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[aprssig] Digipeating behavior

Curt, WE7U archer at eskimo.com
Mon Mar 13 15:44:58 UTC 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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