TCP and UDP — How Data Is Delivered in a Network

Up to this point in the Network Fundamentals section, we have covered:

• how a device joins a network
• how it receives an IP address
• what subnet mask and gateway do
• how DNS works
• how data travels across the Internet

But one essential question remains: How is data actually delivered?

IP determines where a packet should go. It does not guarantee how it gets there. That responsibility belongs to the transport layer — TCP and UDP.

What IP Does — and Where It Stops

IP (Internet Protocol) handles addressing.

It defines:

• the source
• the destination
• the direction of the packet

But IP does not guarantee:

• successful delivery
• correct order
• retransmission if lost
• delivery confirmation

Those functions are handled by TCP or UDP.

TCP — Reliable Delivery

TCP (Transmission Control Protocol) is a connection-oriented protocol.

Before transmitting data, TCP:

1. Establishes a connection.
2. Breaks data into segments.
3. Sends them in order.
4. Waits for confirmation.
5. Retransmits lost segments.

What Is ACK?

ACK (Acknowledgment) is a confirmation message.

When a device receives a segment, it sends back: "ACK — data received."

If no acknowledgment is received, TCP retransmits the segment. This acknowledgment mechanism is what makes TCP reliable.

Where TCP Is Used

• Websites (HTTP / HTTPS)
• File downloads
• Email
• Online banking
• Software updates

Where accuracy and reliability matter, TCP is used.

UDP — Fast Delivery

UDP (User Datagram Protocol) is a connectionless protocol.

It:

• does not establish a session
• does not wait for acknowledgments
• does not retransmit lost packets
• sends data continuously

UDP minimizes delay.

Where UDP Is Used

• Video calls
• Online gaming
• Streaming
• VoIP
• DNS queries

Where speed and low latency matter more than perfect delivery, UDP is used.

Practical Example

A common situation:

Internet speed shows 500 Mbps, but video calls freeze.

The issue may not be speed.

Possible causes:

• packet loss
• high latency
• unstable Wi-Fi signal

Video communication typically uses UDP. Lost packets are not retransmitted.

File downloads use TCP. Lost packets are retransmitted, which reduces speed but preserves accuracy.

This is why high bandwidth does not automatically mean stable performance.

TCP vs UDP — Comparison

Relation to the OSI Model

Within the Network Fundamentals section, we covered the practical layers involved in a home network:

• Physical layer — cable, Wi-Fi signal
• Data link layer — MAC addressing, local transmission
• Network layer — IP addressing and routing
• Transport layer — TCP and UDP

Without the transport layer, the architecture is incomplete.

IP determines direction. TCP or UDP determine delivery behavior.

Conclusion

The fundamental architecture of a home network is now fully defined.

A device connects and receives its configuration through DHCP. An IP address identifies its position within the network. The subnet mask defines local boundaries. The default gateway enables communication beyond the local segment. DNS resolves domain names into IP addresses. Routers forward packets between networks. TCP or UDP determine how data is delivered.

These mechanisms form the operational foundation of Internet communication.

If a connection becomes unstable, the cause typically lies within one of these layers — physical transmission, addressing, routing, or transport behavior.

The Network Fundamentals category establishes the baseline architecture of how a network operates within a home infrastructure.