End-to-End Communication (TCP vs UDP)

The Problem: Delivery Is Still Not Guaranteed

So far, we have learned:

• Layer 1: moves bits
• Layer 2: delivers frames locally
• Layer 3: routes packets between networks

At this point, packets can travel across the world.

But a huge problem still exists.

Imagine Sending a Book Page by Page

You send a 500-page book by mail.

Each page travels independently.

What could go wrong?

• Page arrive out of order
• Some pages are lost
• Some arrive twice
• Some get damaged
• Receiver doesn't know when book ends

This is exactly how IP works.

Reality of IP (Layer 3)

IP provides:

• Addressing
• Routing
• Best-effort delivery

But IP does NOT guarantee:

• Delivery
• Order
• No duplication
• Reliability
• Speed control

IP simply says:

“I will try my best.”

Applications need much more.

The Need for End-to-End Control

We need a system that ensures:

• Data arrives completely
• Data arrives correctly
• Data arrives in order
• Sender does not overwhelm receiver
• Multiple apps share one connection safely

This responsibility belongs to:

The Tansport Layer (Layer 4)

What the Transport Layer Does

The transport layer provides:

Communication between applications, not just devices.

Layer 3: computer -> computer

Layer 4: application -> application

Example

Your laptop runs:

• Browser
• WhatsApp
• Spotify
• Email client

All using the Internet simultaneously.

How does data go to the correct application?

Transport layer solves this.

Ports – Talking to the Right Application

Transport layer introduces:

Port Numbers

Think of IP address as a building address.

Ports are apartment numbers.

Example Address

IP: 142.250.183.14
Port: 443

Meaning:

• Device = Google server
• Application = HTTPS web service

Common Ports

Now routers deliver to device.

Transport layer delivers to correct program.

Segmentation – Breaking Data into Pieces

Applications send large data.

Transport layer splits it into segments.

Example:

Large File
   ↓
Segment 1
Segment 2
Segment 3
...

Each segment travels independently.

Later reassembled.

Two Different Philosophies

Transport layer offers TWO communication styles:

1. TCP – Reliable
2. UDP – Fast

They solve different problems.

TCP (Transmission Control Protocol)

TCP Philosophy

TCP says:

Reliability first.

It guarantees:

• Ordered delivery
• No data loss
• Error recovery
• Congestion control

TCP Connection – The Three-Way Handshake

Before sending data, TCP establishes a connection.

Step 1: SYN

Client -> Server:

“Can we talk?”

Step 2: SYN-ACK

Server -> Client:

“Yes, I am ready.”

Step 3: ACK

Client -> Server:

“Connection established.”

Connection ready.

Reliably Delivery

Every TCP segment has a sequence number.

Receiver sends acknowledgment (ACK):

Received up to byte 5000

If sender doesn't receive ACK:

Retransmit data.

Ordering Guarantee

Even if packets arrive:

3 → 1 → 2

TCP reorders them:

1 → 2 → 3

Application never sees disorder.

Flow Control

Receiver may be slow.

TCP asks:

“How much data can you handle?”

Receiver advertises a window size.

Prevents overload.

Congestion Control (Internet Safety System)

TCP monitors network congestion.

If packets drop:

• Reduce sending speed
• Gradually increase again

This prevents Internet collapse.

Where TCP Is Used

UDP (User Datagram Protocol)

UDP Philosophy

UDP says:

Speed over reliability.

No connection.

No guarantees.

Just send.

UDP Characteristics

• Very fast
• Low overhead
• No handshake
• Minimal delay

But:

• No retransmission
• No ordering
• No congestion control

UDP Analogy

TCP = phone call

UDP = shouting messages in a crowd

Where UDP Is Used

TCP vs UDP Comparison