What happens when a UDP packet is lost?

When a UDP packet is lost, nothing "special" happens - it is simply lost. UDP does not try to detect the loss and does not recover the data.

In short: the packet is gone, and UDP moves on as if nothing happened.

---

#What exactly happens, step by step

1. The sender sends a UDP packet
2. On the network, the packet can:

• get lost
• be dropped due to congestion
• get corrupted

3. The receiver does not get the packet
4. UDP does nothing about it:

• no ACK
• no timeouts
• no retransmission

That is all.

---

#What does NOT happen when a UDP packet is lost

Important for the interview. UDP does not:

• retransmit
• notify the sender
• restore the order
• compensate for the loss

UDP works on a best effort basis.

---

#Who is responsible for the consequences of the loss

If data loss is critical, it has to be handled by:

#Application logic

The application can:

• resend the data
• ignore the loss
• interpolate the data
• request an updated state

UDP deliberately shifts responsibility to the application.

---

#Why this is considered normal

For many scenarios:

• losing one packet is not critical
• delay is worse than loss

Examples:

• voice → a small "dropout"
• video → a dropped frame
• a game → a missed position update

Retransmission would be worse than the loss.

---

#Comparison with TCP

• TCP:
  • detects loss
  • retransmits data
  • slows down transmission
• UDP:
  • does not detect loss
  • does not retransmit
  • works as fast as possible