There may be zero or several fields that follow the actual data. These comprise the trailer. In Fig. 23.3.1, the trailer consists of one field that contains a checksum which allows the receiver to determine that the packet was not altered in transmission. Error detecting and error correcting codes used in these checksums form a fascinating subfield of networking and are supported by well-known mathematical theory. We can but mention a few highlights.

The simplest form of error detecting code is parity, which adds a 1 or a 0 in order to make the number of 1s either even or odd, depending upon whether even parity or odd parity is desired. Parity can detect single errors, i.e. occurrences of a single bit in a packet being changed from 1 to 0 or 0 to 1, but it cannot fix such an error. Nor can it detect a double bit error, although it can detect a triple or any odd-numbered error. Thus parity is quite limited.