UDP

User Datagram Protocol (UDP) is a transport layer protocol

• Bare minimum transport protocol
• “Best effort” service, UDP segments may be lost or delivered out of order
• Connectionless:
  • No handshaking between UDP client and server
  • Each UDP segment handled independently of others
• Datagram messaging service
  • Multiplexing based on port numbers
  • Detects corruption using checksum
• To achieve reliable transfer over UDP, add reliability at application layer
  • Application-specific error recovery

Uses for UDP:

• Streaming multimedia apps
• Simple query-response protocols:
  • DNS
  • DHCP

Advantages of UDP

• Fine-grained control: UDP sends as soon as the application writes
• No connection set-up delay
• No connection state: No buffers, parameters, sequence numbers, etc.
• Small header overhead: UDP header only 8 bytes

What does UDP provide over IP?

• Checksum
• Multiplexing based on port number

Checksum

Goal: detect “flipped bits” in transmitted segment

To compute:

• Treat segment contents as sequence of 16-bit numbers
• Checksum is 1’s complement sum of segment contents

Receiver checks if computed checksum equals checksum field value