Fork of as well as custom CVS import with proper author name/email ('cvsimport' branch)
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# $Id: README.LZS,v 1.4 1999/06/21 13:28:31 hipp Exp $

*** ALPHA *** ALPHA *** ALPHA *** ALPHA *** ALPHA *** ALPHA *** ALPHA ***

LZS (aka Stac aka HiFn) compression support for I4L SyncPPP

Some months ago I picked up a question in the de.alt.isdn4linux newsgroup
asking "Is there Stac compression support for I4L ?". Due to a strange
coincidence, I had found out before on this same day that RFC1974 is a
somewhat complete documentation of the LZS CCP stuff and contains enough
information to code a decompressor and compressor. Thus I answered the
question like "a decompressor requires only some effort, a compressor
is somewhat more complicated but clearly possible". At this time the real
problem was that the isdn_ppp stuff in I4L was not ready for CCP modules at
all, most hooks were still missing. But then Michael Hipp implemented
the hooks in the newer CVS source of I4L and got BSD compression basically
working, so the ball was at my foot again. Thus I decided to code that
decompressor and eventually code a compressor, too.


This code is very early alpha. The LZS compressor itself is quite complete
IMHO, but we had to expand and change some portions of isdn_ppp, mainly
those dealing with Reset Requests and -Acks. This code is raw and not yet
finished, primarily because I didn't want to hack around in isdn_ppp but
just wanted to write the LZS module. But LZS CCP is more complicated than
most other CCP engines and required those changes. I hope someone with more
insight into isdn_ppp will adopt and complete them. The API to the com-
pressor could also get a general facelift, but one thing after the other.


The easiest way to build the module is to copy isdn_lzscomp.c to
linux/drivers/isdn/ and to edit the Makefile in this directory, adding
a line for LZS like:

O_OBJS += isdn_ppp.o
M_OBJS += isdn_bsdcomp.o
+ M_OBJS += isdn_lzscomp.o

The module will be built on the next "make modules". If you want to use
any CCP method with I4L, you must first load the relevant module(s). Then
you start the connection. You may need to give options to ipppd in order
to make sure negotiations succeed, see below.

Supported Modes

This set of sources supports LZS check mode 3 (the default of RFC1974)
and LZS check mode 4 (the derivative used by Micro$oft in Win0.95). If your
peer is an Ascend Max, the correct options are "Stac-9" for mode 3 and
"MS-Stac" for mode 4. Ask your ISP for what their NAS speaks. Either mode
is Ok, but if you insist on non-M$, I'm virtually yours.

Note: If your peer sends you CCP with the LZS identity (0x11) but a
length of 6 bytes, it is not compatible with RFC1974 but uses
a proprietary predecessor of it. Typical case is an Ascend Max
that has never been set to something else but the default "Stac"
in "Answer" and doesn't override this per-profile, either. Ask
your ISP about changing this, nobody (but other Ascend boxes)
will ever be able to use this compression. Especially not Win95,
if the ISP wants interoperability with Win95 he needs to set
MS-Stac, and then this module works as well.

If the NAS you are talking to will not negotiate gracefully you may want
to wire the exact mode ipppd is requesting. For instance, an Ascend Max
with a profile set to Stac-9, when asked for any number of histories but
one or asked for check mode 4 will not NAK these options with the ones
it likes better, but will reject LZS altogether. Your only chance to get
it to negotiate LZS is to use the exact options it wants in the first
place. To achieve that, ipppd got a new parameter:

lzs [recv-hists[:recv-mode[,xmit-hists[:xmit-mode]]]]

recv-hists Downstream # of Histories [8]
recv-mode Mode used downstream [3]
xmit-hists Upstream # of Histories [recv-hists]
xmit-mode Upstream mode [recv-mode]

This option is, as always, accompanied by


to prevent any LZS negotiations from happening. Some examples:

lzs 1 Mode 3 with 1 hist in both directions. This
will negotiate against a Max at Stac-9

lzs 1:4 Mode 4 with 1 hist in both directions. This
will negotiate against a Max at MS-Stac

BTW, I don't know whether multi-history support works. I have no peer to
test against. If your peer can send more than one history please inform
me of the results. LZS4I4L cannot support more than one history in the
compressor, at least not for some time to come.

