Why Layer 3 Isn’t Enough: Reliability Problems and the Rise of TCP/UDP

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Why Layer 3 Isn’t Enough: Reliability Problems and the Rise of TCP/UDP

At first glance, the Network Layer (Layer 3 in the OSI model) seems powerful enough. It can move packets from one device to another across the globe using IP addresses and routing.

So why do we need another layer on top — the Transport Layer (Layer 4) — with protocols like TCP and UDP?

The answer lies in the limitations of Layer 3. Let’s unpack them.

What Layer 3 Does Well

At Layer 3, the Internet Protocol (IP) is responsible for:

Moving data between different networks
Assigning logical addresses (IP addresses)
Finding the right path via routing

This ensures your packet can travel from your laptop in Vienna all the way to a server in California.

But here’s the catch: IP only guarantees “best effort” delivery.

The Problems with Layer 3

Unreliable Delivery
- Packets can get lost in transit.
- IP won’t try again; the data just disappears.
Out-of-Order Packets
- Because of different routes in the network, packet #3 might arrive before packet #2.
- IP doesn’t care — it just delivers whatever arrives.
No Application Separation
- At the IP level, every packet looks the same.
- There’s no built-in way to say: this packet belongs to your browser tab, while that packet belongs to your video call.
No Flow Control
- A fast sender can overwhelm a slow receiver.
- Packets pile up, get dropped, and the communication breaks down.

In short: IP is like mailing postcards — you drop them in the box and hope they arrive, but you don’t know when or in what order.

Enter the Transport Layer (Layer 4)

The Transport Layer was built to solve these problems. Two main protocols dominate:

TCP (Transmission Control Protocol)
UDP (User Datagram Protocol)

TCP: Reliability and Order

TCP adds features that IP doesn’t provide:

Connection-oriented → Before sending data, TCP sets up a connection using the 3-way handshake (SYN, SYN-ACK, ACK).
Acknowledgments → Every packet is confirmed by the receiver. If it’s lost, TCP resends it.
Sequencing → Keeps track of the order so packets are reassembled correctly.
Flow control → Uses a window size to avoid overwhelming the receiver.

This makes TCP reliable — perfect for web browsing, file downloads, and emails where accuracy is critical.

UDP: Speed Over Reliability

Not all applications need strict reliability. For example, in video streaming or gaming:

If a frame of video or one game update is lost, it’s better to skip it than wait.
Latency matters more than guaranteed delivery.

That’s where UDP shines:

No handshake
No acknowledgments
Just fast, lightweight transmission

UDP is often called the “fire-and-forget” protocol.

Why Both TCP and UDP Exist

Think of it this way:

TCP = Registered Mail → Every delivery is confirmed, tracked, and guaranteed.
UDP = Normal Mail → Cheap, fast, but no guarantees.

Both are essential because different applications need different trade-offs.

Final Thoughts

Layer 3 (IP) is great for getting data across networks, but it leaves too many questions unanswered: Did it arrive? In what order? Can the receiver handle the speed?

That’s why the Transport Layer exists. With TCP and UDP, we finally get reliability where it’s needed and speed where it’s preferred.

The next time you stream a movie or download a file, remember: IP gets your data across the world, but TCP or UDP makes sure it arrives in a way your app can actually use.