What fields make up a UDP header?

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.

---

#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

---

#2. Destination Port

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

---

#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)

---

#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

---

#Size of the UDP header

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

---

#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

---

#Comparison with TCP (briefly)

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

A smaller header means lower latency and higher speed.