Subject: why name "extended mode"

From: [email-masked] (Bhavesh Pathak - BasisComm)
Date: Fri, 2 Jul 1999 12:43:10 -0700 (PDT)

dear everybody,
after going through the STAC-LZS rfc (rfc1974), I am not able to
understand the meaning of giving name as "extened mode" for 
checkmode-4? What does check-mode-4 mode try to extend?

with regards,
bhavesh

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From: John Shriver <[email-masked]>
Date: Fri, 2 Jul 1999 16:22:43 -0400

Check mode 4 for LZS isn't really an extended mode.  It's really a
documented Microsoft propietary extension.  Pro and con-wise, it uses
one more byte of overhead than check mode 3.  (The "compressed" bit
does make for easier code logic in handling data expansion.)  Oh, one
other advantage of MPPC is less overhead if you combine it with MPPE
(encryption).

Microsoft first wrote MPPC CCP compression for Windows 95 Dial-Up
Networking (DUN), because NT RAS (Remote Access Server) only supported
MPPC and MPPE.  When Microsoft decided to add LZS, the insisted on
using the same data packet headers for LZS and MPPC in DUN.  So they
devised check mode 4.  After release, they let the PPP working group
know what they had done, since they didn't support check mode 3.

MPPC is really Microsoft's preferred compression protocol, because
it's designed to run fast in C.  Windows NT 4.0 does not do LZS,
because there was no particuarly efficient C implementation of LZS at
that time, and Windows NT 4.0 had a "no assembly language" policy for
reasons of portability.  Since Windows 95 allowed assembly language,
it has LZS.

Of course, MPPC didn't take the world by storm, partly because
Microsoft got into a "little" patent problem with Stac.  Also, they
couldn't document it before the Windows 95 release, due to the legal
hassles with Stac.  (This meant that no third-party could implement
MPPC at the time, which was another reason we wanted Microsoft to do
LZS.)

Stac (hi/fn) now has an efficient C implementation of LZS.  Hopefully
this means that Windows 2000 (a/k/a Windows NT 5.0) will have LZS.
But I haven't putzed with the Beta releases to see.