TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are two of the most widely used transport layer protocols on the Internet.

Both are used to transfer data between applications, but they have different characteristics and are designed for different purposes.

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TCP establishes a connection before transmitting data.

It ensures that:

• Data arrives successfully.
• Packets are delivered in order.
• Lost packets are retransmitted.
• Duplicate packets are discarded.

This reliability makes TCP suitable for applications where accuracy is more important than speed.

Examples:

• HTTP / HTTPS
• FTP
• SSH
• SMTP
• IMAP
• POP3

UDP sends packets without establishing a connection.

It does not guarantee:

• Delivery
• Ordering
• Retransmission

Instead, it focuses on speed and low latency.

This makes UDP ideal for applications where occasional packet loss is acceptable.

Examples:

• DNS
• DHCP
• VoIP
• Video conferencing
• Online gaming
• Live streaming
• NTP

Before exchanging data, TCP performs a connection setup called the Three-Way Handshake.

Client          Server

SYN      ─────►

        ◄───── SYN-ACK

ACK      ─────►

Connection Established

Only after this process begins does application data flow.

UDP has no connection setup.

The sender immediately transmits packets.

Client

Packet 1 ─────►

Packet 2 ─────►

Packet 3 ─────►

Server

This reduces delay but provides no guarantee that packets arrive.

TCP detects missing packets and retransmits them.

If packet number 3 is lost:

1 ✓
2 ✓
3 ✗
4 ✓

↓

TCP retransmits packet 3.

UDP simply continues sending.

The application must decide how to handle missing data.

Because TCP performs acknowledgements, retransmissions, and congestion control, it generally has more overhead.

UDP is usually:

• Faster
• Lower latency
• Simpler

This is why many real-time applications prefer UDP.

• HTTP
• HTTPS
• SSH
• FTP
• SMTP
• POP3
• IMAP

• DNS
• DHCP
• NTP
• TFTP
• SNMP
• RTP
• VoIP

Many services use different port numbers.

Examples:

Use TCP when:

• Data must not be lost.
• Correct packet order is important.
• Reliability is more important than speed.

Use UDP when:

• Low latency is critical.
• Small packet loss is acceptable.
• Real-time communication is required.

Several Telenegar tools distinguish between TCP and UDP.

Port Check

A port may accept:

• TCP
• UDP
• Both
• Neither

For example:

53

TCP ✓
UDP ✓

443

TCP ✓
UDP ✗

Knowing which protocol a service uses is essential when troubleshooting connectivity.

• UDP is better than TCP.
  • False. Each protocol is designed for different applications.

• TCP is always slower.
  • Not necessarily. For many applications, reliability outweighs the additional overhead.

• DNS always uses UDP.
  • False. DNS usually uses UDP, but TCP is used for large responses and zone transfers.

• IP Addresses
• HTTP vs HTTPS
• Port Check
• DNS Check

tcp udp transport-layer networking ports protocols