J1939 Documentation¶

Overview / What Is J1939¶

SAE J1939 defines a higher layer protocol on CAN. It implements a more sophisticated addressing scheme and extends the maximum packet size above 8 bytes. Several derived specifications exist, which differ from the original J1939 on the application level, like MilCAN A, NMEA2000, and especially ISO-11783 (ISOBUS). This last one specifies the so-called ETP (Extended Transport Protocol), which has been included in this implementation. This results in a maximum packet size of ((2 ^ 24) - 1) * 7 bytes == 111 MiB.

Specifications used¶

SAE J1939-21 : data link layer
SAE J1939-81 : network management
ISO 11783-6 : Virtual Terminal (Extended Transport Protocol)

Motivation¶

Given the fact there’s something like SocketCAN with an API similar to BSD sockets, we found some reasons to justify a kernel implementation for the addressing and transport methods used by J1939.

Addressing: when a process on an ECU communicates via J1939, it should not necessarily know its source address. Although, at least one process per ECU should know the source address. Other processes should be able to reuse that address. This way, address parameters for different processes cooperating for the same ECU, are not duplicated. This way of working is closely related to the UNIX concept, where programs do just one thing and do it well.
Dynamic addressing: Address Claiming in J1939 is time critical. Furthermore, data transport should be handled properly during the address negotiation. Putting this functionality in the kernel eliminates it as a requirement for _every_ user space process that communicates via J1939. This results in a consistent J1939 bus with proper addressing.
Transport: both TP & ETP reuse some PGNs to relay big packets over them. Different processes may thus use the same TP & ETP PGNs without actually knowing it. The individual TP & ETP sessions _must_ be serialized (synchronized) between different processes. The kernel solves this problem properly and eliminates the serialization (synchronization) as a requirement for _every_ user space process that communicates via J1939.

J1939 defines some other features (relaying, gateway, fast packet transport, ...). In-kernel code for these would not contribute to protocol stability. Therefore, these parts are left to user space.

The J1939 sockets operate on CAN network devices (see SocketCAN). Any J1939 user space library operating on CAN raw sockets will still operate properly. Since such a library does not communicate with the in-kernel implementation, care must be taken that these two do not interfere. In practice, this means they cannot share ECU addresses. A single ECU (or virtual ECU) address is used by the library exclusively, or by the in-kernel system exclusively.

J1939 concepts¶

Data Sent to the J1939 Stack¶

The data buffers sent to the J1939 stack from user space are not CAN frames themselves. Instead, they are payloads that the J1939 stack converts into proper CAN frames based on the size of the buffer and the type of transfer. The size of the buffer influences how the stack processes the data and determines the internal code path used for the transfer.

Handling of Different Buffer Sizes:

Buffers with a size of 8 bytes or less:
- These are handled as simple sessions internally within the stack.
- The stack converts the buffer directly into a single CAN frame without fragmentation.
- This type of transfer does not require an actual client (receiver) on the receiving side.
Buffers up to 1785 bytes:
- These are automatically handled as J1939 Transport Protocol (TP) transfers.
- Internally, the stack splits the buffer into multiple 8-byte CAN frames.
- TP transfers can be unicast or broadcast.
- Broadcast TP: Does not require a receiver on the other side and can be used in broadcast scenarios.
- Unicast TP: Requires an active receiver (client) on the other side to acknowledge the transfer.
Buffers from 1786 bytes up to 111 MiB:
- These are handled as ISO 11783 Extended Transport Protocol (ETP) transfers.
- ETP transfers are used for larger payloads and are split into multiple CAN frames internally.
- ETP transfers (unicast): Require a receiver on the other side to process the incoming data and acknowledge each step of the transfer.
- ETP transfers cannot be broadcast like TP transfers, and always require a receiver for operation.

Non-Blocking Operation with `MSG_DONTWAIT`:

The J1939 stack supports non-blocking operation when used in combination with the MSG_DONTWAIT flag. In this mode, the stack attempts to take as much data as the available memory for the socket allows. It returns the amount of data that was successfully taken, and it is the responsibility of user space to monitor this value and handle partial transfers.

If the stack cannot take the entire buffer, it returns the number of bytes successfully taken, and user space should handle the remainder.
Error handling: When using MSG_DONTWAIT, the user must rely on the error queue to detect transfer errors. See the SO_J1939_ERRQUEUE section for details on how to subscribe to error notifications. Without the error queue, there is no other way for user space to be notified of transfer errors during non-blocking operations.

Behavior and Requirements:

Simple transfers (<= 8 bytes): Do not require a receiver on the other side, making them easy to send without needing address claiming or coordination with a destination.
Unicast TP/ETP: Requires a receiver on the other side to complete the transfer. The receiver must acknowledge the transfer for the session to proceed successfully.
Broadcast TP: Allows sending data without a receiver, but only works for TP transfers. ETP cannot be broadcast and always needs a receiving client.

