rights reserved.  Permission is granted to any individual 
Xor institution to use, copy, or redistribute this software so 
Xlong as it is not sold for profit, provided this copyright 
Xnotice is retained.
X
XK7enc prepares a compressed file to be transferred by Kermit in
Xbinary mode and decoded by k7dec.  As a filter, its typical
Xusage with the compressed file.Z might be:
X
X	k7enc < file.Z | kermit -is -     (sender)
X	kermit -ik | k7dec > file.Z       (receiver)
X
XWith a text file the following is also useful:
X
X	compress < file.txt | k7enc | kermit -is -     (sender)
X	kermit -ik | k7dec | uncompress > file.txt     (receiver)
X
XK7enc and k7dec feature a special non-linear mapping which avoids
Xall ASCII control characters and makes optimal use of the Kermit's 
Xentire 7-bit ASCII-printable code space.  Thus Kermit has no need
Xof control or 8th-bit prefixing (except for the control prefix itself
Xwhich is doubled).
X*/
X
X/* REVISION HISTORY:
XSat Feb  8 22:15:47 PST 1992	initial release -g.t.
X*/
X
X/* call coroutine */
X#define A_CALL_B(x,Ax) do{An=x;goto B;Ax:;}while(0)	/* K&R C version */
X#define B_CALL_A(x,Bx) do{Bn=x;goto A;Bx:;}while(0)	/* K&R C version */
X
X#define min(x,y) ((x)>(y))?(y):(x)
X#define MAP(x) ((x)+0x20)	/* map to ASCII printables */
X
X#include <stdio.h>
X
Xvoid
Xmain()
X{
X    int An, Bn;
X    int c, m, h, w, inbuf, u, eof;
X    static int rmask[]={0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff};
X
X    /* initialize coroutine linkage */
X    An = 0;
X    Bn = 0;
X    goto A;
X
X    /* input 1-8 bits as per request */
XA0:
X    h = 0;
X    eof = 0;
X    inbuf = 0;
X    while(1)
X    {
X	A_CALL_B(1,A1);
X	if (h < w)
X	{
X	    u = getchar();
X	    eof = feof(stdin);
X	    inbuf <<= 8;
X	    if(!eof) inbuf += u;
X	    h += 8;
X	}
X	c = (inbuf >> (h - w)) & rmask[w];
X	h -= w;
X    }
X
X    /* assemble and output encoded bytes */
XB0:
X    w = 8;
X    B_CALL_A(1,B1);
X    while(1)
X    {
X	if (c < 0x88)
X	{
X	    m = c;
X	    putchar(MAP(m >> 2));
X	    if (eof && h < 6) exit(0);
X	    w = 6;
X	    B_CALL_A(2,B2);
X	    c += (m & 0x03) << 6;
X	}
X	else if (c < 0xfe)
X	{
X	    m = c;
X	    putchar(MAP((m >> 1) - 0x22));
X	    if (eof && h < 7) exit(0);
X	    w = 7;
X	    B_CALL_A(3,B3);
X	    c += (m & 0x01) << 7;
X	}
X	else
X	{
X	    putchar(MAP(c - 0xa1));
X	    if (eof && h < 8) exit(0);
X	    w = 8;
X	    B_CALL_A(4,B4);
X	}
X    }
X    exit(0);
X
X/* more coroutine stuff */
X
XA: 
X    switch(An)
X    {
X    case 0: goto A0;
X    case 1: goto A1;
X    }
X
XB: 
X    switch(Bn)
X    {
X    case 0: goto B0;
X    case 1: goto B1;
X    case 2: goto B2;
X    case 3: goto B3;
X    case 4: goto B4;
X    }
X}
END_OF_FILE
if test 2881 -ne `wc -c <'k7enc.c'`; then
    echo shar: \"'k7enc.c'\" unpacked with wrong size!
fi
# end of 'k7enc.c'
fi
if test -f 'k7dec.c' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'k7dec.c'\"
else
echo shar: Extracting \"'k7dec.c'\" \(2585 characters\)
sed "s/^X//" >'k7dec.c' <<'END_OF_FILE'
X/* k7dec.c - decode for Kermit's 7-bit window (0x20-0x7e).
XSpeeds Kermit transfer of compressed files for 7-bit channels, use 
Xwith k7enc.  For 8-bit channels with no parity use k8enc and k8dec.
X
XAuthor:	Glenn E. Thobe	<thobe@getunx.info.com>
