Chapter 11 User Datagram Protocol (UDP)

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Chapter 11 User Datagram Protocol (UDP)
Mi-Jung Choi Dept. of Computer Science,
KNU mjchoi_at_kangwon.ac.kr
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Contents

• 11.1 PROCESS-TO-PROCESS COMMUNICATION
• 11.2 USER DATAGRAM
• 11.3 CHECKSUM
• 11.4 UDP OPERATION
• 11.5 USE OF UDP
• 11.6 UDP PACKAGE

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Objectives

• Be able to explain process-to-process
  communication
• Know the format of a UDP user datagram
• Be able to calculate a UDP checksum
• Understand the operation of UDP
• Know when it is appropriate to use UDP
• Understand the modules in a UDP package

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Position of UDP in TCP/IP
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UDP protocol duties

• To create a process-to-process communication UDP
  port number
• Error control to some extent
• If UDP detects an error in the received packet,
  it silently drops it
• No flow control and no acknowledgement for
  received data
• Connectionless, unreliable transport protocol
• A very simple protocol using minimum overhead
• The disadvantages come some advantages
• Sending a small messages b/w UDP

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11.1 PROCESS-TO-PROCESS COMMUNICATION

• Before we examine UDP, we must first understand
  host-to-host communication and process-to-process
  communication and the difference between them.
• The topics discussed in this section include
• Port Numbers
• Socket Addresses

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11.1 PROCESS-TO-PROCESS COMMUNICATION
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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)

• Prot Number
• Process-to-process communication client-server
  paradigm
• Both process (client and server) have the same
  name
• Daytime client process / daytime server
• Today OS supports multi-user and multi-processors
  
• A remote computer can run several server programs
  at same time
• A local computer can run several client programs
  at same time
• For communication, we must define the
• local host
• local process
• remote host
• remote process

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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)

• Port number for process communication
• Local host and remote host IP address
• Process port number
• Range of port number integer b/w 0 65,535
• well-known port number (1 1023)
• registered port (1,024 49,151)
• ephemeral port number(49,152 65,535)

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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)

• IP address vs. port number

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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)

• IANA(Internet Assigned Numbers Authority) range
• Well-known port 0 1,023
• Registered port 1,024 49,151
• Ephemeral port number(dynamic port) 49,152
  65,535

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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)

• Well-known port in UDP

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 a quote of the day
19 Chargen Returns a string of characters
53 Nameserver Domain Name Service
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
162 SNMP Simple Network Management Protocol (trap)
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Example 1
In UNIX, the well-known ports are stored in a
file called /etc/services. Each line in this file
gives the name of the server and the well-known
port number. We can use the grep utility to
extract the line corresponding to the desired
application. The following shows the port for
TFTP. Note TFTP can use port 69 on either UDP or
TCP.
grep tftp /etc/services tftp 69/tcp
tftp 69/udp
See Next Slide ?
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Example 1 (cont.)
SNMP uses two port numbers (161 and 162), each
for a different purpose, as we will see in
Chapter 21
grep snmp /etc/services snmp 161/tcp
Simple Net Mgmt Proto snmp 161/udp Simple
Net Mgmt Proto snmptrap 162/udp Traps for
SNMP
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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)

• Socket Address
• Combination of IP address and port number

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11.2 USER DATAGRAM

• UDP packets are called user datagrams and have a
  fixed-size header of 8 bytes.

UDP length IP length - IP headers length
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11.2 USER DATAGRAM

• Format
• source port number
• destination port number
• length header data (065,535)
• checksum to detect error for entire user
  datagram (Pseudoheader header data)

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11.3 CHECKSUM
UDP checksum calculation is different from the
one for IP and ICMP. Here the checksum includes
three sections a pseudoheader, the UDP header,
and the data coming from the application layer.
The topics discussed in this section include
Checksum Calculation at Sender Checksum
Calculation at Receiver Optional Use of the
Checksum
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11.3 Checksum (cont.)

• Pseudoheader added to UDP header

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11.3 Checksum (cont.)

• Checksum calculation at receiver
• Add the peudoheader to UDP user datagram
• Fill the checksum field with 0s
• Divide the total bits into 16-bit (2 bytes)
  sections
• If the total number of bytes is not even, add 1
  byte of padding (all 0s). The padding is only for
  the purpose of calculating the checksum and will
  be discarded afterwards.
• Add all 16-bit sections using ones complement
  arithmetic
• Complement the result, which is a 16-bit number,
  and insert in the checksum field
• Drop the peudoheader and any added padding
• Deliver the user datagram to the IP layer for
  encapsulation

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11.3 Checksum (cont.)

• Checksum calculation of a UDP user datagram

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11.3 Checksum (cont.)

• Checksum calculation at receiver
• Add the peudoheader to UDP user datagram
• Add padding if needed
• Divide the total bits into 16-bit (2 bytes)
  sections
• Add all 16-bit sections using ones complement
  arithmetic
• Complement the result
• If the result is all 0s, drop the peudoheader and
  any added padding and accept the user datagram.
  If the result anything else, discard the user
  datagram

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11.4 UDP OPERATION
UDP uses concepts common to the transport layer.
These concepts will be discussed here briefly,
and then expanded in the next chapter on the TCP
protocol.
The topics discussed in this section include
Connectionless Services Flow and Error
Control Encapsulation and Decapsulation Queuing Mu
ltiplexing and Demultiplexing
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11.4 UDP OPERATION

• Connectionless services
• No flow control and a simple error check
• Encapsulation and Decapsulation

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11.4 UDP OPERATION (cont.)

