6.2 The TCP Three-Way Handshake & Connection Teardown

6.1 TCP vs UDP: Protocols & Characteristics

The Transport Layer (Layer 4) provides direct logical communication between application processes running on distinct hosts using 16-bit Port Numbers.

Connection-oriented: 3-way handshake • Reliable: ACKs & automatic retransmissions • In-order delivery: Byte sequence numbers • Congestion & Flow Control: Built-in cwnd & rwnd • Header Size: 20 - 60 bytes Ideal for: HTTP, HTTPS, FTP, SSH, SMTP (Where data correctness is paramount) UDP (User Datagram Protocol) • Connectionless: Fire-and-forget datagrams • Unreliable: No acknowledgments or retries • No ordering: Datagrams can arrive out of order • Zero flow control: Blazingly fast, lowest latency • Header Size: Exactly 8 bytes fixed Ideal for: DNS, VoIP, Video Streaming, Gaming (Where real-time speed beats retransmission)

Figure 6.1: Technical comparison between connection-oriented TCP and lightweight connectionless UDP.

Before exchanging payload data, TCP establishes a virtual connection using a Three-Way Handshake to synchronize Initial Sequence Numbers (ISNs) and negotiate buffer window sizes.

Figure 6.2: The classic TCP 3-way handshake establishing bidirectional sequence synchronization between client and server.

6.3 TCP Congestion Control (Slow Start & Sawtooth Curve)

While TCP Flow Control protects the receiver from buffer overflow, TCP Congestion Control protects the intermediate routers and network fabric from collapse due to excessive load.

Figure 6.3: TCP AIMD (Additive Increase, Multiplicative Decrease) producing the characteristic sawtooth throughput profile.

MODULE COMPREHENSIVE QUIZ

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