X
XCopyright (C) 1992, Glenn E. Thobe, Canoga Park, California, USA.
XAll rights reserved.  Permission is granted to any individual 
Xor institution to use, copy, or redistribute this software so 
Xlong as it is not sold for profit, provided this copyright 
Xnotice is retained.
X
XK7dec recovers a compressed file which has been encoded by k7enc 
Xand transferred by Kermit in binary mode.  As a filter, its typical
Xusage with the compressed file.Z might be:
X
X	k7enc < file.Z | kermit -is -     (sender)
X	kermit -ik | k7dec > file.Z       (receiver)
X
XWith a text file the following is also useful:
X
X	compress < file.txt | k7enc | kermit -is -     (sender)
X	kermit -ik | k7dec | uncompress > file.txt     (receiver)
X
XK7enc and k7dec feature a special non-linear mapping which avoids
Xall ASCII control characters and makes optimal use of the Kermit's 
Xentire 7-bit ASCII-printable code space.  Thus Kermit has no need
Xof control or 8th-bit prefixing (except for the control prefix itself
Xwhich is doubled).
X*/
X
X/* REVISION HISTORY:
XSat Feb  8 22:15:47 PST 1992	initial release -g.t.
X*/
X
X/* call coroutine */
X#define A_CALL_B(x,Ax) do{An=x;goto B;Ax:;}while(0)	/* K&R C version */
X#define B_CALL_A(x,Bx) do{Bn=x;goto A;Bx:;}while(0)	/* K&R C version */
X
X#define UNMAP(x) ((x)-0x20)	/* unmap from ASCII printables */
X
X#include <stdio.h>
X#include <assert.h>
X
Xvoid
Xmain()
X{
X    int An, Bn;
X    int c, m, h, w, t, u, inbuf;
X    static int rmask[]={0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff};
X
X    /* initialize coroutine linkage */
X    An = 0;
X    Bn = 0;
X    goto A;
X
X    /* input 1-8 bits as per request */
XA0:
X    h = 0;
X    inbuf = 0;
X    while(1)
X    {
X	A_CALL_B(1,A1);
X	while (h < w)
X	{
X	    t = getchar();
X	    if (feof(stdin)) exit(0);
X	    u = UNMAP(t);
X	    if (u < 0x22)
X	    {
X		inbuf <<= 6;
X		inbuf += u;
X		h += 6;
X	    }
X	    else if (u < 0x5d)
X	    {
X		u += 0x22;
X		inbuf <<= 7;
X		inbuf += u;
X		h += 7;
X	    }
X	    else
X	    {
X		u += 0xa1;
X		inbuf <<= 8;
X		inbuf += u;
X		h += 8;
X	    }
X	}
X	c = (inbuf >> (h - w)) & rmask[w];
X	h -= w;
X    }
X
X    /* dissemble encoded bytes and output */
XB0:
X    while(1)
X    {
X	w = 8;
X	B_CALL_A(1,B1);
X	putchar(c);
X    }
X
X/* more coroutine stuff */
X
XA: 
X    switch(An)
X    {
X    case 0: goto A0;
X    case 1: goto A1;
X    }
X
XB: 
X    switch(Bn)
X    {
X    case 0: goto B0;
X    case 1: goto B1;
X    }
X}
END_OF_FILE
if test 2585 -ne `wc -c <'k7dec.c'`; then
    echo shar: \"'k7dec.c'\" unpacked with wrong size!
fi
# end of 'k7dec.c'
fi
if test -f 'k8enc.c' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'k8enc.c'\"
else
echo shar: Extracting \"'k8enc.c'\" \(2737 characters\)
sed "s/^X//" >'k8enc.c' <<'END_OF_FILE'
X/* k8enc.c - encode for Kermit's 8-bit window (0x20-0x7e, 0xa0-0xfe).
XSpeeds Kermit transfer of compressed files for 8-bit channels, use 
Xwith k8enc.  For 7-bit channels with parity use k7enc and k7dec.
X
XAuthor:	Glenn E. Thobe	<thobe@getunx.info.com>
X
XCopyright (C) 1992, Glenn E. Thobe, Canoga Park, California, USA.
XAll rights reserved.  Permission is granted to any individual 
Xor institution to use, copy, or redistribute this software so 
