UDP is a transport layer protocol that provides an unreliable datagram service. It is positioned directly above IP in the TCP/IP protocol stack. UDP packets contain a header with source and destination port numbers as well as length fields, but do not establish connections, provide sequencing, or guarantee delivery like TCP. Well-known ports are assigned to common UDP applications like DNS, time synchronization, and trivial file transfer.

1. User Datagram Protocol (UDP)

2. Learning Outcome
• At the end of this lecture, students will be
able to,
– Explain the format of a UDP packet.
– Identify the services provided by the UDP.
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3. • Where UDP is positioned in the TCP/IP
protocol suite?
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Recall
SCTP TCP UDP
Fig. 1. Transport Layer

4. User Datagram Protocol (UDP)
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Characteristics:
• End-to-End: An application sends/receives data to/from
another application.
• Connectionless: Application does not need pre-established
connection before sending data; not need to terminate the
connection when finished.
• Application Identification: Each application assigned
with an Integer number called PROTOCOL PORT
NUMBER.
• Service: UDP Provides unreliable datagram service
– Packets may be lost or delivered out of order.
– No buffering before transmission.(accept and transmit immediately)

5. User Datagram Protocol (UDP)
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• Transport-level protocol in addition to TCP
• Supports Network Applications
• Placed directly on the top of the Internet Protocol (IP)
• Main purpose is to outline the network traffic in the form
of datagrams.
• Datagram = first 8 bytes contain header information +
remaining bytes contain data.
Fig. 2. UDP User Datagram
DataHeader
8 bytes
8 to 65535 bytes

6. UDP Header Format
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Fig. 3. UDP Datagram Header Format
Source port number Destination port number
Total length Checksum
0 16 31
• Source Port Number - 16 bits port number
• Destination Port Number- 16 bits port number
• Total Length (UDP header + data) - 16 bits count of octets
• Checksum - 16 bits field.
– If 0, then there is no checksum
– If 1, then it is a checksum over a pseudo header + UDP data area

7. UDP Vs TCP
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• Performance:
– TCP uses window based flow control scheme.
– TCP’s slow start algorithm reduces throughput.
– TCP has extra overhead per segment.
– UDP sends small, inefficient Datagrams.
• Reliability:
– TCP provides reliable, in-order transfer.
– UDP provides unreliable service.
• Packet loss due to overflows and errors.
• Out-of-order datagrams.
• Multicast and Broadcast:
– Only supported by UDP.
• Data Size:
– UDP datagram limited to 64KB

8. UDP Multiplexing & Demultiplexing
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Fig. 4. UDP Demultiplexing
• Sender:
– Multiplexing of UDP
Datagrams.
– UDP datagrams are
received from
multiple application
programs.
– A single sequence of
UDP datagrams is
passed to IP Layer.
• Receiver:
– Demultiplexing of UDP Datagrams.
– A single sequence of UDP datagrams is received from IP Layer.
– UDP datagram received is passed to appropriate application.
PORT 1 PORT 2 PORT 3
IP Layer
UDP Datagram arrives
UDP : Demultiplexing
Based on PORT

9. Ports used in UDP
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Port Protocol Description
7 Echo Echoes a received datagram back to the sender
9 Discard Discards any datagram that is received
11 Users Active users
13 Daytime Returns the date and the time
17 Quote Returns the quote of the day
19 Chargen Return a string of characters
56 Domain Domain Name Service(DNS)
67 Bootps Server port to download bootstrap information
68 Bootpc Client port to download bootstrap information
69 TFTP Trivial File transfer protocol
111 RPC Remote Procedure Call
123 NTP Network Time Protocol
161 SNMP Simple Network Management Protocol
Table. 1. Well-Known Ports used in UDP

10. • The following is a dump of a UDP header in
hexadecimal format.
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CB84000D001C001C
THINK…
• What is the source port number?
• What is the destination port number?
• What is the total length of the user datagram?
• What is the length of the data?
• Is the packet directed from a client to a server
or vice versa?
• What is the client process?

11. Solution
• The source port number is the first four hexadecimal digits
(CB84)16 or 52100.
• The destination port number is the second four
hexadecimal digits (000D)16 or 13.
• The third four hexadecimal digits (001C)16 define the
length of the whole UDP packet as 28 bytes.
• The length of the data is the length of the whole packet
minus the length of the header, or 28 – 8 = 20 bytes.
• Since the destination port number is 13 (well-known port),
the packet is from the client to the server.
• The client process is the Daytime (see Table 1).
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12. References
• “Computer Networks” by Andrew S.
Tanenbaum 4th edition by Pearson.
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13. Thank You
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