ISO 15765-2 (ISO-TP)

Overview

ISO 15765-2, also known as ISO-TP, is a transport protocol specifically defined for diagnostic communication on CAN. It is widely used in the automotive industry, for example as the transport protocol for UDSonCAN (ISO 14229-3) or emission-related diagnostic services (ISO 15031-5).

ISO-TP can be used both on CAN CC (aka Classical CAN) and CAN FD (CAN with Flexible Datarate) based networks. It is also designed to be compatible with a CAN network using SAE J1939 as data link layer (however, this is not a requirement).

Specifications used

  • ISO 15765-2:2024 : Road vehicles - Diagnostic communication over Controller Area Network (DoCAN). Part 2: Transport protocol and network layer services.

Addressing

In its simplest form, ISO-TP is based on two kinds of addressing modes for the nodes connected to the same network:

  • physical addressing is implemented by two node-specific addresses and is used in 1-to-1 communication.

  • functional addressing is implemented by one node-specific address and is used in 1-to-N communication.

Three different addressing formats can be employed:

  • “normal” : each address is represented simply by a CAN ID.

  • “extended”: each address is represented by a CAN ID plus the first byte of the CAN payload; both the CAN ID and the byte inside the payload shall be different between two addresses.

  • “mixed”: each address is represented by a CAN ID plus the first byte of the CAN payload; the CAN ID is different between two addresses, but the additional byte is the same.

Transport protocol and associated frame types

When transmitting data using the ISO-TP protocol, the payload can either fit inside one single CAN message or not, also considering the overhead the protocol is generating and the optional extended addressing. In the first case, the data is transmitted at once using a so-called Single Frame (SF). In the second case, ISO-TP defines a multi-frame protocol, in which the sender provides (through a First Frame - FF) the PDU length which is to be transmitted and also asks for a Flow Control (FC) frame, which provides the maximum supported size of a macro data block (blocksize) and the minimum time between the single CAN messages composing such block (stmin). Once this information has been received, the sender starts to send frames containing fragments of the data payload (called Consecutive Frames - CF), stopping after every blocksize-sized block to wait confirmation from the receiver which should then send another Flow Control frame to inform the sender about its availability to receive more data.

How to Use ISO-TP

As with others CAN protocols, the ISO-TP stack support is built into the Linux network subsystem for the CAN bus, aka. Linux-CAN or SocketCAN, and thus follows the same socket API.

Creation and basic usage of an ISO-TP socket

To use the ISO-TP stack, #include <linux/can/isotp.h> shall be used. A socket can then be created using the PF_CAN protocol family, the SOCK_DGRAM type (as the underlying protocol is datagram-based by design) and the CAN_ISOTP protocol:

s = socket(PF_CAN, SOCK_DGRAM, CAN_ISOTP);

After the socket has been successfully created, bind(2) shall be called to bind the socket to the desired CAN interface; to do so:

  • a TX CAN ID shall be specified as part of the sockaddr supplied to the call itself.

  • a RX CAN ID shall also be specified, unless broadcast flags have been set through socket option (explained below).

Once bound to an interface, the socket can be read from and written to using the usual read(2) and write(2) system calls, as well as send(2), sendmsg(2), recv(2) and recvmsg(2). Unlike the CAN_RAW socket API, only the ISO-TP data field (the actual payload) is sent and received by the userspace application using these calls. The address information and the protocol information are automatically filled by the ISO-TP stack using the configuration supplied during socket creation. In the same way, the stack will use the transport mechanism when required (i.e., when the size of the data payload exceeds the MTU of the underlying CAN bus).

The sockaddr structure used for SocketCAN has extensions for use with ISO-TP, as specified below:

struct sockaddr_can {
    sa_family_t can_family;
    int         can_ifindex;
    union {
        struct { canid_t rx_id, tx_id; } tp;
    ...
    } can_addr;
}

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

  • can_addr.tp.rx_id specifies the receive (RX) CAN ID and will be used as a RX filter.

  • can_addr.tp.tx_id specifies the transmit (TX) CAN ID

ISO-TP socket options

When creating an ISO-TP socket, reasonable defaults are set. Some options can be modified with setsockopt(2) and/or read back with getsockopt(2).

General options

General socket options can be passed using the CAN_ISOTP_OPTS optname:

struct can_isotp_options opts;
ret = setsockopt(s, SOL_CAN_ISOTP, CAN_ISOTP_OPTS, &opts, sizeof(opts))

where the can_isotp_options structure has the following contents:

struct can_isotp_options {
    u32 flags;
    u32 frame_txtime;
    u8  ext_address;
    u8  txpad_content;
    u8  rxpad_content;
    u8  rx_ext_address;
};

  • flags: modifiers to be applied to the default behaviour of the ISO-TP stack. Following flags are available:

    • CAN_ISOTP_LISTEN_MODE: listen only (do not send FC frames); normally used as a testing feature.

    • CAN_ISOTP_EXTEND_ADDR: use the byte specified in ext_address as an additional address component. This enables the “mixed” addressing format if used alone, or the “extended” addressing format if used in conjunction with CAN_ISOTP_RX_EXT_ADDR.

    • CAN_ISOTP_TX_PADDING: enable padding for transmitted frames, using txpad_content as value for the padding bytes.

    • CAN_ISOTP_RX_PADDING: enable padding for the received frames, using rxpad_content as value for the padding bytes.

    • CAN_ISOTP_CHK_PAD_LEN: check for correct padding length on the received frames.

    • CAN_ISOTP_CHK_PAD_DATA: check padding bytes on the received frames against rxpad_content; if CAN_ISOTP_RX_PADDING is not specified, this flag is ignored.