Xlong as it is not sold for profit, provided this copyright 
Xnotice is retained.
X
XK8enc prepares a compressed file to be transferred by Kermit in
Xbinary mode and decoded by k8dec.  As a filter, its typical
Xusage with the compressed file.Z might be:
X
X	k8enc < file.Z | kermit -is -     (sender)
X	kermit -ik | k8dec > file.Z       (receiver)
X
XWith a text file the following is also useful:
X
X	compress < file.txt | k8enc | kermit -is -     (sender)
X	kermit -ik | k8dec | uncompress > file.txt     (receiver)
X
XK8enc and k8dec feature a special non-linear mapping which avoids
Xall ASCII control characters and makes optimal use of the Kermit's 
Xentire 8-bit ASCII-printable code space.  Thus Kermit has no need
Xof control prefixing (except for the control prefix itself which 
Xis doubled).
X*/
X
X/* REVISION HISTORY:
XSat Feb  8 22:15:47 PST 1992	initial release -g.t.
X*/
X
X/* call coroutine */
X#define A_CALL_B(x,Ax) do{An=x;goto B;Ax:;}while(0)	/* K&R C version */
X#define B_CALL_A(x,Bx) do{Bn=x;goto A;Bx:;}while(0)	/* K&R C version */
X
X#define min(x,y) ((x)>(y))?(y):(x)
X#define MAP(x) ((x)+(((x)<0x5f)?0x20:0x41))	/* map to printables */
X
X#include <stdio.h>
X
Xvoid
Xmain()
X{
X    int An, Bn;
X    int c, m, h, w, inbuf, u, eof, old_eof;
X    static int rmask[]={0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff};
X
X    /* initialize coroutine linkage */
X    An = 0;
X    Bn = 0;
X    goto A;
X
X    /* input 1-8 bits as per request */
XA0:
X    h = 0;
X    old_eof = eof = 0;
X    inbuf = 0;
X    while(1)
X    {
X	A_CALL_B(1,A1);
X	if (h < w)
X	{
X	    u = getchar();
X	    eof = feof(stdin);
X	    inbuf <<= 8;
X	    if(!eof) inbuf += u;
X	    h += 8;
X	}
X	c = (inbuf >> (h - w)) & rmask[w];
X	h -= w;
X    }
X
X    /* assemble and output encoded bytes */
XB0:
X    w = 8;
X    B_CALL_A(1,B1);
X    while(1)
X    {
X	if (c < 0x84)
X	{
X	    m = c;
X	    putchar(MAP(m >> 1));
X	    if (eof && h < 7) exit(0);
X	    w = 7;
X	    B_CALL_A(2,B2);
X	    c += (m & 0x01) << 7;
X	}
X	else
X	{
X	    putchar(MAP(c-0x42));
X	    if (eof && h < 8) exit(0);
X	    w = 8;
X	    B_CALL_A(3,B3);
X	}
X	old_eof = eof;
X    }
X    exit(0);
X
X/* more coroutine stuff */
X
XA: 
X    switch(An)
X    {
X    case 0: goto A0;
X    case 1: goto A1;
X    }
X
XB: 
X    switch(Bn)
X    {
X    case 0: goto B0;
X    case 1: goto B1;
X    case 2: goto B2;
X    case 3: goto B3;
X    }
X}
END_OF_FILE
if test 2737 -ne `wc -c <'k8enc.c'`; then
    echo shar: \"'k8enc.c'\" unpacked with wrong size!
fi
# end of 'k8enc.c'
fi
if test -f 'k8dec.c' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'k8dec.c'\"
else
echo shar: Extracting \"'k8dec.c'\" \(2500 characters\)
sed "s/^X//" >'k8dec.c' <<'END_OF_FILE'
X/* k8dec.c - decode for Kermit's 8-bit window (0x20-0x7e, 0xa0-0xfe).
XSpeeds Kermit transfer of compressed files for 8-bit channels, use 
Xwith k8enc.  For 7-bit channels with parity use k7enc and k7dec.
X
XAuthor:	Glenn E. Thobe	<thobe@getunx.info.com>
X
XCopyright (C) 1992, Glenn E. Thobe, Canoga Park, California, USA.
XAll rights reserved.  Permission is granted to any individual 
Xor institution to use, copy, or redistribute this software so 
Xlong as it is not sold for profit, provided this copyright 
