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Initial sequence numbers (ISN) refers to the unique 32-bit sequence number assigned to each new connection on a Transmission Control Protocol (TCP)-based data communication. It helps with the allocation of a sequence number that does not conflict with other data bytes transmitted over a TCP connection. An ISN is unique to each connection and separated by each device.
An ISN is designed to randomly select a sequence number for the first byte of data transmitted in a new TCP connection. The ISN can be any number from 0 to 4,294,967,295. Each byte can select any ISN unless it is in use not by a current connection.
The TCP protocol assigns an ISN to each new byte, beginning with 0 and incrementally adding a number every four seconds until the limit is exhausted. In continuous communication, it takes up to four hours to consume all available ISN options. Thus, when the TCP returns to the beginning, it generally starts with ISN options that are released from the completed/closed connections.