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

Heikki Hannikainen hessu at hes.iki.fi
Tue Dec 3 05:01:47 CST 2013


On Tue, Dec 3, 2013 at 3:17 AM, Lee Bengston <lee.bengston at gmail.com> wrote:
> On Mon, Dec 2, 2013 at 1:15 PM, Heikki Hannikainen <hessu at hes.iki.fi> wrote:
>>
>> On Mon, Dec 2, 2013 at 6:50 PM, Steve Dimse <steve at dimse.com> wrote:
>> > Yes, I was assuming IGates would use a range filter specifically to be
>> > sure they would receive the messages for all stations in that area. You
>> > don’t want that, though, do you?
>>
>> Most igates don't send range filters (or any other filters), since
>> they don't need to see all packets in the area. The "heard list" alone
>> should nicely give them the necessary packets for messaging to work.
>
> 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?

To support that, the server gives the igate all Internet-originated
(TCPIP*) packets of stations which have been heard by that igate
(within the "heard" table expiration time).

i.e. if OH7LZB-5 appears on RF behind igate OH2RCH, and then there are
OH7LZB-5>APRS,TCPIP*,... packets on the APRS-IS, the servers will give
those packets to OH2RCH for the next 1 or 3 hours so that the igate
knows the station is also on the Internet.

http://aprs-is.net/IGateDetails.aspx says:

"Gate message packets and associated posits to RF if all of the
following are true:
...
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)."

I don't think this is separately documented in the "server design"
docs, but the required functionality is implied by the igate details
page. The aprsc code relevant to this is here:

https://github.com/hessu/aprsc/blob/master/src/filter.c#L2473

- Hessu


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