Xnotice is retained.
X
XK8dec recovers a compressed file which has been encoded by k8enc 
Xand transferred by Kermit in binary mode.  As a filter, its typical
Xusage with the compressed file.Z might be:
X
X	k8enc < file.Z | kermit -is -     (sender)
X	kermit -ik | k8dec > file.Z       (receiver)
X
XWith a text file the following is also useful:
X
X	compress < file.txt | k8enc | kermit -is -     (sender)
X	kermit -ik | k8dec | uncompress > file.txt     (receiver)
X
XK8enc and k8dec feature a special non-linear mapping which avoids
Xall ASCII control characters and makes optimal use of the Kermit's 
Xentire 8-bit ASCII-printable code space.  Thus Kermit has no need
Xof control prefixing (except for the control prefix itself
Xwhich is doubled).
X*/
X
X/* REVISION HISTORY:
XSat Feb  8 22:15:47 PST 1992	initial release -g.t.
X*/
X
X/* call coroutine */
X#define A_CALL_B(x,Ax) do{An=x;goto B;Ax:;}while(0)	/* K&R C version */
X#define B_CALL_A(x,Bx) do{Bn=x;goto A;Bx:;}while(0)	/* K&R C version */
X
X#define UNMAP(x) ((x)-(((x)<0x80)?0x20:0x41))	/* unmap from printables */
X
X#include <stdio.h>
X#include <assert.h>
X
Xvoid
Xmain()
X{
X    int An, Bn;
X    int c, m, h, w, t, u, inbuf;
X    static int rmask[]={0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff};
X
X    /* initialize coroutine linkage */
X    An=0;
X    Bn=0;
X    goto A;
X
X    /* input 1-8 bits as per request */
XA0:
X    h = 0;
X    inbuf = 0;
X    while(1)
X    {
X	A_CALL_B(1,A1);
X	while (h < w)
X	{
X	    t = getchar();
X	    if (feof(stdin)) exit(0);
X	    u = UNMAP(t);
X	    if (u < 0x42)
X	    {
X		inbuf <<= 7;
X		inbuf += u;
X		h += 7;
X	    }
X	    else
X	    {
X		u += 0x42;
X		inbuf <<= 8;
X		inbuf += u;
X		h += 8;
X	    }
X	}
X	c = (inbuf >> (h - w)) & rmask[w];
X	h -= w;
X    }
X
X    /* dissemble encoded bytes and output */
XB0:
X    while(1)
X    {
X	w = 8;
X	B_CALL_A(1,B1);
X	putchar(c);
X    }
X
X/* more coroutine stuff */
X
XA: 
X    switch(An)
X    {
X    case 0: goto A0;
X    case 1: goto A1;
X    }
X
XB: 
X    switch(Bn)
X    {
X    case 0: goto B0;
X    case 1: goto B1;
X    }
X}
END_OF_FILE
if test 2500 -ne `wc -c <'k8dec.c'`; then
    echo shar: \"'k8dec.c'\" unpacked with wrong size!
fi
# end of 'k8dec.c'
fi
if test -f 'k7verify.ksh' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'k7verify.ksh'\"
else
echo shar: Extracting \"'k7verify.ksh'\" \(90 characters\)
sed "s/^X//" >'k7verify.ksh' <<'END_OF_FILE'
Xfor i
X{
X    echo "testing on file $i"
X    k7enc < $i | k7dec | cmp - $i
X}
Xecho "finished"
END_OF_FILE
if test 90 -ne `wc -c <'k7verify.ksh'`; then
    echo shar: \"'k7verify.ksh'\" unpacked with wrong size!
fi
chmod +x 'k7verify.ksh'
# end of 'k7verify.ksh'
fi
if test -f 'k8verify.ksh' -a "${1}" != "-c" ; then 
  echo shar: Will not clobber existing file \"'k8verify.ksh'\"
else
echo shar: Extracting \"'k8verify.ksh'\" \(90 characters\)
sed "s/^X//" >'k8verify.ksh' <<'END_OF_FILE'
Xfor i
X{
X    echo "testing on file $i"
X    k8enc < $i | k8dec | cmp - $i
X}
Xecho "finished"
END_OF_FILE
if test 90 -ne `wc -c <'k8verify.ksh'`; then
    echo shar: \"'k8verify.ksh'\" unpacked with wrong size!
fi
chmod +x 'k8verify.ksh'
# end of 'k8verify.ksh'
fi
echo shar: End of shell archive.
exit 0

