1. DICT 301: Computer Networks
Open Systems Interconnection(OSI) Reference Model
Engr. Md. Nazim Uddin
B.Sc. Engg., M.Sc. Eng., Ph.D. (on going)

2. Content TCP/IP Protocol Model OSI Reference Model
Data Encapsulation and Decapsulation
Communication over the Network
Binary Number System and Conversion
Hands on

3. Protocol Layers (TCP/IP Model)

4. Network Protocols Pre-determined rules that govern communications
A group of inter-related protocols that are necessary to perform a communication function is called a protocol suite

5. Role of Protocols
The format or structure of the message, such as how much data to put into each segment
The process by which intermediary devices share information about the path to the destination
The method to handle error and system messages between intermediary devices
The process to setup and terminate communications or data transfers between hosts

6. Layered Task Networks are complex!
We use the concept of layers in our daily life.
Networks are complex!
Many “pieces/ devices/ parts”:
hosts
routers
links of various media
applications
protocols
hardware, software

7. Organization of air travel
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
a series of steps

8. Layering of airline functionality
ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departure
airport
arrival
intermediate air-traffic
control centers
ticket (complain)
baggage (claim
gates (unload)
runway (land)
ticket
baggage
gate
takeoff/landing
Layers: each layer implements a service
via its own internal-layer actions
relying on services provided by layer below

9. Why a layered model? Easier to teach communication process.
Speeds development, changes in one layer does not affect how the other levels works.
Standardization across manufactures.
Allows different hardware and software to work together.
Reduces complexity

10. Internet protocol stack (TCP/IP model)
application: supporting network applications
FTP, SMTP, HTTP
transport: host-host data transfer
TCP, UDP
network: routing of datagrams from source to destination
IP, routing protocols
link: data transfer between neighboring network elements
PPP, Ethernet
physical: bits “on the wire”
application
transport
network
link
physical

11. TCP/IP Reference Model
Layer
Protocols
Application
HTTP
TELNET
FTP
SMTP
SNMP
Transport
TCP
UDP
Internet IP
ICMP
Network Access (Host-to-network)
ETHERNET
PACKET RADIO

12. Encapsulation destination source application transport network link
message M
application
transport
network
link
physical
segment Ht M
datagram Ht Hn M
frame Ht Hn Hl M
link
physical Ht Hn Hl M Ht Hn Hl M
switch
destination
network
link
physical Ht Hn M Ht Hn M M
application
transport
network
link
physical Ht Hn Hl M Ht Hn Hl M Ht M Ht Hn M
router Ht Hn Hl M

13. Protocols at the application layer
HTTP:
browser and web server communication
FTP :
file transfer protocol
TELNET:
remote login protocol
POP3: Retrieve
POP3 is designed to delete mail on the server as soon as the user has downloaded it
IMAP (Internet Message Access Protocol )
Retrieve s,
retaining on the server and for organizing it in folders on the serve

14. Protocols at the transport layer
Transmission control protocol (TCP),
Connection oriented
Connection established before sending data
Reliable
user datagram protocol (UDP)
Connectionless
Sending data without establishing connection
Fast but unreliable

15. Protocol at the network layerIP
Path selection ,
routing and addressing
ICMP (Internet Control Message Protocol )
sends error messages relying on IP
a requested service is not available
a host or router could not be reached
Protocols at the link layer
Ethernet
Uses CSMA/CD
Token Ring

16. Data Formats Application data data data data data data message
layer
transport
layer
TCP
header
data
TCP
header
data
TCP
header
data
segment
network
layer IP
header
TCP
header
data
packet
data link
layer
Ethernet
header IP
header
TCP
header
data
Ethernet
trailer
frame

17. Packet Encapsulation (TCP/IP)
The data is sent down the protocol stack
Each layer adds to the data by pretending headers
22Bytes
20Bytes
20Bytes
4Bytes
64 to 1500 Bytes

