Network Layer Concepts

1. Network Layer Concepts Network Layer Concepts

2. IP protocol • addressing conventions • datagram format • packet handling conventions Routing protocols • path selection • RIP, OSPF, BGP routing table ICMP protocol • error reporting • router “signaling” Network Layer Transport layer: TCP, UDP Network layer Link layer physical layer Network Layer Concepts

3. Network Layer • Network layer is concerned with getting packets from the source to the destination across LAN as well as WAN links. • In contrast, data link layer moves frames from one end of the wire to another. Network Layer Concepts

4. Functions of Network Layer • Routing - must know about the topology of the communication subnet. • Services to Transport Layer • Congestion control • Internetworking. Network Layer Concepts

5. Services to Transport Layers • To be independent of subnet technology. • Transport layer should be shielded from the number, type and topology of subnet present. • Network addresses made available to the transport layer should use a uniform numbering plan, even across LANs and WANs. Network Layer Concepts

6. Intermediate Systems • Forwards data not destined to itself. • Unicast Vs. Multicast forwarding. • Implementation - Standalone box (CISCO, BAY NETWORKS) or UNIX host software. Network Layer Concepts

7. The Scenario : Store and Forward Network Layer Concepts

8. Connection Oriented • Connection Less Network Layer Concepts

9. Connection Oriented Packet Switching • Connection identifiers are not global. They are valid only for particular links and may change. • Connection - data structures at hosts and intermediate nodes that are required to be synchronized for data to flow correctly. • If one system reboots then communication does not work anymore. Network Layer Concepts

10. Connection Oriented packet switching contd... • The data structure used by intermediate system is the label swapping table. • The packet forwarding function is simple, based on table lookup. • Control method - • connection setup and release. • connection routing. Network Layer Concepts

11. Connection Oriented Routing Within Virtual Circuit Subnet Network Layer Concepts

12. Connectionless Packet Switching • Packets are called datagrams. • Intermediate systems are called routers. They know how to forward packets to structured sets of addresses. • Every router needs a rule to handle every possible packet that it may ever receive. • Destination address is structured in order to aggregate routing information. Network Layer Concepts

13. Connectionless packet switching • Packet forwarding is based on destination address or routing information contained in every packet. • Control method involves mainly maintenance of routing tables. Network Layer Concepts

14. Connectionless: Routing within a Datagram Subnet Network Layer Concepts

15. Connection oriented (X.25, Frame relay, ATM) - : • better control over individual flows. • required for bandwidth requirements. • tight co-operation of all systems required. • control method is complex. • Individual packets forwarding is simpler. Network Layer Concepts

16. Connectionless (Internet, Appletalk) –: • loose co-operation of all systems. • simple control method. • more complex individual packet forwarding. Network Layer Concepts

17. Network Layer Concepts

18. Where Should the Complexity Be • Connection oriented - in network layer. • Reliable trouble free service with automatic flow control. • Services like audio and video are much easier to provide on top of a CO network layer. • Connection less - in transport layer. • Subnet should not be cluttered up with features, but simply move bits around. • Emphasis on speed rather than accuracy. Network Layer Concepts

19. Service offered by network layer (connection oriented or connection less) is a separate issue from subnet structure. • In theory all 4 combinations are possible • virtual circuits implementation of CO. • datagram implementation of CLS. • Implementing CO using datagrams. • CLs service on top of virtual circuit subnet. Network Layer Concepts

20. Routing • Intermediate systems forward packets -. • Based on layer 3 IP address: routers and layer 3 switches. • Based on layer 2 MAC address: bridges and layer 2 switches. Network Layer Concepts

21. Routing in a Packet Switched Network • Performed on every packet in the network. • Most important function in a packet switched network. • Decides the efficiency of the network. • Decides the speed of the network. Network Layer Concepts

22. Characteristics of Routing • Correctness. • Simplicity. • Robustness: routing algorithm must cope with the changes of topology and traffic. • Stability: algorithm must converge to equilibrium. • Fairness and optimality. • Efficiency. Network Layer Concepts

23. Optimality Principle • If router J is on the optimal path from router I to router K, then the optimal path from J to K also falls along the same route. • Consequence: the set of optimal routes from all the sources to a given destination form a tree rooted at the destination: sink tree. • Goal of routing algorithms is to discover and use the sink trees for all routers. • Sink trees not easy to build and use, with routers going down etc. Network Layer Concepts

24. Subnet and Sink Tree Network Layer Concepts

25. Routing Algorithms • Static or non-adaptive algorithms: not based on measurements or estimates of the current traffic or topology. • Shortest path routing. • Flooding. • Flow based routing. • Adaptive or dynamic routing. • Distance vector routing. • Link state routing. Network Layer Concepts