These different behaviors depend heavily on the size of the buffer provided to the stack, and the appropriate transport mechanism (TP or ETP) is selected based on the payload size. The stack automatically manages the fragmentation and reassembly of large payloads and ensures that the correct CAN frames are generated and transmitted for each session.

PGN¶

The J1939 protocol uses the 29-bit CAN identifier with the following structure:

29 bit CAN-ID

Bit positions within the CAN-ID

28 ... 26

25 ... 8

7 ... 0

Priority

PGN

SA (Source Address)

The PGN (Parameter Group Number) is a number to identify a packet. The PGN is composed as follows:

PGN

Bit positions within the CAN-ID

25

24

23 ... 16

15 ... 8

R (Reserved)

DP (Data Page)

PF (PDU Format)

PS (PDU Specific)

In J1939-21 distinction is made between PDU1 format (where PF < 240) and PDU2 format (where PF >= 240). Furthermore, when using the PDU2 format, the PS-field contains a so-called Group Extension, which is part of the PGN. When using PDU2 format, the Group Extension is set in the PS-field.

PDU1 Format (specific) (peer to peer)

Bit positions within the CAN-ID

23 ... 16

15 ... 8

00h ... EFh

DA (Destination address)

PDU2 Format (global) (broadcast)

Bit positions within the CAN-ID

23 ... 16

15 ... 8

F0h ... FFh

GE (Group Extension)

On the other hand, when using PDU1 format, the PS-field contains a so-called Destination Address, which is _not_ part of the PGN. When communicating a PGN from user space to kernel (or vice versa) and PDU1 format is used, the PS-field of the PGN shall be set to zero. The Destination Address shall be set elsewhere.

Regarding PGN mapping to 29-bit CAN identifier, the Destination Address shall be get/set from/to the appropriate bits of the identifier by the kernel.

Addressing¶

Both static and dynamic addressing methods can be used.

For static addresses, no extra checks are made by the kernel and provided addresses are considered right. This responsibility is for the OEM or system integrator.

For dynamic addressing, so-called Address Claiming, extra support is foreseen in the kernel. In J1939 any ECU is known by its 64-bit NAME. At the moment of a successful address claim, the kernel keeps track of both NAME and source address being claimed. This serves as a base for filter schemes. By default, packets with a destination that is not locally will be rejected.

Mixed mode packets (from a static to a dynamic address or vice versa) are allowed. The BSD sockets define separate API calls for getting/setting the local & remote address and are applicable for J1939 sockets.

Filtering¶

J1939 defines white list filters per socket that a user can set in order to receive a subset of the J1939 traffic. Filtering can be based on:

SA
SOURCE_NAME
PGN

When multiple filters are in place for a single socket, and a packet comes in that matches several of those filters, the packet is only received once for that socket.

How to Use J1939¶

API Calls¶

On CAN, you first need to open a socket for communicating over a CAN network. To use J1939, #include <linux/can/j1939.h>. From there, <linux/can.h> will be included too. To open a socket, use:

s = socket(PF_CAN, SOCK_DGRAM, CAN_J1939);

J1939 does use SOCK_DGRAM sockets. In the J1939 specification, connections are mentioned in the context of transport protocol sessions. These still deliver packets to the other end (using several CAN packets). SOCK_STREAM is not supported.

After the successful creation of the socket, you would normally use the bind(2) and/or connect(2) system call to bind the socket to a CAN interface. After binding and/or connecting the socket, you can read(2) and write(2) from/to the socket or use send(2), sendto(2), sendmsg(2) and the recv*() counterpart operations on the socket as usual. There are also J1939 specific socket options described below.

In order to send data, a bind(2) must have been successful. bind(2) assigns a local address to a socket.

Different from CAN is that the payload data is just the data that get sends, without its header info. The header info is derived from the sockaddr supplied to bind(2), connect(2), sendto(2) and recvfrom(2). A write(2) with size 4 will result in a packet with 4 bytes.

The sockaddr structure has extensions for use with J1939 as specified below:

struct sockaddr_can {
   sa_family_t can_family;
   int         can_ifindex;
   union {
      struct {
         __u64 name;
                  /* pgn:
                   * 8 bit: PS in PDU2 case, else 0
                   * 8 bit: PF
                   * 1 bit: DP
                   * 1 bit: reserved
                   */
         __u32 pgn;
         __u8  addr;
      } j1939;
   } can_addr;
}

can_family & can_ifindex serve the same purpose as for other SocketCAN sockets.

can_addr.j1939.pgn specifies the PGN (max 0x3ffff). Individual bits are specified above.

can_addr.j1939.name contains the 64-bit J1939 NAME.

can_addr.j1939.addr contains the address.

The bind(2) system call assigns the local address, i.e. the source address when sending packages. If a PGN during bind(2) is set, it’s used as a RX filter. I.e. only packets with a matching PGN are received. If an ADDR or NAME is set it is used as a receive filter, too. It will match the destination NAME or ADDR of the incoming packet. The NAME filter will work only if appropriate Address Claiming for this name was done on the CAN bus and registered/cached by the kernel.

