User Datagram Protocol UDP

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User Datagram Protocol (UDP)

• Chapter 11

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• Know TCP/IP transfers datagrams around
• Forwarded based on destinations IP address
• At IP layer, destination address identifies host
• Made no further distinction about which
  application on the host gets the datagram
• This chapter extends TCP/IP protocol suite
• Adds mechanism to distinguish among hosts
• Multiple applications on a given computer can
  send and receive datagrams independently

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The Ultimate Destination

• Most OSs support multiprogramming
• Multiple applications execute simultaneously
• In OS terms, each executing program is a process,
  task, application program, or user level process
• Systems are called multitasking systems

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• Is a process the ultimate destination?
• Seems natural but is a bit misleading
• Processes are created and destroyed dynamically
• Want to replace processes without notifying
  senders
• Want to identify destinations by function
• Not by process that implements them
• Single process may handle multiple functions
• Want process to decide which function the sender
  wants
• So, we will abstract the problem
• Imagine each machine has a set of protocol ports
• Each port identified by a positive integer
• Local OS provides means to specify or access a
  port

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• Most OSs provide synchronous port access
• Computation stops during port access operation
• Block process extracting data if no data there
• Most ports are buffered
• Puts waiting data in a finite queue
• To communicate with a foreign port
• Sender must know IP address protocol port
• Messages must carry
• Destination port on the machine receiving the msg
• Source port on the machine sending the msg

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User Datagram Protocol (UDP)

• Provides primary mechanism for application
  programs to swap datagrams
• Provides protocol ports to distinguish among
  multiple executing programs
• Uses IP to transport messages between machines
• Provides same unreliable, connectionless image
• No acknowledgements no ordering of incoming msgs
• No feedback for rate control can lose messages

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• Application programs have full responsibility
  for
• Reliability
• Message loss
• Duplication
• Out-of-order delivery
• Loss of connectivity
• Unfortunately, most application programmers
  ignore this responsibility
• Testing is usually done in controlled, reliable
  LANs
• Fail dramatically when used in TCP/IP internet

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Format of UDP Messages

• Each UDP message is a user datagram
• Has a header and data area

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UDP Pseudo-Header

• Checksum can cover data in datagram
• Need to ensure more than that
• Datagram only has protocol port number
• Correct destination includes specific machine
• Want to compute checksum on destination IP as
  well as UDP datagram
• Constructs pseudo-header and includes in checksum
  computation

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Pseudo-header
UDP datagram

• Sender computes checksum over pseudo-header and
  datagram
• Only datagram gets sent
• Receiver reconstructs pseudo-header by extracting
  fields from the IP header

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UDP Encapsulation Protocol Layering

• UDP is our first transport protocol example

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• Layering above UDP means complete UDP message
  (header and data) is encapsulated in an IP
  datagram
• UDP datagram in IP datagram data area
• IP datagram in frame data area
• Layering provides division of duties
• IP layer identifies source destination hosts
• UDP layer identifies source or destination ports
  within a host

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Figure 11.4
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Layering UDP Checksum

• Violation of layering rules with checksum
• Pseudo-header needs source dest IP address
• UDP must interact with IP layer to get these
• Regardless of method, still interacts
• Violates separation of functionality by layering
• UDP is tightly integrated with IP
• Compromise made for practical reasons

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UDP Multiplexing, Demultiplexing, Ports

• Thru the layers, SW must mux and demux among
  multiple objects at next layer
• UDP provides another example of this
• Accepts UDP datagrams from many application
  programs
• Passes to IP for transmission
• Takes arriving datagrams
• Passes each to appropriate application program

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Figure 11.5
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• Conceptually, mux demux occurs at ports
• In practice, little more involved
• Applications negotiate with OS to obtain a port
  protocol and associated port number
• One assigned, application puts the port number in
  all outgoing UDP datagrams
• Think of ports as queues
• OS creates internal queue when provides port
• UDP receives datagram, see if port matches one
• If not, sends port unreachable ICMP error msg,
  discards
• If is, enqueues datagram at port for application
  program
• If port full, get error and datagram discarded

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Reserved Available Ports

• How should port numbers be assigned?
• Two computers must agree on port numbers
• A wants file from B needs to know what port the
  file transfer program on B uses
• Two fundamental approaches
• Central authority
• Dynamic binding

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• Central Authority
• Everyone agrees that central authority assigns
• List of assignments will be published
• All software will be built according to the list
• Also called universal assignment
• Called well-known port assignments
• Dynamic Binding
• Ports not globally known
• Software assigns ports when needed
• Must send request to computer to learn of
  assignments
• Target machine replies with the port number to use

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• TCP/IP uses hybrid approach
• Some ports globally assigned a priori
• Many left available for local sites or
  applications
• Low values going up are assigned
• Higher values left open

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Summary

• Can have multiple applications executing
• UDP distinguishes among these processes
• Uses two 16-bit integers for port numbers
• Some are pre-assigned (well-known)
• Some available for arbitrary use

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• UDP is a thin protocol
• Not add significantly to semantics of IP
• Provides way for applications to use IP to
  communicate
• Unreliable, connectionless
• UDP lies in transport layer
• Conceptual independence between it and internet
  layer is somewhat violated