Packet Serialization

This topic provides detailed information about about MAVLink packet serialization, including the over-the-wire formats for MAVLink v1 and v2 packets, the ordering of fields in the message payload, and the CRC_EXTRA used for ensuring that the sender and reciever share a compatible message definition.

It is primarily intended for developers who are creating/maintaining a MAVLink generator

MAVLink users do not typically need to understand the serialization format, as encoding/decoding is handled by the MAVLink libraries.

Packet Format

This section shows the serialized message format of MAVLink packets (the format is inspired by the CAN and SAE AS-4 standards).

Below is the over-the-wire format for a MAVLink 1 packet (the in-memory representation might differ).

MAVLink v1 packet

Byte IndexC versionContentValueExplanation
0uint8_t magicPacket start marker0xFEProtocol-specific start-of-text (STX) marker used to indicate the beginning of a new packet. Any system that does not understand protocol version will skip the packet.
1uint8_t lenPayload length0 - 255Indicates length of the following payload section.
2uint8_t seqPacket sequence number0 - 255Used to detect packet loss. Components increment value for each message sent.
3uint8_t sysidSystem ID1 - 255ID of system (vehicle) sending the message. Used to differentiate systems on network.
4uint8_t compidComponent ID0 - 255ID of component sending the message. Used to differentiate components in a system (e.g. autopilot and a camera).
5uint8_t msgidMessage ID0 - 255ID of message type in payload. Used to decode data back into message object.
6 to (n+6)uint8_t payload[max 255]PayloadMessage data. Content depends on message type (i.e. Message ID).
(n+7) to (n+8)uint16_t checksumChecksum (low byte, high byte)X.25 CRC for message (excluding magic byte). Includes CRC_EXTRA byte.
  • The minimum packet length is 8 bytes for acknowledgment packets without payload.
  • The maximum packet length is 263 bytes for full payload.

MAVLink 2 Packet Format

Below is the over-the-wire format for a MAVLink 2 packet (the in-memory representation might differ).

MAVLink v2 packet

Byte IndexC versionContentValueExplanation
0uint8_t magicPacket start marker0xFDProtocol-specific start-of-text (STX) marker used to indicate the beginning of a new packet. Any system that does not understand protocol version will skip the packet.
1uint8_t lenPayload length0 - 255Indicates length of the following payload section.
2uint8_t incompat_flagsIncompatibility FlagsFlags that must be understood for MAVLink compatibility (implementation discards packet if it does not understand flag).
3uint8_t compat_flagsCompatibility FlagsFlags that can be ignored if not understood (implementation can still handle packet even if it does not understand flag).
4uint8_t seqPacket sequence number0 - 255Used to detect packet loss. Components increment value for each message sent.
5uint8_t sysidSystem ID (sender)1 - 255ID of system (vehicle) sending the message. Used to differentiate systems on network.
6uint8_t compidComponent ID (sender)0 - 255ID of component sending the message. Used to differentiate components in a system (e.g. autopilot and a camera).
7 to 9uint32_t msgid:24Message ID (low, middle, high bytes)0 - 16777215ID of message type in payload. Used to decode data back into message object.
10 to (n+10)uint8_t payload[max 255]PayloadMessage data. Depends on message type (i.e. Message ID) and contents.
(n+11) to (n+12)uint16_t checksumChecksum (low byte, high byte)X.25 CRC for message (excluding magic byte). Includes CRC_EXTRA byte.
(n+12) to (n+26)uint8_t signature[13]Signature(Optional) Signature to ensure the link is tamper-proof.
  • The minimum packet length is 11 bytes for acknowledgment packets without payload.
  • The maximum packet length is 279 bytes for a signed message that uses the whole payload.

Checksum

The packet format includes a 2-byte CRC to allow detection of message corruption. The checksum is the same as used in ITU X.25 and SAE AS-4 standards (CRC-16-CCITT), documented in SAE AS5669A. See the MAVLink source code for the documented C-implementation.

The CRC covers the whole message, excluding magic byte and the signature (if present). The CRC includes the CRC_EXTRA byte for ensuring the message definition matches the current version.

Payload Format

Messages are encoded within the MAVLink packet format(s)

  • The message id field identifies the actual message to be sent
  • The payload field contains the data appropriate to the particular message.