17-Mar-93 21:42:31-GMT,2861;000000000001
Return-Path: <mtdcr!ptm@mtune.att.com>
Received: from att-out.att.com by watsun.cc.columbia.edu (5.59/FCB/jba)
	id AA07725; Wed, 17 Mar 93 16:42:24 EST
Message-Id: <9303172142.AA07725@watsun.cc.columbia.edu>
From: mtdcr!ptm@mtune.att.com
Date: Wed, 17 Mar 93 16:40 EST
To: fdc@watsun.cc.columbia.edu
Subject: Compression FYI

Frank -  (from Peter Mauzey)   March 17, 1993

You may recall months ago asking about a compression algorithm that
is free of patent infringements.  Hence the following quote, FYI.
I'm not sure Keith is correct in his assertion about "patented algorithms."

------ quote start -----
Date: Sun, 14 Mar 1992 20:41:10 EST [ Really 1993 ]
From: w8sdz@TACOM-EMH1.Army.Mil (Keith Petersen)
To: MSDOS-Ann@TACOM-EMH1.Army.Mil
Subject: GZIP106.ZIP - GNU zip: Unix compress/uncompress replacement
Message-Id: <9303142041.12979.w8sdz@TACOM-EMH1.Army.Mil>

I have uploaded to WSMR-SIMTEL20.Army.Mil and OAK.Oakland.Edu:

pd1:<msdos.compress>
GZIP106.ZIP     GNU zip: Unix compress/uncompress replacement

gzip (GNU zip) is a compression utility designed to be a replacement
for 'compress'.  Its main advantages over compress are much better
compression and freedom from patented algorithms.  The GNU Project
uses it as the standard compression program for its system. 

gzip currently uses by default the LZ77 algorithm used in zip 1.9 (the
portable pkzip compatible archiver).  The gzip format was however
designed to accommodate several compression algorithms. 

gunzip can currently decompress files created by gzip, zip (with
restrictions), compress or pack.  (The SCO 'compress -H' format will be
supported in a future version).  The detection of the input format is
automatic.  When using the first two formats, gunzip checks a 32 bit
CRC.  For pack, gunzip checks the uncompressed length.  The 'compress'
format was not designed to allow consistency checks.  However gunzip is
sometimes able to detect a bad .Z file because there is some redundancy
in the .Z compression format.  If you get an error when uncompressing
a .Z file, do not assume that the .Z file is correct simply because the
standard uncompress does not complain.  This generally means that the
standard uncompress does not check its input, and happily generates
garbage output. 

Those who need the source in order to compile this program on other
platforms please see files GZIP106.TAR-Z and GZIPREAD.ME in SIMTEL20
directory PD8:<MISC.UNIX> or OAK directory /pub/misc/unix.

