The OSI Model
OSI Model Consider the network models we have already covered Whenever data is transferred from PC to PC or PC to Server it will travel through the Layers of the OSI Model
OSI Model
OSI Model Why do you need to Learn about OSI? Network professionals use the OSI to communicate with each other in the workplace! OSI = Open Systems Interconnection Developed by Computer Networking industry at ISO (International Standardization Organization) meetings to allow for equipment from different manufacturers to work together.
OSI Layers
OSI Model Mnemonic Layer 7 All A > Application Layer 6 People P > Presentation Layer 5 Seem S > Session Layer 4 To T > Transport Layer 3 Need N > Network Layer 2 Data D > Data Link Layer 1 Processing P > Physical
Software Engineers v s Network Engineers
Software Engineer Layers
Software Engineering Layers
Layer 7 Application Layer
Layer 7 Application Layer Layer 7 = Application Layer What If we had no GUI? We would need to write code\commands! For Example: The HTTP Get Request: GET /index.html HTTP/1.1 Host: www.google.co.uk
Layer 6 Presentation Layer Format the Data Correctly! Ensure it is Readable: Compression - LZ (Lossless) MP3 (Lossy) Encryption - SSL, IPSec File Formats ASCII, MIME, XML, RTF (Text) JPEG, GIF, PNG (Picture) MPEG, MP4 (Video) MP3 (Audio)
Layer 5 Session Layer Layer 5 = Session Layer Start a Session! TCP 3 Way Handshaking > 1/. Client 1 Q: Would You Like to Have a Conversation? 2/. Client 2 A: Yes! 3/. Client 1 A: OK Lets Start Talking!
Layer 5 Session Layer Layer 5 = Session Layer Multiple Sessions Session Authentication e.g. Logging In To Online Banking or estudy Consider Browser Tabs 1 x Applications, 3 x Sessions Open!
Day in the life of a Packet Pt.1
Network Engineer Layers
Layer 4 - Transport A Day in the Life of a Packet! Email Example Part 2 Layer 4 = Transport Layer Logical Port Numbers Encapsulation = Split the Data into Segments Encapsulation = Label Logical Port Numbers Get the Data to the Correct Application From: SMTP Port#25 (Send or Receive?) To: POP3 Port#110
Layer 3 Network Layer A Day in the Life of a Packet! Email Example Part 3 Encapsulation = Packets Remember: The IP Address is looked up using DNS Label From: Source IP Address e.g. 192.168.1.10 Label To: Destination IP Address e.g. 192.168.2.10 Using a Mask of 255.255.255.0 Are these Clients in the Same Network?
Layer 2 Data Link Layer Layer 2 = Data Link Layer Media Independence Different Type of Frames for Different Media e.g. Copper or Fibre optic Cabling A Day in the Life of a Packet! Email Example Part 4 Encapsulation = Frames Label From: Source MAC Address e.g. 00-11-22-33-44-55 Label To: Destination MAC Address e.g. 66-77-88-99-AA-BB (Note: MAC is a Globally Unique Identifier on all NICs it is made up of Hexadecimal digits 0-9 and A-F) Layer 2 vs Layer 3 Addressing Local LAN (MAC) vs Remote WAN (IP) Addressing IP Address Stays the Same! MAC Changes Every Step
Layer 2 Data Link Layer Q: Layer 2 = Data Link Layer How Does the Client Know the MAC Address? A: ARP (Address Resolution Protocol) (Cover Later!)
Layer 2 Data Link Layer ARP is used to perform IP address resolution which is the linking of a Layer 3 IP addresses to a Layer 2 MAC (Media Access Control) hardware addresses. ARP uses a broadcast to do this by asking the host that has the given IP address to respond to the broadcast (ARP REQUEST = WHO HAS XX.XX.XX.XX.XX.XX TELL 172.19.13.1) with its MAC address (ARP REPLY = I HAVE XX.XX.XX.XX.XX.XX @ 172.19.13.50). Broadcast traffic goes to all hosts in the LAN, so all hosts will receive the ARP request and the specific host will be the only one to reply. Entries in ARP cache are kept for a maximum of 10 minutes. An entry that is not used within 2 minutes is deleted. This allows for faulty hardware being removed from the network and the IP address being assigned to different physical hardware.
Layer 1 Physical Layer
Layer 1 Physical Layer Represent the Data as Electrical or Light Pulses
OSI Model