DLMS Block Transfer (Get-Request-Next) Pagination & Window Flow Control

[david_miller]

Managing Large PDU Payloads via Block Transfer

When a response exceeds the maximum PDU size negotiated in AARQ (max_pdu_size, typically 1024 or 2048 bytes), the meter fragments the response into blocks using Get-Response-With-Block (Tag 0xC4 0x02).

Block Transfer Sequence

1. Client -> Get-Request-Normal (Requesting Association View 0.0.40.0.0.255)
2. Meter -> Get-Response-With-Block:
   • Last Block Flag: 0x00 (More blocks follow)
   • Block Number: 0x00000001
   • Raw Data Chunk: (e.g. 1000 bytes)
3. Client -> Get-Request-Next:
   • Block Number: 0x00000001 (Acknowledging block 1 and requesting block 2)
4. Meter -> Get-Response-With-Block:
   • Last Block Flag: 0x01 (Final block)
   • Block Number: 0x00000002

If the client receives a corrupted block (HCS/CRC error), it must re-send Get-Request-Next with the last successfully received block number to request a retransmission.

[elena_rostova]

In the meter firmware, we maintain a 30-second block transfer inactivity timer (t_inactivity). If the client does not acknowledge the current block within 30 seconds, the meter cleans up the buffer allocation to prevent memory leaks.