CRC32

Data frames use a CRC32 checksum to verify payload integrity. The algorithm is standard CRC-32/ISO-HDLC.

Algorithm Parameters

Parameter	Value
Polynomial	0x04C11DB7
Initial Value	0xFFFFFFFF
XOR Out	0xFFFFFFFF
Reflect Input	true
Reflect Output	true
Size	4 bytes
Byte Order	Little-endian

This is the same CRC-32 used by Ethernet, PKZIP, and many other protocols (often called CRC-32b or CRC-32/ISO-HDLC).

CRC Scope

The bytes included in the CRC calculation depend on the message key:

D2

CRC scope: MsgKey → StatusPayload
├── MsgKey (1B)
├── ... all elements ...
├── StatusByte (1B)      ← included
└── StatusPayload (4B)   ← included

The CRC covers everything from the Message Key byte through the StatusPayload (inclusive).

B1 and D1

CRC scope: MsgKey → last data byte
├── MsgKey (1B)
├── ... all elements ...
└── last signal DATA byte
    StatusByte (1B)      ← NOT included

The CRC covers from the Message Key byte through the last signal data byte. StatusByte is not included.

Position in Frame

The CRC32 is always the last 4 bytes of the elements section, immediately before the /BLAECK> footer.

... : StatusPayload(4B) : CRC32(4B) /BLAECK>\r\n

Verification

To verify a frame:

1. Extract the CRC32 from the last 4 bytes of the elements.
2. Compute CRC-32/ISO-HDLC over the appropriate byte range (see scope above).
3. Compare the computed value with the extracted CRC32.
4. If they don't match, discard the frame.

Reference Implementation

Python (using crcmod):

import crcmod

crc32_func = crcmod.predefined.mkCrcFun('crc-32')

def verify_d2_crc(msg_key_byte, elements_without_crc):
    """Verify CRC for a D2 frame.
    
    elements_without_crc: bytes from MsgKey through StatusPayload (excluding CRC32)
    """
    computed = crc32_func(elements_without_crc)
    return computed

See Also

• Frames — D2 payload layout
• Frames — B1 and D1 CRC scope
• Status Codes — StatusByte values included in D2 CRC