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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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