1. Chapter 11 User Datagram Protocol (UDP)
Mi-Jung Choi
Dept. of Computer Science and Engineering

2. Contents 11.1 PROCESS-TO-PROCESS COMMUNICATION 11.2 USER DATAGRAM
11.3 CHECKSUM
11.4 UDP OPERATION
11.5 USE OF UDP
UDP PACKAGE

3. Objectives 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

4. Position of UDP in TCP/IP

5. UDP protocol duties
To create a process-to-process communication: UDP port number
Error control to some extent
If UDP detects an error in the received packet, it silently drops it
No flow control and no acknowledgement for received data
Connectionless, unreliable transport protocol
A very simple protocol using minimum overhead
The disadvantages come some advantages
Sending a small messages b/w UDP

6. 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

7. 11.1 PROCESS-TO-PROCESS COMMUNICATION

8. 11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
Prot Number
Process-to-process communication: client-server paradigm
Both process (client and server) have the same name
Daytime client process / daytime server
Today OS supports multi-user and multi-processors
A remote computer can run several server programs at same time
A local computer can run several client programs at same time
For communication, we must define the
local host
local process
remote host
remote process

9. 11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
Port number for process communication
Local host and remote host: IP address
Process: port number
Range of port number : integer b/w 0 ~ 65,535
well-known port number (1 ~ 1023)
registered port (1,024 ~ 49,151)
ephemeral port number(49,152 ~ 65,535)

10. 11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
IP address vs. port number

11. 11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
IANA(Internet Assigned Numbers Authority) range
Well-known port: 0 ~ 1,023
Registered port: 1,024 ~ 49,151
Ephemeral port number(dynamic port): 49,152 ~ 65,535

12. 11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
Well-known port in UDP
Port
Protocol
Description 7
Echo
Echoes a received datagram back to the sender 9
Discard
Discards any datagram that is received 11
Users
Active users 13
Daytime
Returns the date and the time 17
Quote
Returns a quote of the day 19
Chargen
Returns a string of characters 53
Nameserver
Domain Name Service 67
Bootps
Server port to download bootstrap information 68
Bootpc
Client port to download bootstrap information 69
TFTP
Trivial File transfer Protocol
111
RPC
Remote Procedure Call
123
NTP
Network Time Protocol
161
SNMP
Simple Network Management Protocol
162
Simple Network Management Protocol (trap)

13. Example 1 See Next Slide 
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 

14. Example 1 (cont.)
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

15. 11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
Socket Address
Combination of IP address and port number

16. UDP length = IP length − IP header’s length
11.2 USER DATAGRAM
UDP packets are called user datagrams and have a fixed-size header of 8 bytes.
UDP length = IP length − IP header’s length

17. 11.2 USER DATAGRAM Format source port number destination port number
length : header + data (0~65,535)
checksum : to detect error for entire user datagram (Pseudoheader + header + data)

18. 11.3 CHECKSUM The topics discussed in this section include:
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

19. 11.3 Checksum (cont.)
Pseudoheader added to UDP header

20. 11.3 Checksum (cont.) Checksum calculation at receiver
Add the peudoheader to UDP user datagram
Fill the checksum field with 0s
Divide the total bits into 16-bit (2 bytes) sections
If the total number of bytes is not even, add 1 byte of padding (all 0s). The padding is only for the purpose of calculating the checksum and will be discarded afterwards.
Add all 16-bit sections using one’s complement arithmetic
Complement the result, which is a 16-bit number, and insert in the checksum field
Drop the peudoheader and any added padding
Deliver the user datagram to the IP layer for encapsulation

21. 11.3 Checksum (cont.)
Checksum calculation of a UDP user datagram

22. 11.3 Checksum (cont.) Checksum calculation at receiver
Add the peudoheader to UDP user datagram
Add padding if needed
Divide the total bits into 16-bit (2 bytes) sections
Add all 16-bit sections using one’s complement arithmetic
Complement the result
If the result is all 0s, drop the peudoheader and any added padding and accept the user datagram. If the result anything else, discard the user datagram

