Transcript Document 7348660

Chapter 11
User Datagram Protocol
(UDP)
Mi-Jung Choi
Dept. of Computer Science and Engineering
[email protected]
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Contents
11.1
PROCESS-TO-PROCESS COMMUNICATION
11.2
USER DATAGRAM
11.3
CHECKSUM
11.4
UDP OPERATION
11.5
USE OF UDP
11.6
UDP PACKAGE
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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
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Position of UDP in TCP/IP
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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
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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
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11.1 PROCESS-TO-PROCESS COMMUNICATION
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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
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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)
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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
 IP address vs. port number
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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
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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
 Well-known port in UDP
Port
Protocol
Description
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Echo
Echoes a received datagram back to the sender
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Discard
Discards any datagram that is received
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Users
Active users
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Daytime
Returns the date and the time
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Quote
Returns a quote of the day
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Chargen
Returns a string of characters
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Nameserver
Domain Name Service
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Bootps
Server port to download bootstrap information
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Bootpc
Client port to download bootstrap information
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TFTP
Trivial File transfer Protocol
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RPC
Remote Procedure Call
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NTP
Network Time Protocol
161
SNMP
Simple Network Management Protocol
162
SNMP
Simple Network Management Protocol (trap)
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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 
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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
snmp
161/udp
snmptrap
162/udp
#Simple Net Mgmt Proto
#Simple Net Mgmt Proto
#Traps for SNMP
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11.1 PROCESS-TO-PROCESS COMMUNICATION (cont.)
 Socket Address
Combination of IP address and port number
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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
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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)
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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
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11.3 Checksum (cont.)
Pseudoheader added to UDP header
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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
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11.3 Checksum (cont.)
 Checksum calculation of a UDP user datagram
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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
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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
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11.4 UDP OPERATION
 Connectionless services
 No flow control and a simple error check
 Encapsulation and Decapsulation
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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.
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11.4 UDP OPERATION (cont.)
 Queues in UDP
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UDP vs. TCP communication
응용
응용
응용
TCP
송신버퍼
TCP
수신버퍼
segment
응용
응용
segment
segment
segment
응용
응용
UDP
다중화
응용
UDP
역다중화
datagram
datagram
datagram
dtatgram
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11.4 UDP OPERATION (cont.)
 Multiplexing and Demultiplexing
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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)
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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
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11.6 UDP PACKAGE (cont.)
 UDP design
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11.6 UDP PACKAGE (cont.)
 Control Block Table
Table to keep track of open ports
Table entries (state, process ID, port number, queue number)
State
Process ID Port Number
Queue Number
--------
------------
--------------
------------------
IN-USE
2,345
52,010
34
IN-USE
3,422
52,011
4,652
52,012
FREE
IN-USE
38
FREE
 Input Queue
One for each process
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11.6 UDP PACKAGE (cont.)
 Control-Block Module
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
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11.6 UDP PACKAGE (cont.)
 Input Module
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
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11.6 UDP PACKAGE (cont.)
 Output module
Responsible for creating and sending user datagram
 Algorithm
Receive: data and information from a process
1. Create a UDP user datagram.
2. Send the user datagram.
3. Return.
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Example 2
 Control-block table at the beginning
State
-------IN-USE
IN-USE
FREE
IN-USE
FREE
Process ID
-----------2,345
3,422
Port Number
-------------52,010
52,011
Queue Number
-----------------34
4,652
52,012
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 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.
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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
-------IN-USE
IN-USE
IN-USE
IN-USE
FREE
Process ID
-----------2,345
3,422
4,978
4,652
Port Number
-------------52,010
52,011
52,014
52,012
Queue Number
-----------------34
38
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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
-------IN-USE
IN-USE
IN-USE
IN-USE
FREE
Process ID
-----------2,345
3,422
4,978
4,652
Port Number
-------------52,010
52,011
52,014
52,012
Queue Number
-----------------34
43
38
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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.
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