1. User Datagram Protocol (UDP)
Lesson 8
User Datagram Protocol (UDP)

2. UDP
TCP/IP protocol suite specifies two protocols for the transport layer:UDP and TCP
Application layer
SMTP
FTP
TFTP
DNS
SNMP
BOOTP
TCP
UDP
Transport layer
IGMP
ICMP
IPv4
Network layer
ARP
RARP
Data link layer
Underlying LAN or WAN technology
Physical layer

3. (Running application program) Process (Running application program)
Port Numbers are integers between 0 and 65,535
Domain of UDP protocol
Process
(Running application program)
Process
(Running application program)
Domain of IP protocol
Internet

4. Registered Processes 49,152 65,535 1,023 1,024 49,151 Well-known
1,024
49,151
Well-known
Dynamic 13
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 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 downlaod 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)
Port number selects the process 13
IP header
UDP header 13
Socket address

5. Port
Application
Status
20/TCP
FTP - data
Official
21/TCP
FTP—control (command)
22/TCP,UDP
Secure Shell (SSH)—used for secure logins, file transfers (scp, sftp) and port forwarding
23/TCP
Telnet protocol—unencrypted text communications
25/TCP
Simple Mail Transfer Protocol (SMTP)
42/TCP,UDP
nameserver, ARPA Host Name Server Protocol
WINS
Unofficial
43/TCP
WHOIS protocol
49/TCP,UDP
TACACS Login Host protocol
52/TCP,UDP
XNS (Xerox Network Services) Time Protocol
53/TCP,UDP
Domain Name System (DNS)
54/TCP,UDP
XNS (Xerox Network Services) Clearinghouse
56/TCP,UDP
XNS (Xerox Network Services) Authentication
RAP (Route Access Protocol)[4]
57/TCP
MTP, Mail Transfer Protocol
58/TCP,UDP
XNS (Xerox Network Services) Mail
67/UDP
Bootstrap Protocol (BOOTP) Server; also used by Dynamic Host Configuration Protocol (DHCP)
68/UDP
Bootstrap Protocol (BOOTP) Client; also used by Dynamic Host Configuration Protocol (DHCP)
69/UDP
Trivial File Transfer Protocol (TFTP)
70/TCP
Gopher protocol
79/TCP
Finger protocol
80/TCP
Hypertext Transfer Protocol (HTTP)

6. UDP packet-User Datagram
UDP datagram format
Header
Data
Source port number (16 bits)
Destination port number (16 bits)
Total length 16 bits
Checksum 16 bits
Source port, Dest. Port: range from 0 to 65,535.
Length: total length of the user datagram, header plus data.
Checksum: contains three sections, a pseudoheader, the UDP header, and the data.

7. Checksum field in UDP packet
pseudoheader
32-bit source IP address
32-bit destination IP address
All 0s
8-bit protocol (17)
16-bit UDP total length
Source port address (16 bits)
Dest. port address (16 bits)
UDP total length (16 bits)
Checksum (16 bits)
Data (padding must be added to make the data a multiple of 16 bits)
All 0s 17 15
1087 13 T E S I N G
Only used in the checksum Calculation
TE:
ST:
IN: E
G0:
Optional use of the Checksum: If the checksum is not calculated, the field is filled with 0s.

8. UDP Operation Process Process a. Encapsulation b. Decapsulation
Message from process
Message from process
UDP header
UDP data
UDP header
UDP data
IP header
IP data
IP header
IP data
Frame header
Frame data
Frame header
Frame data
a. Encapsulation
b. Decapsulation
UDP is connectionless services, this means that each user datagram sent by
UDP is an independent datagram. So a process that uses UDP cannot send a stream
Of data to UDP and expect UDP to chop them into different related user datagrams.
Instead each request must be small enough to fit into one user datagram.
No flow control means the receive may overflow with incoming messages.
No error control in UDP except for the checksum. This means that the sender does not
Know if a message has been lost or duplicated. When the receiver detects an error using
The checksum, the user datagram is silently discarded.
So process which uses UDP must provide for these mechanisms.

