UDP is a connectionless transport protocol that provides an unreliable "best effort" service where packets may be lost or delivered out of order. It has no handshaking between sender and receiver and each packet is handled independently. UDP has a small 8-byte header and no congestion control, making it useful for applications that can tolerate some data loss like streaming media. Each UDP packet, called a datagram, carries data that is encapsulated within an IP datagram for transmission. The UDP header includes source and destination port numbers as well as a checksum to detect errors in transmission.

1. UDP: User Datagram Protocol [RFC 768]
 “no frills,” “bare bones”
Internet transport
protocol
 “best effort” service, UDP
segments may be:
 lost
 delivered out of order
to app
 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

2. Datagram Format
 UDP packets, called user datagrams, have a
fixed-size header of 8 bytes.
User datagram format

3. UDP operation
 Connectionless
Services
UDP provides a
connectionless
service. This means
that each user
datagram sent by
UDP is an
independent
datagram.
 Flow and Error Control
UDP is a very simple,
unreliable transport
protocol. There is no
flow control and hence
no window mechanism.
3-3
• Encapsulation and Decapsulation
To send a message from one process to
another, the UDP protocol encapsulates
and decapsulates messages in an IP
datagram.

4. UDP: more
 often used for streaming
multimedia apps
 loss tolerant
 rate insensitive
 other UDP uses
 DNS
 SNMP
 reliable transfer over UDP:
add reliability at
application layer
 application-specific
error recovery!
source port # dest port #
32 bits
Application
data
(message)
UDP segment format
length checksum
Length, in
bytes of UDP
segment,
including
header

5. 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.
But maybe errors
nonetheless? More later
….
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment

6. Internet Checksum Example
 Note: when adding numbers, a carryout from
the most significant bit needs to be added
to the result
 Example: add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0
1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0
1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sum
checksum

7. Checksum calculation
 UDP uses 1’s complement for their
checksums. Suppose you have the following
8-bit bytes: 01010011, 01010100, 01110100.
What is the 1’s complement of the sum of
these 8-bit bytes?
 How does the receiver detect errors?

8. Use of UDP
 UDP is a suitable transport protocol for
multicasting.
 UDP is suitable for a process with internal
flow and error control mechanisms.
 UDP is suitable for a process that requires
simple request-response communication
with little concern for flow and error
control.
 UDP is used for management processes
such as SNMP.