need DFU description explanations

Question

Hello, 
 I'm started implementing DFU over serial port and see documentation from this link: https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.sdk5.v14.2.0%2Flib_dfu_transport_serial.html 
 And for me is misunderstanding about little and big endian in your descriptions. In examples I see is big endian format (PRN number is 02 01 00 - it means 256) but in description is mentioned little endian format ("Value (uint16, little endian) "). What I should believe? Example or description? 
 Same situation is with MTU message, where is the true? 
 And other think if in other messages not mentioned little endian format it does it mean that it is in big endian format? 
 Then how it understand that some messages value (Create and PRN) fields is in little endian format and another all messages is in big endian format? 
 It does it mean that DFU protocol mixing endian between messages? 
 Wait for comments! 
 
 FW developer 
 Remigijus

ivansanchez · Answer

What I should believe? Example or description? 
 I suggest you base your implementation in the already existing reference implementation: pc-nrfutil. See https://github.com/NordicSemiconductor/pc-nrfutil/tree/master/nordicsemi/dfu 
 In examples I see is big endian format (PRN number is 02 01 00 - it means 256) but in description is mentioned little endian format ("Value (uint16, little endian) ") 
 That example is wrong, in fact. A PRN value 0x0100 means decimal 1, not decimal 256. 
 Refer to https://github.com/NordicSemiconductor/pc-nrfutil/blob/7f20bb89b1df082990c94bc58ceb26393fe52341/nordicsemi/dfu/dfu_transport_serial.py#L299 , it looks like: 
 def __set_prn(self):
 self.dfu_adapter.send_message([DfuTransportSerial.OP_CODE['SetPRN']]
 + map(ord, struct.pack('<H', self.prn))) 
 If you check https://docs.python.org/2/library/struct.html#byte-order-size-and-alignment , you'll see that struct.pack('<H', self.prn) means that PRN is a little-endian unsigned short (16-bit) integer. The same goes for all other fields involved in the DFU procedure, you'll find similar struct.pack() calls. 
 If I remember correctly, some of the metadata (in particular the crc32 hashes and sha256 hashes) might be in big-endian due to libraries used to calculate those values. Please be aware that you should check those separately.