This part of ISO 10681 specifies the requirements for a communication protocol tailored to meet the requirements of FlexRay-based vehicle network systems as specified in the FlexRay Communications Systems Protocol Specification. As the communication protocol combines the network layer and transport layer functionality (OSI layers 3 and 4), this part of ISO 10681 does not explicitly distinguish between these layer services. The technical features of this communication protocol are as follows: - transmit messages with known data length; - transmit messages with unknown but finite data length; - additional acknowledgement with retry mechanism; - routing data on the fly; - support of dynamic frame length.