1 .. SPDX-License-Identifier: (GPL-2.0 OR MIT)
2
3 ===================
4 J1939 Documentation
5 ===================
6
7 Overview / What Is J1939
8 ========================
9
10 SAE J1939 defines a higher layer protocol on CAN. It implements a more
11 sophisticated addressing scheme and extends the maximum packet size above 8
12 bytes. Several derived specifications exist, which differ from the original
13 J1939 on the application level, like MilCAN A, NMEA2000, and especially
14 ISO-11783 (ISOBUS). This last one specifies the so-called ETP (Extended
15 Transport Protocol), which has been included in this implementation. This
16 results in a maximum packet size of ((2 ^ 24) - 1) * 7 bytes == 111 MiB.
17
18 Specifications used
19 -------------------
20
21 * SAE J1939-21 : data link layer
22 * SAE J1939-81 : network management
23 * ISO 11783-6 : Virtual Terminal (Extended Transport Protocol)
24
25 .. _j1939-motivation:
26
27 Motivation
28 ==========
29
30 Given the fact there's something like SocketCAN with an API similar to BSD
31 sockets, we found some reasons to justify a kernel implementation for the
32 addressing and transport methods used by J1939.
33
34 * **Addressing:** when a process on an ECU communicates via J1939, it should
35 not necessarily know its source address. Although, at least one process per
36 ECU should know the source address. Other processes should be able to reuse
37 that address. This way, address parameters for different processes
38 cooperating for the same ECU, are not duplicated. This way of working is
39 closely related to the UNIX concept, where programs do just one thing and do
40 it well.
41
42 * **Dynamic addressing:** Address Claiming in J1939 is time critical.
43 Furthermore, data transport should be handled properly during the address
44 negotiation. Putting this functionality in the kernel eliminates it as a
45 requirement for _every_ user space process that communicates via J1939. This
46 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 actually
50 knowing it. The individual TP & ETP sessions _must_ be serialized
51 (synchronized) between different processes. The kernel solves this problem
52 properly and eliminates the serialization (synchronization) as a requirement
53 for _every_ user space process that communicates via J1939.
54
55 J1939 defines some other features (relaying, gateway, fast packet transport,
56 ...). In-kernel code for these would not contribute to protocol stability.
57 Therefore, these parts are left to user space.
58
59 The J1939 sockets operate on CAN network devices (see SocketCAN). Any J1939
60 user space library operating on CAN raw sockets will still operate properly.
61 Since such a library does not communicate with the in-kernel implementation, care
62 must be taken that these two do not interfere. In practice, this means they
63 cannot share ECU addresses. A single ECU (or virtual ECU) address is used by
64 the library exclusively, or by the in-kernel system exclusively.
65
66 J1939 concepts
67 ==============
68
69 PGN
70 ---
71
72 The J1939 protocol uses the 29-bit CAN identifier with the following structure:
73
74 ============ ============== ====================
75 29 bit CAN-ID
76 --------------------------------------------------
77 Bit positions within the CAN-ID
78 --------------------------------------------------
79 28 ... 26 25 ... 8 7 ... 0
80 ============ ============== ====================
81 Priority PGN SA (Source Address)
82 ============ ============== ====================
83
84 The PGN (Parameter Group Number) is a number to identify a packet. The PGN
85 is composed as follows:
86
87 ============ ============== ================= =================
88 PGN
89 ------------------------------------------------------------------
90 Bit positions within the CAN-ID
91 ------------------------------------------------------------------
92 25 24 23 ... 16 15 ... 8
93 ============ ============== ================= =================
94 R (Reserved) DP (Data Page) PF (PDU Format) PS (PDU Specific)
95 ============ ============== ================= =================
96
97 In J1939-21 distinction is made between PDU1 format (where PF < 240) and PDU2
98 format (where PF >= 240). Furthermore, when using the PDU2 format, the PS-field
99 contains a so-called Group Extension, which is part of the PGN. When using PDU2
100 format, 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-ID
106 ----------------------------------------
107 23 ... 16 15 ... 8
108 ============== ========================
109 00h ... EFh DA (Destination address)
110 ============== ========================
111
112 ============== ========================
113 PDU2 Format (global) (broadcast)
114 ----------------------------------------
115 Bit positions within the CAN-ID
116 ----------------------------------------
117 23 ... 16 15 ... 8
118 ============== ========================
119 F0h ... FFh GE (Group Extension)
120 ============== ========================
121
122 On the other hand, when using PDU1 format, the PS-field contains a so-called
123 Destination Address, which is _not_ part of the PGN. When communicating a PGN
124 from user space to kernel (or vice versa) and PDU2 format is used, the PS-field
125 of the PGN shall be set to zero. The Destination Address shall be set
126 elsewhere.
