TCP offers reliability by providing connection-oriented, end-to-end reliable packet delivery through an internetwork. It does this by sequencing bytes with a forwarding acknowledgment number that indicates to the destination the next byte the source expects to receive. Bytes not acknowledged within a specified time period are retransmitted. The reliability mechanism of TCP allows devices to deal with lost, delayed, duplicate, or misread packets. A time-out mechanism allows devices to detect lost packets and request retransmission.
TCP offers efficient flow control, which means that, when sending acknowledgments back to the source, the receiving TCP process indicates the highest sequence number it can receive without overflowing its internal buffers.
Full-duplex operation means that TCP processes can both send and receive at the same time.
TCP Packet Format
source Port and Destination Port—Identifies points at which upper-layer source and destination processes receive TCP services.
Sequence Number—Usually specifies the number assigned to the first byte of data in the current message. In the connection-establishment phase, this field also can be used to identify an initial sequence number to be used in an upcoming transmission.
Acknowledgment Number—Contains the sequence number of the next byte of data the sender of the packet expects to receive.
Data Offset—Indicates the number of 32-bit words in the TCP header.
Reserved—Remains reserved for future use.
Flags—Carries a variety of control information, including the SYN and ACK bits used for connection establishment, and the FIN bit used for connection termination.
Window—Specifies the size of the sender's receive window (that is, the buffer space available for incoming data).
Checksum—Indicates whether the header was damaged in transit.
Urgent Pointer—Points to the first urgent data byte in the packet.
Options—Specifies various TCP options.
Data—Contains upper-layer information.