UDP

Internet Protocols
• Internet transport services:
• reliable, in-order unicast delivery (TCP)
congestion
flow control
connection setup
• unreliable (“best-effort”), unordered unicast or multicast delivery: UDP
services not available:
real-time
bandwidth guarantees
reliable multicast
UDP
•“no frills,” “bare bones” Internet transport protocol
• “best effort” service, UDP segments may be:
– lost
– delivered out of order to applications
connectionless:
– no handshaking between UDP sender, receiver
– each UDP segment handled independently of others
Why is there a UDP?
• no connection establishment (which can add delay)
• simple: no connection state at sender, receiver
• small segment header
• no congestion control: UDP can blast away as fast as desired

UDP header


Header details
• Source and destination port numbers
• The source and destination processes
• Length = length of header + data
• Checksum covers header and data
• Optional in UDP but mandatory in TCP
UDP Checksum
Sender:
• treat segment contents as sequence of 16-bit integers
• checksum: addition (1’s complement sum) of segment contents
• sender puts checksum value into UDP checksum field
Receiver:
• compute checksum of received segment
• check if computed checksum equals checksum field value:
– NO - error detected
– YES - no error detected

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