23. 11.4 UDP OPERATION The topics discussed in this section include:
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

24. 11.4 UDP OPERATION Connectionless services
No flow control and a simple error check
Encapsulation and Decapsulation

25. 11.4 UDP OPERATION (cont.) Queuing of UDP
A client site, when a process starts, some implements create both an incoming and outgoing queue associated with each process identified by ephemeral port number.
When process terminates, the queues are destroyed.
If an outgoing queue is overflow, the OS ask the client process to wait before sending any more messages.
When message arrived for a client, UDP checks to have been created an incoming queue for the port number of arrived user datagram. If there is no such incoming queue, UDP discard the user datagram, ask the ICMP to send a port unreachable message to the server.

26. 11.4 UDP OPERATION (cont.)
Queues in UDP

27. UDP vs. TCP communication응용
UDP
역다중화
datagram
dtatgram
다중화

28. 11.4 UDP OPERATION (cont.)
Multiplexing and Demultiplexing

29. 11.5 USE OF UDP
A simple request-response communication with little concern for flow and error control (not to send bulk data: ftp…)
A process with internal flow and error control mechanism.
Transport protocol for multicasting and broadcasting.
For management process such as SNMP
For some route updating protocols such as RIP (Routing Information Protocol)

30. 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 queue
Control-block module
Input module
Output module

31. 11.6 UDP PACKAGE (cont.)
UDP design

32. 11.6 UDP PACKAGE (cont.) Control Block Table Input Queue
Table to keep track of open ports
Table entries (state, process ID, port number, queue number)
Input Queue
One for each process
State Process ID Port Number Queue Number
IN-USE 2,345 52,
IN-USE 3,422 52,011
FREE
IN-USE 4,652 52,

33. 11.6 UDP PACKAGE (cont.) Control-Block Module Algorithm
Management of the control block table
When process starts, it asks for a port number from the OS
OS assigns well-know port numbers to server and ephemeral port numbers to client
The process passes the process ID and port number to the control block module to create an entry in the table for the process
Algorithm
Receive: a process ID and a port number.
1. Search the control block table for a FREE entry.
1. If (not found)
1. Delete an entry using a predefined strategy.
2. Create a new entry with the state IN-USE.
3. Enter the process ID and the port number.
2. Return

34. 11.6 UDP PACKAGE (cont.) Input Module Algorithm
It receives a user datagram from IP layer
It searches the control block table to find same port number as this user datagram
If the entry is found, the module uses the information in the entry to enqueue the data in the corresponding queue
If the entry is not found, it generates an ICMP message
Algorithm
Receive: a user datagram from IP
1. Look for the corresponding entry in the control-block table.
1. If (found)
1. Check the queue field to see if a queue is allocated.
1. If (no)
1. Allocate a queue.
2. Enqueue the data in the corresponding queue.
2. If (not found)
1. Ask the ICMP module to send an “unreachable port” message.
2. Discard the user datagram.
2. Return

35. 11.6 UDP PACKAGE (cont.) Output module Algorithm
Responsible for creating and sending user datagram
Algorithm
Receive: data and information from a process
Create a UDP user datagram.
Send the user datagram.
Return.

36. Example 2 Control-block table at the beginning 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.
State Process ID Port Number Queue Number
IN-USE 2, ,
IN-USE 3, ,011
FREE
IN-USE 4, ,

37. 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 does not allocate a queue at this moment because no user datagrams have arrived for this destination
State Process ID Port Number Queue Number
IN-USE 2, ,
IN-USE 3, ,011
IN-USE 4, ,014
IN-USE 4, ,
FREE

38. 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).
State Process ID Port Number Queue Number
IN-USE 2, ,
IN-USE 3, ,
IN-USE 4, ,014
IN-USE 4, ,
FREE

39. Examples 5 & 6
Example 5: After a few seconds, a user datagram arrives for port 52,222. The input module checks the table and cannot find the 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.
Example 6: After a few seconds, a process needs to send a user datagram. It delivers the data to the output module which adds the UDP header and sends it.