[aprssig] APRS protocol replacement ideas: protobuf

Weston Bustraan wbustraan at gmail.com
Sat Feb 12 09:44:07 EST 2022

Honestly, from my perspective, we have plenty of technical people in this
community that could come up with a very nice, elegant protocol. But I
think that's the easy part; in order for it to go anywhere, we would need
an "ambassador" with enough charisma and influence to convince the big
companies like Kenwood, Icom, Yaesu, to adopt a new protocol in their
hardware. That's the magic that Bob brought to the table, I think; he had
the technical know-how combined with the charisma to pitch his invention to
the world.

This is where I struggle with where to help, personally. I feel like I
could help in the technical space, but when it comes to approaching a
company like Kenwood, I confess that idea intimidates me a lot.

- Wes W8WJB

On Sat, Feb 12, 2022 at 3:04 AM Heikki Hannikainen <hessu at hes.iki.fi> wrote:

> On Fri, 11 Feb 2022, Weston Bustraan wrote:
> > I can say that it was not easy to navigate the many exceptions and and
> > oddities in the APRS 1.0.1 spec when implementing a parser, and OpenTRAC
> > seems much more consistent, but the OpenTRAC spec does not seem to be
> > fully complete. For example, I would expect that any new protocol that
> > would supplant APRS would be supporting a UTF-8 character set instead of
> > just ASCII.
> Yes! Implementing all variations and details of APRS is quite difficult
> indeed, as there are so many completely different position formats, and
> various quirks in which order extension fields should be decoded in, and
> the specifications do not detail all of those. It is unnecessarily complex
> and an awful lot of work.
> Just last week there was a question on the Facebook APRS group, where one
> user was sending uncompressed APRS position packets, *without* a
> timestamp, but with a '@' packet type identifier which indicates a
> timestamp would follow. Their igate appliance requires the user configure
> a raw packet string template, and it's a little bit difficult to do that
> correctly for someone new to APRS. Aprsdirect parsed the position from the
> packet just fine, even though it was technically incorrect. aprs.fi, and
> likely many other decoders, would try to parse out a timestamp after the
> '@' and fail. From the user's point of view, it looked like aprs.fi is at
> fault. To add to the confusion, the remainder of the packet (after a
> suitable number of invalid timestamp bytes) happened to look like a
> perfectly valid *compressed* packet and aprs.fi managed to decode that to
> some random coordinates on the correct continent!  Now I'm wondering
> whether I should ignore all data after an invalid timestamp, or just keep
> skipping the invalid timestamp like it's been doing so far.
> We have a fairly complete decoder (by OH2KKU and myself), but still not
> fully complete (Area Objects? DF?), and quirks like this come up every now
> and then.  It would not be very motivating to get to implement another
> custom packet encoder and decoder, and it is likely that slightly
> incompatible encoders and decoders would be produced by different
> developers.
> If a replacement is actually planned, I would propose battling against the
> NIH syndrome, and using an existing low-level serializer which is widely
> used elsewhere, such as Protocol Buffers
> (https://developers.google.com/protocol-buffers). It is widely used
> outside Google and it's open source. The encoded on-the-wire (well,
> on-air) format is tight with little overhead and message formats can be
> extended without breaking existing decoders. Integer types use efficient
> variable-length encoding, so for a timestamp one could use int64 which
> would not actually use 64 bits in the packet until we actually reach such
> a date which requires it.
> With Protocol Buffers one writes a schema file which describes the data
> format. A supplied compiler (protoc) compiles the schema file to C code,
> which is then included in the application to encode and decode packets.
> Compilers are available for Java, Python, Objective-C, C++, Kotlin, Dart,
> Go, Ruby, and C#, with more languages to come. A very simple example
> format would look like this:
> message aprspacket {
>          string srccall = 1;
>          int64 timestamp = 2;
>          optional float latitude = 3;
>          optional float longitude = 4;
>          optional int32 course = 5;
>          optional int32 heading = 6;
>          optional int32 altitude = 7;
>          optional string comment = 10;
>          optional string object_name = 11; // set a value to make it an
> object
> }
> Naturally you can have nested structures too - in a real world there would
> be a top-level packet structure with optional position, text message,
> weather and telemetry structures below it.
> The upside of this is that whoever implements an application does not need
> to actually write the low-level encoder or decoder - the protobuf compiler
> builds it from the schema file. Developers download the schema file,
> compile their application with it, call the generated encoder/decoder
> functions/methods (aprspacket_unpack_message(), aprspacket_pack_message())
> and get nice structures out.
> Adding support for new fields in a new protocol version? Download a new
> schema file and recompile, find decoded value in output structure.
> Adding a new custom field to experiment with? Add it in the schema field
> with a suitable large random field identifier in a "reserved for
> experimentation" range and experiment - other apps will ignore it.  Want
> to deploy it in the field for others to use? Submit a request to the
> Protocol Committee which allocates an identifier and releases a new
> version of the schema file.
> The decoders will ignore any new fiels that might be added to packets by
> apps which have been built with a new version of the schema, so old apps
> will not blow up when they receive packets from new apps. Protobuf 3.5
> also retains these unknown fields in the decoder output structure, and
> re-encodes them if the structure is serialized again, so an old app can
> even re-encode and forward a packet with fields it does not implement.
> For embedded systems there's https://github.com/nanopb/nanopb : "It is
> especially suitable for use in microcontrollers, but fits any memory
> restricted system.".
> That said, I find it quite unlikely to find momentum for a completely new
> protocol encoding format, given the large amount of legacy hardware out
> there, and the difficulty of migration and backwards compatibility on RF
> side. Fragmentation would be really bad and a lot of old things would
> never be upgraded. Maybe this sort of thing could be used in the LoRa
> world which is still in development - it would speed up the development of
> new things quite a lot if the encoder/decoder code would be generated, not
> written by hand.
>    - Hessu
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