A Transport Layer protocol in the TCP/IP protocol suite, TCP is a connection-oriented protocol designed to provide reliable transmission across inherently unreliable Internet Protocol (IP) networks such as the Internet. Defined in IETF RFC 793,TCP evolved from the ARPANET Network Control Protocol (NCP), which was developed to provide reliable transmission across the analog links of unshielded twisted pair (UTP) and packet radio (e.g., AlohaNet). TCP is a connection-oriented protocol that employs virtual circuits in support of byte-stream-oriented communications. TCP provides for file segmentation into packets prior to transmission, and for reassembly upon receipt.TCP also provides for packet sequencing, end-to-end flow control, and error control, thereby guaranteeing delivery. Each packet in a stream of packets received by the destination device is either acknowledged as having been received correctly, or is retransmitted. The standard size of the TCP header is 20 octets, as illustrated in Figure T-2, although 4 additional octets may be used to accommodate options.The header fields are as follows: