What is TCP full form?

TCP (Transmission Control Protocol) is one of the core protocols that defines how data is exchanged over the internet and corporate networks. It works in conjunction with the Internet Protocol (IP) to send and receive data packets between devices. Understanding what TCP is and how it works is critical for setting up and troubleshooting network connectivity issues.

What is TCP full form?

What Does TCP Stand For?

TCP stands for Transmission Control Protocol. It is a communication protocol that defines how data should be formatted, addressed, transmitted, routed, and received at the destination.

TCP enables two hosts to establish a connection and exchange streams of data packets in an organized manner. It guarantees reliable and ordered delivery of data between applications running on hosts.

Some key responsibilities of TCP include:

  • Flow control
  • Segmentation
  • Transmission and retransmission
  • Error checking
  • Congestion control

These mechanisms allow TCP to provide reliable end-to-end connections that can transmit very long messages reliably and accurately.

How Does TCP Work?

TCP works using the concept of a three-way handshake to establish a connection, transmit data, and close the connection after the transmission is complete. This exchange of protocol messages ensures that two devices are live and ready to receive before any application data is sent.

TCP Three-Way Handshake

The TCP three-way handshake works as follows:

  1. The client device sends a TCP SYN (synchronize) packet to the server indicating it wants to initiate a new TCP connection. This SYN packet contains TCP configuration parameters like maximum segment size.
  2. Upon receiving the SYN, the server replies by sending a SYN-ACK (synchronize-acknowledge) response packet. This indicates the server has received the client’s SYN packet and is acknowledging it.
  3. Finally, the client acknowledges back to the server by sending an ACK packet.

This final ACK confirms that the client received the earlier SYN-ACK from the server. This completes the three-way handshake, and a TCP connection is formally established between the client and server.

After completing the three-way handshake, application data can be transmitted between the client and server using TCP packets over the connection.

TCP three-way handshake (Image source: firewall.cx)

TCP Packet Structure

Once a TCP connection is established, the application data being transmitted is broken down into TCP packets or segments before being sent.

Each TCP packet contains the following key elements in its segment header:

  • Source port – The port number of the device sending the data, for example, port 80 for a web server.
  • Destination port – The port number of the device receiving the data, for example, port 3390 for an RDP server.
  • Sequence number – The number assigned to each TCP packet in the current session. This allows segments to be identified and reordered on the receiving end.
  • Acknowledgment number – The sequence number that the destination is expecting next, allowing packets to be tracked.
  • Header length – The length of the TCP header in 32-bit blocks.
  • Window size – The amount of data the receiver is willing to accept from the sender.
  • Checksum – A CRC checksum computed by the sender and checked by the receiver to confirm the packet wasn’t corrupted.
  • Urgent pointer – A pointer that indicates certain TCP packets are considered urgent and should be prioritized.
  • Options – Various TCP options that can be set, for example, enabling quality of service.
  • Payload – The actual upper-layer application data being carried inside the TCP packet.

This TCP segment structure allows data to be addressed, transmitted reliably, tracked for errors, and reassembled in the correct order at the destination.

Reassembly happens by arranging TCP packets back based on the assigned sequence numbers in each packet. Acknowledgment packets indicate when packets have been successfully received on the other end.

TCP Connection Termination

Once the applications have finished transmitting data over TCP, the established connection needs to be closed gracefully. This is done using a four-way handshake between the client and server:

  1. The application client sends a FIN packet to inform the server it has no more data to send. It requests the server to close the connection.
  2. The server replies with an ACK to the FIN packet, confirming it received the close request.
  3. When the server is ready to close the connection, it sends its own FIN packet informing the client about this.
  4. Finally, the client replies with a final ACK back to the server, confirming the connection can be closed.

This four-way handshake ensures a reliable connection shutdown, giving the two hosts a chance to transmit any final packets before formally closing things down.

Key Functions of TCP

Now that we understand the basics of how TCP connections are established and terminated, let’s explore some of the key mechanisms TCP provides to enable reliable end-to-end communication between applications:

Flow Control Using Sliding Window

TCP uses the concept of a sliding window to implement reliable flow control between transmitting and receiving devices. Flow control regulates the amount of data that can be sent before requiring an acknowledgement. TCP flow control works as follows:

  1. The receiver specifies a window size indicating the volume of data it is willing to manage at any given point.
  2. The sender can only transmit up to the allowed window size before requiring an ACK from the receiver.
  3. As the receiver processes and frees up buffer space, it keeps advancing its window allowing more data to be sent.
  4. By sliding the window forward as capacity becomes available, smooth data flow is achieved.

