Notes On Chapter Twenty-Four -- UDP: Datagram Transport Service

• 24.1 Introduction
  • This chapter discusses:
    • UDP Packet Format,
    • Applications of UDP, and
    • Protocol Port Numbers
  
  
• 24.2 The Need For End-to-End Transport Protocols
  
  
  • IP datagrams have address fields only for the source and destination hosts.
    
    
  • Multiple programs may be running on a host.
    
    
  • UDP is an end-to-end protocol (transport protocol) that allows individual application programs (processes) to communicate.
    
    
  • IP is not an end-to-end protocol.
    
    
• 24.3 The User Datagram Protocol
  
  
  • UDP characteristics are:
    • End-to-end.
    • Connectionless
    • Message-oriented
    • Best-effort
    • Arbitrary interaction - [send to or receive from any application(s)]
    • Operating system independent - [identify a process without using the "pid's" used by the local OS]
  
  
• 24.4 The Connectionless Paradigm
  
  
  • UDP is "one-shot" communication - like sending a postcard.
  • No connection setup or tear-down.
  • No control messages.
  • Low Overhead
  
  
• 24.5 Message-Oriented Interface
  
  
  • Each UDP packet is distinct - packet boundaries are preserved - receivers have no problem knowing where the data from one UDP packet ends and another starts.
    
    
  • UDP can lead to inefficient use of the network.
    
    
  • The ratio of header to payload may be high.
    
    
  • Large UDP packets may be fragmented - even on the sending host.
    
    
• 24.6 UDP Communication Semantics
  
  
  • UDP uses IP for delivery
  • Delivery problems may occur
  • Applications must be immune to delivery problems, OR
  • Programmers must add code to handle such errors
  
  
• 24.7 Arbitrary Interaction
  
  
  • UDP allows four styles of interaction:
    • 1-to-1
    • 1-to-many
    • many-to-1
    • many-to-many
  
  
• 24.8 Support For Unicast, Multicast, and Broadcast
  
  
  • UDP packets may be encapsulated in IP broadcast or multicast packets.
  
  
• 24.9 Endpoint Identification With Protocol Port Numbers
  
  
  • UDP identifies source and destination processes using port numbers
    
    
  • Each OS provides a mapping between port numbers and local process identifiers.
    
    
  • "Standard" servers listen for clients on well-known port numbers. (e.g. timeserver on port 37)
    
    
  • An application using UDP can specify a particular remote host and port from which to accept datagrams, OR
    
    
  • An application using UDP can accept packets from any host and port.
    
    
  • There's an example in chapter 29 that shows how to specify port numbers using the socket API.
    
    
  
  
• 24.10 UDP Datagram Format
  
  
  • The UDP packet is called a user datagram.
    
    
  • The format of a user datagram is as shown at right.
  
  
• 24.11 The UDP Checksum And The Pseudo Header
  
  
  • The UDP checksum is optional.
    
    
  • There is no place in the user datagram for IP addresses - they go in the header of the encapsulating IP datagram.
    
    
  • As a guard against possible errors at the IP level, UDP incorporates certain fields from the IP header into the UDP checksum:
    • source IP address,
    • destination IP address, and
    • IP type field.
  
  
• 24.12 UDP Encapsulation
  
  
  • The figure at right illustrates encapsulation of a UDP user datagram.
  
  
• 24.13 Summary
  
  

  ---