On the other hand connect(2) assigns the remote address, i.e. the destination address. The PGN from connect(2) is used as the default PGN when sending packets. If ADDR or NAME is set it will be used as the default destination ADDR or NAME. Further a set ADDR or NAME during connect(2) is used as a receive filter. It will match the source NAME or ADDR of the incoming packet.

Both write(2) and send(2) will send a packet with local address from bind(2) and the remote address from connect(2). Use sendto(2) to overwrite the destination address.

If can_addr.j1939.name is set (!= 0) the NAME is looked up by the kernel and the corresponding ADDR is used. If can_addr.j1939.name is not set (== 0), can_addr.j1939.addr is used.

When creating a socket, reasonable defaults are set. Some options can be modified with setsockopt(2) & getsockopt(2).

RX path related options:

SO_J1939_FILTER - configure array of filters
SO_J1939_PROMISC - disable filters set by bind(2) and connect(2)

By default no broadcast packets can be send or received. To enable sending or receiving broadcast packets use the socket option SO_BROADCAST:

int value = 1;
setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value));

The following diagram illustrates the RX path:

               +--------------------+
               |  incoming packet   |
               +--------------------+
                         |
                         V
               +--------------------+
               | SO_J1939_PROMISC?  |
               +--------------------+
                        |  |
                    no  |  | yes
                        |  |
              .---------'  `---------.
              |                      |
+---------------------------+        |
| bind() + connect() +      |        |
| SOCK_BROADCAST filter     |        |
+---------------------------+        |
              |                      |
              |<---------------------'
              V
+---------------------------+
|      SO_J1939_FILTER      |
+---------------------------+
              |
              V
+---------------------------+
|        socket recv()      |
+---------------------------+

TX path related options: SO_J1939_SEND_PRIO - change default send priority for the socket

recvmsg(2)¶

In most cases recvmsg(2) is needed if you want to extract more information than recvfrom(2) can provide. For example package priority and timestamp. The Destination Address, name and packet priority (if applicable) are attached to the msghdr in the recvmsg(2) call. They can be extracted using cmsg(3) macros, with cmsg_level == SOL_J1939 && cmsg_type == SCM_J1939_DEST_ADDR, SCM_J1939_DEST_NAME or SCM_J1939_PRIO. The returned data is a uint8_t for priority and dst_addr, and uint64_t for dst_name.

uint8_t priority, dst_addr;
uint64_t dst_name;

for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
        switch (cmsg->cmsg_level) {
        case SOL_CAN_J1939:
                if (cmsg->cmsg_type == SCM_J1939_DEST_ADDR)
                        dst_addr = *CMSG_DATA(cmsg);
                else if (cmsg->cmsg_type == SCM_J1939_DEST_NAME)
                        memcpy(&dst_name, CMSG_DATA(cmsg), cmsg->cmsg_len - CMSG_LEN(0));
                else if (cmsg->cmsg_type == SCM_J1939_PRIO)
                        priority = *CMSG_DATA(cmsg);
                break;
        }
}

setsockopt(2)¶

The setsockopt(2) function is used to configure various socket-level options for J1939 communication. The following options are supported:

`SO_J1939_FILTER`¶

The SO_J1939_FILTER option is essential when the default behavior of bind(2) and connect(2) is insufficient for specific use cases. By default, bind(2) and connect(2) allow a socket to be associated with a single unicast or broadcast address. However, there are scenarios where finer control over the incoming messages is required, such as filtering by Parameter Group Number (PGN) rather than by addresses.

For example, in a system where multiple types of J1939 messages are being transmitted, a process might only be interested in a subset of those messages, such as specific PGNs, and not want to receive all messages destined for its address or broadcast to the bus.

By applying the SO_J1939_FILTER option, you can filter messages based on:

Source Address (SA): Filter messages coming from specific source addresses.
Source Name: Filter messages coming from ECUs with specific NAME identifiers.
Parameter Group Number (PGN): Focus on receiving messages with specific PGNs, filtering out irrelevant ones.

This filtering mechanism is particularly useful when:

You want to receive a subset of messages based on their PGNs, even if the address is the same.
You need to handle both broadcast and unicast messages but only care about certain message types or parameters.
The bind(2) and connect(2) functions only allow binding to a single address, which might not be sufficient if the process needs to handle multiple PGNs but does not want to open multiple sockets.

To remove existing filters, you can pass optval == NULL or optlen == 0 to setsockopt(2). This will clear all currently set filters. If you want to update the set of filters, you must pass the updated filter set to setsockopt(2), as the new filter set will replace the old one entirely. This behavior ensures that any previous filter configuration is discarded and only the new set is applied.

Example of removing all filters:

setsockopt(sock, SOL_CAN_J1939, SO_J1939_FILTER, NULL, 0);

Maximum number of filters: The maximum amount of filters that can be applied using SO_J1939_FILTER is defined by J1939_FILTER_MAX, which is set to 512. This means you can configure up to 512 individual filters to match your specific filtering needs.

