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RFC 793 somewhat redundantly describes TCP as a “connection-oriented, end-to-end reliable protocol.” TCP provides the following features:

 Reliable delivery

 Ordering

 Error control

 Flow control

 Congestion avoidance

The phrase “connection oriented” refers to TCP's attempt to simulate the properties of a physical connection. Sound familiar? An ideal connection provides reliability and order. The data the sender sends is exactly what the receiver receives and in the same order.

TCP guarantees both order and reliability through the use of sequence numbers and acknowledgments. The sender marks each TCP segment with a sequence number that increments in a predictable fashion. This allows the receiver to reassemble the segments in the correct order should they arrive out of order. The receiver sends an acknowledgment (ACK) in response to each segment, indicating the sequence number of the segment it received. If the sender fails to receive an ACK after a period of time, it will retransmit the segment.