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.

Enable j1939 (obsolete!)

CAN has no protocol id field. The can-j1939 stack only activates when a socket opens for a network device.

The methods described here existed in earlier implementations.

netlink

ip link set can0 j1939 on

This method is obsoleted in favor of on socket connect.

procfs for legacy kernel (2.6.25)

This API is dropped for kernels with netlink support!

echo can0 > /proc/net/can-j1939/net

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

iproute2 (obsolete!)

Older versions of can-j1939 used a modified iproute2 for manipulating the kernel lists of current addresses.

Static addressing

ip addr add j1939 0x80 dev can0

Dynamic addressing

ip addr add j1939 name 0x012345678abcdef dev can0

kurt-vd / test-can-j1939