[aprssig] I-gate loops for telemetry support text messages

[Lynn W Deffenbaugh (Mr)]

An IGate should not gate messages to RF addressed to stations that have 
also been heard on the APRS-IS. This is precisely why the APRS-IS 
servers provide packets originated by stations that have been gated by a 
given connection for some period of time after said station was last 
heard on RF and gated to the server.  So that the IGate can see the 
TCPIP* in the path from the server and know that particular station is 
on both RF and the -IS and should not have any messages gated to it.

At least, I believe that's what you were describing as the "loop".

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

PS.  This is rule # 4 from http://www.aprs-is.net/IGateDetails.aspx:

> Gate message packets and associated posits to RF if all of the 
> following are true:
>
>  1. the receiving station has been heard within range within a
>     predefined time period (range defined as digi hops, distance, or
>     both).
>  2. the sending station has not been heard via RF within a predefined
>     time period (packets gated from the Internet by other stations are
>     excluded from this test).
>  3. the sending station does not have TCPXX, NOGATE, or RFONLY in the
>     header.
>  4. the receiving station has not been heard via the Internet within a
>     predefined time period.
>     A station is said to be heard via the Internet if packets from the
>     station contain TCPIP* or TCPXX* in the header or if gated
>     (3rd-party) packets are seen on RF gated by the station and
>     containing TCPIP or TCPXX in the 3rd-party header (in other words,
>     the station is seen on RF as being an IGate).
>



On 9/22/2021 5:44 PM, Andrew Pavlin via aprssig wrote:
> Greetings.
>
> I just discovered an interesting "loop" caused by I-gating. My Tx 
> I-gate received a text message from APRS-IS that was originally sent 
> by a digipeater that I received both by Internet and by long-range 
> digpeater hop on RF. So, I've heard the station on RF (but not _local_ 
> RF, because there were one to three digipeaters in the path by the 
> time I received it on RF). It is one of the self-addressed text 
> messages that go with telemetry messages to say how to interpret the 
> telemetry (EQNS, BITS, UNIT, PARM).
>
> So, my Tx I-gate, having received from the Internet a text message 
> addressed to a station it has heard over RF, proceeded to transmit the 
> text message to RF (but without enough digipeat aliases to actually 
> reach the station).
>
> This sounds like a bug in my I-gate. So is the correct solution:
>
> 1. never Tx I-gate self-addressed text messages.
>
> 2. never Tx I-gate to RF stations that require more hops than I allow 
> on my Tx I-gate digipeat path (i.e., for Tx I-gates that can put a 
> WIDE1-1 or WIDE2-1 on their retransmissions). For the common case of 
> no digipeat path for Tx I-gating, the RF station would have to be 
> heard directly (not by digipeater) by the I-gate.
>
> So how should I fix my I-gate?
>
> Andrew, KA2DDO
> author of YAAC ("Yet Another APRS Client")
>
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