On Sat, Jul 31, 1999 at 01:40:08PM +0100, Robin Gilks wrote:
> ie. bit 0 is set to say this is the end of the address field -
> by this means, AX25 (and X25 on which it is based) already
> support variable length address fields.
>
> That most implementations don't support it is another matter
> entirely!!!
Well, this is not exactly true. If you read the AX.25
specification (2.0 or 2.1), you will see that what you assert is
true, but you overlook the technical hurdle. Bit 0 is 0
throughout ALL addresses except for the last octet. This is to
comply with the SDLC addressing style to facilitate hardware
computation of the CRC FCS. The difficulty lies in sifting out
the calls. The 2.x specification states that the addressing
fields must be an even multiple of seven octets. This currently
rules out variable length IDs. Having some sort of flag within
each address to mark its end, or having a length field as was
proposed earlier, makes the most sense. Unfortunately, it would
mean that these stations would be unable to connect with stations
which do not understand the new addressing scheme.
73
-Thomas
ke6cbr