Practical use case: Monitoring Address Claiming

One practical use case is monitoring the J1939 address claiming process by filtering for specific PGNs related to address claiming. This allows a process to monitor and handle address claims without processing unrelated messages.

Example:

struct j1939_filter filt[] = {
    {
        .pgn = J1939_PGN_ADDRESS_CLAIMED,
        .pgn_mask = J1939_PGN_PDU1_MAX,
    }, {
        .pgn = J1939_PGN_REQUEST,
        .pgn_mask = J1939_PGN_PDU1_MAX,
    }, {
        .pgn = J1939_PGN_ADDRESS_COMMANDED,
        .pgn_mask = J1939_PGN_MAX,
    },
};
setsockopt(sock, SOL_CAN_J1939, SO_J1939_FILTER, &filt, sizeof(filt));

In this example, the socket will only receive messages with the PGNs related to address claiming: J1939_PGN_ADDRESS_CLAIMED, J1939_PGN_REQUEST, and J1939_PGN_ADDRESS_COMMANDED. This is particularly useful in scenarios where you want to monitor and process address claims without being overwhelmed by other traffic on the J1939 network.

`SO_J1939_PROMISC`¶

The SO_J1939_PROMISC option enables socket-level promiscuous mode. When this option is enabled, the socket will receive all J1939 traffic, regardless of any filters set by bind() or connect(). This is analogous to enabling promiscuous mode for an Ethernet interface, where all traffic on the network segment is captured.

However, `SO_J1939_FILTER` has a higher priority compared to SO_J1939_PROMISC. This means that even in promiscuous mode, you can reduce the number of packets received by applying specific filters with SO_J1939_FILTER. The filters will limit which packets are passed to the socket, allowing for more refined traffic selection while promiscuous mode is active.

The acceptable value size for this option is sizeof(int), and the value is only differentiated between 0 and non-zero. A value of 0 disables promiscuous mode, while any non-zero value enables it.

This combination can be useful for debugging or monitoring specific types of traffic while still capturing a broad set of messages.

Example:

int value = 1;
setsockopt(sock, SOL_CAN_J1939, SO_J1939_PROMISC, &value, sizeof(value));

In this example, setting value to any non-zero value (e.g., 1) enables promiscuous mode, allowing the socket to receive all J1939 traffic on the network.

`SO_BROADCAST`¶

The SO_BROADCAST option enables the sending and receiving of broadcast messages. By default, broadcast messages are disabled for J1939 sockets. When this option is enabled, the socket will be allowed to send and receive broadcast packets on the J1939 network.

Due to the nature of the CAN bus as a shared medium, all messages transmitted on the bus are visible to all participants. In the context of J1939, broadcasting refers to using a specific destination address field, where the destination address is set to a value that indicates the message is intended for all participants (usually a global address such as 0xFF). Enabling the broadcast option allows the socket to send and receive such broadcast messages.

The acceptable value size for this option is sizeof(int), and the value is only differentiated between 0 and non-zero. A value of 0 disables the ability to send and receive broadcast messages, while any non-zero value enables it.

Example:

int value = 1;
setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value));

In this example, setting value to any non-zero value (e.g., 1) enables the socket to send and receive broadcast messages.

`SO_J1939_SEND_PRIO`¶

The SO_J1939_SEND_PRIO option sets the priority of outgoing J1939 messages for the socket. In J1939, messages can have different priorities, and lower numerical values indicate higher priority. This option allows the user to control the priority of messages sent from the socket by adjusting the priority bits in the CAN identifier.

The acceptable value size for this option is sizeof(int), and the value is expected to be in the range of 0 to 7, where 0 is the highest priority, and 7 is the lowest. By default, the priority is set to 6 if this option is not explicitly configured.

Note that the priority values 0 and 1 can only be set if the process has the CAP_NET_ADMIN capability. These are reserved for high-priority traffic and require administrative privileges.

Example:

int prio = 3;  // Priority value between 0 (highest) and 7 (lowest)
setsockopt(sock, SOL_CAN_J1939, SO_J1939_SEND_PRIO, &prio, sizeof(prio));

In this example, the priority is set to 3, meaning the outgoing messages will be sent with a moderate priority level.

`SO_J1939_ERRQUEUE`¶

The SO_J1939_ERRQUEUE option enables the socket to receive error messages from the error queue, providing diagnostic information about transmission failures, protocol violations, or other issues that occur during J1939 communication. Once this option is set, user space is required to handle MSG_ERRQUEUE messages.

Setting SO_J1939_ERRQUEUE to 0 will purge any currently present error messages in the error queue. When enabled, error messages can be retrieved using the recvmsg(2) system call.

When subscribing to the error queue, the following error events can be accessed:

``J1939_EE_INFO_TX_ABORT``: Transmission abort errors.
``J1939_EE_INFO_RX_RTS``: Reception of RTS (Request to Send) control frames.
``J1939_EE_INFO_RX_DPO``: Reception of data packets with Data Page Offset (DPO).
``J1939_EE_INFO_RX_ABORT``: Reception abort errors.

