Introduction

TCP/IP (Transmission Control Protocol/Internet Protocol) is the backbone of modern networking, enabling reliable communication over the internet. This post explores the key essentials of TCP/IP protocols, focusing on the three-way handshake and four-way wavelet processes that ensure reliable data transmission. TCP/IP is a suite of protocols that govern how data is transmitted over the internet. It consists of multiple layers, each responsible for different aspects of communication. The two most important protocols in this suite are TCP and IP.

Three-Way Handshake

The TCP three-way handshake establishes a reliable connection between two endpoints (e.g., Client A and Server B) over an unreliable network. It ensures synchronization of sequence numbers and confirms bidirectional communication readiness.

Process Flow

1. SYN, seq = x
A -----------------> B
    (Client initiates connection with SYN flag and initial sequence number *x*)

2. SYN-ACK, ACK = x + 1, seq = y
A <----------------- B
    (Server responds with SYN-ACK flag, acknowledges *x+1*, and sends its initial sequence *y*)

3. ACK = y + 1
A -----------------> B
    (Client confirms server's sequence *y+1*; connection established)

Key Purposes

  • Sequence Synchronization: Both parties exchange initial sequence numbers (x and y) to track data order and detect duplicates.
  • Reliability: Prevents stale/duplicate connections (e.g., delayed SYN packets) by requiring explicit ACKs.
  • Why Not Two-Way? If a delayed SYN packet arrives after a previous connection closes, a two-way handshake would allow the server to accept it erroneously. The third ACK ensures the client actively validates the connection.

Four-Way Wavelet

TCP connections are full-duplex; closing requires independent termination of each direction. The four-way wavelet ensures graceful shutdown without data loss.

Process Flow

1. FIN, seq = x+3, ACK = y+1
A -----------------> B
    (Client initiates termination with FIN flag, ending its data stream)

2. ACK = x+3
A <----------------- B
    (Server acknowledges client's FIN)

3. FIN, seq = y+1
A <----------------- B
    (Server sends its FIN after finishing data transmission)

4. ACK = y+2
A -----------------> B
    (Client confirms server's FIN; connection fully closed)

Key Reasons

  • Full-Duplex Closure: Each party independently signals termination (FIN) and acknowledges the other’s FIN.
  • TIME_WAIT State: After sending the final ACK, the client waits for 2MSL (Maximum Segment Lifetime) to handle delayed packets and prevent old data from interfering with new connections.

Technical Mechanisms Supporting Reliability

  1. Sequence & Acknowledgment Numbers
    • Track byte order and confirm receipt.
  2. Flow Control
    • Sliding window adjusts transmission rates based on receiver buffer capacity.
  3. Error Detection
    • Checksums validate header and data integrity; corrupted segments are discarded.
  4. Retransmission
    • Unacknowledged packets trigger retransmission after timeout.
  5. Congestion Control
    • Algorithms (slow start, fast recovery) prevent network overload.

Summary

The three-way handshake and four-way wavelet are foundational to TCP’s reliability. By synchronizing sequence numbers, validating bidirectional readiness, and ensuring graceful closure, TCP mitigates risks in untrusted networks. Combined with mechanisms like sliding windows and checksums, these processes enable robust data transmission for applications requiring high integrity.