This back-and-forth mechanism prevents the sender from overwhelming the receiver with an uncontrolled flood of packets. The sliding window protocol provides fully reliable and buffered transmission control.

Reliable Data Transfer Using Sequence Numbers, Acknowledgments and Checksums

TCP implements various mechanisms that confirm if packets are being delivered error-free between the sender and receiver:

  • Sequence numbers uniquely identify each data byte sent, allowing the receiver host to reassemble segments in the correct order.
  • Acknowledgement (ACK) packets confirm receipt of data back to the sender.
  • Checksums like CRC allow error checking on the packet data.

If issues are detected, automatic retransmissions of lost segments are triggered using these tracking numbers. Duplicate ACKs or failed checksum verifications serve as indicators to retransmit dropped packets.

These capabilities guarantee reliable end-to-end data transfer between applications. Data integrity is preserved by correcting any problems during transmission.

Automatic Retransmissions for Reliability

As discussed above, TCP uses sequence numbers, acknowledgments, and checksums to detect lost or corrupt data segments.

If certain TCP packets don’t arrive correctly or get dropped along the network path, the destination host will automatically trigger retransmissions. TCP handles these resends automatically without informing the actual applications.

For example, if a browser is receiving an HTML file from a web server using TCP, and some segments go missing, TCP will handle requesting the retransmission of that data. This control protocol machinery preserves reliability automatically.

Adaptive Data Rate Using Congestion Control

TCP implements advanced congestion control algorithms that dynamically adapt the rate of data transmission based on current network conditions between the hosts.

By closely watching acknowledgment patterns and connection statistics, TCP can detect impending congestion along the path. When this potential bottleneck is sensed, the congestion avoidance logic automatically throttles transmission rates. This prevents congestive collapse and buffer overruns.

As congestion clears up, TCP is able to steadily raise transmission speeds again. This self-regulating mechanism keeps data flowing smoothly regardless of changing cross-traffic patterns over the connection’s lifespan.

Advanced TCP congestion control methods like CUBIC, BBR, and others optimize throughput while minimizing latency for modern high-speed networks.

Packet Segmentation and Reassembly

TCP breaks down large chunks of application data into smaller segments that can be individually packaged and transmitted through underlying networks. This is known as TCP segmentation.

The TCP maximum segment size (MSS) determines how much application payload data is packed into each transmitted packet. On IPv4 networks, the maximum size of a TCP packet is around 1460 bytes.

Conversely at the receiving end, all TCP segments are put together by the destination host. The sequence numbers allow smooth reassembly despite out-of-order arrival of segments. Packet fragmentation and reassembly are seamless transparent processes handled automatically by TCP.


TCP is one of the core Internet protocols, however, it is not a panacea for all types of network communications. Its connection-oriented design and reliability mechanisms come with a certain degree of overhead traffic and packet processing complexity.

For certain types of real-time communication apps, the User Datagram Protocol (UDP) offers a simpler unreliable but low-latency alternative transport protocol.

Let’s compare some key technical differences between TCP vs UDP:

Parameter TCP UDP
Reliable Yes No
Ordered Yes No
Heavy error checking Yes No
Transmission control Advanced algorithms None
Congestion control Adaptive and optimized None
Packet sequencing Yes No
Connection-oriented Yes No, connectionless
Segmentation Yes Apps handle themselves
Lightweight overhead No Extremely lean
Suitable apps Web, databases, email, etc Video streaming, VoIP etc

As seen above, TCP offers total reliability while UDP focuses on providing barebones best effort delivery with the lowest latency. Choosing between both protocols depends on the application requirements like tolerance to packet loss.

TCP Port Numbers

While transferring data using TCP packets between two hosts, how does TCP identify which exact process the data is meant for at the destination?

