CAN-J1939 on linux

CAN on linux

J1939 networking in short

Add addressing on top of CAN (destination address & broadcast)
Any (max 1780) length packets. Packets of 9 or more use Transport Protocol (fragmentation) Such packets use different CANid for the same PGN.
only 29bit, non-RTR CAN frames
CAN id is composed of
- 0..8: SA (source address)
- 9..26:
- PDU1: PGN+DA (destionation address)
- PDU2: PGN
- 27..29: PRIO
SA / DA may be dynamically assigned via j1939-81 Fixed rules of precedence in Specification, no master necessary

J1939 on SocketCAN

J1939 is just another protocol that fits in the Berkely sockets.

socket(AF_CAN, SOCK_DGRAM, CAN_J1939)

differences from CAN_RAW

addressing

SA, DA & PGN are used, not CAN id.

Berkeley socket API is used to communicate these to userspace:

SA+PGN is put in sockname (getsockname)
DA+PGN is put in peername (getpeername) PGN is put in both structs

PRIO is a datalink property, and irrelevant for interpretation Therefore, PRIO is not in sockname or peername.

The data that is [recv][recvfrom] or [send][sendto] is the real payload. Unlike CAN_RAW, where addressing info is data.

Packet size

J1939 handles packets of 8+ bytes with Transport Protocol fragmentation transparently. No fixed data size is necessary.

send(sock, data, 8, 0);

will emit a single CAN frame.

send(sock, data, 9, 0);

will use fragementation, emitting 1+ CAN frames.

Using J1939

BSD socket implementation

socket
bind / connect
recvfrom / sendto
getsockname / getpeername

Modified struct sockaddr_can

struct sockaddr_can {
    sa_family_t can_family;
    int         can_ifindex;
    union {
        struct {
            __u64 name;
            __u32 pgn;
            __u8 addr;
        } j1939;
    } can_addr;
}

can_addr.j1939.pgn is PGN
can_addr.j1939.addr & can_addr.j1939.name determine the ECU
- receiving address information, addr is always set, name is set when available.
- When providing address information, name != 0 indicates dynamic addressing

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