Any bit sequence can be transmitted in the data field. The character set of the applied code is used as a basis. However, the bit sequence can be structured completely independently of a fixed character structure.
Known data fields used in IP-based protocols are the address field( AF) and the source address field, the type field and option field, the version field, time-to-live field, next header field, priority field and the checksum field.
The size of the data fields is protocol dependent and varies from protocol to protocol. It depends entirely on the functions and mechanisms that the protocol is intended to perform.