MAVLink is a binary telemetry protocol designed for resource-constrained systems and bandwidth-constrained links. MAVLink is deployed in two major versions: v1.0 and v2.0, which is backwards-compatible (v2.0 implementations can parse and send v1.0 packets). Telemetry data streams are sent in a multicast design while protocol aspects that change the system configuration and require guaranteed delivery like the mission protocol or parameter protocol are point-to-point with retransmission.
MAVLink 1 Packet Format
Below is the over-the-wire format for a MAVLink 1 packet. The in-memory representation might differ.
uint8_t magic; ///< protocol magic marker uint8_t len; ///< Length of payload uint8_t seq; ///< Sequence of packet uint8_t sysid; ///< ID of message sender system/aircraft uint8_t compid; ///< ID of the message sender component uint8_t msgid; ///< ID of message in payload uint8_t payload[max 255]; ///< A maximum of 255 payload bytes uint16_t checksum; ///< X.25 CRC
MAVLink 2 Packet Format
Below is the over-the-wire format for a MAVLink 2 packet. The in-memory representation might differ.
uint8_t magic; ///< protocol magic marker uint8_t len; ///< Length of payload uint8_t incompat_flags; ///< flags that must be understood uint8_t compat_flags; ///< flags that can be ignored if not understood uint8_t seq; ///< Sequence of packet uint8_t sysid; ///< ID of message sender system/aircraft uint8_t compid; ///< ID of the message sender component uint8_t msgid 0:7; ///< first 8 bits of the ID of the message uint8_t msgid 8:15; ///< middle 8 bits of the ID of the message uint8_t msgid 16:23; ///< last 8 bits of the ID of the message uint8_t target_sysid; ///< Optional field for point-to-point messages, used for payload else uint8_t target_compid; ///< Optional field for point-to-point messages, used for payload else uint8_t payload[max 253]; ///< A maximum of 253 payload bytes uint16_t checksum; ///< X.25 CRC uint8_t signature; ///< Signature which allows ensuring that the link is tamper-proof
The over-the-wire format of MAVLink is optimized for resource-constrained systems and hence the field order is not the same as in the XML specification. The over-the-wire generator sorts all fields of the message according to size, with the largest fields (uint64_t) first, then down to smaller fields. The sorting is done using a stable sorting algorithm, which ensures that fields that do not need to be reordered stay in the same order. This prevents alignment issues on the encoding / decoding systems and allows for very efficient packing / unpacking.
Multicast Streams vs. Guaranteed Delivery
MAVLink is built for hybrid networks where high-rate data streams from data sources (commonly drones) flow to data sinks (commonly ground stations), but are mixed with transfers requiring guaranteed delivery. The key insight is that for most telemetry streams there is not a known or single recipient: Instead, typically an onboard computer, a ground control station and a cloud system all need the same data stream.
On the other hand configuring the onboard mission or changing the system configuration with onboard parameters requires point-to-point communication with guaranteed delivery. MAVLink achieves very high efficiency by allowing both modes of operation.
Topic Mode (publish-subscribe)
In topic mode the protocol will not emit a target system and component ID for messages to save link bandwidth. Typical examples for this communication mode are all autopilot data streams like position, attitude, etc.
The main benefit of this multicast mode is that no additional overhead is generated and multiple subscribers can all receive this data.
In point-to-point mode MAVLink uses a target ID and target component. In most cases where these fields are used the sub-protocol also ensures guaranteed delivery (missions, parameters, commands).
Integrity Checks / Checksum
MAVLink implements two integrity checks: The first check is on the integrity of the packet during transmission using the X.25 checksum (CRC-16-CCITT). This however only ensures that the data has not been altered on the link - it does not ensure consistency with the data definition. The second integrity check is on the data description to ensure that two messages with the same ID are indeed containing the same information. To achieve this the data definition itself is run through CRC-16-CCITT and the resulting value is used to seed the packet CRC. Most reference implementations store this constant in an array named CRC_EXTRA.