[aprssig] are write-only APRS-IS clients valid?

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Tue Dec 3 05:54:03 CST 2013

On 12/2/2013 8:17 PM, Lee Bengston wrote:
> Yes, the heard list should give the IGate the necessary packets, but 
> what about the ability of the IGate to determine if a station heard on 
> RF is also directly connected to the Internet/APRS-IS?  With no range 
> filter (or a very small range filter), wouldn't that create the 
> possibility of gating a message via RF addressed to a station heard on 
> RF that is also connected to the Internet?  Or would the IS server 
> take care of this by not sending the message to IGates that have gated 
> RF beacons from the message recipient - instead only send the message 
> to the addressee?

The APRS-IS server(s) not only send messages addressed to the heard list 
stations, but also any posits for those heard stations as well.  This 
allows the IGate to determine #4 in (from 
http://www.aprs-is.net/IGateDetails.aspx, emphasis mine):

> 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).

To provide for the determination of #4, my understanding (and 
observation) is that the APRS-IS servers forward any packets sourced by 
the heard stations to the IGate client on the filter port.  This is why 
IGates with no filters will STILL receive packets from the APRS-IS that 
are NOT intended to be transmitted to RF, but provide visibility for the 
IGate to make local gating decisions.

There's (at least) one IGate implementation out there (don't remember 
which one) that blindly transmits 3rd party packets via RF for every 
packet received from the APRS-IS server.  In my understanding, this is 
not proper as the APRS-IS servers forward information-providing packets 
not intended for transmission, but instead rely on local IGate 
intelligence to determine which packets should actually be gated from 
-IS to RF.

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

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

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