127
128 Regarding PGN mapping to 29-bit CAN identifier, the Destination Address shall
129 be get/set from/to the appropriate bits of the identifier by the kernel.
130
131
132 Addressing
133 ----------
134
135 Both static and dynamic addressing methods can be used.
136
137 For static addresses, no extra checks are made by the kernel and provided
138 addresses are considered right. This responsibility is for the OEM or system
139 integrator.
140
141 For dynamic addressing, so-called Address Claiming, extra support is foreseen
142 in the kernel. In J1939 any ECU is known by its 64-bit NAME. At the moment of
143 a successful address claim, the kernel keeps track of both NAME and source
144 address being claimed. This serves as a base for filter schemes. By default,
145 packets with a destination that is not locally will be rejected.
146
147 Mixed mode packets (from a static to a dynamic address or vice versa) are
148 allowed. The BSD sockets define separate API calls for getting/setting the
149 local & remote address and are applicable for J1939 sockets.
150
151 Filtering
152 ---------
153
154 J1939 defines white list filters per socket that a user can set in order to
155 receive a subset of the J1939 traffic. Filtering can be based on:
156
157 * SA
158 * SOURCE_NAME
159 * PGN
160
161 When multiple filters are in place for a single socket, and a packet comes in
162 that matches several of those filters, the packet is only received once for
163 that socket.
164
165 How to Use J1939
166 ================
167
168 API Calls
169 ---------
170
171 On CAN, you first need to open a socket for communicating over a CAN network.
172 To use J1939, ``#include <linux/can/j1939.h>``. From there, ``<linux/can.h>`` will be
173 included too. To open a socket, use:
174
175 .. code-block:: C
176
177 s = socket(PF_CAN, SOCK_DGRAM, CAN_J1939);
178
179 J1939 does use ``SOCK_DGRAM`` sockets. In the J1939 specification, connections are
180 mentioned in the context of transport protocol sessions. These still deliver
181 packets to the other end (using several CAN packets). ``SOCK_STREAM`` is not
182 supported.
183
184 After the successful creation of the socket, you would normally use the ``bind(2)``
185 and/or ``connect(2)`` system call to bind the socket to a CAN interface. After
186 binding and/or connecting the socket, you can ``read(2)`` and ``write(2)`` from/to the
187 socket or use ``send(2)``, ``sendto(2)``, ``sendmsg(2)`` and the ``recv*()`` counterpart
188 operations on the socket as usual. There are also J1939 specific socket options
189 described below.
190
191 In order to send data, a ``bind(2)`` must have been successful. ``bind(2)`` assigns a
192 local address to a socket.
193
194 Different from CAN is that the payload data is just the data that get sends,
195 without its header info. The header info is derived from the sockaddr supplied
196 to ``bind(2)``, ``connect(2)``, ``sendto(2)`` and ``recvfrom(2)``. A ``write(2)`` with size 4 will
197 result in a packet with 4 bytes.
198
199 The sockaddr structure has extensions for use with J1939 as specified below:
200
201 .. code-block:: C
202
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 0
211 * 8 bit: PF
212 * 1 bit: DP
213 * 1 bit: reserved
214 */
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 are
224 specified above.
225
226 ``can_addr.j1939.name`` contains the 64-bit J1939 NAME.
227
228 ``can_addr.j1939.addr`` contains the address.
229
230 The ``bind(2)`` system call assigns the local address, i.e. the source address when
231 sending packages. If a PGN during ``bind(2)`` is set, it's used as a RX filter.
232 I.e. only packets with a matching PGN are received. If an ADDR or NAME is set
233 it is used as a receive filter, too. It will match the destination NAME or ADDR
234 of the incoming packet. The NAME filter will work only if appropriate Address
235 Claiming for this name was done on the CAN bus and registered/cached by the
236 kernel.
237
238 On the other hand ``connect(2)`` assigns the remote address, i.e. the destination
239 address. The PGN from ``connect(2)`` is used as the default PGN when sending
240 packets. If ADDR or NAME is set it will be used as the default destination ADDR
241 or NAME. Further a set ADDR or NAME during ``connect(2)`` is used as a receive
242 filter. It will match the source NAME or ADDR of the incoming packet.
243
244 Both ``write(2)`` and ``send(2)`` will send a packet with local address from ``bind(2)`` and the
245 remote address from ``connect(2)``. Use ``sendto(2)`` to overwrite the destination
246 address.
