[aprssig] APRS message-handling algorithms / libraries

John Goerzen jgoerzen at complete.org
Sun Apr 8 15:09:44 CDT 2012

Hi Lynn,

On the one hand, you raise good points; on the other hand, what I'm 
proposing is nothing particularly new.  We already have:

    * PC keyboard to RF bidirectional messaging (Xastir, whatever is out
      there for Windows)
    * Bidirectional email to APRS messaging (APRSLink, other gateways)

So I guess I don't understand why XMPP is a concern whereas the others 
aren't.  Honestly I think it's LESS of a concern than email, as XMPP is 
more strongly authenticated than SMTP is, etc.  What does the iGate 
operator do about someone on Winlink sending messages that they shouldn't?

-- John

On 04/08/2012 02:27 PM, Lynn W. Deffenbaugh (Mr) wrote:
> Please consider that the APRS channel is 1200 baud and shared in any 
> given area.  I considered a Skype chat IM to APRS gateway early on, 
> but it is trivially easy to completely overwhelm the APRS channel with 
> just a simple 2 or 3 person keyboard chat.
> Going from APRS to IM is easy, the RF side is initiating and providing 
> the throttle.  Going from IM to APRS is my concern, in additional to 
> the authentication that "friend B" may not be a licensed amateur, even 
> though approved by "friend A" who may be.  Friend B is still not 
> entitled to transmit on amateur frequencies, and it's not "friend A" 
> taking the license risk, it's the IGate operator in "Friend A"'s 
> neighborhood.
> Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32
> On 4/8/2012 1:24 PM, John Goerzen wrote:
>> Hi folks,
>> I am looking to write some APRS software, and I'm hoping to avoid 
>> re-inventing the wheel.  I am particularly interested in libraries in 
>> any cross-platform language (C, Perl, Python, Java, etc.) that can be 
>> used to receive and generate APRS messages.
>> In my research, there are quite a few libraries that receive APRS 
>> messages, but I have found none that are helpful with generating 
>> them.  This seems to be something with some complexity; such as:
>>     * Splitting data up into packets of the appropriate size
>>       (APRSLink for Winlink appears to somehow detect the type of
>>       device in use and generates shorter messages for TH-D7A. 
>>       Anyone know how to do this, and what are the appropriate sizes
>>       for the D7A, D72A, 710, and VX-8GR?)
>>     * Processing/generating ACKs, REJs, etc.
>>     * Proper timing for retransmit of packets that weren't ACKed.
>> None of this is hugely complicated, and the APRS spec is fairly clear 
>> on the ACK, REJ, etc. process.  The maximum message sizes appear to 
>> be undocumented and I'm hoping someone can help me out there.
>> My project, incidentally, is to build a bidirectional APRS-XMPP 
>> bridge.  XMPP is the protocol behind the instant messaging tools such 
>> as Jabber and Google Talk, and is available for free on every modern 
>> platform.  In the spirit of both amateur radio and the Free Software 
>> communities I'm a part of, full source code to this project will be 
>> available.  I actually already wrote a much simpler piece of related 
>> software, letting someone take a connected AX.25 session and bridge 
>> it to XMPP, [1] so I do have some experience under my belt.  This is 
>> obviously a more complex project, but XMPP looks like an excellent 
>> way to bridge APRS to other systems.  I think it is a better fit than 
>> the email gateways out there, and in fact, XMPP already has fields 
>> for carrying presence information and location information with 
>> presence packets.
>> Are there any particular notes available to those implementing 
>> gateways between APRS and non-APRS sources?  I have taken note of 
>> what sites such as OpenAPRS or Winlink do to validate people for 
>> access, and plan to do something similar here.
>> Thanks,
>> -- John
>> KR0L
>> [1] source code at https://github.com/jgoerzen/ax25xmpp
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