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1.. SPDX-License-Identifier: (GPL-2.0 OR MIT)2 3===================4J1939 Documentation5===================6 7Overview / What Is J19398========================9 10SAE J1939 defines a higher layer protocol on CAN. It implements a more11sophisticated addressing scheme and extends the maximum packet size above 812bytes. Several derived specifications exist, which differ from the original13J1939 on the application level, like MilCAN A, NMEA2000, and especially14ISO-11783 (ISOBUS). This last one specifies the so-called ETP (Extended15Transport Protocol), which has been included in this implementation. This16results in a maximum packet size of ((2 ^ 24) - 1) * 7 bytes == 111 MiB.17 18Specifications used19-------------------20 21* SAE J1939-21 : data link layer22* SAE J1939-81 : network management23* ISO 11783-6 : Virtual Terminal (Extended Transport Protocol)24 25.. _j1939-motivation:26 27Motivation28==========29 30Given the fact there's something like SocketCAN with an API similar to BSD31sockets, we found some reasons to justify a kernel implementation for the32addressing and transport methods used by J1939.33 34* **Addressing:** when a process on an ECU communicates via J1939, it should35 not necessarily know its source address. Although, at least one process per36 ECU should know the source address. Other processes should be able to reuse37 that address. This way, address parameters for different processes38 cooperating for the same ECU, are not duplicated. This way of working is39 closely related to the UNIX concept, where programs do just one thing and do40 it well.41 42* **Dynamic addressing:** Address Claiming in J1939 is time critical.43 Furthermore, data transport should be handled properly during the address44 negotiation. Putting this functionality in the kernel eliminates it as a45 requirement for _every_ user space process that communicates via J1939. This46 results in a consistent J1939 bus with proper addressing.47 48* **Transport:** both TP & ETP reuse some PGNs to relay big packets over them.49 Different processes may thus use the same TP & ETP PGNs without actually50 knowing it. The individual TP & ETP sessions _must_ be serialized51 (synchronized) between different processes. The kernel solves this problem52 properly and eliminates the serialization (synchronization) as a requirement53 for _every_ user space process that communicates via J1939.54 55J1939 defines some other features (relaying, gateway, fast packet transport,56...). In-kernel code for these would not contribute to protocol stability.57Therefore, these parts are left to user space.58 59The J1939 sockets operate on CAN network devices (see SocketCAN). Any J193960user space library operating on CAN raw sockets will still operate properly.61Since such a library does not communicate with the in-kernel implementation, care62must be taken that these two do not interfere. In practice, this means they63cannot share ECU addresses. A single ECU (or virtual ECU) address is used by64the library exclusively, or by the in-kernel system exclusively.65 66J1939 concepts67==============68 69PGN70---71 72The J1939 protocol uses the 29-bit CAN identifier with the following structure:73 74 ============ ============== ====================75 29 bit CAN-ID76 --------------------------------------------------77 Bit positions within the CAN-ID78 --------------------------------------------------79 28 ... 26 25 ... 8 7 ... 080 ============ ============== ====================81 Priority PGN SA (Source Address)82 ============ ============== ====================83 84The PGN (Parameter Group Number) is a number to identify a packet. The PGN85is composed as follows:86 87 ============ ============== ================= =================88 PGN89 ------------------------------------------------------------------90 Bit positions within the CAN-ID91 ------------------------------------------------------------------92 25 24 23 ... 16 15 ... 