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SMP Transport Specification

The documents specifies information needed for implementing server and client side SMP transports.

BLE (Bluetooth Low Energy)

MCUmgr Clients need to use following BLE Characteristics, when implementing SMP client:

  • Service UUID: 8D53DC1D-1DB7-4CD3-868B-8A527460AA84

  • Characteristic UUID: DA2E7828-FBCE-4E01-AE9E-261174997C48

All SMP communication utilizes a single GATT characteristic. An SMP request is sent via a GATT Write Without Response command. An SMP response is sent in the form of a GATT Notification

If an SMP request or response is too large to fit in a single GATT command, the sender fragments it across several packets. No additional framing is introduced when a request or response is fragmented; the payload is simply split among several packets. Since GATT guarantees ordered delivery of packets, the SMP header in the first fragment contains sufficient information for reassembly.

UART/serial and console

SMP protocol specification by MCUmgr subsystem of Zephyr uses basic framing of data to allow multiplexing of UART channel. Multiplexing requires prefixing each frame with two byte marker and terminating it with newline. Currently MCUmgr imposes a 127 byte limit on frame size, although there are no real protocol constraints that require that limit. The limit includes the prefix and the newline character, so the allowed payload size is actually 124 bytes.

Although no such transport exists in Zephyr, it is possible to implement MCUmgr client/server over UART transport that does not have framing at all, or uses hardware serial port control, or other means of framing.

Frame fragmenting

SMP protocol over serial is fragmented into MTU size frames; each frame consists of two byte start marker, body and terminating newline character.

There are four types of types of frames: initial, partial, partial-final and initial-final; each frame type differs by start marker and/or body contents.

Frame formats

Initial frame requires to be followed by optional sequence of partial frames and finally by partial-final frame. Body is always Base64 encoded, so the body size, here described as MTU - 3, is able to actually carry N = (MTU - 3) / 4 * 3 bytes of raw data.

Body of initial frame is preceded by two byte total packet length, encoded in Big Endian, and equals size of a raw body plus two bytes, size of CRC16; this means that actual body size allowed into an initial frame is N - 2.

If a body size is smaller than N - 4, than it is possible to carry entire body with preceding length and following it CRC in a single frame, here called initial-final; for the description of initial-final frame look below.

Initial frame format:

Content

Size

Description

0x06 0x09

2 bytes

Frame start marker

<base64-i>

no more than MTU - 3 bytes

Base64 encoded body

0x0a

1 byte

Frame termination

<base64-i> is Base64 encoded body of format:

Content

Size

Description

total length

2 bytes

Big endian 16-bit value representing total length of body + 2 bytes for CRC16; note that size of total length field is not added to total length value.

body

no more than MTU - 5

Raw body data fragment

Initial-final frame format is similar to initial frame format, but differs by <base64-i> definition.

<base64-i> of initial-final frame, is Base64 encoded data taking form:

Content

Size

Description

total length

2 bytes

Big endian 16-bit value representing total length of body + 2 bytes for CRC16; note that size of total length field is not added to total length value.

body

no more than MTU - 7

Raw body data fragment

crc16

2 bytes

CRC16 of entire packet body, preceding length not included.

Partial frame is continuation after previous initial or other partial frame. Partial frame takes form:

Content

Size

Description

0x04 0x14

2 bytes

Frame start marker

<base64-i>

no more than MTU - 3 bytes

Base64 encoded body

0x0a

1 byte

Frame termination

The <base64-i> of partial frame is Base64 encoding of data, taking form:

Content

Size

Description

body

no more than MTU - 3

Raw body data fragment

The <base64-i> of partial-final frame is Base64 encoding of data, taking form:

Content

Size

Description

body

no more than MTU - 3

Raw body data fragment

crc16

2 bytes

CRC16 of entire packet body, preceding length not included.

CRC Details

The CRC16 included in final type frames is calculated over only raw data and does not include packet length. CRC16 polynomial is 0x1021 and initial value is 0.