TCP Features : Reliability

One of the most important features of TCP is reliable end-to-end data
delivery. In order to provide reliability, TCP must recover from data
that is damaged, lost, duplicated, or delivered out of order by the
Network Layer. TCP uses the Positive Acknowledgment Retransmission
(PAR) scheme for achieving reliability.

TCP implements PAR by assigning a sequence number to each octet that
is transmitted and by requiring a positive acknowledgment (ACK) from
the receiving TCP module. If the ACK is not received within a time-out
interval, the data is retransmitted. At the receiver TCP module, the
sequence numbers are used to correctly order segments that may have
arrived out of order and to eliminate duplicates. Corruption of data
is detected by using a checksum field filed in the TCP packet header.
Data segments that are received with a bad checksum field are
discarded. Unless a physical break in the link causes physical
partitioning of the network, TCP recovers from most Internet
communications system errors.