893 ============ ============== ================= =================94 R (Reserved) DP (Data Page) PF (PDU Format) PS (PDU Specific)95 ============ ============== ================= =================96 97In J1939-21 distinction is made between PDU1 format (where PF < 240) and PDU298format (where PF >= 240). Furthermore, when using the PDU2 format, the PS-field99contains a so-called Group Extension, which is part of the PGN. When using PDU2100format, the Group Extension is set in the PS-field.101 102 ============== ========================103 PDU1 Format (specific) (peer to peer)104 ----------------------------------------105 Bit positions within the CAN-ID106 ----------------------------------------107 23 ... 16 15 ... 8108 ============== ========================109 00h ... EFh DA (Destination address)110 ============== ========================111 112 ============== ========================113 PDU2 Format (global) (broadcast)114 ----------------------------------------115 Bit positions within the CAN-ID116 ----------------------------------------117 23 ... 16 15 ... 8118 ============== ========================119 F0h ... FFh GE (Group Extension)120 ============== ========================121 122On the other hand, when using PDU1 format, the PS-field contains a so-called123Destination Address, which is _not_ part of the PGN. When communicating a PGN124from user space to kernel (or vice versa) and PDU1 format is used, the PS-field125of the PGN shall be set to zero. The Destination Address shall be set126elsewhere.127 128Regarding PGN mapping to 29-bit CAN identifier, the Destination Address shall129be get/set from/to the appropriate bits of the identifier by the kernel.130 131 132Addressing133----------134 135Both static and dynamic addressing methods can be used.136 137For static addresses, no extra checks are made by the kernel and provided138addresses are considered right. This responsibility is for the OEM or system139integrator.140 141For dynamic addressing, so-called Address Claiming, extra support is foreseen142in the kernel. In J1939 any ECU is known by its 64-bit NAME. At the moment of143a successful address claim, the kernel keeps track of both NAME and source144address being claimed. This serves as a base for filter schemes. By default,145packets with a destination that is not locally will be rejected.146 147Mixed mode packets (from a static to a dynamic address or vice versa) are148allowed. The BSD sockets define separate API calls for getting/setting the149local & remote address and are applicable for J1939 sockets.150 151Filtering152---------153 154J1939 defines white list filters per socket that a user can set in order to155receive a subset of the J1939 traffic. Filtering can be based on:156 157* SA158* SOURCE_NAME159* PGN160 161When multiple filters are in place for a single socket, and a packet comes in162that matches several of those filters, the packet is only received once for163that socket.164 165How to Use J1939166================167 168API Calls169---------170 171On CAN, you first need to open a socket for communicating over a CAN network.172To use J1939, ``#include <linux/can/j1939.h>``. From there, ``<linux/can.h>`` will be173included too. To open a socket, use:174 175.. code-block:: C176 177 s = socket(PF_CAN, SOCK_DGRAM, CAN_J1939);178 179J1939 does use ``SOCK_DGRAM`` sockets. In the J1939 specification, connections are180mentioned in the context of transport protocol sessions. These still deliver181packets to the other end (using several CAN packets). ``SOCK_STREAM`` is not182supported.183 184After the successful creation of the socket, you would normally use the ``bind(2)``185and/or ``connect(2)`` system call to bind the socket to a CAN interface. After186binding and/or connecting the socket, you can ``read(2)`` and ``write(2)`` from/to the187socket or use ``send(2)``, ``sendto(2)``, ``sendmsg(2)`` and the ``recv*()`` counterpart188operations on the socket as usual. There are also J1939 specific socket options189described below.190 191In order to send data, a ``bind(2)`` must have been successful. ``bind(2)`` assigns a192local address to a socket.193 194Different from CAN is that the payload data is just the data that get sends,195without its header info. The header info is derived from the sockaddr supplied196to ``bind(2)``, ``connect(2)``, ``sendto(2)`` and ``recvfrom(2)``. A ``write(2)`` with size 4 will197result in a packet with 4 bytes.198 