MAVLink reorders the message fields in the payload (from the original XML Message Definitions) for over-the-wire transmission.

As MAVLink does not include information about the message structure in the payload (in order to reduce overhead), so the meaning, order and size of message fields in the over-the-wire must be understood/agreed by both the sender and receiver.

One extra CRC (CRC_EXTRA) is added to the message checksum that allows a receiver to detect changes in message specifications that might make the over-the-wire format incompatible: new/removed fields, or changes to field name, data type, order, or array length.

A MAVLink library should notify a bad CRC during decoding if a message specification is incompatible (e.g. the C library mavlink_parse_char() gives a status MAVLINK_FRAMING_BAD_CRC).

Field Reordering

Message payload fields are reordered for transmission as follows:

  • Fields are sorted according to their native data size, first (u)int64_t and double, then (u)int32_t, float, (u)int16_t, (u)int8_t.
  • If two fields have the same length, their order is preserved as it was present before the data field size ordering
  • Arrays are handled based on the data type they use, not based on the total array size
  • The over-the-air order is the same as for the struct and thus represents the reordered fields
  • The CRC_EXTRA field is calculated after the reordering, to ensure that a mistake during field reordering will be caught by a faulty CRC. The provided Python, C and C# reference implementations are tested to have the correct field reordering, this is only a concern for custom implementations.

The only exception to the above reordering is for MAVLink 2 extension fields. Extension fields are sent in XML-declaration order and are not included in the CRC_EXTRA calculation. This allows new extension fields to be added without breaking binary compatibility.

This ordering is unique and can be easily implemented in a protocol generator by using a stable sorting algorithm. The alternative to using sorting would be either to use inefficient alignment, which is bad for the target architectures for typical MAVLink applications, or to have function calls in the order of the variable size instead of the application context. This would lead to very confusing function signatures of serialization functions.

CRC_EXTRA Calculation

The CRC_EXTRA CRC is required to verify that the sender and receiver have a shared understanding of the over-the-wire format of a particular message.

When the MAVLink code generator runs, it takes a checksum of the XML structure for each message and creates an array define MAVLINK_MESSAGE_CRCS. This is used to initialise the mavlink_message_crcs[] array in the C/C++ implementation, and is similarly used in the Python (or any other, such as the C# and JavaScript) implementation.

When the sender calculates the checksum for a message it adds the CRC_EXTRA byte onto the end of the data that the checksum is calculated over. The recipient calculates a checksum for the received message and adds its own CRC_EXTRA for the particular message id. If the CRC_EXTRA for the sender and receiver are different the checksums will not match.

This approach ensures that only messages where the sender and recipient are using the same message structure will be decoded (or at least it makes a mistake much more unlikely, as for any checksum application).

If you are doing your own implementation of MAVLink you can get this checksum in one of two ways: you can include the generated headers and use MAVLINK_MESSAGE_CRCS to get the right seed for each message type, or you can re-implement the code that calculates the seed.

As MAVLink internally reorders the message fields according to their size to prevent word / halfword alignment issues (see Data structure alignment (Wikipedia) for further reference), and a wrongly implemented reordering potentially can cause inconsistencies as well, the CRC_EXTRA is calculated based on the over-the-air message layout rather than the XML order.

As discussed in the previous section, MAVLink 2 extension fields are not included in the CRC_EXTRA calculation.

This is the Python code that calculates the CRC_EXTRA seed:

def message_checksum(msg):
    '''calculate a 8-bit checksum of the key fields of a message, so we
       can detect incompatible XML changes'''
    from .mavcrc import x25crc
    crc = x25crc()
    crc.accumulate_str(msg.name + ' ')
    # in order to allow for extensions the crc does not include
    # any field extensions
    crc_end = msg.base_fields()
    for i in range(crc_end):
        f = msg.ordered_fields[i]
        crc.accumulate_str(f.type + ' ')
        crc.accumulate_str(f.name + ' ')
        if f.array_length:
            crc.accumulate([f.array_length])
    return (crc.crc&0xFF) ^ (crc.crc>>8)

This uses the same x25 checksum that is used at runtime. It calculates a CRC over the message name (such as “RAW_IMU”) followed by the type and name of each field, space separated. The order of the fields is the order they are sent over the wire. For arrays, the array length is also added.

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