• Queuing of UDP
• A client site, when a process starts, some
  implements create both an incoming and outgoing
  queue associated with each process identified by
  ephemeral port number. (other create only
  incoming queue)
• When process terminates, the queues are
  destroyed.
• If an outgoing queue is overflow, the OS ask the
  client process to wait before sending any more
  messages.
• When message arrived for a client, UDP checks to
  have been created an incoming queue for the port
  number of arrived user datagram. If there is no
  such incoming queue, UDP discard the user
  datagram, ask the ICMP to send a port unreachable
  message to the server.

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11.4 UDP OPERATION (cont.)

• Queues in UDP

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UDP vs. TCP communication
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11.4 UDP OPERATION (cont.)

• Multiplexing and Demultiplexing

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11.5 USE OF UDP

• A simple request-response communication with
  little concern for flow and error control (not to
  send bulk data ftp)
• A process with internal flow and error control
  mechanism.
• Transport protocol for multicasting and
  broadcasting.
• For management process such as SNMP
• For some rout updating protocols such as
  RIP(Routing Information Protocol)

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11.6 UDP PACKAGE

• To show how UDP handles the sending and receiving
  of UDP packets, we present a simple version of
  the UDP package. The UDP package involves five
  components a control-block table, input queues,
  a control-block module, an input module, and an
  output module.
• The topics discussed in this section include
• Control-Block Table
• Input queue
• Control-block module
• Input module
• Output module

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11.6 UDP PACKAGE (cont.)

• UDP design

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11.6 UDP PACKAGE (cont.)

• Control Block Table
• Table to keep track of open ports
• Table entries (state, process ID, port number,
  queue number)
• Input Queue
• One for each process

State Process ID Port Number Queue
Number -------- ------------ -------------- -----
------------- IN-USE 2,345 52,010 34 IN-USE 3,42
2 52,011 FREE IN-USE 4,652 52,012 38 FREE
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11.6 UDP PACKAGE (cont.)

• Control-Block Module
• Management of the control block table
• When process starts, it asks for a port number
  from the OS
• OS assigns well-know port numbers to server and
  ephemeral port numbers to client
• The process passes the process ID and port number
  to the control block module to create an entry in
  the table for the process
• Algorithm
• Receive a process ID and a port number.
• 1. Search the control block table for a FREE
  entry.
• 1. If (not found)
• 1. Delete an entry using a predefined
  strategy.
• 2. Create a new entry with the state IN-USE.
• 3. Enter the process ID and the port number.
• 2. Return

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11.6 UDP PACKAGE (cont.)

• Input Module
• It receives a user datagram from IP layer
• It searches the control block table to find same
  port number as this user datagram
• If the entry is found, the module uses the
  information in the entry to enqueue the data in
  the corresponding queue
• If the entry is not found, it generates an ICMP
  message
• Algorithm
• Receive a user datagram from IP
• 1. Look for the corresponding entry in the
  control-block table.
• 1. If (found)
• 1. Check the queue field to see if a queue is
  allocated.
• 1. If (no)
• 1. Allocate a queue.
• 2. Enqueue the data in the corresponding
  queue.
• 2. If (not found)
• 1. Ask the ICMP module to send an unreachable
  port message.
• 2. Discard the user datagram.
• 2. Return

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11.6 UDP PACKAGE (cont.)

• Output module
• Responsible for creating and sending user
  datagram
• Algorithm
• Receive data and information from a process
• Create a UDP user datagram.
• Send the user datagram.
• Return.

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Example 2

• Control-block table at the beginning
• Example 2
• The first activity is the arrival of a user
  datagram with destination port number 52,012. The
  input module searches for this port number and
  finds it. Queue number 38 has been assigned to
  this port, which means that the port has been
  previously used. The input module sends the data
  to queue 38. The control-block table does not
  change.

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Example 3

• After a few seconds, a process starts. It asks
  the operating system for a port number and is
  granted port number 52,014. Now the process sends
  its ID (4,978) and the port number to the
  control-block module to create an entry in the
  table. The module does not allocate a queue at
  this moment because no user datagrams have
  arrived for this destination

State Process ID Port Number Queue
Number -------- ------------ -------------- -----
------------- IN-USE 2,345 52,010 34 IN-USE
3,422 52,011 IN-USE 4,978 52,014 IN-USE
4,652 52,012 38 FREE
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Example 4

• A user datagram now arrives for port 52,011. The
  input module checks the table and finds that no
  queue has been allocated for this destination
  since this is the first time a user datagram has
  arrived for this destination. The module creates
  a queue and gives it a number (43).

State Process ID Port Number Queue
Number -------- ------------ -------------- -----
------------- IN-USE 2,345 52,010
34 IN-USE 3,422 52,011 43 IN-USE 4,978 52,014
IN-USE 4,652 52,012 38 FREE
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Examples 5 6

• Example 5 After a few seconds, a user datagram
  arrives for port 52,222. The input module checks
  the table and cannot find the entry for this
  destination. The user datagram is dropped and a
  request is made to ICMP to send an unreachable
  port message to the source.
• Example 6 After a few seconds, a process needs
  to send a user datagram. It delivers the data to
  the output module which adds the UDP header and
  sends it.