199The sockaddr structure has extensions for use with J1939 as specified below:200 201.. code-block:: C202 203 struct sockaddr_can {204 sa_family_t can_family;205 int can_ifindex;206 union {207 struct {208 __u64 name;209 /* pgn:210 * 8 bit: PS in PDU2 case, else 0211 * 8 bit: PF212 * 1 bit: DP213 * 1 bit: reserved214 */215 __u32 pgn;216 __u8 addr;217 } j1939;218 } can_addr;219 }220 221``can_family`` & ``can_ifindex`` serve the same purpose as for other SocketCAN sockets.222 223``can_addr.j1939.pgn`` specifies the PGN (max 0x3ffff). Individual bits are224specified above.225 226``can_addr.j1939.name`` contains the 64-bit J1939 NAME.227 228``can_addr.j1939.addr`` contains the address.229 230The ``bind(2)`` system call assigns the local address, i.e. the source address when231sending packages. If a PGN during ``bind(2)`` is set, it's used as a RX filter.232I.e. only packets with a matching PGN are received. If an ADDR or NAME is set233it is used as a receive filter, too. It will match the destination NAME or ADDR234of the incoming packet. The NAME filter will work only if appropriate Address235Claiming for this name was done on the CAN bus and registered/cached by the236kernel.237 238On the other hand ``connect(2)`` assigns the remote address, i.e. the destination239address. The PGN from ``connect(2)`` is used as the default PGN when sending240packets. If ADDR or NAME is set it will be used as the default destination ADDR241or NAME. Further a set ADDR or NAME during ``connect(2)`` is used as a receive242filter. It will match the source NAME or ADDR of the incoming packet.243 244Both ``write(2)`` and ``send(2)`` will send a packet with local address from ``bind(2)`` and the245remote address from ``connect(2)``. Use ``sendto(2)`` to overwrite the destination246address.247 248If ``can_addr.j1939.name`` is set (!= 0) the NAME is looked up by the kernel and249the corresponding ADDR is used. If ``can_addr.j1939.name`` is not set (== 0),250``can_addr.j1939.addr`` is used.251 252When creating a socket, reasonable defaults are set. Some options can be253modified with ``setsockopt(2)`` & ``getsockopt(2)``.254 255RX path related options:256 257- ``SO_J1939_FILTER`` - configure array of filters258- ``SO_J1939_PROMISC`` - disable filters set by ``bind(2)`` and ``connect(2)``259 260By default no broadcast packets can be send or received. To enable sending or261receiving broadcast packets use the socket option ``SO_BROADCAST``:262 263.. code-block:: C264 265 int value = 1;266 setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value));267 268The following diagram illustrates the RX path:269 270.. code::271 272 +--------------------+273 | incoming packet |274 +--------------------+275 |276 V277 +--------------------+278 | SO_J1939_PROMISC? |279 +--------------------+280 | |281 no | | yes282 | |283 .---------' `---------.284 | |285 +---------------------------+ |286 | bind() + connect() + | |287 | SOCK_BROADCAST filter | |288 +---------------------------+ |289 | |290 |<---------------------'291 V292 +---------------------------+293 | SO_J1939_FILTER |294 +---------------------------+295 |296 V297 +---------------------------+298 | socket recv() |299 +---------------------------+300 301TX path related options:302``SO_J1939_SEND_PRIO`` - change default send priority for the socket303 304Message Flags during send() and Related System Calls305^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^306 307``send(2)``, ``sendto(2)`` and ``sendmsg(2)`` take a 'flags' argument. Currently308supported flags are:309 310* ``MSG_DONTWAIT``, i.e. non-blocking operation.311 312recvmsg(2)313^^^^^^^^^^314 315In most cases ``recvmsg(2)`` is needed if you want to extract more information than316``recvfrom(2)`` can provide. For example package priority and timestamp. The317Destination Address, name and packet priority (if applicable) are attached to318the msghdr in the ``recvmsg(2)`` call. They can be extracted using ``cmsg(3)`` macros,319with ``cmsg_level == SOL_J1939 && cmsg_type == SCM_J1939_DEST_ADDR``,320``SCM_J1939_DEST_NAME`` or ``SCM_J1939_PRIO``. The returned data is a ``uint8_t`` for321``priority`` and ``dst_addr``, and ``uint64_t`` for ``dst_name``.322 323.. code-block:: C324 