# 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)

The ISO-TP standard describes three different kinds of addressing: normal, extended, and mixed addressing. 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

A2

Normal fixed addressing, 29-bit CAN identifier

A3

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

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

$$2^{11} \times 2^8 = 2^{19}$$

$$524288$$

## ISOTP communication#

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.