    • CAN_ISOTP_HALF_DUPLEX: force ISO-TP socket in half duplex mode (that is, transport mechanism can only be incoming or outgoing at the same time, not both).

    • CAN_ISOTP_FORCE_TXSTMIN: ignore stmin from received FC; normally used as a testing feature.

    • CAN_ISOTP_FORCE_RXSTMIN: ignore CFs depending on rx stmin; normally used as a testing feature.

    • CAN_ISOTP_RX_EXT_ADDR: use rx_ext_address instead of ext_address as extended addressing byte on the reception path. If used in conjunction with CAN_ISOTP_EXTEND_ADDR, this flag effectively enables the “extended” addressing format.

    • CAN_ISOTP_WAIT_TX_DONE: wait until the frame is sent before returning from write(2) and send(2) calls (i.e., blocking write operations).

    • CAN_ISOTP_SF_BROADCAST: use 1-to-N functional addressing (cannot be specified alongside CAN_ISOTP_CF_BROADCAST).

    • CAN_ISOTP_CF_BROADCAST: use 1-to-N transmission without flow control (cannot be specified alongside CAN_ISOTP_SF_BROADCAST). NOTE: this is not covered by the ISO 15765-2 standard.

    • CAN_ISOTP_DYN_FC_PARMS: enable dynamic update of flow control parameters.

  • frame_txtime: frame transmission time (defined as N_As/N_Ar inside the ISO standard); if 0, the default (or the last set value) is used. To set the transmission time to 0, the CAN_ISOTP_FRAME_TXTIME_ZERO macro (equal to 0xFFFFFFFF) shall be used.

  • ext_address: extended addressing byte, used if the CAN_ISOTP_EXTEND_ADDR flag is specified.

  • txpad_content: byte used as padding value for transmitted frames.

  • rxpad_content: byte used as padding value for received frames.

  • rx_ext_address: extended addressing byte for the reception path, used if the CAN_ISOTP_RX_EXT_ADDR flag is specified.

Flow Control options

Flow Control (FC) options can be passed using the CAN_ISOTP_RECV_FC optname to provide the communication parameters for receiving ISO-TP PDUs.

struct can_isotp_fc_options fc_opts;
ret = setsockopt(s, SOL_CAN_ISOTP, CAN_ISOTP_RECV_FC, &fc_opts, sizeof(fc_opts));

where the can_isotp_fc_options structure has the following contents:

struct can_isotp_options {
    u8 bs;
    u8 stmin;
    u8 wftmax;
};

  • bs: blocksize provided in flow control frames.

  • stmin: minimum separation time provided in flow control frames; can have the following values (others are reserved):

    • 0x00 - 0x7F : 0 - 127 ms

    • 0xF1 - 0xF9 : 100 us - 900 us

  • wftmax: maximum number of wait frames provided in flow control frames.

Transmission stmin

The transmission minimum separation time (stmin) can be forced using the CAN_ISOTP_TX_STMIN optname and providing an stmin value in microseconds as a 32bit unsigned integer; this will overwrite the value sent by the receiver in flow control frames:

uint32_t stmin;
ret = setsockopt(s, SOL_CAN_ISOTP, CAN_ISOTP_TX_STMIN, &stmin, sizeof(stmin));

Reception stmin

The reception minimum separation time (stmin) can be forced using the CAN_ISOTP_RX_STMIN optname and providing an stmin value in microseconds as a 32bit unsigned integer; received Consecutive Frames (CF) which timestamps differ less than this value will be ignored:

uint32_t stmin;
ret = setsockopt(s, SOL_CAN_ISOTP, CAN_ISOTP_RX_STMIN, &stmin, sizeof(stmin));

Multi-frame transport support

The ISO-TP stack contained inside the Linux kernel supports the multi-frame transport mechanism defined by the standard, with the following constraints:

  • the maximum size of a PDU is defined by a module parameter, with an hard limit imposed at build time.

  • when a transmission is in progress, subsequent calls to write(2) will block, while calls to send(2) will either block or fail depending on the presence of the MSG_DONTWAIT flag.

  • no support is present for sending “wait frames”: whether a PDU can be fully received or not is decided when the First Frame is received.

Errors

Following errors are reported to userspace:

RX path errors

-ETIMEDOUT

timeout of data reception

-EILSEQ

sequence number mismatch during a multi-frame reception

-EBADMSG

data reception with wrong padding

TX path errors

-ECOMM

flow control reception timeout

-EMSGSIZE

flow control reception overflow

-EBADMSG

flow control reception with wrong layout/padding

Examples

Basic node example

Following example implements a node using “normal” physical addressing, with RX ID equal to 0x18DAF142 and a TX ID equal to 0x18DA42F1. All options are left to their default.

int s;
struct sockaddr_can addr;
int ret;

s = socket(PF_CAN, SOCK_DGRAM, CAN_ISOTP);
if (s < 0)
    exit(1);

addr.can_family = AF_CAN;
addr.can_ifindex = if_nametoindex("can0");
addr.tp.tx_id = 0x18DA42F1 | CAN_EFF_FLAG;
addr.tp.rx_id = 0x18DAF142 | CAN_EFF_FLAG;

ret = bind(s, (struct sockaddr *)&addr, sizeof(addr));
if (ret < 0)
    exit(1);

/* Data can now be received using read(s, ...) and sent using write(s, ...) */

Additional examples

More complete (and complex) examples can be found inside the isotp* userland tools, distributed as part of the can-utils utilities at: https://github.com/linux-can/can-utils