This role is fulfilled by TCP port numbers. Let’s learn how they work:

  • Port numbers allow a single host with a single physical IP address to uniquely address application processes running internally using TCP.
  • Common logical TCP ports have been standardized like port 80 for HTTP or port 25 for email. This allows easy interoperability between clients and servers.
  • There are 65535 TCP ports available, ranging from 0 to 65535. Only ports 0 to 1023 are reserved for common protocols like DNS, SSH, FTP etc.
  • Client applications randomly select any unused high-number source port above 1023 to identify themselves.
  • Well-known registered server processes like a mail server or database listener remains fixed on permanently defined ports below 1023.

This source-destination port addressing provides easy application multiplexing, allowing a single server to uniquely communicate with hundreds of clients at once.

Port numbers transform a basic network hardware host into a multi-function capable server running several services using TCP/IP for communication.

Sample Use Cases of TCP

Now that we understand the basics of what TCP is and how it works, let’s check out some sample applications that leverage TCP/IP connectivity for communication:

Web and Mobile App Access

The front-facing protocol of the web, HTTP rides on top of TCP to fetch web pages. Secure HTTP web traffic leverages TLS encryption applied over TCP. This makes the web intrinsically reliant on TCP.

Similarly, mobile apps also use TCP packets over Wi-Fi or cellular data networks to access back-end cloud APIs and transfer data. The reliability mechanisms of TCP become critical to preserve full client-server sync for apps.

Database Access

Relational database processes like MySQL, Oracle, MS SQL Server etc all allow TCP/IP network connectivity to enable remote data access. Database client programs leverage TCP packets to connect to listeners running on database servers. Queries and responses are exchanged using TCP reliable streams.

File Transfer

Protocols like FTP and SFTP rely on TCP for establishing sessions and transferring files between hosts. File metadata and content get divided into TCP segments during transit. Sequence numbers ensure lossless transmission enabling even huge files spanning gigabytes to be sent reliably.

Media Streaming

While live video streaming uses barebones UDP packets, on-demand media streaming platforms like YouTube, Netflix heavily depend on TCP. This ensures viewers receive 100% of their video without stuttering or artifacts despite fluctuating internet speeds at home.

Cloud Computing

Today’s cloud computing paradigm is completely based on TCP/IP connectivity over the internet. Cloud APIs, virtual desktops, SSH, and database servers running in cloud VPC networks all leverage TCP for allowing remote access into cloud hosted systems.

Critical Network Infrastructure

Core routing protocols like BGP uses TCP to exchange network routes and tables between peers over the public Internet. Voice signaling in VoIP uses TCP enabling advanced telephony capabilities. STOMP messaging queues use TCP for reliability too.

Advantages of TCP

After reviewing TCP’s goals and capabilities, let’s summarize the key advantages offered by the Transmission Control Protocol:

  • Reliable – Retransmissions and intelligent error checking provides full reliability.
  • Ordered – Sequence numbers maintain correct packet order.
  • Adaptive – Dynamic congestion control adjusts data rate automatically.
  • Segmented – No IP fragmentation allows fast transfers.
  • Guaranteed delivery – Positive tracking ensures zero data loss.
  • Streamlined – Stream transmission model ideal for networking apps.
  • Compatible – Works over nearly all network types like Wi-Fi, Ethernet, cellular internet etc.
  • Interoperable – Client-server compatibility guaranteed between any TCP implementation.
  • Universal – Supported by every modern computing device, OS, switch, router etc.
  • Secure – TLS encryption can be layered over TCP for security.

These useful features establish TCP as one of the most foundational and versatile protocols powering modern networks and applications today.


TCP implements advanced connection-oriented transmission mechanisms that perfected end-to-end communication over packet-switched networks like the Internet.

The TCP three-way handshake, intelligent error recovery, congestion control and other innovations allow this protocol to transport application data streams reliably between distant networked hosts.

Nearly every forms of digital communication happening today relies on TCP/IP connectivity for delivery. Understanding the workings of the TCP protocol provides crucial insight into the inner workings of modern computer networks and the Internet itself.

While new protocols like QUIC aim to replace TCP, the transmission control protocol has demonstrated proven stability powering the Internet for over three decades already. The heart of reliable networking continues to beat strong using TCP!

Frequently Asked Questions 

  1. What is the key benefit offered by TCP?
    The main benefit of TCP is the guaranteed reliable and ordered delivery of data packets between applications over the Internet. TCP ensures zero data loss using intelligent retransmissions, acknowledgments and checksums during transmission.

