Routing

This topic explains how messages should be routed by MAVLink systems.

Overview

A MAVLINK network is made up of systems (vehicles, ground stations, antenna trackers, etc.), which may be composed from one or more components (autopilot, camera, servos, etc.).

Each system has a network-unique system id, and each component has a system-unique component id that can be used for addressing/routing:

  • The system id has a value between 1 and 255.
    • The default autopilot system id is usually 1. Users should allocate unique increasing id values when adding new autopilots to a network.
    • GCS systems and developer APIs typically use an ID at the top of the numeric range to reduce ID clashes (e.g. 255). Often their system ID is configurable to allow multi-GCS systems.
  • The component id is allocated by type and number from MAV_COMPONENT.

Messages can be intended for all systems, specific systems, all components in a system, or specific components within a system. The protocol defines two 8-bit fields that can (optionally) be specified in the message payload to indicate where the message should be sent/routed. If the ids are omitted or set to zero then the message is considered a broadcast (intended for all systems/components).

  • target_system: System that should execute the command
  • target_component: Component that should execute the command (requires target_system).

MAVLink components are expected to process messages that have a matching system/component id and broadcast messages. They are expected to route/resend messages that are intended for other (or all) recipients to other active channels (i.e. MAVLink systems may be connected across different transports, connected by a MAVLink system that routes the messages). Broadcast messages are forwarded to all channels that haven't seen the message. Addressed messages are resent on a new channel iff the system has previously seen a message from the target on that channel (messages are not resent if the addressee is not known or is on the original/incoming channel).

Forwarded messages must not be changed/repackaged by the forwarding system (the original message is passed to the new link).

Routing Detail

Systems/components should process a message locally if any of these conditions hold:

  • It is a broadcast message (target_system field omitted or zero).
  • The target_system matches its system id and target_component is broadcast (target_component omitted or zero).
  • The target_system matches its system id and has the component's target_component
  • The target_system matches its system id and the component is unknown (i.e. this component has not seen any messages on any link that have the message's target_system/target_component).

Systems should forward messages to another link if any of these conditions hold:

  • It is a broadcast message (target_system field omitted or zero).
  • The target_system does not match the system id and the system knows the link of the target system (i.e. it has previously seen a message from target_system on the link).
  • The target_system matches its system id and has a target_component field, and the system has seen a message from the target_system/target_component combination on the link.

Non-broadcast messages must only be sent (or forwarded) to known destinations (i.e. a system must previously have received a message from the target system/component).

Systems should also check for SYSTEM_TIME messages with a decrease in time_boot_ms, as this indicates that the system has rebooted. In this case it should clear stored routing information (and might perform other actions that are useful following a reboot - e.g. re-fetching parameters and home position etc.).

Library Support

C Library (mavgen)

The generated code for the MAVLink v1 C Library has no specific support for routing or working with target_system and target_component. To extract this information you will need to use the normal methods provided for reading payload fields, and match on the field names.

The MAVLink v2 generator for the C library has been updated to make it easier to get the destination system and component ID from the payload (when these are assigned). Specifically, the mavlink_msg_entry_t structure contains flags to tell you if the message contains target system/component information (FLAG_HAVE_TARGET_SYSTEM, FLAG_HAVE_TARGET_COMPONENT) and offsets into the payload that you can use to get these ids (target_system_ofs and target_system_ofs, respectively). The MAVLink helper method const mavlink_msg_entry_t* mavlink_get_msg_entry(uint32_t msgid) can be used to get this structure from the message id.

Unsigned MAVLink 2 packets are routed in the same way as MAVLink 1 packets. Signed packets may have some special routing requirements (see Routing Signed Packets below).

Routing Signed Packets

Routing signed packets is largely undefined at time of writing (for example, it is not clear whether a router should simply forward a signed message or decode and recode with its own keys).

The discussion can be tracked in MAVLink/#984

Router Implementation

The MAVLink Router created by Intel allows to mix-and-match different IP protocols with serial ports and route MAVLink traffic.

Further Information

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