325 uint8_t priority, dst_addr;326 uint64_t dst_name;327 328 for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {329 switch (cmsg->cmsg_level) {330 case SOL_CAN_J1939:331 if (cmsg->cmsg_type == SCM_J1939_DEST_ADDR)332 dst_addr = *CMSG_DATA(cmsg);333 else if (cmsg->cmsg_type == SCM_J1939_DEST_NAME)334 memcpy(&dst_name, CMSG_DATA(cmsg), cmsg->cmsg_len - CMSG_LEN(0));335 else if (cmsg->cmsg_type == SCM_J1939_PRIO)336 priority = *CMSG_DATA(cmsg);337 break;338 }339 }340 341Dynamic Addressing342------------------343 344Distinction has to be made between using the claimed address and doing an345address claim. To use an already claimed address, one has to fill in the346``j1939.name`` member and provide it to ``bind(2)``. If the name had claimed an address347earlier, all further messages being sent will use that address. And the348``j1939.addr`` member will be ignored.349 350An exception on this is PGN 0x0ee00. This is the "Address Claim/Cannot Claim351Address" message and the kernel will use the ``j1939.addr`` member for that PGN if352necessary.353 354To claim an address following code example can be used:355 356.. code-block:: C357 358 struct sockaddr_can baddr = {359 .can_family = AF_CAN,360 .can_addr.j1939 = {361 .name = name,362 .addr = J1939_IDLE_ADDR,363 .pgn = J1939_NO_PGN, /* to disable bind() rx filter for PGN */364 },365 .can_ifindex = if_nametoindex("can0"),366 };367 368 bind(sock, (struct sockaddr *)&baddr, sizeof(baddr));369 370 /* for Address Claiming broadcast must be allowed */371 int value = 1;372 setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value));373 374 /* configured advanced RX filter with PGN needed for Address Claiming */375 const struct j1939_filter filt[] = {376 {377 .pgn = J1939_PGN_ADDRESS_CLAIMED,378 .pgn_mask = J1939_PGN_PDU1_MAX,379 }, {380 .pgn = J1939_PGN_REQUEST,381 .pgn_mask = J1939_PGN_PDU1_MAX,382 }, {383 .pgn = J1939_PGN_ADDRESS_COMMANDED,384 .pgn_mask = J1939_PGN_MAX,385 },386 };387 388 setsockopt(sock, SOL_CAN_J1939, SO_J1939_FILTER, &filt, sizeof(filt));389 390 uint64_t dat = htole64(name);391 const struct sockaddr_can saddr = {392 .can_family = AF_CAN,393 .can_addr.j1939 = {394 .pgn = J1939_PGN_ADDRESS_CLAIMED,395 .addr = J1939_NO_ADDR,396 },397 };398 399 /* Afterwards do a sendto(2) with data set to the NAME (Little Endian). If the400 * NAME provided, does not match the j1939.name provided to bind(2), EPROTO401 * will be returned.402 */403 sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr));404 405If no-one else contests the address claim within 250ms after transmission, the406kernel marks the NAME-SA assignment as valid. The valid assignment will be kept407among other valid NAME-SA assignments. From that point, any socket bound to the408NAME can send packets.409 410If another ECU claims the address, the kernel will mark the NAME-SA expired.411No socket bound to the NAME can send packets (other than address claims). To412claim another address, some socket bound to NAME, must ``bind(2)`` again, but with413only ``j1939.addr`` changed to the new SA, and must then send a valid address claim414packet. This restarts the state machine in the kernel (and any other415participant on the bus) for this NAME.416 417``can-utils`` also include the ``j1939acd`` tool, so it can be used as code example or as418default Address Claiming daemon.419 420Send Examples421-------------422 423Static Addressing424^^^^^^^^^^^^^^^^^425 426This example will send a PGN (0x12300) from SA 0x20 to DA 0x30.427 428Bind:429 430.. code-block:: C431 432 struct sockaddr_can baddr = {433 .can_family = AF_CAN,434 .can_addr.j1939 = {435 .name = J1939_NO_NAME,436 .addr = 0x20,437 .pgn = J1939_NO_PGN,438 },439 .can_ifindex = if_nametoindex("can0"),440 };441 442 bind(sock, (struct sockaddr *)&baddr, sizeof(baddr));443 444Now, the socket 'sock' is bound to the SA 0x20. Since no ``connect(2)`` was called,445at this point we can use only ``sendto(2)`` or ``sendmsg(2)``.446 447Send:448 449.. code-block:: C450 451 const struct sockaddr_can saddr = {452 .can_family = AF_CAN,453 .can_addr.j1939 = {454 .name = J1939_NO_NAME;455 .addr = 0x30,456 .pgn = 0x12300,457 },458 };459 460 sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr));461