[aprssig] Digipeating behavior
Curt, WE7U
archer at eskimo.com
Mon Mar 13 10:44:58 EST 2006
On Sat, 11 Mar 2006 scott at opentrac.org wrote:
> The AX.25 spec (version 2.2, page 13) has what I assume to be a typo in its
> digipeating description - an 'or' that seems to make no sense.
I seem to remember running into something goofy in the spec there
too when implementing code, but don't remember the details anymore.
I think I posted to APRSSIG with a question about it at the time.
> 1. When first digipeater field (starting after the source call) with H = 0
> matches my callsign or alias: Set H bit, retransmit frame.
>
> Bob suggests that this should not be limited to the first H = 0 field.
> Comments?
That would be pre-emptive digipeating. In that case you'd set the
H-bit for all fields up to and including the last one that exactly
matches your call and send the packet back out.
> 2. When digipeater field matches ALIASn-n, SSID is non-zero, and
> payload+address is unique in past S seconds, decrement SSID and set H = 1 if
> SSID is now zero. Optionally insert MYCALL prior to ALIASn-n.
>
> Should this apply only to the first H = 0 field?
Yes, unless preemptive digipeating.
> What else am I missing?
A hop-count maximum, to prevent people from doing things that are
too goofy with the n-N numbers. Check each of them, if either is
beyond the max specified by a config parameter, drop the packet.
You might also check whether n<N and drop it in that case as well as
it means someone is trying to circumvent the local maximums.
--
Curt, WE7U. APRS Client Comparisons: http://www.eskimo.com/~archer
"Lotto: A tax on people who are bad at math." -- unknown
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