TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) both operate at the Transport Layer of the OSI model. Their primary role is to deliver application data—such as files, web content, audio, and video—between hosts in a network.
Despite serving the same layer, TCP and UDP make fundamentally different trade-offs between reliability, latency, and overhead.
🔍 Key Differences Between TCP and UDP #
| Feature | TCP | UDP |
|---|---|---|
| Connection Model | Connection-oriented | Connectionless |
| Reliability | Reliable, ordered, lossless | Best-effort, possible loss |
| Overhead | Higher (state tracking, ACKs) | Very low |
| Latency | Higher | Lower |
| Typical Use Cases | File transfer, email, HTTP/HTTPS | Streaming, VoIP, DNS, VPNs |
Intuitive analogy:
- UDP is like sending a letter—once sent, there is no guarantee it arrives or arrives intact.
- TCP is like a phone call—you establish a connection, receive feedback, and ensure everything is heard in order.
🔄 The TCP Communication Process #
TCP provides reliable communication through well-defined connection management and data tracking mechanisms. Its operation can be divided into three phases.
🧩 Three-Way Handshake (Connection Establishment) #
To establish a TCP connection, the client and server exchange three packets:
- SYN — Client: “I want to connect.”
- SYN + ACK — Server: “I agree, let’s connect.”
- ACK — Client: “Confirmed.”
After this exchange, both sides agree on initial sequence numbers and the connection enters the ESTABLISHED state.
Why three steps?
The handshake prevents errors caused by delayed or duplicated packets. A stale SYN alone is not enough to consume server resources—the client must explicitly confirm the connection.
📦 Data Transfer (Reliability and Ordering) #
TCP ensures reliable delivery using sequence numbers and acknowledgments:
- Each byte of data is assigned a sequence number.
- The receiver sends an ACK indicating the next expected sequence number.
- Missing or delayed ACKs trigger retransmission.
- Out-of-order packets are reordered before being delivered to the application.
This mechanism guarantees in-order, lossless delivery, regardless of network conditions.
🔚 Four-Way Wavehand (Connection Termination) #
TCP closes connections gracefully using a four-step exchange:
- FIN — Client: “I’m done sending.”
- ACK — Server: “Acknowledged.”
- FIN — Server: “I’m done too.”
- ACK — Client: “Confirmed, closing.”
After the final ACK, the client enters the TIME_WAIT state.
Why TIME_WAIT matters:
It ensures that delayed packets or retransmitted FINs are handled correctly, preventing data corruption in future connections using the same socket pair.
🚀 UDP Protocol Characteristics #
UDP is a stateless, connectionless protocol. It simply adds a lightweight header and sends packets without tracking their delivery.
Advantages:
- Minimal latency
- Low CPU and memory overhead
- Ideal for high-throughput or real-time traffic
Limitations:
- No delivery guarantees
- No ordering
- No congestion control
Because of these properties, UDP underpins technologies such as:
- Live video streaming
- Online gaming
- VoIP
- DNS
- Overlay and tunneling protocols (VXLAN, VPNs)
Many modern protocols build reliability on top of UDP when needed.
🧾 Summary #
TCP prioritizes accuracy, reliability, and correctness, making it ideal for data integrity–critical applications.
UDP prioritizes speed and responsiveness, making it the foundation of real-time and latency-sensitive systems.
Choosing between TCP and UDP is not about which is “better,” but which trade-off best matches the application’s requirements.