Keith
--
Keith Petersen
Maintainer of the MS-DOS archive at WSMR-SIMTEL20.Army.Mil [192.88.110.20]
Internet: w8sdz@TACOM-EMH1.Army.Mil     or      w8sdz@Vela.ACS.Oakland.Edu
Uucp: uunet!umich!vela!w8sdz                         BITNET: w8sdz@OAKLAND
------ quote end -----

Peter Mauzey   908 957-3887       ptm@mtdcr.att.com
Bell Labs, 200 Laurel Avenue, Room 1A-301, Middletown, NJ  07748-4801

 1-Dec-93 21:34:56-GMT,9258;000000000001
Return-Path: <beebe@math.utah.edu>
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	id AA11988; Wed, 1 Dec 93 16:34:53 EST
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	id AA06920; Wed, 1 Dec 93 14:34:50 MST
Date: Wed, 1 Dec 93 14:34:50 MST
From: "Nelson H. F. Beebe" <beebe@math.utah.edu>
To: fdc@watsun.cc.columbia.edu
Cc: beebe@math.utah.edu, bowman@math.utah.edu
X-Us-Mail: "Center for Scientific Computing, South Physics, University of
        Utah, Salt Lake City, UT 84112"
X-Telephone: +1 801 581 5254
X-Fax: +1 801 581 4148
Subject: Kermit and dynamic compression
Message-Id: <CMM.0.90.2.754781688.beebe@solitude.math.utah.edu>

During the last year, implementations of the UNIX ftpd have become
available that support dynamic archiving (tar, arc, zip, zoo) and
compression (compress, gzip) for ftp "get" and "mget" commands.  For
compression, the data transfer volume is often reduced to 1/3 to 1/4
of what it would be without that compression; some representative
figures are given below.

For compression at least, the code changes are very very small: just
replace

        fp = fopen(filename,"r");
        ...
        (void)fclose(fp);

with
        char buffer[MAXFILENAME + xxx];
        sprintf(command,"gzip < %s",filename);
        fp = popen(command,"r");
        ...
        (void)pclose(fp);

popen()/pclose() are defined in POSIX and X/Open, so most UNIX
implementations already have them.  Otherwise, they can be synthesized
from fork() and exec(), and I'm confident that freely-distribution
versions of suitable code could be found on the Internet, or
straightforwardly written.  [xarchie found these a few seconds ago:

irisa.irisa.fr:/News/Jukebox/comp.sources.atari.st/volume04/popen
spud.hyperion.com:/Atari/sources/volume4/popen

]

ftpd is somewhat fancier, since it allows the command lines to be
specified in an external file, along with matching extensions, making
it possible to extend the choice of archive and compression techniques
without code modification.  A typical example is appended to this
message.

The current Kermit implementation, 5A(189), for UNIX uses run-length
encoding, as implemented in function encode() in ckcfns.c.  For
certain types of files (e.g. those with long strings of blanks or
NULs, or binary images with constant backgrounds), this works quite
well.  However, the compression algorithms of gzip at present seem to
outperform almost everything that I've tried (gzip often beats
compress by about 25%, and both are much better than the dynamic
Huffman encoding of UNIX pack/unpack).

Here is a single comparison of gzip and compress on the UNIX (Sun
SunOS 4.1.3) executable, /usr/local/bin/reduce, the largest one in
that directory on our system:

-rwxrwxr-x  1 bowman    5414912 Jun 10  1991 /usr/local/bin/reduce
-rw-rw-r--  1 beebe      601721 Dec  1 14:06 /tmp/reduce.Z
-rw-rw-r--  1 beebe      294912 Dec  1 14:06 /tmp/reduce.gz

compress decreased the file size by a factor of 601721/5414912 = 0.11,
while gzip decreased it by a factor of 294912/5414912 = 0.05.

For a large text file, the Steele/Raymond `New Hacker's Dictionary',
we have

-rw-rw-r--  1 beebe     1367403 Jul 27 21:04 jargon.info
-rw-rw-r--  1 beebe      587751 Dec  1 14:19 /tmp/jargon.info.Z
-rw-rw-r--  1 beebe      523845 Dec  1 14:20 /tmp/jargon.info.gz

compress reduces the file by a factor of 0.43, and gzip, by
0.35.