9. UDP Operation c. Queuing Outgoing queue Outgoing queue Incoming queue
Daytime client
Daytime server
Outgoing queue
Incoming queue
Outgoing queue
Incoming queue
UDP
UDP
Port 52000
Port 13
Queuing: In UDP, queues are associated with ports.
At the client site, when a process starts, it requests a port number from the operating system. Some implementations create both an incoming and an outgoing queue associated with each process. If a process wants communicate with multiple processes, it obtains only one port number and eventually one outgoing and one incoming queue.
When a message arrives for a client, UDP checks to see if an incoming queue has been created for the port number . If yes, put into the queue, if not, send ICMP a port unreachable.

10. UDP Operation d. Multiplexing and Demultiplexing UDP (Multiplexer) UDP
Processes
Processes
UDP
(Multiplexer)
UDP
(Demultiplexer) IP IP
Only one UDP but possibly several processes that may want to use the services of UDP.
Multiplexing
At the sender site, there may be several processes that need to send user datagrams. However, there is only one UDP. This is a many-to-one relationship and requires multiplexing.
Demultiplexing
At the receiver site, there is only one UDP. However, t here may be several processes that need to reveive user datagrams.. This is a one-to-many relationship and requires demultiplexing.

11. Use of UDP
UDP is suitable for a process that requires simple request- response communication and with little concern for flow and error control. Not usually for a protocol that needs to send bulk data, such as FTP.
UDP is suitable for a process with internal flow and error- control mechanisms. For example, the Trivial File Transfer Protocol(TFTP).
UDP is a suitable transport protocol for multicasting and broadcasting.
UDP is used for management processes such as SNMP.
UDP is used for some route updating protocols such as RIP.

12. UDP IP UDP Design Processes (when started) Data Queues
Control-block module
Input module …
Output module
Control-block table
UDP User datagram IP
UDP User datagram
Five components: (two data-structures plus three modules)
Control-block table: keep tracks of the open ports. Four fields: the state, the Process ID, the port number, and the corresponding queue number.
Input Queues: s set of input queues, one for each process.

13. Control-Block Module operation
Receive: a process ID and a port number
1. Search the control block table for a FREE entry
1) If (not found), 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.
Input Module operation
Receive: a user datagram from IP
1. Look for the corresponding entry in the control-block table.
1) If (found), check the queue field to see if a queue is allocated If (no) , allocate a queue, then enqueue the data. 2) if (not found) ask the ICMP module to send an “unreachable port” message.
discard the user datagram.
2. Return.
Output Module operation
Receive: data and information from a process
1. Create a UDP user datagram.
2. Send the user datagram.
3. Return.

14. Examples
The control-block table at the beginning of examples
State
Process ID
Port Number
Queue Number
IN-USE
2,345
52,010 34
3,422
52,011
FREE
4,652
52,012 38
Example 1: arrival of a user datagram with destination port number 52,012.
Ans: the input module searches for this port number and finds that Queue number 38 has been assigned to this port. The input module sends the data to Queue 38.
Example 2: After a few seconds, a process ID: 4,978 starts. Get port number 52,014 from OS.
Ans: Now the process sends its ID 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.

15. Examples Example 3: A user datagram now arrives for port 52,011.
The control-block table at the beginning of examples
State
Process ID
Port Number
Queue Number
IN-USE
2,345
52,010 34
3,422
52,011
4,978
52,014
4,652
52,012 38
FREE
Example 3: A user datagram now arrives for port 52,011.
Ans: the input module searches for this port number and finds that No Queue number has been assigned. The input module creates a queue and gives it a number (43).
Example 4: A user datagram now arrives for port 52,223.
Ans: the input module searches for this port number and finds that No entry for this port number. The user datagram is dropped and a request is made to ICMP to send an “unreachable port” message to the source.
Example 5: A process needs to send a user datagram:
Ans: It delivers the data to the output module which adds the UDP header and sends it.

