What is Transmission Control Protocol (TCP)

Transmission Control Protocol (TCP) is a fundamental protocol of the Internet protocol suite. It provides a reliable and connection-oriented communication service between devices over an IP network. In this article, we will explore TCP, how it works, its congestion control mechanisms, reliability features, flow control, and its differences from User Datagram Protocol (UDP). We will also touch upon TCP implementations and versions.


Key features of TCP

Transmission Control Protocol: TCP offers several key features that make it widely used for various applications:


  • Reliability: TCP ensures reliable data delivery by using sequence numbers, acknowledgments, and retransmission mechanisms. 
  • Connection-oriented: TCP establishes a connection between the sender and receiver before data transfer. 
  • Flow control: TCP regulates the flow of data to prevent overwhelming the receiving device. 
  • Congestion control: TCP dynamically adjusts the sending rate based on network conditions to avoid congestion. 
  • Ordered data delivery: TCP guarantees that data packets arrive in the same order they were sent.

How Transmission Control Protocol (TCP) Works

When two devices want to establish a TCP connection, they go through a three-way handshake process:


SYN: The initiating device sends a synchronization (SYN) segment to the receiving device, indicating the desire to establish a connection.


SYN-ACK: The receiving device responds with an SYN-ACK segment, acknowledging the request and indicating its readiness to establish the connection.


ACK: Finally, the initiating device sends an acknowledgment (ACK) segment, confirming the connection establishment.


TCP segments and data transfer

Once the connection is established, data transfer occurs using TCP segments. The sender breaks the data into smaller chunks, encapsulates them into TCP segments, and sends them to the receiver. The receiver acknowledges the receipt of each segment and requests a retransmission if necessary. This process ensures reliable data delivery.


TCP Congestion Control

Congestion window

TCP utilizes a congestion window mechanism to manage network congestion. The congestion window determines the number of unacknowledged segments that can be in transit at any given time. It dynamically adjusts its size based on network conditions.


Slow start

During the initial phase of a connection, TCP employs a slow start algorithm to avoid overwhelming the network with too many packets at once. It gradually increases the congestion window size, probing the network’s capacity.


Congestion avoidance

Once the congestion window reaches a certain threshold, TCP switches to congestion avoidance mode. It increases the congestion window more cautiously to prevent congestion collapse. If congestion is detected, TCP reduces the congestion window size and enters congestion recovery.


Fast retransmit and fast recovery

If TCP detects missing segments, it assumes that congestion occurred and performs a fast retransmit. It resends the missing segment without waiting for a timeout. Fast recovery enables TCP to resume data transmission without performing a full slow start.


TCP Reliability

Sequence numbers and acknowledgments TCP ensures reliable data transfer by assigning sequence numbers to each transmitted segment. The receiver acknowledges the receipt of segments by sending back acknowledgment numbers. Sequence numbers and acknowledgments enable the detection of missing or out-of-order segments.



If a segment is lost or arrives out of order, TCP retransmits it. The receiver detects missing segments by identifying gaps in the received sequence numbers and requests retransmission for those segments.


Selective acknowledgments

To optimize performance, TCP allows the receiver to selectively acknowledge individual segments instead of waiting for missing segments to trigger retransmission. This mechanism reduces unnecessary retransmissions.


Transmission Control Protocol (TCP) Flow Control

Sliding window: TCP uses a sliding window mechanism to manage the flow of data between the sender and receiver. It enables the sender to transmit multiple segments without waiting for acknowledgments for each individual segment.

Receive window: The receiver maintains a receive window, indicating the maximum amount of data it can accept. The sender adjusts the flow of data based on the size of the receiver’s window, preventing data overflow at the receiver.


Differences between TCP and UDP

While TCP provides reliable and ordered data delivery, User Datagram Protocol (UDP) offers a connectionless and unreliable service. Here are the key differences:

  • TCP is connection-oriented, while UDP is connectionless.
  • TCP provides reliability and error-checking, while UDP does not.
  • TCP guarantees ordered data delivery, while UDP does not enforce any order.
  • TCP includes flow control and congestion control mechanisms, while UDP does not. 

When to use TCP or UDP?

The choice between TCP and UDP depends on the specific requirements of the application. TCP is suitable for applications that require reliable and ordered data delivery, such as web browsing, email, and file transfer. UDP is preferred for real-time applications like video streaming, VoIP, and online gaming, where speed and lower overhead are more critical than reliability.


  • TCP Implementations and Versions
  • TCP/IP stack
  • TCP is an integral part of the TCP/IP stack, which consists of various protocols enabling internet communication. It operates in conjunction with IP (Internet Protocol), forming the foundation of modern Internet communication.

TCP versions: TCPv4 and TCPv6

There are two versions of TCP: TCPv4 and TCPv6. TCPv4 is the most widely used version and has been in use since the early days of the internet. TCPv6 is designed to address the limitations of IPv4 and support the growing number of internet-connected devices with its larger address space.


Transmission Control Protocol (TCP) is a vital protocol for reliable and ordered data transmission over IP networks. It offers features such as reliability, flow control, congestion control, and ordered delivery. Understanding TCP’s working principles, congestion control mechanisms, reliability features, and its differences from UDP is crucial for anyone working with network protocols. Now that you have a solid understanding of TCP, you can explore its various implementations, versions, and applications. Keep in mind that TCP’s reliability comes with some overhead, so it’s essential to choose the right protocol based on the specific requirements of your application.

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