The error queue can be used to correlate errors with specific message transfer sessions using the session ID (tskey). The session ID is assigned via the SOF_TIMESTAMPING_OPT_ID flag, which is set by enabling the SO_TIMESTAMPING option.

If SO_J1939_ERRQUEUE is activated, the user is required to pull messages from the error queue, meaning that using plain recv(2) is not sufficient anymore. The user must use recvmsg(2) with appropriate flags to handle error messages. Failure to do so can result in the socket becoming blocked with unprocessed error messages in the queue.

It is recommended that SO_J1939_ERRQUEUE be used in combination with SO_TIMESTAMPING in most cases. This enables proper error handling along with session tracking and timestamping, providing a more detailed analysis of message transfers and errors.

The acceptable value size for this option is sizeof(int), and the value is only differentiated between 0 and non-zero. A value of 0 disables error queue reception and purges any existing error messages, while any non-zero value enables it.

Example:

int enable = 1;  // Enable error queue reception
setsockopt(sock, SOL_CAN_J1939, SO_J1939_ERRQUEUE, &enable, sizeof(enable));

// Enable timestamping with session tracking via tskey
int timestamping = SOF_TIMESTAMPING_OPT_ID | SOF_TIMESTAMPING_TX_ACK |
                   SOF_TIMESTAMPING_TX_SCHED |
                   SOF_TIMESTAMPING_RX_SOFTWARE | SOF_TIMESTAMPING_OPT_CMSG;
setsockopt(sock, SOL_SOCKET, SO_TIMESTAMPING, &timestamping,
           sizeof(timestamping));

When enabled, error messages can be retrieved using recvmsg(2). By combining SO_J1939_ERRQUEUE with SO_TIMESTAMPING (with SOF_TIMESTAMPING_OPT_ID and SOF_TIMESTAMPING_OPT_CMSG enabled), the user can track message transfers, retrieve precise timestamps, and correlate errors with specific sessions.

For more information on enabling timestamps and session tracking, refer to the SO_TIMESTAMPING section.

`SO_TIMESTAMPING`¶

The SO_TIMESTAMPING option allows the socket to receive timestamps for various events related to message transmissions and receptions in J1939. This option is often used in combination with SO_J1939_ERRQUEUE to provide detailed diagnostic information, session tracking, and precise timing data for message transfers.

In J1939, all payloads provided by user space, regardless of size, are processed by the kernel as sessions. This includes both single-frame messages (up to 8 bytes) and multi-frame protocols such as the Transport Protocol (TP) and Extended Transport Protocol (ETP). Even for small, single-frame messages, the kernel creates a session to manage the transmission and reception. The concept of sessions allows the kernel to manage various aspects of the protocol, such as reassembling multi-frame messages and tracking the status of transmissions.

When receiving extended error messages from the error queue, the error information is delivered through a struct sock_extended_err, accessible via the control message (cmsg) retrieved using the recvmsg(2) system call.

There are two typical origins for the extended error messages in J1939:

serr->ee_origin == SO_EE_ORIGIN_TIMESTAMPING:

In this case, the serr->ee_info field will contain one of the following timestamp types:
- SCM_TSTAMP_SCHED: This timestamp is valid for Extended Transport Protocol (ETP) transfers and simple transfers (8 bytes or less). It indicates when a message or set of frames has been scheduled for transmission.
  - For simple transfers (8 bytes or less), it marks the point when the message is queued and ready to be sent onto the CAN bus.
  - For ETP transfers, it is sent after receiving a CTS (Clear to Send) frame on the sender side, indicating that a new set of frames has been scheduled for transmission.
  - The Transport Protocol (TP) case is currently not implemented for this timestamp.
  - On the receiver side, the counterpart to this event for ETP is represented by the J1939_EE_INFO_RX_DPO message, which indicates the reception of a Data Page Offset (DPO) control frame.
- SCM_TSTAMP_ACK: This timestamp indicates the acknowledgment of the message or session.
  - For simple transfers (8 bytes or less), it marks when the message has been sent and an echo confirmation has been received from the CAN controller, indicating that the frame was transmitted onto the bus.
  - For multi-frame transfers (TP or ETP), it signifies that the entire session has been acknowledged, typically after receiving the End of Message Acknowledgment (EOMA) packet.
serr->ee_origin == SO_EE_ORIGIN_LOCAL:

