Chapter 1 - Network Models

Question 1

What is a computer network?

Answer 1

An interconnection of hosts (computer devices) over a network medium (cables or wireless signals) to share data.

Question 2

What is the OSI Model, why is it needed? What layers are involved?

Answer 2

The Open Systems Interconnection (OSI) model is a way of separating the duties of different network protocols needed for hosts to communicate in a computer network.

For eg. One network protocol might have the duty of transmitting the data across the physical medium, while another protocol might have the duty of ensuring all data is received in the correct order. These duties are each given “layers” in the OSI model. The model can be described as a protocol suite or protocol stack.

L7 Application
L6 Presentation
L5 Session
L4 Transport
L3 Network
L2 Data Link - [LLC / MAC]
L1 Physical

Question 3

Describe the Application layer of the OSI model.

Answer 3

• The actual services provided to end-users
  
  • Applications can make use of networks via API calls provided to the programmer by the OS

Question 4

Describe the Presentation layer of the OSI model.

Answer 4

Translates data from lower layers into a form usable for the application layer

Question 5

Describe the Session layer of the OSI model.

Answer 5

• Tracks sessions and names them
• Combinations of IP Addresses + Ports for Tx and Rx form a single session
• Eg. TCP 192.168.0.1:9999 192.168.0.2:443 Established [This looks like a single web session]

Question 6

Describe the Transport layer of the OSI model.

Answer 6

• Segmentation, reassembly and reliability
• Connection oriented protocol is Transmission Control Protocol which sends Segments
• Connectionless protocol is User Datagram Protocol which sends Datagrams

Question 7

Describe the Network layer of the OSI model.

Answer 7

• Packets are created to move data between networks
• The most widely used logical addressing (not fixed to hardware, independent of hardware) protocol is Internet Protocol
• Within the Internet Protocol an IP Address is used
• IP Addresses exist as;
  
  • IPv4 (older, more widespread)
  • IPv6 (newer, less adopted)
• IP Addresses are network-unique within a network [no two hosts have identical IPs]

Question 8

Describe the Data Link layer of the OSI model.

Answer 8

• For moving data between devices (hop-to-hop)
• 48 bit value stored on ROM within NIC
• This value is a unique address for the computing system
• 48 bits = 48/8 Bytes = 6 Bytes = 6 * 2 hex digits = 12 hex digits
• 6 hex digits Organisationally Unique Identifier vendor || 6 hex digits Device ID
• Pairs of hex digits delimited by : Mac/Linux or - Windows
• This MAC Address is also known as MAC-48 or EUI-48
• Frames (discrete chunks of bits) are transmitted to the physical layer
  
  • Frames are used so that no one host can “hog” the connection medium
  • Rx MAC Address , Tx MAC Address Header
  • Type
  • Data Payload
  • Frame Check Sequence Trailer
• Frames sent to a Central Box
  
  • Hub Sends frame to all non-Tx connected devices Considered a physical layer device as it cannot process MAC Addresses, just repeats the signal to all non-input interfaces
  • Switch Sends frame across interface (connection) associated with the Rx MAC Address only
• Broadcast frames are used if the Rx MAC Address is unknown to the Tx (in a protocol known as Address Resolution Protocol)
  
  1. Tx sends frame FF:FF:FF:FF:FF:FF (equivalent to 111111….1 48 bits worth) to the central box, alongside the Rx IP Address
  2. All hosts on the network receive the broadcast frame and check the Rx IP Address to see if it is theirs
  3. Discard if not their IP Address // If is their IP Address reply to Tx host with their respective MAC Address
• The Data Link layer actually comprises two sublayers;
• Logical Link Control
  
  • NIC ←→ OS
  • Network protocols
  • Flow Control
• Media Access Control
  
  • Creates frames
  • Attaches MAC addresses to frames
  • Evaluates the FCS
  • Transmission of frames across the physical medium i.e. NIC ←→ Physical Medium

Question 9

Describe the Physical layer of the OSI model.

Answer 9

• Cabling
• Hubs (Can’t process MAC Addresses, dumb repeaters)
• How to encode bits in the medium (eg. high/low voltage etc.)

Question 10

Compare the concepts of encapsulation and decapsulation.

Answer 10

• Encapsulation = Appending extra data to a payload in order to move it down the OSI layers (adding headers or trailers)
• Decapsulation = Reading and stripping extra data from a payload to move it up the OSI layers (removing headers or trailers)

Question 11

Commands to output Physical Address on different OS.

Answer 11

Linux ip a (or ifconfig)
Mac ifconfig
Windows ipconfig