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[aprssig] AX.25 'H' flag

Curt, WE7U archer at eskimo.com
Mon Nov 14 16:04:35 UTC 2005


On Mon, 14 Nov 2005, Wes Johnston wrote:

> passing thru a mishmash of digipeaters, you can end up with a packet
> like this:
> digi1*,digi2,digi3*,digi4
>
> In either of these cases, digi4 should figure out that it's the one
> supposed to digi next...  This point drove me nuts when I wrote my kiss
> digipeater years ago.  Best to parse packet paths from the END and work
> toward the first path element looking for any H flag to be set.  This
> also plays well with pre-emptive digipeating.

Wes, if you're really talking KISS packets (or raw AX.25 packets)
then you should never see the operation you describe else it's a bug
in the software in those TNC's and not per spec.  That bit _must_
get set in order to show that the slot is used up.  Havoc will reign
otherwise.

For command-mode TNC's then the '*' really can't be relied upon to
indicate state of each digipeater field.  The most common format
would be a single asterisk at the last used-up digipeater slot.  The
other format I know of would be an asterisk for every one from the
beginning to the last used-up slot, which is more indicative of the
state of the bit for each slot than the first format.

--
Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
"Lotto:    A tax on people who are bad at math." -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
"The world DOES revolve around me:  I picked the coordinate system!"




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