18. Encapsulation Data usually transferred in blocks
Protocol data units (PDUs)
Each PDU contains data and control information
Some PDUs only control
Three categories of control
Address
Of sender and/or receiver
Error-detecting code
e.g. frame check sequence
Protocol control
Additional information to implement protocol functions
Addition of control information to data is encapsulation
Data accepted or generated by entity and encapsulated into PDU
Containing data plus control information
e.g. TFTP, HDLC, frame relay, ATM, AAL5, LLC, IEEE 802.3, IEEE
[Advance]

19. Benefits of a Layered Model
Assists in protocol design
protocols that operate at a specific layer have defined information that they act upon and a defined interface to the layers above and below
Fosters competition
products from different vendors can work together
Prevents technology or capabilities changes in one layer from affecting other layer above and below
Provides a common language to describe networking functions and capabilities
There are benefits to using a layered model to describe network protocols and operations.
[Advance]

20. OSI Reference Model

21. Protocol and Reference Models
2 types of networking models
A protocol model provides a model that closely matches the structure of a particular protocol suite
Transmission Control Protocol/Internet Protocol (TCP/IP)
A reference model provides a common reference for maintaining consistency within all types of network protocols and services
it is not intended to be an implementation specification or to provide a sufficient level of detail to define precisely the services of the network architecture
the primary purpose is to aid in clearer understanding of the functions and processes involved
Open Systems Interconnection (OSI) model

22. The OSI Reference Model
The OSI reference model is the primary model for network communications.
Allows you to view the network functions that occur at each layer.
It is a framework that you can use to understand how information travels throughout a network
7 layers -- each of which illustrates a particular network function.
APS TN DP

23. Summary of layers

24. TCP/IP and OSI Models
The TCP/IP model describes the functions that occur at layer of protocols within the TCP/IP suite (Protocol Model)
The OSI model is used for network design, operation specifications and troubleshooting (Reference Model)

25. Comparing the OSI and TCP/IP Models
The functions of the Application, Presentation and Session layers of the OSI model are combined into one Application layer in the TCP/IP model
The Data Link and Physical layers of the OSI model combine to make the Network Access layer of the TCP/IP model
The key parallel between the two models occur at layers 3 and 4 of the OSI model
TCP operates at the transport layer
IP operates at the Internet layer

26. Communicating over the Network

27. Communication Process
Creation of data at the application layer of the originating source end device
Segmentation and encapsulation of data as it passes down the protocol stack in the source end device
Generation of the data onto the media at the network access layer of the stack
Transportation of the data through the internetwork, which consists of media and any intermediary devices
Reception of the data at the network access layer of the destination end device
Decapsulation and reassembly of the data as it passes up the stack in the destination device
Passing this data to the destination application at the application layer of the destination end device
The above seven steps in the communication process are indicated in the next slide.

28. Communication Process cont…1 2 3 4 5 6 7
Source
Destination
This slide shows the seven steps in the communication process.

29. Communication with OSI Model
Data
(Port) Segment
(IP) Packet
(MAC) Frame

30. Device Operation Layer
Devices
Operate at Layer
Computer
Layer-7 (6,5,4)
Router
Layer-3
Switch
Layer-2
Hub
Layer-1

31. The virtual interaction between layers
[Advance]

32. Protocol Data Unit and Encapsulation
Protocol data unit (PDU) is the generic term for “data” at each level
Encapsulation is the process of adding control information as it passes through the layered model

33. Sending Process
The Application layer begins the process by delivering the data to the Transport layer
The Application data is broken into TCP segments and each segment is given a label, called a header
the header contains information about which process running on the destination computer should receive the message
it also contains the information to enable the destination process to reassemble the segments
The TCP segment is sent to the Internet layer and encapsulated within an IP packet, which adds an IP header
the IP header contains the source and destination IP addresses
In the web server example, the TCP/IP model is used to illustrate the process of sending an HTML web page to a client.

34. Sending Process cont…
The IP packet is sent to the Network Access layer where it is encapsulated with a frame header and trailer
each frame header contains the source and destination physical addresses
the trailer contains error checking information
Finally the bits are encoded onto the Ethernet media by the end- device network interface card (NIC)

35. Sending Process cont…

36. Receiving Process
This process is reversed at the receiving host. The data is decapsulated as it moves up the stack toward the end user application.

