Companion app doesn't take into account 4-byte overhead for region scoped messages

[weebl2000]

@liamcottle see discussion in #troubleshooting on Discord.

tl;dr is I'm suspecting the app doesn't take into account 4-bytes of overhead when sending region scoped messages, and any messages close to the limit hit MAX_FRAME_SIZE on any repeaters so they get dropped silently.

Do we want a fix in the firmware or just reduce 4-byte when region scope is active?

[liamcottle]

Howdy!

The messages should still get transmit, and still show up as a message on other devices. The issue is as you described, the MAX_FRAME_SIZE.

I have some previous discussions on this exact issue in the Discord from some time ago now. Ideally, we would raise the MAX_FRAME_SIZE so RX Log is able to pass these larger packets to client apps.

Because the larger packets don't get passed to RX Log, the app is unable to get the channel message data to decrypt and increase the "heard repeats" counter etc...

The issue with raising the max frame size, is now you need to increase the BLE MTU. This comes with its own issues, such as some older iPhones or specific devices not supporting MTUs greater than ~185 bytes.

Currently, our max frame size is 172 bytes, however our max packet size can be larger than this. A raw LoRa frame can be up to ~255 bytes, so anything over 172 will not be passed to RX Log.

Most Android devices have supported up to 512 MTU for sometime now, and I believe most of the newer iPhones also support higher MTU.

It would be nice to increase the BLE MTU and MAX_FRAME_SIZE, but there could be unintended side effects of cutting off support for some BLE hardware.

For now, I have added an additional limit on channel messages when the user has a region scope active. If a scope is active, the message must be 10 bytes shorter. If you remove the scope, you can send an additional 10 bytes in your message.

I understand the overhead is 4 bytes for 2x 16-bit transport codes, but I still had to remove more bytes before RX Log would receive anything.

I'll close this one as completed, since it's patched in the app. But increasing MAX_FRAME_SIZE and BLE MTU is something to explore further.

[weebl2000]

Yeah I dove into increase MAX_FRAME_SIZE but it's going to affect a lot of places & ram usage here and there so I didn't touch it yet.

[weebl2000]

@liamcottle I think just increasing by 4 bytes is the easiest fix for now, then the companion just has to check and decrement 4 bytes for region scoped messages (when sufficient repeaters have this fix) - see PR #2022

Increasing by 4 bytes shouldn't increase RAM too much overall and be well within other MTU limits.