The document discusses the seven layers of the OSI model and provides details about each layer. It describes the functions and protocols associated with each layer, including the physical, data link, network, transport, session, presentation, and application layers. Examples are provided to illustrate concepts like encapsulation and the OSI model's role in network communication.

1. YARMOUK UNIVERSITY
CS 492: Special Topics
Preliminaries to Network Monitoring
Dr. Ahmed Manasrah

2. Table of Contents
OSI Model (Open System Interconnection Model) ................................................................... 4
Layer 1 - Physical Layer .......................................................................................................... 4
Layer 2 - Data Link Layer (LLC and MAC) ............................................................................... 4
Functions ........................................................................................................................... 5
Layer 3 - Network Layer............................................................................................................. 5
Functions ........................................................................................................................... 5
Protocols ............................................................................................................................ 5
Layer 4 - Transport Layer ....................................................................................................... 5
Functions ........................................................................................................................... 6
Protocols ............................................................................................................................ 6
Layer 5 - Session Layer........................................................................................................... 6
Protocols ................................................................................................................................ 6
Layer 6 - Presentation Layer .................................................................................................. 6
Protocols ............................................................................................................................ 7
Layer 7 - Application Layer .................................................................................................... 7
Protocols ............................................................................................................................ 7
Network Communication through OSI: an Example ................................................................. 8
Network Protocols Header, size, format and encapsulation .................................................. 11
IP Header Format ................................................................................................................ 11
TCP Header Format ............................................................................................................. 12
UDP Header Format ............................................................................................................ 13
ARP operation and header structure (Format) ................................................................... 13
ICMP header structure (Format) ......................................................................................... 14
Ethernet Frame Format ....................................................................................................... 15
Examples .................................................................................................................................. 16
IP and ICMP Header format ................................................................................................. 16
Ethernet and ARP Format .................................................................................................... 17
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3. List of Figures
Figure 1: OSI Reference Model .................................................................................................. 4
Figure 2: Network Communication Example (OSI emulation) .................................................. 9
Figure 3: Data Encapsulation example .................................................................................... 10
Figure 4. IP packet Format. ...................................................................................................... 11
Figure 5. The basic structure of a TCP packet ......................................................................... 12
Figure 6. The basic structure of a UDP packet ........................................................................ 13
Figure 7. ARP is used to map a device's data-link identifier to its IP address (ARP format) ... 13
Figure 8. ICMP packet format and Types. ............................................................................... 14
Figure 9: Ethernet Frame Format ............................................................................................ 15
Figure 10: IP and ICMP Encapsulation Example ...................................................................... 16
Figure 11: : Ethernet frame and ARP Encapsulation Example ................................................. 17
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4. OSI Model (Open System Interconnection Model)
The OSI, or Open System Interconnection, model defines a network
framework
for implementing protocols in seven layers. Control is passed from one layer to the
next, starting at the application layer laye
in one station, and proceeding to the bottom
layer, over the channel to the next station and back up the hierarchy.
Layer 1 - Physical Layer
Physical layer defines the cable or physical medium itself
Functions
networking framewor
· Transmits raw bit stream over physical cable.
· Defines cables, cards, and physical aspects.
· Defines NIC attachments to hardware, how cable is attached to NIC.
· Defines techniques to transfer bit stream to cable.
Layer 2 - Data Link Layer (LLC and MAC)
Data Link layer defines the format of data on the network. A network data frame
includes checksum, source and destination address, and data. The largest packet that
can be sent through a data link layer defines
Ethernet addresses a host using a unique, 48
Media Access Control (MAC) address.
4
Figure 1: OSI Reference Model
itself.
ata the Maximum Transmission Unit (MTU).
48-bit address called its Ethernet address or
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Functions
· Turns packets into raw bits 100101 and at the receiving end turns bits into
packets.
· Handles data frames between the Network and Physical layers.
· The receiving end packages raw data from the Physical layer into data frames
for delivery to the Network layer.
· This layer defines the methods used to transmit and receive data on the
network.
Layer 3 - Network Layer
this layer uses Internetwork Protocol (IP) as its network layer interface. IP is
responsible for routing, directing datagrams from one network to another.
Functions
· Translates logical network address and names to their physical address (e.g.
computername ==> MAC address).
· Responsible for addressing, determining routes for sending and managing
network problems such as packet switching, data congestion and routing.
· If router can’t send data frame as large as the source computer sends, the
network layer compensates by breaking the data into smaller units. At the
receiving end, the network layer reassembles the data.
Protocols
IP
ARP
RARP
ICMP
Layer 4 - Transport Layer
Transport layer subdivides user-buffer into network-buffer sized datagrams and
enforces desired transmission control. Two transport protocols, Transmission Control
Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer.
Reliability and speed are the primary difference between these two protocols. TCP
establishes connections between two hosts on the network through 'sockets' which are
determined by the IP address and port number. TCP keeps track of the packet delivery
order and the packets that must be resent. Maintaining this information for each
connection makes TCP a stateful protocol. UDP on the other hand provides a low
overhead transmission service, but with less error checking.