37. Data Encapsulation and Decapsulation

38. An exchange using the OSI model
[Advance]

39. How the OSI and TCP/IP Models Relate in a Networking Environment

40. Addressing

41. Addressing
Various types of addresses are needed to successfully deliver the data from a source application running on one host to the correct destination application running on another
Data Link physical addresses (MAC)
Network logical addresses (IP)
Transport service port numbers (Port)
MAC Address
IP Address
Port Number

42. MAC Addresses (6 Byte unique)
A MAC address is a unique 6-byte address that is burned into each network interface or more specifically, directly into the PROM chip on the NIC.

43. IPv4 Address Each device on a network must be uniquely defined.
The packets of the communication need to be identified with the source and destination addresses of the two end systems
IPv4 is 32 bits in length and difficult for human to remember. Therefore, represent IPv4 addresses using dotted decimal format.
( )
IP has two portion: network and host
network
host

44. Layer 2 Address (MAC address)
This is the physical address of the host (or end device)
in a LAN using Ethernet, this address is called the Media Access Control (MAC) address
Layer 2 addresses are used to communicate between devices on a single local network
(packet)

45. Layer 3 Address (IP address)
This is the logical address
Layer 3 addresses are primarily used to move data from one local network to another local network
enable intermediary network devices to locate hosts on different networks
(segment)
Network
Device
230

46. Why MAC and IP both Address?
Source and destination MAC change each time to transport packet from one device to another device.
Source and destination IP address is fixed.
[Advance]

47. Layer 4 Service Port Number
At layer 4, the information contained in the PDU header identifies the specific process or service running on the destination host device
the separately running programs are examples of individual processes

48. Service Port Example Bank Different Service, different Counter
Remittance Service Counter
Cash Pay Counter
Cash Receive Counter
Electric Bill Counter
Information Desk
………
Different Service, different Counter
Networking (PC)
File Transfer/ FTP
/ SMTP
Web/ HTTP
Telnet
……….
Different Service, different Port

49. Service Port List Service Protocol Port Mail SMTP 25 POP3 POP, POP3
110
Web
HTTP 80
File Transfer
FTP
20, 21
Telnet 23
Domain Name Resolution
DNS 53
Dynamic Host Configuration
DHCP
67, 68

50. Port numbers Well known ports (Numbers 0 to 1023)
Registered Ports (Numbers 1024 to 49151)
Dynamic or Private Ports (Numbers to 65535)
The Internet Assigned Numbers Authority (IANA) assigns port numbers.
netstat

51. Relationship of layers and addresses in TCP/IP
[Advance]

52. Getting Data to the End Device
The host physical address, is contained in the header of the Layer 2 PDU, called a frame.
Layer 2 is concerned with the delivery of messages on a single local network.
The Layer 2 address is unique on the local network and represents the address of the end device on the physical media.
In a LAN using Ethernet, this address is called the Media Access Control (MAC) address.
When two end devices communicate on the local Ethernet network, the frames that are exchanged between them contain the destination and source MAC addresses.
Once a frame is successfully received by the destination host, the Layer 2 address information is removed as the data is decapsulated and moved up the protocol stack to Layer 3.
[Advance]

53. Getting the Data Through the Internetwork
Layer 3 protocols are primarily designed to move data from one local network to another local network within an internetwork.
Layer 3 addresses must include identifiers that enable intermediary network devices to locate hosts on different networks
At the boundary of each local network, an intermediary network device, usually a router, decapsulates the frame to read the destination host address contained in the header of the packet, the Layer 3 PDU
Routers use the network identifier portion of this address to determine which path to use to reach the destination host.
[Advance]

54. Getting Data to the Right Application
Think about a computer that has only one network interface on it. How to differentiate various type of data?
Each application or service is represented at Layer 4 by a port number
When the data is received at the host, the port number is examined to determine which application or process is the correct destination for the data
Example of popular port numbers?
[Advance]

55. OSI Reference Model at a Glance

56. Division of Layers 7. Application 6. Presentation Upper Layers
5. Session
4. Transport
Middle Layer
3. Network
2. Data Link
Lower Layers
1. Physical

57. OSI – The Application Layer
Provides network services to the user's applications.
It does not provide services to any other OSI layer
***Think of any network application you use daily

58. OSI – The Presentation Layer
It ensures that the information that the application layer of one system sends out is readable by the application layer of another system.
*** Think of any common file formats (JPEG, txt etc)

59. OSI – The Session Layer
*** After you prepare your data, you need to establish the communication channels to send data
This layer establishes, manages, and terminates sessions between two communicating hosts.
It also synchronizes dialogue between the two hosts' presentation layers and manages their data exchange.

