initial sequence number (TCP) (ISN)

The TCP protocol works with a sequence number that determines the order of the data packets. When establishing a connection, the sending station calculates the first sequence number according to a certain rule, this is the Initial Sequence Number ( ISN).

The receiving station in turn determines its own Initial Sequence Number, which it manages itself, as does the sending station. The first data packet transmitted by the sending station has the following Initial Sequence Number ISN+1.

The Initial Sequence Number is a 32-bitdigit that is incremented with a clock signal. The clock time is 4 microseconds and increments the ISN digit by 1. With 32 bits, the maximum number is 4,294,967,295. At this digit, the reset to zero occurs.