What Is TCP (Transmission Control Protocol)?

Updated on August 4, 2025

TCP is a connection-oriented, reliable transport layer protocol that provides full-duplex, byte-stream delivery between applications on different hosts. It ensures all bytes are received in the same order as sent and notifies the sender if data fails to arrive. 

Operating at Layer 4 of the OSI model, between the Internet Protocol (IP) and application layers, TCP breaks data into smaller pieces, tracks their delivery, and reassembles them at the destination.

Definition and Core Concepts

TCP establishes a logical connection before any data transfer begins. This connection-oriented approach differs from connectionless protocols like User Datagram Protocol (UDP), which send data without establishing a dedicated path.

Connection-Oriented Communication

TCP requires a handshake process before data flows between sender and receiver. This establishes parameters like sequence numbers and window sizes. Both endpoints must agree to communicate before any application data moves across the network.

Reliable Delivery Guarantee

TCP provides absolute reliability through acknowledgments and retransmissions. Every byte sent receives confirmation of delivery. If acknowledgment doesn’t arrive within a specified timeout, TCP automatically retransmits the data.

Ordered Delivery Service

Data segments may arrive out of order due to network routing variations. TCP uses sequence numbers to track each byte and reassemble segments in their original order before delivering them to the application.

Full-Duplex Operations

Both endpoints can send and receive data simultaneously. This bidirectional communication allows efficient protocols like HTTP, where clients send requests while servers prepare responses.

Byte-Stream Service

TCP treats application data as a continuous stream of bytes rather than discrete messages. Applications can send data in any size chunks. TCP handles segmentation and reassembly transparently.

Transport Layer Position

As a Layer 4 protocol, TCP bridges the gap between network-layer routing (IP) and application-layer services (HTTP, SMTP, FTP). It provides reliability services that IP lacks while remaining independent of specific applications.

TCP Segments and Sockets

The TCP segment represents the basic data unit, consisting of a header with control information and a data payload. Sockets serve as communication endpoints, uniquely identified by an IP address and port number combination.

How It Works

TCP’s reliability mechanisms involve several coordinated processes that manage connection lifecycle, data transfer, and network resource utilization.

Connection Establishment (Three-Way Handshake)

TCP connections begin with a precise three-step process that synchronizes both endpoints.

• SYN (Synchronize): The client sends a SYN packet containing its initial sequence number (ISN). This random number prevents connection confusion and security attacks.
• SYN-ACK (Synchronize-Acknowledge): The server responds with both acknowledgment of the client’s SYN and its own SYN containing the server’s ISN. This confirms the server’s willingness to establish connection.
• ACK (Acknowledge): The client sends final acknowledgment of the server’s SYN-ACK. The connection becomes established and data transfer can begin.

This handshake synchronizes sequence numbers, establishes initial window sizes, and confirms both sides are ready for communication.

Data Transfer Mechanisms

Once established, TCP manages ongoing data flow through several integrated systems.

• Segmentation Process: TCP accepts data from applications and divides it into segments sized appropriately for the network path. Each segment receives a TCP header with control information.
• Sequence Number Assignment: Every data byte receives a unique sequence number. This enables proper ordering and duplicate detection at the receiver.
• Acknowledgment System: Receivers send ACK packets to confirm successful data receipt. These acknowledgments reference sequence numbers to specify exactly which data arrived safely.
• Retransmission Logic: Senders maintain timers for each transmitted segment. If acknowledgment doesn’t arrive before timeout expires, TCP assumes loss and retransmits the segment.
• Duplicate Detection: Receivers use sequence numbers to identify and discard duplicate segments that may arrive due to unnecessary retransmissions.
• Out-of-Order Handling: When segments arrive out of sequence, TCP buffers them until missing segments arrive. Then it reassembles the complete data stream before delivering to the application.

Flow Control Implementation

TCP prevents fast senders from overwhelming slower receivers through the sliding window mechanism.

• Receiver Window (rwnd): Each ACK packet includes the receiver’s current available buffer space. This advertises how much additional data the receiver can accept.
• Transmission Control: Senders limit outstanding unacknowledged data to the receiver’s advertised window size. This prevents buffer overflow at the destination.

Congestion Control Algorithms

TCP dynamically adjusts transmission rates to prevent network overload and ensure fair bandwidth sharing.

• Congestion Window (cwnd): Senders maintain a congestion window that limits the amount of data in flight based on perceived network capacity.
• Slow Start Algorithm: New connections begin with small congestion windows and double the window size with each successful round-trip until reaching a threshold or detecting congestion.
• Congestion Avoidance: After slow start, TCP increases the congestion window more gradually to probe for additional available bandwidth without causing network overload.
• Fast Retransmit and Fast Recovery: When duplicate ACKs indicate probable packet loss, TCP quickly retransmits the missing segment and adjusts its congestion window without waiting for timeout.

Connection Termination (Four-Way Handshake)

TCP connections close through a coordinated four-step process that ensures both sides finish sending data.

• FIN (Finish): Either endpoint sends a FIN packet to indicate it has no more data to transmit.
• ACK Response: The other side acknowledges the FIN packet.
• Return FIN: When the second endpoint finishes sending its data, it sends its own FIN packet.
• Final ACK: The first endpoint acknowledges the second FIN, completing the connection termination.

TCP supports half-close operations where one side stops sending while continuing to receive data from its peer.