60. Application Layer Protocols
Domain Name Service (DNS)
used to resolve Internet names to IP addresses
Hypertext Transfer Protocol (HTTP)
used to transfer files that make up the web pages of the world wide web
Simple Mail Transfer Protocol (SMTP)
used for the transfer of mail messages and attachments
Telnet (terminal emulation protocol)
used to provide remote access to servers and networking devices
File Transfer Protocol (FTP)
used for interactive file transfers between systems
These protocols specify the format and control information necessary for many of the common Internet communication functions.

61. Application Layer Software
Network-aware applications
implement the application layer protocols and are able to communicate directly with the lower layers of the protocol stack
clients and web browsers
Application layer services
are programs that interface with the network and prepare the data for transfer
different types of data (text, graphics or video) require different network services to ensure that it is properly prepared for processing by the functions occurring at the lower layers of the OSI model
Within the Application layer, there are two forms of software programs or processes that provide access to the network: applications and services.

62. DNS Service and Protocol
DNS uses TCP/UDP port 53
Domain names were created to convert the numeric IP address into a simple, recognizable name
domain names are easier to remember than actual numeric addresses
any change to the address is transparent to the user since the domain name is the same

63. DNS Directory Structure
Uses a hierarchical structure to create a name database
hierarchy is an inverted tree with the root at the top and branches below
The different top-level domains (TLD) represent either the country origin or the type of organization
After TLD are second-level domain names, and below them are other lower level domains
[Advance]

64. WWW Service and Protocol
The web browser establishes a connection to the web service running on the server using the Hypertext Transfer Protocol (HTTP)
request a page using an URL (uniform resource locator) or web address
a web browser is a client application running on a client device
HTTP uses TCP port 80
An URL consists of three parts
the protocol – hypertext transfer protocol (http)
the server name
the specific file name requested
Browsers can interpret and present many data types
plain text and Hypertext Markup Language (HTML)
other data types require another service or program such as plug-ins or add-ons

65. WWW Service and HTTP cont…
The web client makes a connection to the HTTP server and requests a page
In response to the request, the HTTP server returns the code for a web page
The browser interprets the HTML code and displays a web page

66. HTTP Protocol HTTP specifies a request/response protocol
HTTP protocol uses three common messages – GET, POST and PUT
HTTP is not a secure protocol
POST messages in plain text can be intercepted and read
HTML pages are not encrypted
HTTP Secure (HTTPS) protocol can use authentication and encryption to secure the data
The Hypertext Transfer Protocol (HTTP), one of the protocols in the TCP/IP suite, was originally developed to publish and retrieve HTML pages and is now used for distributed, collaborative information systems.

67. E-mail Services and SMTP/POP3
User composes an using an application called a mail user agent (MUA) or client
Client sends s to a server using Simple Mail Transfer Protocol (SMTP) and receives s using Post Office Protocol version 3 (POP3)
SMTP uses TCP port 25
POP uses UDP port 110

68. Mail Transfer Agent The MTA process is used to forward e-mail
the MTA receives messages from a MUA or another MTA
based on the message header, it determines how a message has to be forwarded to reach its destination
if the recipient resides on the local server, the mail is passed to the mail delivery agent (MDA)
if the recipient is not on the local server, the MTA routes the to the MTA on the appropriate server
The server operates two separate processes:
Mail Transfer Agent (MTA)
Mail Delivery Agent (MDA)

69. Mail Delivery Agent
The MDA receives the inbound mail from the MTA and delivers the mail to the appropriate users’ mailboxes
It can also resolve final delivery issues, such as virus scanning, spam filtering and return-receipt handling
The server operates two separate processes:
Mail Transfer Agent (MTA)
Mail Delivery Agent (MDA)