247
248 If ``can_addr.j1939.name`` is set (!= 0) the NAME is looked up by the kernel and
249 the corresponding ADDR is used. If ``can_addr.j1939.name`` is not set (== 0),
250 ``can_addr.j1939.addr`` is used.
251
252 When creating a socket, reasonable defaults are set. Some options can be
253 modified with ``setsockopt(2)`` & ``getsockopt(2)``.
254
255 RX path related options:
256
257 - ``SO_J1939_FILTER`` - configure array of filters
258 - ``SO_J1939_PROMISC`` - disable filters set by ``bind(2)`` and ``connect(2)``
259
260 By default no broadcast packets can be send or received. To enable sending or
261 receiving broadcast packets use the socket option ``SO_BROADCAST``:
262
263 .. code-block:: C
264
265 int value = 1;
266 setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value));
267
268 The following diagram illustrates the RX path:
269
270 .. code::
271
272 +--------------------+
273 | incoming packet |
274 +--------------------+
275 |
276 V
277 +--------------------+
278 | SO_J1939_PROMISC? |
279 +--------------------+
280 | |
281 no | | yes
282 | |
283 .---------' `---------.
284 | |
285 +---------------------------+ |
286 | bind() + connect() + | |
287 | SOCK_BROADCAST filter | |
288 +---------------------------+ |
289 | |
290 |<---------------------'
291 V
292 +---------------------------+
293 | SO_J1939_FILTER |
294 +---------------------------+
295 |
296 V
297 +---------------------------+
298 | socket recv() |
299 +---------------------------+
300
301 TX path related options:
302 ``SO_J1939_SEND_PRIO`` - change default send priority for the socket
303
304 Message Flags during send() and Related System Calls
305 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
306
307 ``send(2)``, ``sendto(2)`` and ``sendmsg(2)`` take a 'flags' argument. Currently
308 supported flags are:
309
310 * ``MSG_DONTWAIT``, i.e. non-blocking operation.
311
312 recvmsg(2)
313 ^^^^^^^^^^
314
315 In most cases ``recvmsg(2)`` is needed if you want to extract more information than
316 ``recvfrom(2)`` can provide. For example package priority and timestamp. The
317 Destination Address, name and packet priority (if applicable) are attached to
318 the msghdr in the ``recvmsg(2)`` call. They can be extracted using ``cmsg(3)`` macros,
319 with ``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`` for
321 ``priority`` and ``dst_addr``, and ``uint64_t`` for ``dst_name``.
322
323 .. code-block:: C
324
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
341 Dynamic Addressing
342 ------------------
343
344 Distinction has to be made between using the claimed address and doing an
345 address claim. To use an already claimed address, one has to fill in the
346 ``j1939.name`` member and provide it to ``bind(2)``. If the name had claimed an address
347 earlier, all further messages being sent will use that address. And the
348 ``j1939.addr`` member will be ignored.
349
350 An exception on this is PGN 0x0ee00. This is the "Address Claim/Cannot Claim
351 Address" message and the kernel will use the ``j1939.addr`` member for that PGN if
352 necessary.
353
354 To claim an address following code example can be used:
355
356 .. code-block:: C
357
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 the
400 * NAME provided, does not match the j1939.name provided to bind(2), EPROTO
401 * will be returned.
402 */
403 sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr));
404
405 If no-one else contests the address claim within 250ms after transmission, the
406 kernel marks the NAME-SA assignment as valid. The valid assignment will be kept
407 among other valid NAME-SA assignments. From that point, any socket bound to the
408 NAME can send packets.
409
410 If another ECU claims the address, the kernel will mark the NAME-SA expired.
411 No socket bound to the NAME can send packets (other than address claims). To
412 claim another address, some socket bound to NAME, must ``bind(2)`` again, but with
413 only ``j1939.addr`` changed to the new SA, and must then send a valid address claim
414 packet. This restarts the state machine in the kernel (and any other
415 participant on the bus) for this NAME.
416
417 ``can-utils`` also include the ``j1939acd`` tool, so it can be used as code example or as
418 default Address Claiming daemon.
419
420 Send Examples
421 -------------
422
423 Static Addressing
424 ^^^^^^^^^^^^^^^^^
425
426 This example will send a PGN (0x12300) from SA 0x20 to DA 0x30.
427
428 Bind:
429
430 .. code-block:: C
431
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
444 Now, the socket 'sock' is bound to the SA 0x20. Since no ``connect(2)`` was called,
445 at this point we can use only ``sendto(2)`` or ``sendmsg(2)``.
446
447 Send:
448
449 .. code-block:: C
450
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));
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