ISO 15765-2,[1] or ISO-TP (Transport Layer), is an international standard for sending data packets over a CAN-Bus.
The protocol allows for the transport of messages that exceed the eight byte maximum payload of CAN frames.
ISO-TP segments longer messages into multiple frames, adding metadata (CAN-TP Header) that allows the interpretation of individual frames and reassembly into a complete message packet by the recipient.
For clarity the protocol description below is based on Normal Addressing with eight byte CAN frames.
The initial field is four bits indicating the frame type, and implicitly describing the PCI length.
With the 0 in the type field, this can also pass as a simpler protocol with a length-data format and is often misinterpreted as such.
Even careful implementations may fail to account for the minor effect of bit-stuffing in the physical layer.
The 12 bit length field (in the FF) allows up to 4095 bytes of user data in a segmented message, but in practice the typical application-specific limit is considerably lower because of receive buffer or hardware limitations.