Key Features and Components

TCP’s reliability and performance depend on several integrated features working together.

Reliability Features

• Error Detection: TCP uses checksums to detect corrupted segments during transmission. Corrupted segments are discarded and trigger retransmission.
• Guaranteed Delivery: The acknowledgment and retransmission system ensures all data eventually reaches its destination or the sender receives notification of permanent failure.
• Duplicate Prevention: Sequence numbers allow receivers to identify and discard duplicate segments that may result from network delays or retransmissions.

Performance Management

• Flow Control: The sliding window mechanism prevents receiver buffer overflow by allowing receivers to control sender transmission rates.
• Congestion Control: Multiple algorithms work together to detect network congestion and adjust transmission rates to maintain optimal performance.
• Full-Duplex Communication: Simultaneous bidirectional data flow maximizes connection efficiency for interactive applications.

Addressing and Multiplexing

• Port Numbers: TCP uses 16-bit port numbers to distinguish between different applications on the same host. Well-known ports (0-1023) are reserved for standard services.
• Socket Identification: Each TCP connection endpoint is uniquely identified by the combination of IP address and port number, enabling multiple simultaneous connections.

Segment Structure

TCP segments contain standardized header fields including source and destination ports, sequence and acknowledgment numbers, control flags, window size, checksum, urgent pointer, optional fields, and application data.

Use Cases and Applications

TCP serves as the foundation for most internet applications that require reliable data delivery.

Web Browsing (HTTP/HTTPS)

Web browsers depend on TCP to ensure complete loading of HTML pages, images, stylesheets, and scripts. HTTP’s request-response model requires reliable delivery to function properly.

Email Services (SMTP, POP3, IMAP)

Email protocols use TCP to guarantee accurate message delivery between mail servers and clients. Lost or corrupted email data would render the service unreliable.

File Transfer (FTP, SFTP)

File transfer applications require absolute data integrity. TCP ensures files arrive without missing bytes or corruption that would render them unusable.

Secure Shell (SSH)

Remote command-line access demands reliable communication for both command transmission and response delivery. TCP provides the foundation for secure remote administration.

Database Communications

Database applications require guaranteed delivery for queries and responses. Data integrity is critical for maintaining consistent database states across network connections.

Message Queue Systems

Enterprise message queues use TCP to ensure reliable message delivery between application components. Lost messages could disrupt business processes.

Advantages and Trade-offs

TCP’s reliability features provide significant benefits but come with inherent costs that affect performance and resource utilization.

TCP Advantages

• Guaranteed Reliability: Applications can trust that data will arrive complete and correct. This eliminates the need for application-layer reliability mechanisms.
• Automatic Ordering: TCP handles segment reordering transparently. Applications receive data in the exact sequence it was sent.
• Network Adaptation: Flow and congestion control automatically adjust to network conditions and receiver capabilities without application intervention.
• Error Recovery: Automatic detection and retransmission of lost or corrupted data ensures robust communication over unreliable networks.
• Universal Support: TCP enjoys widespread implementation across all operating systems and network devices, ensuring broad compatibility.

Performance Trade-offs

• Connection Overhead: The three-way handshake adds initial latency before data transfer begins. Short-lived connections pay this cost repeatedly.
• Header Overhead: TCP headers consume 20-60 bytes per segment, reducing available bandwidth for application data compared to UDP’s 8-byte header.
• Retransmission Delays: Lost segments trigger timeouts and retransmissions that can significantly increase data delivery latency, especially over high-latency networks.
• Head-of-Line Blocking: Lost segments block delivery of subsequently received segments until retransmission completes. This can delay time-sensitive data unnecessarily.
• Resource Consumption: TCP connections require memory for buffers and state information on both endpoints. High connection counts can strain system resources.
• Processing Complexity: TCP’s algorithms require significant CPU resources for sequence tracking, acknowledgment processing, and congestion control calculations.

Key Terms Appendix

• TCP (Transmission Control Protocol): A reliable, connection-oriented transport layer protocol that guarantees ordered data delivery.
• IP (Internet Protocol): The network layer protocol that TCP uses to route packets across networks.
• TCP/IP Suite: The fundamental set of internet protocols combining TCP’s reliability with IP’s routing capabilities.
• Connection-Oriented: Communication model requiring connection establishment before data transfer begins.
• Three-Way Handshake: TCP’s connection establishment process using SYN, SYN-ACK, and ACK packets.
• Reliable Delivery: Guaranteed data arrival through acknowledgments, retransmissions, and error detection.
• Ordered Delivery: Ensuring data segments are reassembled in their original transmission sequence.
• Full-Duplex: Simultaneous bidirectional data transmission capability.
• Flow Control: Mechanism preventing sender from overwhelming receiver’s buffer capacity.
• Congestion Control: Algorithms that adjust transmission rates to prevent network overload.
• Sequence Number: Unique identifier for each data byte enabling proper ordering and duplicate detection.
• Acknowledgment Number: Receiver’s confirmation of successfully received data segments.
• TCP Segment: Basic TCP data unit consisting of header and payload.
• Socket: Connection endpoint identified by IP address and port number combination.
• Port Number: 16-bit identifier distinguishing different applications on the same host.
• Head-of-Line (HOL) Blocking: Performance issue where delayed packets block subsequently received packets.
• UDP (User Datagram Protocol): Connectionless, unreliable transport protocol often contrasted with TCP.