A more specialized definition of state is used for computer programs that operate serially or sequentially on streams of data, such as parsers, firewalls, communication protocols and encryption. Serial programs operate on the incoming data characters or packets sequentially, one at a time. In some of these programs, information about previous data characters or packets received is stored in variables and used to affect the processing of the current character or packet. This is called a stateful protocol and the data carried over from the previous processing cycle is called the "state". In others, the program has no information about the previous data stream and starts fresh with each data input; this is called a stateless protocol.