Demystifying TCP: How Reliable Data Transfer Works, Illustrated with Examples

TCP (Transmission Control Protocol) uses various control flags within its header to manage communication between two devices .....

by Vikash Kumawat
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TCP (Transmission Control Protocol) uses various control flags within its header to manage communication between two devices.

Certainly! Let’s explain how TCP (Transmission Control Protocol) works with the help of an example:

Scenario: Sending Data Using TCP

Suppose you have two computers, a sender (Computer A) and a receiver (Computer B), connected over a network. Computer A wants to send a file to Computer B using TCP.

1. Connection Establishment:

  • Computer A initiates the communication by sending a TCP packet with the SYN (synchronize) flag set to Computer B.

Computer A –> SYN –> Computer B

  • Computer B receives the SYN packet, acknowledges it, and responds with a TCP packet containing both the SYN and ACK (acknowledge) flags set. This indicates its readiness to establish a connection.

Computer B –> SYN-ACK –> Computer A

  • Computer A acknowledges the response from Computer B.

Computer A –> ACK –> Computer B

  • Now, a connection is established, and both computers can exchange data.

2. Data Transfer:

  • Computer A begins sending the file to Computer B in smaller chunks called TCP segments. Each segment is assigned a sequence number.

Computer A –> [Data Segment 1, Sequence 1] –> Computer B

  • Computer B receives the segment, acknowledges its receipt by sending an acknowledgment (ACK) for sequence number 2, indicating it is ready for the next segment.

Computer B –> ACK for Sequence 2 –> Computer A

  • This process continues until all segments are sent and acknowledged. If Computer B misses any segment or detects an error, it will request retransmission of the missing or erroneous data.

3. Flow Control:

  • TCP employs flow control mechanisms to manage the rate of data transfer. Each computer advertises a “receive window” indicating how much data it can receive and buffer. The sender ensures it doesn’t overwhelm the receiver by sending data within the advertised window.

4. Error Handling and Retransmission:

  • If any TCP segment is lost or corrupted during transmission, TCP will handle retransmission. For example, if Computer B fails to acknowledge a segment within a certain time (timeout), Computer A will resend that segment.

5. Connection Termination:

  • When the file transfer is complete, Computer A initiates the connection termination by sending a FIN (finish) packet.

Computer A –> FIN –> Computer B

  • Computer B acknowledges the FIN packet.

Computer B –> ACK –> Computer A

  • Now, Computer B decides it’s also done and sends its own FIN.

Computer B –> FIN –> Computer A

  • Computer A acknowledges Computer B’s FIN.

Computer A –> ACK –> Computer B

  • The connection is now gracefully terminated.

In this example, TCP ensures that data is delivered reliably and in the correct order, handles congestion control, retransmits lost data, and manages the connection’s establishment and termination. TCP provides a reliable and ordered data transfer service over potentially unreliable networks, making it suitable for applications where data integrity is essential, such as web browsing, email, and file transfers.

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