For a much more rigorous test, here are the compression results for
our last night's file system backup with amanda (Advanced Maryland
Automatic Network Disk Archiver), which uses the standard UNIX
compress utility.  The COMP% column shows the degree of compression
achieved for each file system, a total of 3.4GB of data:

STATISTICS:
                          Total       Full      Daily
                        --------   --------   --------
Dump Time (hrs:min)        6:57       4:27       0:39   (0:32 start, 1:19 idle)
Output Size (meg)        1550.2      995.8      554.4
Original Size (meg)      3430.4     2389.0     1041.5
Avg Compressed Size (%)    39.9       41.7       34.2
Tape Used (%)              36.9       23.7       13.2   (level:#disks ...)
Filesystems Dumped           23          3         20   (1:15 2:4 3:1)
Avg Dump Rate (k/s)        51.0       61.6       38.9
Avg Tp Write Rate (k/s)    86.4       63.7      241.6

DUMP SUMMARY:
                                   DUMPER STATS                  TAPER STATS
  HOSTNAME   DISK   LV  ORIG-KB   OUT-KB  COMP%  MMM:SS   KB/s  MMM:SS   KB/s
  --------------------  --------------------------------------  -------------
  alfred     rz0a    1      198      100   50.8    0:12    8.2    0:16    8.0
  alfred     rz0h    1    70152    40085   57.1   13:18   50.2    1:33  433.2
  alfred     rz2a    1     4967     1020   20.5    1:48    9.5    0:12   85.6
  alfred     rz3a    0  1649058   630695   38.2  230:27   45.6  230:36   45.6
  alfred     rz5a    2   154888    58652   37.9   28:05   34.8    2:20  417.9
  csc-sun    xd0a    1    18412    18412    --     2:05  147.8    0:46  403.9
  csc-sun    xd0g    1      272      272    --     1:22    3.3    0:08   34.9
  csc-sun    xd0h    2    57012    57012    --     9:03  105.0    2:05  455.1
  csc-sun    xd1g    1    67337    50299   74.7   17:09   48.9   17:17   48.5
  csc-sun    xd1h    1    34372    34372    --    12:25   46.1    1:28  391.7
  csc-sun    xd3c    2    26432    26432    --     6:28   68.1    1:10  376.9
  csc-sun    xd4c    1    33992    33992    --     7:14   78.2    1:26  395.4
  csc-sun    xd5c    3   138462   138462    --    25:30   90.5    5:22  430.6
  eros       erosb   0   138333    28705   20.8   10:26   45.8    1:18  370.7
  eros       root    1    25940     8855   34.1    3:54   37.8    0:26  340.0
  geronimo   geroni  1   355416    59176   16.6   12:17   80.3    2:14  441.4
  geronimo   root    0   658900   360265   54.7   34:58  171.7   35:06  171.0
  honeycomb  root    2    14495     1910   13.2    1:07   28.6    0:13  145.0
  saturna    root    1      412      131   31.9    0:29    4.5    0:07   21.5
  sparky     sd0a    1     6076     1237   20.4    3:10    6.5    0:19   67.4
  sparky     sd0g    1      107       48   44.9    0:28    1.7    0:07    9.1
  sparky     sd1c    1    55902    36969   66.1   96:50    6.4    1:33  399.8
  todd       root    1     1642      308   18.7    0:30   10.2    0:08   38.9


So, the question arises, have the Kermit developers considered
supporting this kind of dynamic compression, at least with a UNIX
kermit server?

The general idea would be that the user would type

        kermit> get foo

to get the original file foo, and

        kermit> get foo.gz
or
        kermit> get foo.Z

to get foo.gz or foo.Z if they exist, but OTHERWISE, to dynamically
invoke gzip or compress via popen(), and send the compressed file to
the user's machine.  If foo is a directory, a "get foo.tar" or "get
foo.tar.gz" or "get foo.tar.Z" could dynamically invoke tar and
optionally, gzip or compress.

ftpd/ftp provide no facility for dynamic decompression or
de-archiving, and I see no strong need to do so for kermit either,
although, clearly, it could be an option, e.g. "get/decompress foo.Z"
or "get/untar foo.tar".  The latter would make it possible for kermit
to move entire directory trees with a single user command.