In this case, the serr->ee_info field will contain one of the following J1939 stack-specific message types:
- J1939_EE_INFO_TX_ABORT: This message indicates that the transmission of a message or session was aborted. The cause of the abort can come from various sources:
  - CAN stack failure: The J1939 stack was unable to pass the frame to the CAN framework for transmission.
  - Echo failure: The J1939 stack did not receive an echo confirmation from the CAN controller, meaning the frame may not have been successfully transmitted to the CAN bus.
  - Protocol-level issues: For multi-frame transfers (TP/ETP), this could include protocol-related errors, such as an abort signaled by the receiver or a timeout at the protocol level, which causes the session to terminate prematurely.
  - The corresponding error code is stored in serr->ee_data (session->err on kernel side), providing additional details about the specific reason for the abort.
- J1939_EE_INFO_RX_RTS: This message indicates that the J1939 stack has received a Request to Send (RTS) control frame, signaling the start of a multi-frame transfer using the Transport Protocol (TP) or Extended Transport Protocol (ETP).
  - It informs the receiver that the sender is ready to transmit a multi-frame message and includes details about the total message size and the number of frames to be sent.
  - Statistics such as J1939_NLA_TOTAL_SIZE, J1939_NLA_PGN, J1939_NLA_SRC_NAME, and J1939_NLA_DEST_NAME are provided along with the J1939_EE_INFO_RX_RTS message, giving detailed information about the incoming transfer.
- J1939_EE_INFO_RX_DPO: This message indicates that the J1939 stack has received a Data Page Offset (DPO) control frame, which is part of the Extended Transport Protocol (ETP).
  - The DPO frame signals the continuation of an ETP multi-frame message by indicating the offset position in the data being transferred. It helps the receiver manage large data sets by identifying which portion of the message is being received.
  - It is typically paired with a corresponding SCM_TSTAMP_SCHED event on the sender side, which indicates when the next set of frames is scheduled for transmission.
  - This event includes statistics such as J1939_NLA_BYTES_ACKED, which tracks the number of bytes acknowledged up to that point in the session.
- J1939_EE_INFO_RX_ABORT: This message indicates that the reception of a multi-frame message (Transport Protocol or Extended Transport Protocol) has been aborted.
  - The abort can be triggered by protocol-level errors such as timeouts, an unexpected frame, or a specific abort request from the sender.
  - This message signals that the receiver cannot continue processing the transfer, and the session is terminated.
  - The corresponding error code is stored in serr->ee_data (session->err on kernel side ), providing further details about the reason for the abort, such as protocol violations or timeouts.
  - After receiving this message, the receiver discards the partially received frames, and the multi-frame session is considered incomplete.

In both cases, if SOF_TIMESTAMPING_OPT_ID is enabled, serr->ee_data will be set to the session’s unique identifier (session->tskey). This allows user space to track message transfers by their session identifier across multiple frames or stages.

In all other cases, serr->ee_errno will be set to ENOMSG, except for the J1939_EE_INFO_TX_ABORT and J1939_EE_INFO_RX_ABORT cases, where the kernel sets serr->ee_data to the error stored in session->err. All protocol-specific errors are converted to standard kernel error values and stored in session->err. These error values are unified across system calls and serr->ee_errno. Some of the known error values are described in the Error Codes in the J1939 Stack section.

When the J1939_EE_INFO_RX_RTS message is provided, it will include the following statistics for multi-frame messages (TP and ETP):

J1939_NLA_TOTAL_SIZE: Total size of the message in the session.

J1939_NLA_PGN: Parameter Group Number (PGN) identifying the message type.

J1939_NLA_SRC_NAME: 64-bit name of the source ECU.

J1939_NLA_DEST_NAME: 64-bit name of the destination ECU.

J1939_NLA_SRC_ADDR: 8-bit source address of the sending ECU.

J1939_NLA_DEST_ADDR: 8-bit destination address of the receiving ECU.

For other messages (including single-frame messages), only the following statistic is included:
- J1939_NLA_BYTES_ACKED: Number of bytes successfully acknowledged in the session.

The key flags for SO_TIMESTAMPING include:

SOF_TIMESTAMPING_OPT_ID: Enables the use of a unique session identifier (tskey) for each transfer. This identifier helps track message transfers and errors as distinct sessions in user space. When this option is enabled, serr->ee_data will be set to session->tskey.
SOF_TIMESTAMPING_OPT_CMSG: Sends timestamp information through control messages (struct scm_timestamping), allowing the application to retrieve timestamps alongside the data.
SOF_TIMESTAMPING_TX_SCHED: Provides the timestamp for when a message is scheduled for transmission (SCM_TSTAMP_SCHED).
SOF_TIMESTAMPING_TX_ACK: Provides the timestamp for when a message transmission is fully acknowledged (SCM_TSTAMP_ACK).
SOF_TIMESTAMPING_RX_SOFTWARE: Provides timestamps for reception-related events (e.g., J1939_EE_INFO_RX_RTS, J1939_EE_INFO_RX_DPO, J1939_EE_INFO_RX_ABORT).

These flags enable detailed monitoring of message lifecycles, including transmission scheduling, acknowledgments, reception timestamps, and gathering detailed statistics about the communication session, especially for multi-frame payloads like TP and ETP.

Example:

// Enable timestamping with various options, including session tracking and
// statistics
int sock_opt = SOF_TIMESTAMPING_OPT_CMSG |
               SOF_TIMESTAMPING_TX_ACK |
               SOF_TIMESTAMPING_TX_SCHED |
               SOF_TIMESTAMPING_OPT_ID |
               SOF_TIMESTAMPING_RX_SOFTWARE;

setsockopt(sock, SOL_SOCKET, SO_TIMESTAMPING, &sock_opt, sizeof(sock_opt));

Dynamic Addressing¶

Distinction has to be made between using the claimed address and doing an address claim. To use an already claimed address, one has to fill in the j1939.name member and provide it to bind(2). If the name had claimed an address earlier, all further messages being sent will use that address. And the j1939.addr member will be ignored.

