J1939 transport protocol breaks up PGs larger than 8 data bytes and up to 1785 bytes, into multiple packets. The transport protocol defines the rules for packaging, transmitting, and reassembling the data.
Messages that have multiple packets are transmitted with a dedicated PGN, and have the same message ID and similar functionality.
The length of each message in the packet must be 8 bytes or fewer.
The first byte in the data field of a message specifies the sequence of the message (one to 255) and the next seven bytes contain the original data.
All unused bytes in the data field are set to zero.
A different PGN controls the message flow.
The data package is passed to the application layer after it is reassembled in the order specified by the first data-field byte.