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Chapter 11

Chapter 11. User Datagram Protocol. Objectives. Upon completion you will be able to:. 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

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Chapter 11

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  1. Chapter 11 User DatagramProtocol Objectives Upon completion you will be able to: • 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 TCP/IP Protocol Suite

  2. Figure 11.1Position of UDP in the TCP/IP protocol suite TCP/IP Protocol Suite

  3. 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 TCP/IP Protocol Suite

  4. Figure 11.2UDP versus IP TCP/IP Protocol Suite

  5. Figure 11.3Port numbers request response TCP/IP Protocol Suite

  6. Figure 11.4IP addresses versus port numbers TCP/IP Protocol Suite

  7. Figure 11.5ICANN ranges ICANN: Internet Corporation for Assigned Names and Numbers TCP/IP Protocol Suite

  8. Note: The well-known port numbers are less than 1024. TCP/IP Protocol Suite

  9. Table 11.1 Well-known ports used with UDP TCP/IP Protocol Suite

  10. 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 TCP/IP Protocol Suite

  11. Example 1 (Continued) 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 TCP/IP Protocol Suite

  12. Figure 11.6Socket address TCP/IP Protocol Suite

  13. 11.2 USER DATAGRAM UDP packets are called user datagrams and have a fixed-size header of 8 bytes. TCP/IP Protocol Suite

  14. Figure 11.7User datagram format TCP/IP Protocol Suite

  15. Note: UDP length = IP length − IP header’s length TCP/IP Protocol Suite

  16. 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 TCP/IP Protocol Suite

  17. Figure 11.8Pseudoheader for checksum calculation TCP/IP Protocol Suite

  18. Figure 11.9 Checksum calculation of a simple UDP user datagram TCP/IP Protocol Suite

  19. Optional Use of the Checksum The checksum is optional. If not used, the field is filled with 0s. TCP/IP Protocol Suite

  20. 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 Multiplexing and Demultiplexing TCP/IP Protocol Suite

  21. Connectionless Services UDP provides connectionless service. This means that each user datagram sent by UDP is an independent datagram. No connection establishment, no connection termination, each datagram can travel on a different path. TCP/IP Protocol Suite

  22. Connectionless Services We cannot send a stream of data to UDP and expect UDP to chop them into different related user datagrams. Instead each datagram must be small enough to fit into one user datagram. Only those processes sending short messages should use UDP. TCP/IP Protocol Suite

  23. Flow and Error Control There is no flow control. There is no error control except for the checksum. TCP/IP Protocol Suite

  24. Figure 11.10Encapsulation and decapsulation TCP/IP Protocol Suite

  25. Figure 11.11Queues in UDP TCP/IP Protocol Suite

  26. Figure 11.12Multiplexing and demultiplexing TCP/IP Protocol Suite

  27. 11.5 USE OF UDP We discuss some uses of the UDP protocol in this section. UDP is suitable for a process that requires simple request-response communication with little concern for flow and error control. UDP is a suitable transport protocol for multicasting. UDP is used for network management such as SNMP. UDP is used in routing protocols such as RIP. TCP/IP Protocol Suite

  28. 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 Queues Control-Block Module Input Module Output Module TCP/IP Protocol Suite

  29. Figure 11.13UDP design TCP/IP Protocol Suite

  30. Table 11.2 The control-block table at the beginning of examples TCP/IP Protocol Suite

  31. 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. TCP/IP Protocol Suite

  32. 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 takes the first FREE entry and inserts the information received. The module does not allocate a queue at this moment because no user datagrams have arrived for this destination (see Table 11.3). See Next Slide TCP/IP Protocol Suite

  33. Table 11.3 Control-block table after Example 3 TCP/IP Protocol Suite

  34. 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). See Table 11.4. See Next Slide TCP/IP Protocol Suite

  35. Table 11.4 Control-block after Example 4 TCP/IP Protocol Suite

  36. Example 5 After a few seconds, a user datagram arrives for port 52,222. The input module checks the table and cannot find an 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. TCP/IP Protocol Suite

  37. Page 273, Exercises: 3, 4, 12, 13 Homework TCP/IP Protocol Suite

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