An exception on this is PGN 0x0ee00. This is the “Address Claim/Cannot Claim Address” message and the kernel will use the j1939.addr member for that PGN if necessary.

To claim an address following code example can be used:

struct sockaddr_can baddr = {
        .can_family = AF_CAN,
        .can_addr.j1939 = {
                .name = name,
                .addr = J1939_IDLE_ADDR,
                .pgn = J1939_NO_PGN,    /* to disable bind() rx filter for PGN */
        },
        .can_ifindex = if_nametoindex("can0"),
};

bind(sock, (struct sockaddr *)&baddr, sizeof(baddr));

/* for Address Claiming broadcast must be allowed */
int value = 1;
setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value));

/* configured advanced RX filter with PGN needed for Address Claiming */
const struct j1939_filter filt[] = {
        {
                .pgn = J1939_PGN_ADDRESS_CLAIMED,
                .pgn_mask = J1939_PGN_PDU1_MAX,
        }, {
                .pgn = J1939_PGN_REQUEST,
                .pgn_mask = J1939_PGN_PDU1_MAX,
        }, {
                .pgn = J1939_PGN_ADDRESS_COMMANDED,
                .pgn_mask = J1939_PGN_MAX,
        },
};

setsockopt(sock, SOL_CAN_J1939, SO_J1939_FILTER, &filt, sizeof(filt));

uint64_t dat = htole64(name);
const struct sockaddr_can saddr = {
        .can_family = AF_CAN,
        .can_addr.j1939 = {
                .pgn = J1939_PGN_ADDRESS_CLAIMED,
                .addr = J1939_NO_ADDR,
        },
};

/* Afterwards do a sendto(2) with data set to the NAME (Little Endian). If the
 * NAME provided, does not match the j1939.name provided to bind(2), EPROTO
 * will be returned.
 */
sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr));

If no-one else contests the address claim within 250ms after transmission, the kernel marks the NAME-SA assignment as valid. The valid assignment will be kept among other valid NAME-SA assignments. From that point, any socket bound to the NAME can send packets.

If another ECU claims the address, the kernel will mark the NAME-SA expired. No socket bound to the NAME can send packets (other than address claims). To claim another address, some socket bound to NAME, must bind(2) again, but with only j1939.addr changed to the new SA, and must then send a valid address claim packet. This restarts the state machine in the kernel (and any other participant on the bus) for this NAME.

can-utils also include the j1939acd tool, so it can be used as code example or as default Address Claiming daemon.

Send Examples¶

Static Addressing¶

This example will send a PGN (0x12300) from SA 0x20 to DA 0x30.

Bind:

struct sockaddr_can baddr = {
        .can_family = AF_CAN,
        .can_addr.j1939 = {
                .name = J1939_NO_NAME,
                .addr = 0x20,
                .pgn = J1939_NO_PGN,
        },
        .can_ifindex = if_nametoindex("can0"),
};

bind(sock, (struct sockaddr *)&baddr, sizeof(baddr));

Now, the socket ‘sock’ is bound to the SA 0x20. Since no connect(2) was called, at this point we can use only sendto(2) or sendmsg(2).

Send:

const struct sockaddr_can saddr = {
        .can_family = AF_CAN,
        .can_addr.j1939 = {
                .name = J1939_NO_NAME;
                .addr = 0x30,
                .pgn = 0x12300,
        },
};

sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr));

Error Codes in the J1939 Stack¶

This section lists all potential kernel error codes that can be exposed to user space when interacting with the J1939 stack. It includes both standard error codes and those derived from protocol-specific abort codes.

