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**The UDP header** is a very simple control part of a UDP datagram.
It contains the **minimum amount of information** needed to deliver data to an application.

In short:
**the UDP header consists of only 4 fields**.

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## Fields of the UDP header

### 1. **Source Port**

- **16 bits**
- indicates **which application** the data was sent from
- can be `0` if the source port is not needed

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### 2. **Destination Port**

- **16 bits**
- indicates **which application** the data is intended for
- a required field

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### 3. **Length**

- **16 bits**
- contains the **total length of the UDP datagram**:
  - the UDP header plus the data
- the minimum value is **8 bytes** (header only)

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### 4. **Checksum**

- **16 bits**
- used to **verify data integrity**
- if the checksum does not match:
  - the datagram is dropped

Important:

- in IPv4, the checksum can be optional
- in IPv6, the checksum is **mandatory**

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## Size of the UDP header

- **Always 8 bytes**
- does not depend on the data
- unlike TCP, **there are no options or extra fields**

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## What is important to remember

- the UDP header:
  - **does not contain** sequence numbers
  - **does not contain** an ACK
  - **does not contain** windows or flags
- so UDP is:
  - fast
  - has minimal overhead
  - has no delivery or order guarantees

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## Comparison with TCP (briefly)

- **UDP** - 8-byte header
- **TCP** - at least 20 bytes

A smaller header means lower latency and higher speed.

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