TCP and UDP

Introduction

Modern computer networks rely on communication protocols to transfer data between devices.

Two of the most important transport layer protocols on the internet are:

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

Both protocols are built on top of IP (Internet Protocol), but they are designed for different goals.

TCP prioritizes:

• Reliability
• Ordered delivery
• Error recovery

UDP prioritizes:

• Speed
• Low latency
• Minimal overhead

Understanding the trade-offs between TCP and UDP is fundamental in networking and distributed systems.

UDP

UDP is a lightweight transport protocol that sends data without establishing a connection first.

Characteristics of UDP

• Connectionless
• No guaranteed delivery
• No ordering guarantees
• No retransmission
• Very low overhead

Example Flow

Sender ---> Receiver

UDP simply sends packets called datagrams.

The protocol does not verify whether:

• Packets arrived successfully
• Packets arrived in order
• Packets were duplicated

Advantages

Low Latency

UDP avoids connection setup and retransmission overhead.

This makes it extremely fast.

Minimal Overhead

UDP headers are small compared to TCP headers.

This reduces network and processing overhead.

Suitable for Real-Time Communication

Applications that prioritize speed over perfect reliability often use UDP.

Examples include:

• Online gaming
• Video streaming
• Voice calls (VoIP)
• DNS queries

In many real-time applications, receiving slightly outdated data is less useful than receiving newer data quickly.

Disadvantages of UDP

UDP provides no built-in reliability.

Problems may include:

• Packet loss
• Packet duplication
• Out-of-order delivery
• No congestion control

Applications using UDP must handle reliability themselves if needed.

TCP

TCP is a connection-oriented transport protocol designed for reliable communication.

Unlike UDP, TCP ensures:

• Reliable delivery
• Ordered packets
• Error detection
• Congestion handling

TCP is the foundation for many internet applications such as:

• Web browsing (HTTP/HTTPS)
• Email
• File transfers
• Database connections

3-Way Handshake

Before data transfer begins, TCP establishes a connection using a process called the 3-way handshake.

Step 1: SYN

Client sends a SYN (synchronize) packet to the server.

Client ---> SYN ---> Server

Step 2: SYN-ACK

Server responds with:

• SYN
• ACK (acknowledgment)

Client <--- SYN-ACK <--- Server

Step 3: ACK

Client sends a final ACK packet.

Client ---> ACK ---> Server

After this process completes:

• Both sides establish a connection
• Data transfer can begin

Reliable Delivery

One of TCP’s primary goals is ensuring reliable delivery.

TCP guarantees:

• Data arrives completely
• Data arrives in order
• Missing packets are retransmitted

Sequence Numbers

TCP assigns sequence numbers to packets.

Example

Packet 1 -> Seq 1000
Packet 2 -> Seq 2000
Packet 3 -> Seq 3000

Sequence numbers allow receivers to:

• Detect missing packets
• Reassemble packets in the correct order

If packets arrive out of order:

• TCP buffers and reorders them before delivering data to applications.

Timeouts and Retransmission

TCP uses acknowledgments (ACKs) to confirm successful delivery.

If the sender does not receive an ACK within a certain timeout:

• Packet is retransmitted

Example

Sender ---> Packet ---> Receiver

No ACK received

Sender ---> Retransmit Packet ---> Receiver

This mechanism helps recover from packet loss.

Flow Control

Flow control prevents a fast sender from overwhelming a slow receiver.

TCP uses a mechanism called the receive window.

The receiver informs the sender how much data it can currently process.

Benefits

• Prevents buffer overflow
• Maintains stable communication
• Matches sender speed to receiver capacity

Congestion Control

Congestion control helps prevent network overload.

If too much traffic is sent into the network:

• Routers may drop packets
• Latency may increase
• Throughput may collapse

TCP dynamically adjusts transmission speed based on network conditions.

Common TCP Congestion Strategies

TCP implementations commonly use:

• Slow start
• Congestion avoidance
• Fast retransmit
• Fast recovery

The goal is balancing:

• High throughput
• Network stability
• Fairness between connections

TCP vs UDP

Both protocols optimize for different goals.

Conclusion

TCP and UDP are two foundational internet transport protocols with very different design goals.

TCP focuses on:

• Reliability
• Ordered delivery
• Error recovery
• Network stability

This makes it ideal for:

• Websites
• Databases
• File transfers
• APIs

UDP focuses on:

• Speed
• Low latency
• Minimal overhead

This makes it ideal for:

• Gaming
• Streaming
• Real-time communication

Understanding the trade-offs between TCP and UDP is essential for designing scalable and efficient networked systems.

References

TCP - MDN Web Docs

UDP - MDN Web Docs

Transmission Control Protocol on Wikipedia

User Datagram Protocol on Wikipedia