Packet Structure

This document describes how the OSDP packets are built/decoded.

Header

All OSDP packets must have the following 5 bytes.

Byte	Name	Meaning	Value
0	SOM	Start of Message	0x53
1	ADDR	Physical Address of the PD	0x00 – 0x7E
2	LEN_LSB	Packet Length Least Significant Byte	Any
3	LEN_MSB	Packet Length Most Significant Byte	Any
4	CTRL	Message Control Information	See Below

Message Control Information

CTRL byte at offset 4 of packet header is a bit mask which is exploded as:

BIT	MASK	NAME	Meaning
0 - 1	0x03	SQN	Packet sequence number
2	0x04	CKSUM/ CRC	Set: 16-bit CRC; Clear: 8-bit CHECKSUM
3	0x08	SCB	Set: SCB is present; Clear: SCB is absent

SCB is Security Control Block presence indicator. This is an optional message block that is present in secure channel messages.

Security Control Block (Optional)

Byte	Name	Meaning	Value
5	SEC_BLK_LEN	Length of Security Control Block	Any
6	SEC_BLK_TYPE	Security Block Type	See Below
7 - m-1	SEC_BLK_DATA	Security Block Data	Based on type

Command/Reply Structure

The Command/Reply block is a mandatory field. If no SCB exist it follows directly after fixed header. If SCB is present, then this is offset by as many bytes.

Byte	Name	Meaning	Value
5	CMND/REPLY	Command or Reply Code	See CMD/REPLY
6 - n	DATA	(optional) Data Block	Based on CMD/REPLY

Message Authentication Code (Optional)

This is an optional block present in secure channel messages. These 4 bytes are just above the CRC/CKSUM bytes.

Byte	Name	Meaning	Value
n-6	MAC[0]	Message Authentication Code Byte-0	Any
n-5	MAC[0]	Message Authentication Code Byte-0	Any
n-4	MAC[0]	Message Authentication Code Byte-0	Any
n-3	MAC[0]	Message Authentication Code Byte-0	Any

Packet Validation

These are the last 2 bytes (1 if CKSUM is was used in Message Control Information bit mask) of the packet.

Byte	Name	Meaning	Value
n-2	CKSUM/CRC_LSB	Checksum, or, CRC-16 Least Significant Byte	CKSUM/CRC
n-1	CRC_MSB	(optional) CRC-16 Most Significant Byte	CKSUM/CRC