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[aprssig] APRS IS question

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Sat Dec 5 07:12:05 UTC 2009


Answers below to keep the context...

Keith VE7GDH wrote:
>> 3. What happens when I gate RF ---> INET packets that have already
>> gone INET --- > RF
>
> When you "gate RF ---> INET" your IGate is doing what it is supposed to
> do. If you gate something that the APRS-IS already had, and someone (you
> or someone else) gates it to RF and it is heard by an IGate again, I
> would think that the APRS-IS would throw it out as it would be a
> duplicate of a packet heard in the previous 30 seconds... unless there
> were any delays of more than 30 seconds.
>
>> My questions are more about the inner workings of the APRS-IS and why
>> it does not simply fall into a giant RF loop
>
> It's a good question. However, I think the APRS-IS would take care of
> that potential problem by discarding packets that it saw as dupes. Any
> comments from server operators or the server software designer?
>

Packets gated from the Internet to RF are transmitted with a 3rd party 
wrapper (page 85 of aprs101.pdf).  When such packets are received from 
RF and injected back into the -IS, they are automatically dropped by the 
first javAPRSsrvr (the core server of the APRS-IS) that sees them.  This 
prevents the loops you are worried about.  Oh, and any packet with TCPIP 
in the path are also not forwarded further into the -IS.

Lynn (D) - KJ4ERJ-7 - Palm Bay, FL javAPRSsrvr IGate/Digi



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