TCP (Transmission Control Protocol)

The Transmission Control Protocol (TCP) is a connection-oriented, stateful protocol that ensures data sent from one application is delivered completely, in order, and without corruption to the receiving application. It operates at the transport layer (Layer 4) of the OSI and TCP/IP models, sitting directly above the Internet Protocol (IP). Unlike its connectionless counterpart, UDP (User Datagram Protocol), TCP establishes a formal connection through a three-way handshake (SYN, SYN-ACK, ACK) before data transfer begins and meticulously manages the session until it is gracefully terminated.

TCP's reliability is achieved through several key mechanisms. It uses sequence numbers and acknowledgment (ACK) packets to confirm data receipt and enable retransmission of lost segments. Flow control, managed via a sliding window, prevents a fast sender from overwhelming a slow receiver. Congestion control algorithms, such as TCP Reno or CUBIC, dynamically adjust the data transmission rate based on network conditions to avoid overloading the network path. These features make TCP the protocol of choice for applications where data integrity is paramount, such as web browsing (HTTP/HTTPS), email (SMTP, IMAP), and file transfers (FTP).

A TCP connection is identified by a socket pair, consisting of the source and destination IP addresses and port numbers. The protocol header includes fields for the aforementioned sequence and acknowledgment numbers, window size, and control flags (like SYN, ACK, FIN, RST). While TCP's guarantees come with overhead in latency and bandwidth due to handshakes and acknowledgments, this trade-off is essential for reliable communication. Its design is foundational to the modern internet, enabling the stable, long-lived connections that underpin most of our daily online interactions.

The Transmission Control Protocol (TCP) is a core protocol of the Internet protocol suite that provides reliable, ordered, and error-checked delivery of a byte stream between applications running on hosts communicating via an IP network. It is connection-oriented, meaning a logical connection must be established between two endpoints before data transfer can begin. This foundational reliability makes TCP the protocol of choice for critical applications like web browsing (HTTP/HTTPS), email (SMTP), and file transfers, where data integrity is paramount.

Connection establishment is achieved through a process known as the three-way handshake. This sequence synchronizes sequence numbers and negotiates connection parameters. First, the client sends a SYN (synchronize) packet to the server. The server responds with a SYN-ACK (synchronize-acknowledge) packet. Finally, the client sends an ACK (acknowledge) packet back to the server. Only after this handshake is complete is the bidirectional communication channel considered open and ready for data transfer, ensuring both sides are prepared to send and receive.

Once connected, TCP manages data flow using a combination of sequence numbers, acknowledgments, and a sliding window. Each byte of data is assigned a sequence number. The receiver sends ACK packets to confirm receipt of data, allowing the sender to know which data has been successfully delivered. The sliding window protocol controls the flow of data by dictating how many bytes can be sent before waiting for an acknowledgment, optimizing throughput and preventing the receiver from being overwhelmed. This mechanism is central to TCP's congestion control algorithms.

For reliability, TCP employs error detection via checksums and retransmission of lost packets. If a sender does not receive an acknowledgment for a sent segment within a calculated timeout period, it assumes the packet was lost and retransmits it. Duplicate packets are discarded by the receiver using sequence numbers. This guarantee of delivery, while introducing some overhead and latency, is what distinguishes TCP from connectionless protocols like UDP (User Datagram Protocol).

Connection termination uses a modified handshake, often called a four-way handshake. One side initiates closure by sending a FIN packet. The other side acknowledges this FIN and may continue sending its own data until it also sends a FIN. Each FIN must be acknowledged by an ACK. This graceful termination ensures that all data in flight is properly delivered before the connection resources are freed, preventing data loss during shutdown.

The Transmission Control Protocol (TCP) header is a 20-byte (minimum) to 60-byte data structure that precedes the payload in every TCP segment, containing the control information necessary for establishing connections, managing data flow, and ensuring reliable delivery. Its fixed format includes critical fields such as source and destination port numbers (which identify the sending and receiving applications), sequence and acknowledgment numbers (for tracking byte order and confirming receipt), and the data offset (which specifies the header length). The header's design is fundamental to TCP's role as a connection-oriented, stateful protocol within the Internet Protocol (IP) suite.

Key fields governing connection state and data integrity include the control flags, a set of 1-bit indicators that manage the TCP session's lifecycle. The SYN, ACK, and FIN flags are central to the three-way handshake for connection establishment and the graceful connection termination process. The checksum field provides error detection for both the header and the data payload, while the window size field communicates the receiver's available buffer space, enabling flow control to prevent the sender from overwhelming the receiver. These mechanisms collectively enforce TCP's reliability guarantees.

Additional fields handle advanced protocol functions and optimizations. The urgent pointer field, used in conjunction with the URG flag, indicates the location of high-priority "urgent" data within the segment. Options and padding allow the header to be extended beyond 20 bytes to support features like Maximum Segment Size (MSS) negotiation, Selective Acknowledgment (SACK), and window scaling for high-bandwidth connections. The variable-length options field is always padded to ensure the header ends on a 32-bit boundary, maintaining alignment for efficient processing by network hardware and operating systems.