------------------------------------------------------------------------

# This is /usr/local/etc/ftpconversions
# File format:
# strip prefix:strip postfix:addon prefix:addon postfix:external command:types:options:description
#
# For example,
# :  :  :.tar.Z:/bin/tar -c --compress -f - %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+COMPRESS
# says that a get of foo.tar.Z should execute the command
# /bin/tar -c --compress -f - foo
# return the piped output as foo.tar.Z.
#
# The line
# :.Z:  :  :/bin/compress -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS
# says that a get of foo for the file foo.Z should run
# /bin/compress -d -c foo.Z
# and return the piped output as foo
#
 :.Z:  :  :/bin/compress -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS
 :-z:  :  :/bin/compress -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS
 :.z:  :  :/bin/gunzip -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS
 :.trz: :.tar:/bin/gunzip -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS
 :.tar: :.trz:/bin/gunzip -d -c %s:T_REG|T_ASCII:O_UNCOMPRESS:UNCOMPRESS
 :  :  :.Z:/bin/compress -c %s:T_REG:O_COMPRESS:COMPRESS
 :  :  :.z:/bin/gzip -c %s:T_REG:O_COMPRESS:COMPRESS
 :  :  :.tar:/bin/tar -c -f - %s:T_REG|T_DIR:O_TAR:TAR
 :  :  :.tar.Z:/bin/tar -c --compress -f - %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+COMPRESS
 :  :  :.tar.z:/bin/tar -c --gzip -f - %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+COMPRESS
 :  :  :.zoo:/bin/ZOO %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+COMPRESS
 :  :  :.zip:/bin/ZIP %s:T_REG|T_DIR:O_COMPRESS|O_TAR:TAR+COMPRESS


========================================================================
Nelson H. F. Beebe                      Tel: +1 801 581 5254
Center for Scientific Computing         FAX: +1 801 581 4148
Department of Mathematics, 105 JWB      Internet: beebe@math.utah.edu
University of Utah
Salt Lake City, UT 84112, USA
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 1-Dec-93 22:02:41-GMT,2148;000000000001
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Date: Wed, 1 Dec 93 17:02:35 EST
From: Frank da Cruz <fdc@watsun.cc.columbia.edu>
To: "Nelson H. F. Beebe" <beebe@math.utah.edu>
Cc: bowman@math.utah.edu
Subject: Re: Kermit and dynamic compression
In-Reply-To: Your message of Wed, 1 Dec 93 14:34:50 MST
Message-Id: <CMM.0.90.4.754783355.fdc@watsun.cc.columbia.edu>

> During the last year, implementations of the UNIX ftpd have become
> available that support dynamic archiving (tar, arc, zip, zoo) and
> compression (compress, gzip) for ftp "get" and "mget" commands...
> ...
> So, the question arises, have the Kermit developers considered
> supporting this kind of dynamic compression, at least with a UNIX
> kermit server?
> 
Yes indeed.  There is a discussion of it in the file kermit/e/compress.txt on
kermit.columbia.edu, where you will find your own message as the latest
entry.  (Well, except for this one.)  And no, nothing as simplistic as running
things through gzip would do the trick.  Sure it's easy in UNIX, but any
higher-order compression method we adopt is going to have to meet certain
important criteria:

 . It must be portable among all OS's and file systems.

 . It must be designed to produce a data stream that contains no
   control or 8-bit characters (depending on Kermit's PARITY and
   SET CONTROL settings), so that Kermit's transparency encoding
   does not undo the effects of compression.

Even if we liked the idea of using popen() to run a file through gzip
before sending it, it still wouldn't be right.  Before compressing a
text file, we need to convert its character set and record format, otherwise
it'll be garbage after decompression on the receiving end.  And vice versa.

So don't worry, compression is high up on the list of items to add to the
Kermit protocol, pretty much right behind checkpoint/restart.  But it's not
simple.

Meanwhile, of course, when transferring between two UNIX systems, you can
already pipe stuff through C-Kermit, and that applies to compress, gzip,
crypt, sort, or anything else that works with stdio.

- Frank