1. 7.1 The Network Layer It provides services to the transport layer. It is concerned with getting packets from the source to the destination, possibly making many hops along the way between intermediate computers called Routers. The layer beneath the network layer is only concerned with each hop along the way. while, the layer above only concerned with communication between the source and destination. The unit of information transfer at the network layer is called a PACKET. The network layer handles the problems of incompatible services, configuration, and architecture using device such as Gateways or Routers. Because of this, Transport layer can provide its services independent of underlying network structure and addressing issues.

2. 7.2 Routing It is the process of finding a path from a source to a destination across the network. Routing is simple when both source and destination are connected to the same LAN. It becomes complicated when the source and destination are separated by WAN. In WAN, the information must pass through the network layer of several intermediate autonomous computers i.e. ROUTERS.

3. Routing Tables Routing Tables are the tables that are present in every routers to tell it where to forward an incoming packets based on the destination address for that packet. R1R2 PC 1 PC 2 R1 – Routing Table 192.168.1.0192.168.2.0192.168.3.0 192.168.1.0 192.168.2.0 192.168.3.0 192.168.1.0 Shared Routing Information

4. Static vs Dynamic Routing In Static Routing, all routes between all possible nodes in a network are computed before the network is up and running. Hence, routing is efficient as there is no overhead of making decisions at routers when packets arrive, as this information is fixed in the routing tables. On the other hand, if the network is big and have redundancy path in network then all the routes information should be input manually or else redundant path would be wasted. In Dynamic Routing, routes are recomputed and configured during the operation of the network, taking into account changing network topology. Advantage of dynamic routing is that it can react to changing traffic patterns on the network.

5. Centralized vs Decentralized Routing In Centralized routing, a single routing control centre (RCC) is used to determine all routes between nodes on network. It can be used from small network but not in Internet scenario. If the Routing centre or the link to it goes down then the network is severely affected. In Distributed routing, router would compute its own routing information, based on information about its neighbors and from its neighbors. Advantage of dynamic routing is that it can react to changing traffic patterns on the network. Though, it solves the major problem with centralized routing, it is much more complicated and location information, like link failure, takes longer to propagate across the network.

6. 7.3 Routing Algorithms Routing algorithms are used to compute the minimum cost path or paths between pair of nodes. The cost of link is usually defined by several attributes of the link, like Physical length of the link, The propagation delay, Communication cost, and Bandwidth. 7.3.1 Shortest Path Algorithms These algorithms are conceptually simple and widely used. Given a network with costs on each link, these algorithms determine the minimum cost route between any two nodes. Advantage: Simple and the packets move along the most efficient path at the time that the algorithms are evaluated. Dis-Advantage: Not robust to link failure and changes to the cost of links i.e. if link breaks, packets get stuck till path are recomputed. Further, as they are single path algorithms, heavy traffic could result in CONGESTION.

7. 7.3.2 Bellman Ford Routing Algorithm Can be used as both Centralized and Distributed algorithm. Every node in network is assigned a cost, and every node has a table that gives the next hop and overall cost for each destination node on the network. Initially, all entries in the table are initialized to empty, except for entries to the neighbors of a node. In each step of this algorithm, every node sends its table to its neighbors, and the information it receives is used to update its own table. When all the tables in the network stop changing, the algorithm terminates.

8. 7.3.2 Bellman Ford Routing Algorithm B A C D 10 30 2 35 5 1ABCD A0A10B30C-- B10A0B5C35D C30A5B0C2D D--35B2C0D

9. 7.3.2 Bellman Ford Routing Algorithm B A C D 10 30 2 35 5 1ABCD A0A10B15B32C B10A0B5C7C C15A5B0C2D D32C7B2C0D

10. 7.3.2 Bellman Ford Routing Algorithm B A C D 10 30 2 35 5 1ABCD A0A10B15B17B B10A0B5C7C C15A5B0C2D D17C7B2C0D

11. 7.3.3 Dijkstra’s Algorithm Typically implemented as a static centralized routing. At each step of the algorithm, the next shortest path is added. This can be made dynamic by re-evaluating the algorithm for all nodes periodically. Each node is labeled with its distance from the source node along the best known path. Initially, no paths are known, so all nodes are labeled with infinity. As the algorithm proceeds, paths are found which can be changed, reflecting better paths. When it is discovered that a label represents the shortest possible path from the source to that node, the label is made permanent and never changed thereafter.

12. Dijkstra’s Algorithm B A C D 10 30 2 35 5 DestinationBCD IterationPcostPredPcostPredPcostPred 1InfinityundefinedInfinityundefinedInfinityundefined 210A30AInfinityundefined 310A15B45B 410A15B17C

13. 7.3.4 Hierarchical Routing It is not feasible in large network to compute entire routing table for every node. Hierarchical routing divides the network into subgroups of nodes, where each subgroup is call a DOMAIN and is treated as separate network. Routing within a domain is carried out using a standard techniques like shortest-path routing. If domain grows very large, then can be further divided into SUB-DOMAINS. For inter-communication between domains there is special node in between, known as Gateways or Routers. DOMAIN 1 DOMAIN 2 R R R R R R R R R

14. 7.3.5 Multipath Routing Shortest path algorithm is not efficient because of single path between two nodes. Whereas Multipath routing spreads traffic over several routes, resulting in improved performance and greater reliability, but with greater complexity. Multipath routing requires each node to maintain a table with several entries for each destination, ranked in order of increasing cost.

15. 7.3.6 Flooding In flooding, routers forward packets to all ports except the incoming port generating large number of duplicate packets. However, this phenomenon is controlled by using HOP COUNTER or SEQUENCE NUMBER assigned to the packets. It is not practical in general. However can be used in Military application, when large numbers of nodes can get destroyed any minute. Advantage: every destination in the network is reachable when network topology is unknown. Disadvantage: wastes link capacity loops must be detected.

16. 7.4 Congestion Control 1. Packet Elimination If a node becomes overload with packets some packets are simply discarded. Later, those packets need to be retransmitted. 2. Flow Control used to control the flow of packets between pairs of nodes. However, it is not the best solution as it can’t fight with the congestion problem. 3. Connection-oriented network layers here, buffers are prelocated along routes that have been computed to perform packet transfer. 4. Isarithmetic This method keeps the total number of packets in the network constant, by circulating permits in the network. A node must capture a permit before it can release a packet into the network.

17. 7.4 Congestion Control contd. 5. Choke Packets It is based on monitoring utilization of the network links. When a node detects utilization on an incoming link above a certain threshold, it sends a choke packet to the source node. 6. Deadlock Detection In this method, each node waits for something to happen on other node and react accordingly.

18. 7.5 Types of Network Services Virtual Circuit (VC) VC do routing when connection is set up and use that route for all packets on the connection. For this, network services must include primitives to set up, manage and disconnect connections. VC can guarantee a certain level of QoS, as connection is already setup – for transaction processing. For instance, Credit Card Validation. However, a VC is more vulnerable to crash of intermediate computer.

19. 7.5 Types of Network Services contd Packet Switching Here, each packet is routed individually i.e. the only services provided are to send and receive packets. It is good for short connections or bursty traffic, while VC are better for long connections or heavy sustained traffic. Besides VC and Packet Switching techniques, there are other CONNECTION-OREINTED (X.25) and CONNECTIONLESS (IP) network services.