OSI Network Layer

1. OSI Network Layer Network Fundamentals – Chapter 5

2. Objectives • Identify the role of the Network Layer, as it describes communication from one end device to another end device. • Examine the most common Network Layer protocol, Internet Protocol (IP), and its features for providing connectionless and best-effort service. • Understand the principles used to guide the division or grouping of devices into networks. • Understand the hierarchical addressing of devices and how this allows communication between networks. • Understand the fundamentals of routes, next hop addresses and packet forwarding to a destination network.

3. Network Layer Protocols and Internet Protocol (IP) • Layer 3 of OSI • Receives segments or PDUs from TL • 4 tasks: • Addressing packets with an IP address • Encapsulation • Routing • Decapsulation

4. Network Layer Protocols and Internet Protocol (IP)

5. Network Layer Tasks • Addressing packets with an IP address • Each sending and receiving device must have unique IP address • Device with IP address = hosts • Sending host = source IP address • Receiving host = destination IP address

6. Network Layer Tasks • Encapsulation • IP header – source and destination IP addresses • Process of adding information = encapsulation • Encapsulated PDU = packet

7. Network Layer Tasks • Routing • Routers =device that connect networks • Routers – understand packets and calculating best path for packets • Routing = process perform by routers : receive packets, analyzing dest add info, select a path and forwarding packets to the next router • Each route to next device = hop • Decapsulation • Process of removing encapsulation data • Actually encap and decap happened at all layers of OSI model

8. Network Layer Protocols

9. IPv4 • Most widely used • Basic characteristics:

10. IPv4 • Connectionless • No established connection • IP simply sends packets without informing receiver • Requires less data to perform required tasks – uses much less processing power and bandwidth = overhead

11. IPv4

12. IPv4 • Best Effort • TCP is reliable • IP is unreliable • IP makes a ‘best effort’ to deliver packets • TCP can be relied on delivery problems • TCP/IP – TL & NL

13. IPv4 • Best Effort

14. IPv4 • Media independent • IP is not concerned with physical medium that carries packet • Internetwork communication – multimedia journey • ex. wireless, ethernet cable, fiber optic

15. IPv4 • Media independent

16. IPv4 • IPv4 encapsulates or packages the TL segment or datagram as packets

17. IPv4 Packet Header • IP Source Address • 32 bits • IP Destination Address • 32 bits • Time to Live (TTL) • 8 bits • Max hops the packet can take before considered lost/undeliverable • Each router decrements TTL field by at leased 1 • If TTL reaches 0 – packet will be dropped

18. IPv4 Packet Header • Type of Service (ToS) • 8 bits • Describes level of throughput • Ex – voice data precede streaming music • Quality of Service - QOS • Protocol • 8 bits • Indicate upper layer protocol • TCP, UDP or ICMP • Flag and Fragment Offset • Packet fragmented – small MTU • Used to reconstruct the packets

19. IPv4 Packet Header • Version • IPv4 or IPv6 • Internet Header Length (IHL) • How long the header - Options may caused different lengths • Packet Length • Total length of datagram including the header • Min 20 bytes, max 65,535 bytes • Identification • Help reassemble any fragments

20. IPv4 Packet Header • Header Checksum • Indicate length of header • Checked by each router • If invalid, packet assumed to be corrupted and is dropped • Relation to TTL? • Options • Special routing services • Padding • Fill bits when header data does not end on 32 bits boundary

21. TBC - Wednesday

22. Grouping Devices into Networks and Hierarchical Addressing • Networks are communities of computers and other hosts • Like human communities • Small town • Easy to find and communicate, not need large roads & expensive traffic signals, not many services, trust each other and considered safer • Large town • Ex..address • Same to computer communities • More planning to address the network so it can be managed efficiently

23. Grouping Devices into Networks and Hierarchical Addressing • Grouping devices into sub-networks

24. Grouping Devices into Networks and Hierarchical Addressing • Large • Computer networks can be separated into internetworks • Departments and groups share computers and servers into common subnetwork or subnet • Geographically, Specific Purpose, Ownership

25. Grouping Devices into Networks and Hierarchical Addressing • Geographically

26. Grouping Devices into Networks and Hierarchical Addressing • Specific Purpose • Different user – different reasons, different tools, different requirements

27. Grouping Devices into Networks and Hierarchical Addressing • Ownership • Main concern – security

28. Why Separate Hosts into Networks? • Performance degradation • Security issues • Address management

29. Why Separate Hosts into Networks? • Performance • Hosts can be chatty devices • Broadcast news about themselves • Broadcast = message sent from one host to all other hosts on the network • Share own information and request information about other hosts • More broadcast = more bandwidth consumed • Broadcast domain

30. Why Separate Hosts into Networks? • Performance

31. Why Separate Hosts into Networks? • Security • Isolating and shielding devices from public access • Better protection • Local network manager can more easily control outside access to the smaller network • Router or firewall at the perimeter of the network • Configured – known & trusted data/user to access network

32. Why Separate Hosts into Networks? • Security

33. Why Separate Hosts into Networks? • Address Management • Gateway router – send/receive messages beyond the network

34. Why Separate Hosts into Networks? • Address Management • Hierarchical Addressing

35. Dividing Networks from Networks • IPv4 address = 32 bits • Two parts • Network = 24 bits - postcode • Host = 8 bits - destination

36. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • Intermediary gateway device allowing devices to communicate across sub-divided networks • A host has a default gateway address defined • Ipconfig command

37. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • IP packet traverses unchanged via routers from sub network to sub-network

38. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • Gateway needed to send packet out of the network • Routers add routes for the connected network to their routing table • Routing table stores information about connected and remote networks • When configured with IP and subnet mask, the interface becomes part of the network • The routing table includes that network as directly connected network.

39. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding

40. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • 3 mains features of routes in routing table • Destination network • Next-hop • Metric • Hop Count • Delay

41. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • Destination network in routing table entry represents a range of host addresses or network and host addresses

42. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • Next Hop – where the packet goes next • Next hop is the address of the device that will process the packet next

43. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • Steps of IP packets as they are routed through several gateways from devices on one sub network to devices on other sub networks

44. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • Static Routing • Manually configured route information on the router • Low router processing overhead, High administrative cost • Dynamic Routing • Routers can learn about routes automatically from other routers • High router processing overhead, Little administrative cost • Routing protocols • Are the set of rules by which router dynamically share their routing information • -Routing Information Protocols (RIP) • Enhance interior Gateway Protocol (EIGRP) • - Open Shortest Path First (OSPF)

45. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • Static Routing

46. Fundamentals of Routes, Next Hop Addresses and Packet Forwarding • Dynamic Routing

47. Summary