TCP number all data bytes that are transmitted in a connection. Numbering is independent in each direction when TCP receives bytes of data a process. It stores them in the sending buffer and number them. The numbering does not necessarily start from0. Instead TCP generated a random number between 0 and 232-1 for the number of the first byte for example if the random numbers happens to be 1057 and the total data to be sent are 6000 bytes, are numbered from 1057 and 7056.
The bytes of data being transferred in each connection are numbered by TCP. The numbering starts with a randomly generated number.