As previously stated, TCP is considered a reliable transport protocol, which means that TCP includes processes to ensure reliable delivery between applications through the use of acknowledged delivery. TCP is analogous to sending packages that are tracked from source to destination. If a FedEx order is broken up into several shipments, a customer can check online to see the order of the delivery.
With TCP, the three basic operations of reliability are:
- Tracking transmitted data segments
- Acknowledging received data
- Retransmitting any unacknowledged data
TCP breaks up a message into small pieces known as segments. The segments are numbered in sequence and passed to the IP process for assembly into packets. TCP keeps track of the number of segments that have been sent to a specific host from a specific application. If the sender does not receive an acknowledgement within a certain period of time, it assumes that the segments were lost and retransmits them. Only the portion of the message that is lost is resent, not the entire message. On the receiving host, TCP is responsible for reassembling the message segments and passing them to the application. The File Transfer Protocol (FTP) and the Hypertext Transfer Protocol (HTTP) are examples of applications that use TCP to ensure data delivery.
Click the Play button in the figure to see an animation of TCP segments being transmitted from sender to receiver.
These reliability processes place additional overhead on network resources due to the processes of acknowledgement, tracking, and retransmission. To support these reliability processes, more control data is exchanged between the sending and receiving hosts. This control information is contained in a TCP header.