Module options

When loading the isdn_lzscomp module, some options are available to tweak
it for ones personal desires:

comp=n n=0..9 [0]
Specifies the level of compression to be used.
The compressor is less complete compared to
the decompressor, play around with care.

0: No compression at all.
1: Absolutely minimal compression.
7: Good compression.
8: Heavy compression.
9: Best compression, but eats more
cycles than most CPUs can dream of.

If you want to try upstream compression,
start with 7. Try 9 only for fun.

debug=n n=0..3
Increasing levels of debug messages from
the LZS code. 3 includes full packet dumps
before and after compression and decompression.
More than 1 is overkill if you don't hunt

tweak=n n=tweak-flag-mask
Tweaks some aspects of the module to behave
strange or even violate RFCs in order to talk
to RFC-impaired peers. For Ascend, use 0x7.
If you want to know why, UTSL.


Please check whether using this code constitutes a crime in your country.
While I just implemented a module that can source and sink the data format
described by Stac/HiFn in RFC1974, they claim to have US Patents on
practically every aspect of this format. Thus I assume that selling this
code violates these US Patents. As this code is

a) Freeware under GPL and
b) Developed in Germany

I hope I wont get any terror from the lawyers of the patent holders. I'm
sure that the availability of this code will actually help HiFn sales
because more users (the Linux community) will ask their ISPs for LZS
support on their NASes and these NASes will contain HiFn chips for that
purpose. Then again, after they claimed even deflate would be covered by
their patents and tried to sue Netscape for using deflate in the PNG code
of Mozilla, they are probably not on the planet for good will games.
We'll see.

FMI, surf and search for "stac".


I said it's ALPHA already, didn't I ? There are bugs for sure. Because
this is a kernel module, chances are that they crash the whole system.
I _have_ seen crashes. They mostly appeared when a connection severly
hung due to non-working reset handling. Seems to be a NULL pointer de-
reference is still hiding somewhere in isdn_ppp.c or even isdn_lzscomp.c.
I've got the feeling those crashes occur when unloading isdn_lzscomp
but dunno exactly. Then again, I've done large downloads and surfed the
net for more than a hour without any problem. Your milage may vary.


- Cleanup MTU weirdness in the compressor
- Implement the remaining modes from RFC1974 provided anyone finds a peer
NAS that speaks one of them. I don't expect that ever happens ;)
- Thoroughly revise and clean up the changes done to isdn_ppp in order
to get in-kernel CCP reset handling. I assume the bug mentioned above
sits there.
- Fix the bugs you find.


The source code isdn_lzscomp.c is heavily commented (so I can still read it
in a month). It contains a thorough description of the compression algo-
rithm used and how I came to it. This in turn involves Credits to the
net.persons who helped me with getting that compressor faster. When we
are at it:


Phil Katz and L. Peter Deutsch
for the zlib and deflate format and specifications

Jean-loup Gailly and Mark Adler
for deflate, gzip and zlib as well as the comp.compression FAQ

David Carr
for a lot of general tips, hints and tricks speeding up a
compressor, pointers to freeze and the sigma buffer and especially
explaining to me the concept of the barrier hash until I finally
got it

Leonid Broukis
for the freeze implementation with the sigma buffer which
inspired my 8-buffer and for pointing out how memcmp(3) can
be used in match searching

Ojala Pasi 'Albert'
for general tips and his website which was the first reading
I had about speeding up a compressor beyond brute force


You can reach me by EMail:
Discussion will take place at de.alt.comm.isdn4linux, the USENET mirror
of the official I4L Mailing List.
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