Define how to pass payload from Compression Layer

[StarBrand]

Compression Layer sends payload which is composed of bytes and a residue made by bits.

Define how to pass this to the fragmentation layer.

[StarBrand]

@felipe-canales Specify this issue

[felipe-canales]

The output of compression, which is the return value of SCHC_Compressor.compress, is a tuple of 2 values:
( schc_packet , padding_length )
The first value schc_packet is the compressed packet in the format specified by RFC 8724 and the SCHC over LoRaWAN draft (1 byte for the rule id) with no indication of where the residue ends and the payload begins.

+-----------+-----------+---------+
|  Rule ID  |  Residue  | Payload |
|  1  byte  | 0-n  bits | m bytes |
+-----------+-----------+---------+

The padding_length value is returned due to the residue not necessarily being byte aligned and the limitation of returning a byte array. It indicates how many bits (from right to left) are added to the packet to deliver an integer number of bytes. For example, if padding_length is 6, then only the first 2 bits of the last byte are part of the message.

The padding bits are by default 0 (specified by SCHC over LoRaWAN), but they may be required to have another value if other LPWAN profile is used.