I'm developing an application for Linux that uses 6LoWPAN over BTLE (Bluetooth Low Energy). I have read that 6LoWPAN over BTLE requires the support of Data Packet Length Extension, that is a feature introduced by the BTLE 4.2 standard and, therefore, should not be available using the common BTLE 4.0 adapters.
This is an issue for me, because I've looked for an USB dongle for PC supporting BTLE 4.2, compatible with BlueZ stack for Linux, but nothing seems to be available at the moment, as it can be read here:
https://groups.google.com/forum/#!topic/btstack-dev/IeA92qNGR9c
However, I have read at the link
MQTT-SN on Raspberry pi3 , 6lowpan over BLE
that it is possible to support 6LoWPAN over BTLE using some Raspberry PI 3.
The BTLE chipset integrated in Raspberry PI 3 is the Broadcom BCM43438 that is declared (only) BTLE 4.1 compatible.
Is it possible to create a working 6LoWPAN connection between 2 RaspPI3 boards, using only BTLE 4.0/4.1-compatible adapters ? How the adapters manage the compatibility with 6LoWPAN using short BTLE frames since the Data Length Extension is not available ?
Can the previous limitation be overcome by a firmware update for Broadcom chipsets that is integrated in the last Linux kernel, thus adding support for Data Packet Length Extension via software ?
Do you know some USB dongle that supports (or that can be updated to support) BTLE 4.2 + Data Length Extension and that is compatible with BlueZ Linux stack ?
I don't know what you got 22/18 bytes from. For each packet, L2CAP data of 27 bytes can be transmitted. L2CAP has a 4 byte header.
L2CAP has built-in fragmentation/reassembly support so an L2CAP packet of more than 27 bytes can be sent even if the Link Layer MTU is 27 bytes, so there should be no problem running 6LoWPAN on a controller that does not support LE Data Packet Length Extension.