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[aprssig] APRS Message Retransmissions

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Thu Dec 3 19:27:01 UTC 2009

I'm trying to determine when an APRS client program should invoke the 
decaying retransmission algorithm (and where that algorithm is detailed).

Page 71 of aprs101.pdf:

"Messages with a message identifier are intended to be acknowledged by the
addressee. The sending station will repeatedly send the message until it
receives an acknowledgment, or it is canceled, or it times out."

This implies that only identified messages (those with a {xxxx ID) are 
"repeatedly" transmitted.  I cannot locate any definition in that 
document of what repeatedly is nor have I found any of the 1.1 and/or 
1.2 pages that actually describe the decay other than some references to 
"fast" as starting at 8 seconds.

This "fast" rate obviously applies to RF-connected clients as the 
APRS-IS dupe detector will not pass through any duplicate messages for 
(by default) 30 seconds.

And should non-identified messages (ack-less messages) be retransmitted 
or sent but a single time?

Interestingly, page 72 of aprs101.pdf recognizes this dupe detector with 
respect to "network components":

"In either of these two situations, multiple message acknowledgments should
be separated by at least 30 seconds (this is because some network
components such as digipeaters will suppress duplicated messages within a
30-second period)."

So I really wonder what benefit the fast 8 second retransmission rate 
accomplishes unless the sender and receiver are in APRS-simplex range.  
Any Digi (provided it heard the first one) will be suppressing the 
retransmissions turning them into local QRM for the remainder of the 
first 30 seconds, or am I missing something here?

Lynn (D) - KJ4ERJ - Looking for a definitive description of expected 
APRS client messaging behavior

PS.  I have read about REPLY-ACKs and do plan to implement those in my 
client as well.

PPS.  I'm asking all of these questions so I code my (currently 
-IS-only) client "correctly", "completely", and not "poorly" before 
adding KISS code to give it direct RF access.

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