[aprssig] APRS message-handling algorithms / libraries

[John Gorkos]

Please review the code found here:
https://github.com/ab0oo/javAPRSlib
The multi-part message capability isn't in there, but the ability to encode
and decode messages is.
Let me know if you'd like any help implementing or testing.

John Gorkos
AB0OO

From:  John Goerzen <jgoerzen at complete.org>
Reply-To:  TAPR APRS Mailing List <aprssig at tapr.org>
Date:  Sun, 08 Apr 2012 12:24:29 -0500
To:  <aprssig at tapr.org>
Subject:  [aprssig] APRS message-handling algorithms / libraries

    
 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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