====== TCP vs UDP ======

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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===== At a Glance =====

^ Feature ^ TCP ^ UDP ^
| Connection | Connection-oriented | Connectionless |
| Reliability | Reliable | Best effort |
| Packet Order | Guaranteed | Not guaranteed |
| Error Recovery | Yes | No |
| Speed | Slower | Faster |
| Overhead | Higher | Lower |
| Typical Uses | Web, Email, File Transfer | DNS, VoIP, Streaming, Gaming |

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===== What Is TCP? =====

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

-----

===== What Is UDP? =====

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

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===== TCP Connection =====

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

<code>
Client          Server

SYN      ─────►

        ◄───── SYN-ACK

ACK      ─────►

Connection Established
</code>

Only after this process begins does application data flow.

-----

===== UDP Communication =====

UDP has no connection setup.

The sender immediately transmits packets.

<code>
Client

Packet 1 ─────►

Packet 2 ─────►

Packet 3 ─────►

Server
</code>

This reduces delay but provides no guarantee that packets arrive.

-----

===== Reliability =====

TCP detects missing packets and retransmits them.

If packet number 3 is lost:

<code>
1 ✓
2 ✓
3 ✗
4 ✓

↓

TCP retransmits packet 3.
</code>

UDP simply continues sending.

The application must decide how to handle missing data.

-----

===== Performance =====

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.

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===== Common Protocols =====

===== TCP =====

  * HTTP
  * HTTPS
  * SSH
  * FTP
  * SMTP
  * POP3
  * IMAP

===== UDP =====

  * DNS
  * DHCP
  * NTP
  * TFTP
  * SNMP
  * RTP
  * VoIP

-----

===== TCP and UDP Port Numbers =====

Many services use different port numbers.

Examples:

^ Service ^ Protocol ^ Default Port ^
| HTTP | TCP | 80 |
| HTTPS | TCP | 443 |
| SSH | TCP | 22 |
| SMTP | TCP | 25 |
| DNS | UDP (usually) | 53 |
| DNS Zone Transfer | TCP | 53 |
| DHCP | UDP | 67 / 68 |
| NTP | UDP | 123 |

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===== Which One Should Be Used? =====

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.

-----

===== TCP vs UDP in Telenegar =====

Several Telenegar tools distinguish between TCP and UDP.

**Port Check**

A port may accept:

  * TCP
  * UDP
  * Both
  * Neither

For example:

<code>
53

TCP ✓
UDP ✓

443

TCP ✓
UDP ✗
</code>

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

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===== Common Misconceptions =====

  * **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.

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===== Related Topics =====

  * [[networking:internet:ip-addresses|IP Addresses]]
  * [[networking:protocols:http-https|HTTP vs HTTPS]]
  * [[tools:portcheck|Port Check]]
  * [[tools:dnscheck|DNS Check]]

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{{tag>tcp udp transport-layer networking ports protocols}}