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Functions
· Additional connection below the session layer.
· Manages the flow control of data between parties across the network.
· Divides streams of data into chunks or packets; the transport layer of the
receiving computer reassembles the message from packets.
· Provides error-checking to guarantee error-free data delivery, with on losses or
duplications
· Provides acknowledgment of successful transmissions; requests retransmission
if some packets don’t arrive error-free.
· Provides flow control and error-handling.
Protocols
TCP
UDP
Layer 5 - Session Layer
The session protocol defines the format of the data sent over the connections. Login
sessions uses TCP.
Functions
· Establishes, maintains and ends sessions across the network.
· Responsible for name recognition (identification) so only the designated
parties can participate in the session.
· Provides synchronization services by planning check points in the data stream
=> if session fails, only data after the most recent checkpoint need be
transmitted.
· Manages who can transmit data at a certain time and for how long.
· Examples are interactive login and file transfer connections, the session would
connect and re-connect if there was an interruption; recognize names in
sessions and register names in history.
Protocols
NetBIOS
Layer 6 - Presentation Layer
External Data Representation (XDR) sits at the presentation level. It converts local
representation of data to its canonical form and vice versa.
Functions
· Translates from application to network format and vice-versa.

7. · All different formats from all sources are made into a common uniform format
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that the rest of the OSI model can understand.
· Responsible for protocol conversion, character conversion,data encryption /
decryption, expanding graphics commands, data compression.
· Sets standards for different systems to provide seamless communication from
multiple protocol stacks.
· Not always implemented in a network protocol.
Protocols
MIME
Layer 7 - Application Layer
Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are
examples of network applications.
Functions
· Used for applications specifically written to run over the network.
· Allows access to network services that support applications.
· Directly represents the services that directly support user applications.
· Handles network access, flow control and error recovery.
· Example apps are file transfer,e-mail, NetBIOS-based applications.
Protocols
DNS

8. Network Communication through OSI: an Example
Assume two networked computers running identical operating systems and
applications and are using identical protocols (or rules) at all OSI layers. Working in
conjunction, the applications, the OS, and the hardware implement the seven
functions described in the OSI model.
Each computer is also running an e-mail program that is independent of the OSI
layers. The e-mail program enables the users of the two computers to exchange
messages. The figure represents the transmission of one message from Sam to
Charlie.
The transmission starts when Sam types in a message to Charlie and presses the
"send" key. Sam's operating system appends to the message (or "encapsulates") a set
of application-layer instructions (OSI Layer 7) that will be read and executed by the
application layer on Charlie's computer. The message with its Layer 7 header is then
transferred to the part of the operating system that deals with presentation issues (OSI
Layer 6) where a Layer 6 header is appended to the message. The process repeats
through all the layers until each layer has appended a header. The headers function as
an escort for the message so that it can successfully negotiate the software and
hardware in the network and arrive intact at its destination.
When the data-link-layer header is added at Layer 2, the data unit is known as a
"frame." The final header, the physical-layer header (OSI Layer 1) tells the hardware
in Sam's computer the electrical specifics of how the message will be sent (which
medium, at which voltage, at which speed, etc.). Although it is the final header to be
added, the Layer 1 header is the first in line when the message travels through the
medium to the receiving computer.
When the message with its seven headers arrives at Charlie's computer, the hardware
in his computer is the first to handle the message. It reads the instructions in the Layer
1 header, executes them, and strips off the header before passing the message to the
Layer 2 components. These Layer 2 components execute those instructions, strip off
the header, and pass the message to Layer 3, and so on. Each layer's header is
successively stripped off after its instructions have been read so that by the time the
message arrives at Charlie's e-mail application; the message has been properly
received, authenticated, decoded, and presented.
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Figure 2: Network Communication Example (OSI emulation)

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The whole encapsulation process appears as the following figure.
Figure 3: Data Encapsulation example

11. Network Protocols Header, size, format and encapsulation
IP Header Format: this header generated at layer 3, the format of the protocol as
follows.
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Another view if the IP header structure,
Figure 4. IP packet Format.

12. TCP Header Format: this header generated at layer 4, the format of the protocol as
follows. TCP is a connection oriented protocol that regulates the flow of data.
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Another view if the TCP header structure,
Figure 5. The basic structure of a TCP packet

13. UDP Header Format: this header generated at layer 4, the format of the protocol as
follows. UDP is a connectionless protocol.
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Figure 6. The basic structure of a UDP packet
ARP operation and header structure (Format) : this protocol is used to map
the IP address to its MAC address
Figure 7. ARP is used to map a device's data-link identifier to its IP address (ARP format)

14. ICMP header structure (Format) : this protocol is used for error reporting
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Figure 8. ICMP packet format and Types.

15. Ethernet Frame Format: look at this as the content of the data inside the cable.
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Figure 9: Ethernet Frame Format

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Examples
IP and ICMP Header format
Figure 10: IP and ICMP Encapsulation Example

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Ethernet and ARP Format
Figure 11: : Ethernet frame and ARP Encapsulation Example