ISO-TP

Protocol Basics (ISO 15765-2)

ISO 15765-2, also known as ISO-TP (ISO-15765 Transport Protocol), is a communication protocol used in the automotive industry to transmit data over a Controller Area Network (CAN) bus. It is designed to provide a reliable and efficient way to transfer large amounts of data, such as software updates and diagnostic data, over the CAN bus.

The protocol defines a set of rules for the data transfer, including the format of the data frames, the flow control mechanisms, and the error handling. The data frames are divided into smaller segments, called segments, which are transmitted over the CAN bus. The protocol also defines a set of flow control mechanisms, such as flow control frames, to handle situations where the receiver’s buffer is full.

ISO 15765-2 uses a multi-frame format, where a large message is divided into multiple smaller frames and sent over the bus. Each frame is identified by a unique ID, which allows the receiver to reassemble the frames in the correct order and to detect missing or duplicate frames.

Summary

• Transport protocol for data transfer over CAN, FlexRay, LIN and MOST

• Point-to-point communication

• Up to \(2^{32}-1\) byte payloads

• Flow-Control management

• No re-transmission functionality

Protocol Control Information (PCI)

• ISO-TP defines four special frame type

  • Frame type 0: SF (Single Frame)

  • Frame type 1: FF (First Frame)

  • Frame type 2: CF (Consecutive Frame)

  • Frame type 3: FC (Flow Control Frame)

Fig. 39 Transport Layer (ISO-TP) frame types

ISOTP-addressing

The ISO-TP standard describes three different kinds of addressing: normal, extended, and mixed addressing[ISOCSecretary16a]. Dependent on the addressing mode, the AI (Address Information) is encoded in different fields of a CAN frame. Also, the position of the protocol control information field varies, depending on the used addressing scheme. For further references in this paper, all addressing schemes will be identified with a label from the table following.

Table 1 Overview of ISOTP addressing schemes with labels for internal reference.

Ref.	Description
A1	Normal addressing, 11-bit CAN identifier
A2	Normal fixed addressing, 29-bit CAN identifier
A3	Extended addressing, 11-bit CAN identifier
A4	Mixed addressing with 29-bit CAN identifier
A5	Mixed addressing with 11-bit CAN identifier

FC structure

Flow-Control status (FS) codes:

• 0 = Clear to send

• 1 = Wait

• 2 = Overflow

Fig. 40 Summary of flow control messages in 11-bit CAN identifier (CAN Id.) frames for normal (A1), extended (A3), and mixed addressing (A5) mode. Address Extension (AE)

Fig. 41 Summary of flow control messages in 29-bit CAN identifier frames for normal fixed (A2) and mixed (A4) addressing mode with physical and functional addressing types.

0x18DA = Const: Normal fixed addressing, physical 0x18DB = Const: Normal fixed addressing, functional 0x18CE = Const: Mixed addressing, physical 0x18CD = Const: Mixed addressing, functional

ISOTP addressing complexities

Table 2 Overview of complexities per ISOTP addressing scheme

Ref.
A1	\(2^{11}\)	\(2048\)
A2	\(2 \times 2^8 \times 2^8 = 2^{17}\)	\(131072\)
A3	\(2^{11} \times 2^8 = 2^{19}\)	\(524288\)
A4	\(2 \times 2^8 \times 2^8 \times 2^8 = 2^{25}\)	\(33554432\)
A5	\(2^{11} \times 2^8 = 2^{19}\)	\(524288\)

FC examples

Fig. 42 Communication example of addressing scheme A1 captured with candump.

Fig. 43 Communication example of addressing scheme A3 or A5 captured with candump.

ISOTP communication

Fig. 44 ISO-TP fragmented communication

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[ISOCSecretary16]

ISO Central Secretary. Road vehicles – Diagnostic communication over Controller Area Network (DoCAN) – Part 2: Transport protocol and network layer services. Standard ISO 15765-2:2016, International Organization for Standardization, Geneva, CH, 2016. URL: https://www.iso.org/standard/66574.html.