16. Practice Quiz for UDP: UDP is acronym for?
What is the maximum and minimum size of a UDP datagram? And what is the practice size of a UDP datagram?
What is the maximum and minimum size of the process data that can be encapsulated in a UDP datagram?
The following is a dump of a UDP header in hexadecimal format D 00 1C E2 17 a) what is the source port number? b) What is the destination port number? c) What is the total length of the user datagram? d) What is the length of the process data? e) Is the packet directed from a client to a server or vice versa?

17. UDP Application – Trivial File Transfer protocol (TFTP)
There are occasions when we need to simply copy a file without
the need for all of the functions of the FTP protocol.

18. TFTP, a protocol quickly copies(writes) the files
TFTP, a protocol quickly copies(writes) the files. used in bootstrap or DHCP.
Reading means copying a file from the server site to the client. Writing means copying a file from the client site to the server site.
TFTP uses the services of UDP on the well-known port 69.
TFTP has five types of message, RRQ, WRQ, DATA, ACK, and ERROR

19. RRQ- read request message
RRQ message format (the client establish a connection for reading data from the server.)
OPcode =1
File name
All 0s
Mode
2 bytes
Variable
1 byte
1 byte
Variable (netascii, (ASCII )or octet (binary file)
WRQ message format (the client establish a connection for writing data to the server.)
OPcode =2
File name
All 0s
Mode
2 bytes
Variable
1 byte
1 byte
Variable (netascii, (ASCII )or octet (binary file)

20. DATA message format (Server or client send blocks of data)
OPcode =3
Block number
Data
2 bytes
0~512 bytes
2 bytes
Block number. The sender of the data (server or client) uses this field for sequencing.
the block number is necessary for acknowledgement.
Data: this block must be exactly 512 bytes in all DATA messages except the last block
which must be between 0 and 511 bytes.
ACK message format
OPcode =4
Block number
2 bytes
2 bytes

21. OPcode =5 Error number Data All 0s Number Meaning 1 2 3 4 5 6 7
ERROR message format: is used when a connection can not be established or when there is
a problem during data transmission. It can e sent as a negative response to PRQ or WRQ. It
Can also be used if the next block can not be transferred during the actual data transfer phase.
ERROR message format
OPcode =5
Error number
Data
All 0s
2 bytes
Variable
2 bytes
1 byte
Number
Meaning
Not defined 1
File not found 2
Access violation 3
Disk full or quota on disk exceeded 4
Illegal operation 5
Unknown port number 6
File already exists 7
No such user

22. TFTP messages encapsulation
TFTP Process
TFTP Process
Message from TFTP
Message from TFTP
UDP header
UDP data
UDP header
UDP data
IP header
IP data
IP header
IP data
Frame header
Frame data
Frame header
Frame data
a. Encapsulation
b. Decapsulation
TFTP’s five types of message are encapsulated in UDP packet. This means TFTP message delivery is unreliable. So within TFTP, there must have its own mechanism to handle flow control and error control.

23. Connection
TFTP uses UDP services. There is no provision for connection establishment and termination in UDP. In TFTP, we may transfer blocks of data as they all belong to the same file. TFTP uses RRQ, WRQ, ACK and ERROR messages To establish connection. It uses the DATA message with a block of data fewer than 512 bytes (0~511) to terminate connection.
READING: TFTP client sends the RRQ message to server. 1) RRQ include the file and the transmission mode. if permits, the server responds positively with a DATA message containing the first block of data. Others, the server responds negatively by sending an ERROR message.
WRITING: TFTP client sends the WRQ message to server. 1) RRQ include the file and the transmission mode. if permits, the server responds positively with a ACK message. Others,
the server responds negatively by sending an ERROR message.
Connection Termination: termination is accomplished by sending the last block of data,
which should be fewer than 512 bytes.

24. Connection OR OR Connection for reading Connection for writing ascii
Client
Server
Client
Server
RRQ
WRQ
ascii
file1 1
ascii
file1 2
DATA
ACK 3 1
First block of data 4 1 OR OR
ERROR
ERROR 5
Error data 5
Error data
Connection for reading
Connection for writing
Connection Termination: termination is accomplished by sending the last block of data, which should be fewer than 512 bytes.

