|  | 12.2 | TCP and UDP
| 12.2.1 | TCP
Transmission Control Protocol (TCP) is a connection-oriented Layer 4 (transport layer) protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack. - Following are the definitions of the fields in the TCP segment: source port -- number of the calling port destination port -- number of the called port sequence number -- number used to ensure correct sequencing of the arriving data acknowledgment number - next expected TCP octet HLEN -- number of 32-bit words in the header reserved -- set to zero code bits -- control functions (such as setup and termination of a session) window -- number of octets that the sender is willing to accept checksum -- calculated checksum of the header and data fields urgent pointer -- indicates the end of the urgent data option-one option -- maximum TCP segment size data -- upper-layer protocol data