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Viscous Digipeater in APRX
The Viscous Digipeater is variation of AX.25 Digipeater, which puts
arriving packets into a time delay probation pipe (up to 9 seconds),
before digipeating them.
One can use a Viscous Digipeater as a fill-in system, which does not
repeat on band a packet that it hears having already been repeated.
It does have also benefits on Tx-IGate, which will not re-transmit
to band something that already was heard there within the small window
of a few seconds that the probation pipe takes.
The trick is that duplicate accounting is done right up front at
arrival packet arrival time, and depending on what kind of source
a similar packet arrives from (or none at all), that delayed packet
may get bumped off the delay pipe, or even be sent out.
The rules are a bit more complicated than for plain simple digipeater,
but not impossibly so:
Packets arriving from non-viscous sources trump those waiting in
viscous queues. First one arriving will be transmitted, unless
the viscous queue has no longer this packet (but it was there.)
( delayed_seen > 0, seen == 1, pbuf == NULL -> drop this )
( delayed_seen > 0, seen == 1, pbuf != NULL -> clean pbuf, transmit this )
( delayed_seen == 0, seen == 1 -> transmit this )
Subsequent packets arriving from non-viscous sources are dropped
as duplicates ( seen > 1 -> drop this )
Packets arriving from any viscous source are dropped, if there
already was some direct delivery packet
( delayed_seen > 0, seen > 0 -> drop )
First packet arriving from any viscous source is put on viscous queue,
unless there was non-viscous packet previously.
( delayed_seen == 1, seen == 0 -> put this on viscous queue )
Then among viscous sources:
- "Transmitter" kind source: an <interface> which is same as
that of <digipeater>'s transmitter <interface>.
- "Elsewhere" kind source: an <interface> which is some other
than that of transmitter's, but has viscous-delay > 0
Account the number of viscous sourced packets sourced from "transmitter"
if (source_is_transmitter)
seen_on_transmitter += 1;
For second and subsequent viscous sourced packets, if any of observed
packets came from transmitter (seen_on_transmitter > 0), then drop
current packet, and clear possible viscous queued pbuf.
Matti Aarnio, OH2MQK, 2009-10-21