TCPSEGMENT

A TCPSegment, commonly called a TCP segment, is the unit of data exchanged by the Transmission Control Protocol (TCP) within the TCP/IP suite. Each segment consists of a TCP header and an optional payload. TCP is a connection-oriented, reliable protocol that provides ordered delivery, flow control, and congestion management for transported data.

The TCP header includes several essential fields: source port and destination port identify the communicating endpoints; sequence number and acknowledgment number enable in-order delivery and reliable retransmission; data offset indicates the header length; reserved bits are unused; control flags (including NS, CWR, ECE, URG, ACK, PSH, RST, SYN, FIN) convey segment state; window size implements flow control; checksum ensures data integrity for the header and payload; urgent pointer marks urgent data; and options with padding may extend the header. The baseline header is 20 bytes; options can lengthen it.

During connection setup, a three-way handshake (SYN, SYN-ACK, ACK) establishes a TCP connection. After establishment, data is transmitted in segments, each carrying a sequence number. The receiver sends an acknowledgment with the ACK flag set and an acknowledgment number equal to the next expected byte. If segments are lost or corrupted, retransmission occurs. Flow control is achieved through the window size, which advertises available receive buffer space.

On receipt, segments are reassembled into a continuous byte stream. Out-of-order segments are buffered until missing ones arrive; duplicates are discarded. The TCP checksum covers the segment header, data, and a pseudo-header derived from IP information to detect corruption. Segment size is bounded by the maximum segment size (MSS); network layers may further constrain transmission through fragmentation, though TCP typically aims to avoid internal fragmentation.