The protocol seems to be split in several layers, which are located in different libraries.

Physical layer

This seems to be IrDA-SIR, with a baud rate of 115.2 kbit/s. As this is a standardized protocol, there should be documentation available on other parts of the internet.

Data link layer

The next layer seems to be some kind of link layer, as it contains an 'unique' address, checksum and packet length.

Frame structure

The names are not official in any means, but I will use them in the rest of the documentation,

Seems to correspond with the packet structure used by IR:USER

Name	offset (bytes)	lenght (bytes)	Description
Magic?	0x0	0x1	Always a5?
ConnectionID	0x1	0x1	Connection ID, 00 if there is no connection made yet. (?)
SetupFlag?	0x2	0x1	First bit: flag of some sort? 1 if no connection was made yet or wants to stop the connection?
LargeFlag?	^	^	Second bit: flag of some sort? 1 if the packet size is larger than 63 bytes?
PacketSize	^	^	Remaing 6 bits: length of the Data (in bytes) if BigFlag is not set.
PacketSize	0x3 (Not present if LargeFlag was not set)	0x1	Size (in the case that BigFlag was set, otherwise data starts here.)
Data	0x3 (0x4 if LargeFlag was set)	Y	Data, see the next layer.
Checksum	0x3 + Y (0x4 + Y if LargeFlag was set)	0x1	8-bit CRC over all the previous bytes