70. SMTP and POP POP and POP3 are inbound mail delivery protocols
MDA listens for a client connection to the server
SMTP governs the transfer of outbound from the sending client to the MDA, as well as the transport of between MTA
enables to be transported across data networks between different types of server and client software
[Advance]

71. File Transfer Protocol
FTP uses TCP ports 20 (data) and 21 (commands and replies)
An FTP client is used to push and pull files from a server running the FTP daemon (FTPd)
Client establishes the first connection to the server on TCP port 21
for control traffic such as client commands and server replies
Client establishes the second connection to the server over TCP port 20
for file transfer in both direction
TCP port 21
TCP port 20

72. Telnet Telnet uses TCP port 23
Provides a method of emulating text-based terminals over the network
allows a local device to access a remote device as if the keyboard and monitor are connected to the remote device directly
A connection using Telnet is called a virtual terminal (VTY) session
The Telnet server runs a service called the Telnet daemon

73. OSI – The Transport Layer
Data will be segmented and send to destination device. Transport layer of destination device will reassemble them.
This layer handles details of reliable transfer. (ensures that the data arrive completely )

74. Basic functions of the Transport layer
Segmentation and Reassembly
Conversation Multiplexing
Plus :
Connection-oriented conversations
Reliable delivery
Ordered data reconstruction
Flow control
Protocols
The two most common Transport layer protocols of TCP/IP protocol suite are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

75. User Datagram Protocol (UDP)
UDP is a simple, connectionless protocol, described in RFC 768.
It has the advantage of providing for low overhead data delivery.
The pieces of communication in UDP are called datagrams.
These datagrams are sent as "best effort" by this Transport layer protocol.
8 bytes overhead

76. User Datagram Protocol
Applications that use UDP include:
Domain Name System (DNS)
Video Streaming
Voice over IP (VoIP)

77. Transmission Control Protocol (TCP)
TCP is a connection-oriented protocol, described in RFC 793.
Each TCP segment has 20 bytes of overhead in the header encapsulating the Application layer data.
Same order delivery
Reliable delivery
Flow control.

78. Transmission Control Protocol
Applications that use TCP are:
Web Browsers
File Transfers

79. OSI – The Network Layer
Many paths to the same destination. So, which path to follow?
Segmented data needs address to reach the destination (network address)
This layer handle 2 above stated issues.

80. Network Layer Processes
Addressing
Network layer must provide a method for addressing the end devices
each device must have a unique address
Encapsulation
Network layer receives the Layer 4 PDU and adds a Layer 3 header
the Layer 3 header contains a source address, destination address and other control information
this Layer 3 PDU is known as a packet
the packet is sent down to the Data Link
layer
Routing
the Network layer must provide services to direct these packets to the destination host
intermediary devices, called routers, are used to direct packets toward the destination
[Advance]

81. Network Layer Processes cont…
Decapsulation
the destination host examines the destination address to verify that the packet was addressed to this device
the packet is decapsulated by the Network layer and the Layer 4 PDU contained in the packet is passed up to the appropriate service at the Transport layer
encapsulation
decapsulation
[Advance]

82. OSI – The Data Link Layer
It provides means for exchanging data frames over a common media
To detect and possibly correct errors that may occur in the Physical layer
Physical Addressing, topologies and flow control

83. OSI – The Physical Layer
It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.
Voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes defined by physical layer specifications.

84. Hands on

85. Binary Number System Decimal: 10 digit 0 1 2 3 4 5 6 7 8 9
Octal: 8 digit
Hexadecimal: 16 digit A B C D E F
Binary: 2 digit 0 1
Computer only understand binary
Decimal:
Binary:
{0,1} each called a bit , Binary Digit = Bit
8 bit = 1 Byte
1000 Byte (1024) = 1 Kilobyte
1000 Kilobyte (1024) = 1 Megabyte
1000 Megabyte (1024) = 1 Gigabyte
1000 Gigabyte (1024) = 1 Terabyte …
MAC- Hexadecimal
IP- Decimal

86. Decimal to Binary Conversion and vise versa

87. Thank you…