This document specifies how the mcumgr Simple Management Procotol (SMP) is transmitted over text consoles.
Mcumgr packets sent over serial are fragmented into frames of 127 bytes or fewer. This 127-byte maximum applies to the entire frame, including header, CRC, and terminating newline.
The initial frame in a packet has the following format:
offset 0: 0x06 0x09
=== Begin base64 encoding ===
offset 2: <16-bit packet-length>
offset ?: <body>
offset ?: <crc16> (if final frame)
=== End base64 encoding ===
offset ?: 0x0a (newline)
All subsequent frames have the following format:
offset 0: 0x04 0x14
=== Begin base64 encoding ===
offset 2: <body>
offset ?: <crc16> (if final frame)
=== End base64 encoding ===
offset ?: 0x0a (newline)
All integers are represented in big-endian. The packet fields are described below:
| Field | Description |
|---|---|
| 0x06 0x09 | Byte pair indicating the start of a packet. |
| 0x04 0x14 | Byte pair indicating the start of a continuation frame. |
| Packet length | The combined total length of the unencoded body plus the final CRC (2 bytes). Length is in Big-Endian format. |
| Body | The actual SMP data (i.e., 8-byte header and CBOR key-value map). |
| CRC16 | A CRC16 of the unencoded body of the entire packet. This field is only present in the final frame of a packet. |
| Newline | A 0x0a byte; terminates a frame. |
The packet is fully received when bytes of body has been received.
The CRC16 should be calculated with the following parameters:
| Field | Value |
|---|---|
| Polynomial | 0x1021 |
| Initial Value | 0 |