The networking foundation

Before any implement can borrow a screen or report a sprayed litre, four things must already work: the wire must carry bits, those bits must form named messages, the node must own an address, and it must be able to move data larger than one frame. That spine is ISO 11783 parts 1–5 standing on SAE J1939 and CAN. This page is the map; each layer has its own chapter.

Why this is the spine

A farmer buys a tractor from one company and a planter from another, ten years apart, and expects them to cooperate the moment the hitch pin drops — no installer, no network admin. Plug-and-play between strangers on a noisy two-wire bus is the demand that shapes every choice below. Get this layer right and everything above it is just conversation; get it wrong and nothing works at all.

The layer cake

Each layer trusts the one beneath it (read bottom-up):

   ISO 11783-5   network management  address claiming (lowest NAME wins)
   ISO 11783-4   network layer       joining segments, routing, loop guard
   ISO 11783-3   data link           TP / ETP / Fast Packet (big payloads)
   SAE J1939     naming              29-bit identifier · PGN · PDU1/PDU2
   ISO 11783-1   general             the NAME · device classes · layering
   ISO 11783-2   physical            250 kbit/s CAN · arbitration

Read it layer by layer

ISO 11783-1 — general and device classes — the architecture and the NAME (with its bit-field).
ISO 11783-2 — the physical layer — the CAN bus and non-destructive arbitration.
SAE J1939 — the heritage — the identifier, PGNs, and the PDU1/PDU2 trap.
ISO 11783-3 — data link and transport — moving more than 8 bytes (TP/ETP/Fast Packet) with the RTS/CTS handshake.
ISO 11783-4 — the network layer — joining CAN segments safely.
ISO 11783-5 — network management and address claiming — the plug-and-play handshake that runs first.

The two ideas to carry away

Lowest number wins, everywhere. CAN arbitration (lowest identifier wins the bus), message priority (lowest value wins), and address claiming (lowest NAME wins the address) are the same rule applied at three levels. Once it clicks, the whole foundation feels inevitable.
Layers above pretend messages are any size and addresses are stable. Transport quietly chops and reassembles; address claiming quietly resolves conflicts. The application services get to ignore both.

How machbus expresses the foundation

You almost never touch it directly — that is the point. Build a node with a NAME and a preferred address; the claim runs first. Send a payload of any size; the network layer picks single-frame / Fast Packet / TP / ETP. Receive fully reassembled Messages, routed to the subsystem that cares. React to AddressClaim and bus/confinement events on the unified stream. See The session facade.

From concept to code

You read about…	Build it with…	See…
Claiming an address	`Session::start()` + drive `poll()`	Address claim
Sending any-size data	`Session::send_raw` / codec send (transport automatic)	Transport Protocol
Routing across segments	`net::niu`	Network routing
Seeing it on the wire	`candump` + replay	Reading candump traces

Failure modes worth knowing (across the foundation)

PDU1 vs PDU2 confusion — a frame that looks right but routes wrong.
Talking before claimed — application sends are rejected until the claim completes.
Transport timeouts/aborts — a stalled CTS or missed packet kills a transfer; retry.
Address loss mid-run — a lower-NAME newcomer can take your address.
Bus-off / error confinement — a flood of CAN errors takes a node off the bus.

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