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

Steve Dimse steve at dimse.com
Mon Dec 2 09:54:52 CST 2013

On Dec 2, 2013, at 10:22 AM, Steve Dimse <steve at dimse.com> wrote:
> This isn’t an academic exercise. Every client program I’ve seen allows one-way gating, and furthermore it will always be possible by turning off TX Enable, not connecting a PTT line, or by using a scanner to feed a TNC. So please, I’m asking as nicely as I can, convince me this is a real problem.

FYI, I did look at the web site when this began to see if something had changed, and see nothing that explains this. To help with your explanation, let me set up the situation as I understand it. If there is anything in my setup that prevents the occurrence please feel free to arrange differently.

The Igates are 1way and 2way. 1way is connected to APRS-IS hubA, 2way is connected to APRS-IS hubB, the two hubs interconnect through hubC. An internet ham called Iham connects (validated) to hubC (again, change the location of connection if this matters).

RF is the ham user, if it matters that there are digis involved feel free to add them, but I don’t think it does do it is direct. There is a 10 second delay in the audio of 2way, allowing 1way’s packets to fully propagate through APRS IS before anyone sees them.

So RF sends a position, it travels through 1way, hubA, to hubC. From hubC it goes to Iham as well as proceeding through hubB to 2way. After all this has happened the same posit arrives at 2way, is sent to hubB if the IGate does its own dup filtering in which case it is filtered at hubB, or else it is filtered by the IGate if it does its own filtering. However, the packet was heard on RF, so 2way recognizes that RF is a local station.

iHam sees RF’s position, wants to say hi, and sends a message. the message hits hubC, goes both to hubA and 1way, where of course nothing happens, and to hubB and, since RF’s position lies within the IGate’s footprint, passes through the filter to 2way.

You list four conditions that must be met for a packet to be met:

> 	• the receiving station has been heard within range within a predefined time period (range defined as digi hops, distance, or both).

A few seconds, direct and a few miles away. PASS.

> 	• 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).

Iham is internet only. PASS

> 	• the sending station does not have TCPXX, NOGATE, or RFONLY in the header.

Iham is validated so no TCPXX. PASS

> 	• 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).

1way IGates do not convert the RF station to TCPIP or TCPXX, the packet is passed just the same as a two-way IGate, nor are 3rd party packets used. PASS

Four for four, so 2way IGates the message to RF. QED

So, again, I ask you to rearrange the above scenario as needed, add or remove pieces, change the interconnections, or alter the paths in order to have the message not make it through.

Thanks, Steve

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