  2. Does TCP provide encryption and security?
    TCP alone does not encrypt application data being transmitted. Security is implemented using additional encryption protocols like TLS (formerly SSL) that apply algorithmic encryption over TCP packet streams.

  3. What network applications use TCP?
    Some major applications relying on TCP include – HTTP web access, database connectivity, email, FTP/SFTP transfers, VPN tunnels, voice/video calls etc. Essentially most network client-server applications use TCP/IP.

  4. Does TCP handle application data segmentation?
    Yes. TCP breaks down large chunks of application data into smaller segments of appropriate sizes that networks can handle, which is known as TCP segmentation. Reassembly happens automatically at receiving host.

  5. Can two devices communicate without TCP?
    Devices on an isolated local network can talk without TCP using bare UDP packets or other custom protocols. However, for global Internet communications, the reliability and routing capabilities of TCP/IP are always required.

  6. Does TCP require IP?
    TCP relies on the Internet Protocol (IP) for the actual delivery of its segmented packets over multiple physical networks and routes. TCP works directly above the IP layer, so IP connectivity is mandatory for TCP to function.

  7. What are the core protocols that define the internet?
    The Internet Protocol Suite has TCP and IP as its core layer of protocols. Built on top of them are application protocols like HTTP, FTP, DNS, SSH etc. Routing happens over IP while TCP enables end-to-end communication.

  8. How many layers form the TCP/IP model?
    The TCP/IP model consists of four key conceptual layers – Application Layer, Transport Layer (TCP), Internet Layer (IP) and Network Access Layer (LAN, WAN etc). Each layer has specific responsibilities in the communication process.

  9. Does every TCP connection perform a handshake?
    Yes, every new TCP session begins with a three-way handshake (SYN, SYN-ACK, ACK) between the client and remote server. This handshake ensures both devices are alive and ready for communication before actual data transfer begins.

  10. Can a single device have multiple TCP ports open?
    Yes. A server can have multiple TCP ports open at the same time, allowing it to uniquely identify and communicate with hundreds of different clients at once using source-port destination-port addressing.

  11. What causes TCP congestion control to activate?
    TCP congestion control algorithms closely watch packet loss patterns and unusual variations in round-trip times to infer congestion. Duplicate ACKs from clients not receiving expected packets reliably hints congestion, triggering rate control.

  12. How does TCP detect packet loss?
    TCP uses unique sequence numbers assigned to each transmitted byte, allowing the receiving host to detect missing segments. Duplicate acknowledgments (ACKs) for the same packet also indicate a segment was lost during transit between the hosts.

  13. How does traceroute map network hops using TCP?
    Traceroute sends TCP packets with intentionally very short TTL values that force intermediate routers to send back ICMP TTL expired messages. This reveals every IP hop the packet traversed between the source and destination.

  14. Can UDP transfer data as reliably as TCP?
    UDP lacks TCP’s advanced reliability mechanisms. For apps that prefer lowest latency over 100% packet delivery, UDP sends best effort bare datagrams without handshakes.

  15. Does TCP work on Layer 2 networks?
    No, TCP relies on IP for the actual routing and delivery of packets. TCP cannot work over Layer 2 networks alone. However, TCP traffic can be encapsulated and tunneled over Layer 2 links using protocols like GRE, L2TP, PPP etc.

  16. Can a TCP connection get established without a handshake?
    No, the opening three-way handshake (SYN, SYN-ACK, ACK) is mandatory before any data can be exchanged between hosts using TCP. This prevents blind data transmission attempts.

  17. Does closing a TCP socket require a handshake?
    Yes, TCP connections use a four-way handshake for graceful closure. The client or server sends a FIN, the other party ACKs, then it sends its own FIN packet which is finally ACKed completing the teardown handshake.

  18. Can TCP packet sequencing happen without sequence numbers?
    TCP’s reliability depends on each byte having a unique sequence number during transmission. Without it, the receiving host cannot track or reassemble segments correctly. So sequence numbers are mandatory.

  19. How does TCP differentiate between applications?
    TCP leverages port numbers to differentiate application processes running on the same server. For example, port 80 is reserved for HTTP traffic while port 25 is used for SMTP email traffic.

  20. What is a TCP socket connection?
    A TCP socket refers to the virtual link between two applications established using TCP ports and IP addressing. Data gets transmitted over this TCP socket connection in a reliable stream-based manner.

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