EAGAIN: Operation would block; retry may succeed. One common reason is that an active TP or ETP session exists, and an attempt was made to start a new overlapping TP or ETP session between the same peers.
ENETDOWN: Network is down. This occurs when the CAN interface is switched to the “down” state.
ENOBUFS: No buffer space available. This error occurs when the CAN interface’s transmit (TX) queue is full, and no more messages can be queued.
EOVERFLOW: Value too large for defined data type. In J1939, this can happen if the requested data lies outside of the queued buffer. For example, if a CTS (Clear to Send) requests an offset not available in the kernel buffer because user space did not provide enough data.
EBUSY: Device or resource is busy. For example, this occurs if an identical session is already active and the stack is unable to recover from the condition.
EACCES: Permission denied. This error can occur, for example, when attempting to send broadcast messages, but the socket is not configured with SO_BROADCAST.
EADDRNOTAVAIL: Address not available. This error occurs in cases such as:
- When attempting to use getsockname(2) to retrieve the peer’s address, but the socket is not connected.
- When trying to send data to or from a NAME, but address claiming for the NAME was not performed or detected by the stack.
EBADFD: File descriptor in bad state. This error can occur if:
- Attempting to send data to an unbound socket.
- The socket is bound but has no source name, and the source address is J1939_NO_ADDR.
- The can_ifindex is incorrect.
EFAULT: Bad address. Occurs mostly when the stack can’t copy from or to a sockptr, when there is insufficient data from user space, or when the buffer provided by user space is not large enough for the requested data.
EINTR: A signal occurred before any data was transmitted; see signal(7).
EINVAL: Invalid argument passed. For example:
- msg->msg_namelen is less than J1939_MIN_NAMELEN.
- addr->can_family is not equal to AF_CAN.
- An incorrect PGN was provided.
ENODEV: No such device. This happens when the CAN network device cannot be found for the provided can_ifindex or if can_ifindex is 0.
ENOMEM: Out of memory. Typically related to issues with memory allocation in the stack.
ENOPROTOOPT: Protocol not available. This can occur when using getsockopt(2) or setsockopt(2) if the requested socket option is not available.
EDESTADDRREQ: Destination address required. This error occurs:
- In the case of connect(2), if the struct sockaddr *uaddr is NULL.
- In the case of send*(2), if there is an attempt to send an ETP message to a broadcast address.
EDOM: Argument out of domain. This error may happen if attempting to send a TP or ETP message to a PGN that is reserved for control PGNs for TP or ETP operations.
EIO: I/O error. This can occur if the amount of data provided to the socket for a TP or ETP session does not match the announced amount of data for the session.
ENOENT: No such file or directory. This can happen when the stack attempts to transfer CTS or EOMA but cannot find a matching receiving socket anymore.
ENOIOCTLCMD: No ioctls are available for the socket layer.
EPERM: Operation not permitted. For example, this can occur if a requested action requires CAP_NET_ADMIN privileges.
ENETUNREACH: Network unreachable. Most likely, this occurs when frames cannot be transmitted to the CAN bus.
ETIME: Timer expired. This can happen if a timeout occurs while attempting to send a simple message, for example, when an echo message from the controller is not received.
EPROTO: Protocol error.
- Used for various protocol-level errors in J1939, including:
  - Duplicate sequence number.
  - Unexpected EDPO or ECTS packet.
  - Invalid PGN or offset in EDPO/ECTS.
  - Number of EDPO packets exceeded CTS allowance.
  - Any other protocol-level error.
EMSGSIZE: Message too long.
ENOMSG: No message available.
EALREADY: The ECU is already engaged in one or more connection-managed sessions and cannot support another.
EHOSTUNREACH: A timeout occurred, and the session was aborted.
EBADMSG: CTS (Clear to Send) messages were received during an active data transfer, causing an abort.
ENOTRECOVERABLE: The maximum retransmission request limit was reached, and the session cannot recover.
ENOTCONN: An unexpected data transfer packet was received.
EILSEQ: A bad sequence number was received, and the software could not recover.

The Linux Kernel

Contents

This Page

J1939 Documentation¶

Overview / What Is J1939¶

Specifications used¶

Motivation¶

J1939 concepts¶

Data Sent to the J1939 Stack¶

PGN¶

Addressing¶

Filtering¶

How to Use J1939¶

API Calls¶

recvmsg(2)¶

setsockopt(2)¶

`SO_J1939_FILTER`¶

`SO_J1939_PROMISC`¶

`SO_BROADCAST`¶

`SO_J1939_SEND_PRIO`¶

`SO_J1939_ERRQUEUE`¶

`SO_TIMESTAMPING`¶

Dynamic Addressing¶

Send Examples¶

Static Addressing¶

Error Codes in the J1939 Stack¶

29 bit CAN-ID
Bit positions within the CAN-ID
28 ... 26	25 ... 8	7 ... 0
Priority	PGN	SA (Source Address)

PGN
Bit positions within the CAN-ID
25	24	23 ... 16	15 ... 8
R (Reserved)	DP (Data Page)	PF (PDU Format)	PS (PDU Specific)

J1939 Documentation¶

Overview / What Is J1939¶

Specifications used¶

Motivation¶

J1939 concepts¶

Data Sent to the J1939 Stack¶

PGN¶

Addressing¶

Filtering¶

How to Use J1939¶

API Calls¶

Message Flags during send() and Related System Calls¶

recvmsg(2)¶

setsockopt(2)¶

SO_J1939_FILTER¶

SO_J1939_PROMISC¶

SO_BROADCAST¶

SO_J1939_SEND_PRIO¶

SO_J1939_ERRQUEUE¶

SO_TIMESTAMPING¶

Dynamic Addressing¶

Send Examples¶

Static Addressing¶

Error Codes in the J1939 Stack¶

`SO_J1939_FILTER`¶

`SO_J1939_PROMISC`¶

`SO_BROADCAST`¶

`SO_J1939_SEND_PRIO`¶

`SO_J1939_ERRQUEUE`¶

`SO_TIMESTAMPING`¶