25. Data Transfer Data Transfer phase occurs between connection.
The file is divided into blocks of data, in which each block except the last one is exactly 512 bytes.
TFTP has to creates a flow-and error-control mechanism to transfer a file.
Flow Control
TFTP sends a block of data using the DATA message and waits for an ACK message. If the sender receives an acknowledge before the time-out, it sends the next block. Thus, flow control is achieved by numbering the data blocks and waiting for an ACK before the next data block is send.
Error Control
Both the sender and the receiver use time-outs. The sender uses a time-out for data messages; the receives uses a time-out for acknowledge messages. If a data message is lost, the sender retransmits it after time-out expiration. So is the same as the receiver.

26. Error Control: is needed in four situations: damaged message, lost message, lost ack, or
Duplicated message.
Damaged message: If a block of data is damaged, it will detected by the receiver and the
Block is discarded. The sender waits for the ACK and does not receive it within the time-out period. The checksum field in the UDP provide the damaged check for the message.
Lost message: If a block is lost, it never reaches the receiver and no ACK is sent. The sender resends.The block after the time-out.
Lost ACK: Two situations can happen. If the timer of the receiver matures before the timer of the sender, the receiver retransmits the ACK; otherwise, the sender retransmits the data.
Duplicate message: Duplication of blocks can be detected by the receiver through block number. If a block is duplicated, it is simply discarded by the receiver.

27. Sorcerer’s Apprentice Bug(魔法師學徒蟲)
Situation occurs when the ACK message for a message is not lost, but delayed.
Client
Server
Block 5
In this figure, ack 5 is delayed. After the time-out, the sender retransmits the fifth block, which will be acknowledged by the receiver again. The sender receives the two ack 5, which triggers it to send the sixth block twice.
ack 5
Block 5
ack 5
ack 5
Block 6
Block 6
ack 6
ack 6
Block 7
Block 7

28. UDP ports Server Client Server Client Server Client
a. Passive open by server
Passive
open
Client
Server 69
b. Active open by client
Active
open
Client
Server
50032 69
a. Rest of Communication 69
Server
Client
50032
62001

29. TFTP Example Damaged ascii file1 1 3 1 1 4 3 2 3 2 2 4 2 4 3 3 4
Client
Server
The client wants to retrieve the content of a 1336-byte file called file1. the client sends an RRQ message. The server sends the first block with 512 byte.
RRQ
ascii
file1 1 3 1
Block 1: 512 bytes 1 4
Lost 3 2
Block 2: 512 bytes 3 2
Block 2: 512 bytes 2 4
Damaged 2 4 3
Block 3: 312 bytes 3 4

30. Security
Client
Server
TELNETclient
TFTP server
TFTP client
TELNETserver
Security in TFTP must combined with other protocol such as TELNET.
The user must first access TELNET. TELNET checks whether the user
Has the right to access the system and the corresponding file. It then calls
The TFTP client and passes the file name to the client.
The client then makes the TFTP connection to TFTP server at the user site.

31. Applications
Client
Server
TFTP client
BOOTP client
BOOTP server
TFTP server 1 2 4 5 3
TFTP is very useful for basic file transfer where security is not a big issue.
It can be used to initialize devices such as bridges or routers. Its main application
Is in conjunction with the BOOTP or DHCP protocols. TFTP requires only a small amount of memory and uses the services of UDP and IP.
It can easily be configured into ROM. When the station is powered on, TFTP will be
Connected to a server ad can download the configuration files from there.
As the figure above shows the powered-on station uses the BOOTP (or DHCP) client to get the name of the configuration file from BOOTP server. The station then passes the name of the file to the TFTP client to get the contents of the configuration file from the TFTP sever.

32. Practice Quiz for TFTP:
Show the encapsulation of a WRQ message in a UDP user datagram. Assume the file name is REPORT and the mode is ASCII. What is the size of the UDP datagram?
Show the encapsulation of a TFTP data message, carrying block number 7, in a UDP user datagram. What is the total size of the user datagram?
Host A uses TFTP to read 2,150 bytes of data from host. Show all the TFTP commands including commands needed for connection establishment and termination. Assume one ACK error.