Transport Layer (TCP IP) - buaamer81/IT130-Networking-Wiki GitHub Wiki

Transport Layer (TCP/IP Model)

📘 Overview

The Transport Layer is the second layer in the TCP/IP Model.
Its job is to handle the delivery of data between devices and ensure it reaches the correct application on the correct device.

Think of it as:
“The layer responsible for how data travels from App A on your computer to App B on another device.”

🔧 Key Responsibilities

Reliable or Unreliable data delivery
Data segmentation and reassembly
Flow control (avoiding overload)
Error checking
Uses Port Numbers to identify applications
Provides end-to-end communication

📦 Data Unit:

Segment → when using TCP
Datagram → when using UDP

🔁 Protocols at the Transport Layer

Protocol	Type	Characteristics
TCP (Transmission Control Protocol)	Reliable	Connection-oriented, error correction, ordered delivery
UDP (User Datagram Protocol)	Unreliable	Connectionless, faster, no error recovery, no order guarantee

🧩 Port Numbers

Port Range	Purpose
0 - 1023	Well-known ports (common protocols)
1024 - 49151	Registered ports (custom services)
49152 - 65535	Dynamic / Private ports (temporary use)

Examples:

HTTP → Port 80
HTTPS → Port 443
DNS → Port 53
FTP → Port 21

🧠 Real-Life Scenario

Downloading a file from a server:

TCP breaks the file into segments.
Adds sequence numbers.
Ensures segments arrive correctly and in order.
Reassembles them on your device.

Streaming a video in real-time:

UDP sends video data fast without waiting for acknowledgments.

🧪 Troubleshooting at Transport Layer

Issue	Possible Cause
Slow connections	TCP windowing problems, congestion
Missing or out-of-order data	UDP transmission (expected behavior)
Port unreachable	Firewall blocking, misconfigured ports
Connection resets	TCP errors, wrong app configuration

Tools to troubleshoot:

netstat
telnet IP port
nc (Netcat)
Wireshark (analyze TCP handshakes)

📊 Visual Aids

🔁 Related Pages

Return to: TCP/IP Model Overview