MNP Auto-Reliable + getty: happy accident or design?

[Jim Rosenberg]

Lately I've discovered that if a UNIX machine is answering the phone at 1200
baud and the caller is calling at 2400 in MNP auto-reliable mode, the MNP
handshake seems to toggle the receiver's getty to 2400 just like that, just
about every time.  The modems in both cases are similar:  Multitech external
and Multitech internal.  Both have MNP, but only the caller is in auto-
reliable mode; the receiver is running plain vanilla.  The manual describes
the MNP handshake as a "Link Request".  MNP functions with no start & stop
bits, so I assume this Link Request is sent with no start & stop bits also.
My understanding is that getty will be toggled by anything that looks like a
null (0x0) -- and that's the *only* way getty toggles.  (A break should appear
as a null with a framing error at any baud rate.)  Is the Link Request
*guaranteed* to toggle getty?  Under what combinations of baud rates?  Or does
this simply happen as a lucky accident?

Lucky accidents do happen.  It appears that if the caller is calling at 1200
and the receiver is answering the phone at 2400, if the caller transmits a CR
the receiver will hear two bytes, one of which is 0x80.  If getty is stripping
parity this will come out as 0x0 and woila, getty will toggle.  (Good luck
going back!)

What exactly is the composition of the Link Request?  What will it look like
to a modem not set up for MNP?  More to the point, where can I read up on MNP?
-- 
Jim Rosenberg                        pitt
Oglevee Computer Systems                 >--!amanue!oglvee!jr
151 Oglevee Lane                      cgh
Connellsville, PA 15425                                #include <disclaimer.h>