User Datagram Protocol UDP

1
User Datagram Protocol (UDP)

• Chapter 12 User Datagram Protocol
• D.E. Comer. Internetworking with TCP/IP
  Principles, Protocols and Architecture, Vol. 1,
  Prentice Hall, 2000
• Presented by Tony Wong

2
Flow of Presentation

• Motivation
• Introduction
• Characteristics of UDP
• UDP Message Format
• UDP Message Delivery
• UDP Port Numbers
• Conclusion

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Motivation Why?

• IP is used to distinguish different machines on
  the internet
• But can we distinguish between different
  application programs within a machine?
• NO!

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Motivation Why? (cont.)

• Operating Systems nowadays support
  Multiprogramming allowing multiple application
  programs, or processes, to run simultaneously
• A mechanism is necessary to
• 1) distinguish among destination within a given
  host
• 2) allow processes within a given computer to
  send and receive datagram independently

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Ultimate Destination I

• Where is the ultimate destination for a message?
• the process? (e.g. FTP get operation process
  1)
• Problems may arise.

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Ultimate Destination II

• 1) Processes are created and destroyed
  dynamically
• Hard for the sender to keep track of all these
  changes
• e.g. System reboot all processes will be
  changed

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Ultimate Destination III

• 2) Sender should not be aware of the
  implementation details of applications
• Violate the information-hiding principle
• e.g. to ask for a file from a file server

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Solution I

• Add an additional level of abstraction between
  processes and messages
• i.e. Port
• Ports are buffered (e.g. arrays of messages)
• Preserve information hiding principle

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Solution II

• To talk to another port, a send needs to know
  both the IP address and the port number at the
  destination
• Each message must carry destination IP and port
  number, as well as its own IP and port number

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UDP (User Datagram Protocol)

• Appeared first in IETF RFC 768
• a procedure for application programs to send
  messages to other programs with a minimum of
  protocol mechanism

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UDP II

• Resides at the same level as TCP
• Connectionless service
• Uses underlying IP to carry messages
• A thin protocol limited functionalities
• NO ACK, NO segmentation, NO sequencing
• Applications NFS, SNMP, DNS

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UDP Message Format

• Consists of
• 1) UDP header
• 2) UDP data area

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Data Integrity I

• IP does not compute a checksum on the data
  portion of an IP datagram
• UDP has to make sure the data portion (IP
  address, actual data.) is correct
• UDP requests IP information from lower level

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Data Integrity II

• Pseudo- header is prepended to an UDP datagram

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Data Integrity III

• 16 bit 1s complement on the entire object
  (pseudo-header, header, data)
• To ensure data integrity
• Sender-side
• Create pseudo-header, then checksum is performed
• Checksum is saved at the checksum field
• Receiver-side
• Re-create the pseudo-header and re-compute the
  checksum

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UDP Datagram Transmission I

• Multiplexes data from different
  applications/processes
• UDP datagram is encapsulated with different
  headers before actual 0s and 1s physical
  transmission

User Data
User Data
UDP header
UDP Datagram
IP header
IP Datagram
Frame header
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UDP Datagram Transmission II

• When a packet arrives, headers are removed layer
  by layer
• UDP is responsible for datagram demultiplexing
  based on the destination port number

Application 1
Application 2
Port 2
Port 1
UDP
IP Layer
.Previous Layers
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Port Assignment I

• How do we assign port numbers to different
  applications?
• Both sender and receiver must agree on port
  numbers before they can interoperate

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Port Assignment II

• Two approaches
• 1) Universal Assignment
• Everyone agrees on some port assignments
• 2) Dynamic Binding
• Port numbers are assigned locally, sender must
  ask the receiver for the current port assignments

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Port Assignment III

• A hybrid approach has been chosen
• Ports 0-1023 are well-known ports
• Rest are dynamic ports

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Conclusion

• UDP provides an unreliable connectionless
  delivery service using IP as the transportation
  medium
• Provides additional ability to distinguish among
  multiple destinations within a given host by
  using the concept of Port
• A thin protocol with limited functionalities
  works well in a local area environment