AX.25 "reserved" bits in SSID bytes

[bruce@beagle.pixar.com]

I'm writing a really picky version of "listen" for Linux. It complains
about anything odd it sees in AX.25 packets, to help debug AX.25 software.

On page 7 of the AX.25 Level 2 Specification from ARRL, there is
explanation of two "reserved" bits in the SSID byte of each address,
that can be used "in an agreed-upon manner in individual networks".
It also says "When not implemented, they should be set to one".

At least three systems in my neighborhood that run some flavor of NOS
set both reserved bits in the destination address to 0, not 1, when
sending to _my_ station. They don't do this all of the time - without
looking very much it appears they only do it when sending to me. My
system doesn't appear to set the bits to 0 in either the source or
destination address. Their systems set the reserved bits in the source
address to 1, as expected.

Does anyone know of any meaning assigned to these bits by any NOS
implementation, or are they spuriously being set to 0?

	Thanks

	Bruce Perens