Software-Engineering

Transmission Control Protocol (TCP)

What is TCP?

Transmission Control Protocol (TCP) is a core protocol of the Internet Protocol Suite (TCP/IP). It operates at the Transport Layer (Layer 4) of the OSI model and provides reliable, ordered, and error-checked delivery of data between applications running on hosts communicating over an IP network.

How Does TCP Work?

TCP establishes a connection-oriented communication channel between two endpoints. It breaks data into segments, adds sequence numbers and acknowledgments, and ensures reliable delivery through:

1. Connection Establishment: Three-way handshake
2. Data Transfer: Reliable, ordered delivery with flow control
3. Connection Termination: Four-way handshake
4. Error Recovery: Retransmission of lost packets
5. Congestion Control: Prevents network congestion

TCP Header Structure

A TCP header contains 20 bytes of mandatory fields plus optional data:

• Source Port (16 bits): Port number of the sender
• Destination Port (16 bits): Port number of the receiver
• Sequence Number (32 bits): Byte number of the first byte in this segment
• Acknowledgment Number (32 bits): Next expected byte number
• Data Offset (4 bits): Size of TCP header in 32-bit words
• Reserved (6 bits): For future use
• Control Flags (6 bits): URG, ACK, PSH, RST, SYN, FIN
• Window Size (16 bits): Number of bytes the receiver can accept
• Checksum (16 bits): Error detection
• Urgent Pointer (16 bits): Points to urgent data
• Options (variable): Various TCP options

Why is TCP Needed?

TCP is essential for applications requiring reliable data delivery:

• Reliability: Guarantees data arrives intact and in order
• Flow Control: Prevents sender from overwhelming receiver
• Congestion Control: Adapts to network conditions to prevent collapse
• Error Recovery: Automatically retransmits lost or corrupted data
• Ordered Delivery: Maintains sequence of data packets

Without TCP, applications like web browsing, email, and file transfers would be unreliable and prone to data loss.

TCP Three-Way Handshake

The three-way handshake establishes a TCP connection:

sequenceDiagram
    participant Client
    participant Server

    Client->>Server: SYN (seq=x)
    Server->>Client: SYN-ACK (seq=y, ack=x+1)
    Client->>Server: ACK (ack=y+1)

1. SYN: Client sends synchronization packet with initial sequence number
2. SYN-ACK: Server acknowledges client’s SYN and sends its own SYN
3. ACK: Client acknowledges server’s SYN

TCP Connection Termination

TCP uses a four-way handshake to close connections gracefully:

sequenceDiagram
    participant Client
    participant Server

    Client->>Server: FIN
    Server->>Client: ACK
    Server->>Client: FIN
    Client->>Server: ACK