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) 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 set-up or tear-down.
  • No control messages.
  • Low Overhead
  
  
• 24.5 Message-Oriented Interface
  • Each UDP packet is distinct - Preservation of packet boundaries - receivers have no problem knowing where the data from one UDP packet ends and another starts.
  • Can lead to inefficient use of the network.
  • 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
  • Application must be immune to delivery problems, or
  • Programmer must add code to handle such errors
  
  
• 24.7 Arbitrary Interaction ... styles:
  • 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 Identifcation With Protocol Port Numbers
  • UDP identifies source and destination processes using port numbers
  • Each OS provides 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 sending host and sending port, or
  • An application using UDP can accept packets from any host and port.
  • There's an example of the use of UDP in chapter 29.
  
  
• 24.10 UDP Datagram Format
  • The UDP packet is called a user datagram
  • The format of a user datagram is as shown above.
  
  
• 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 can use the UDP Checksum Field as a "Pseudo Header" ... a hash value based on the IP addresses and the UDP header fields.
  
  
• 24.12 UDP Encapsulation
  • The figure above illustrates encapsulation of a UDP user datagram.
• 24.13 Summary

  ---