Routing Protocols notes document .pptx.pdf
- 1. Routing
- 2. Introduction ● The main role of the network layer is to move packets from the source host to the receiving host. This function is done by the routers. There are two important network layer functions: ○ Packet forwarding ○ Routing ● Packet Forwarding: the process of transferring packet from an incoming link to an outgoing link within a single router ● Routing: The process of deciding the best paths that a packet should follow from source to destination. The process is performed by routing protocols which are within the router processor. The routing protocols are responsible for calculating the best paths. The process involves interaction of many or all network routers 2
- 4. Router Architecture The router has four main components 1. Input ports: Performs the following functions ○ terminates physical layer link at a router ○ Performs look up. The lookup involves consultation of forwarding table to determine the output port to which an incoming packet will be forwarded. 2. Switching fabric: The role of switching fabric is to connect input ports to output ports. Example is crossbar switching fabric 4
- 5. Router Architecture Crossbar switching fabric 5
- 6. Router Architecture Output Ports: stores packet received from input ports through the switching fabric and transmits these packets on the outgoing link by performing necessary link layer and physical layer functions. Routing processor: This part executes the routing protocols. It maintains routing protocols, the link state information and computes forwarding tables The routers input ports, output ports and the switching fabric together they implement the forwarding function also known as router forwarding plane (Data plane). It is implemented in hardware 6
- 7. Router Architecture The router processor forms what is known the router control plane. Router management functions are done in the routing processor. The control plane is implemented inform of software 7
- 8. Forwarding This is router local action of moving packets from an input link interface to the appropriate output link interface. Every router has a forwarding table. Router forwards packets by examining the value of a field in the arriving packet header and the using this value to index into the router forwarding table. The value stored in router forwarding table for that header indicates the link outgoing interface where the packet will be forwarded 8
- 9. Routing Routing is a network wide process that determines end to end paths that packets should follow from source to destination. The routing functionality is executed in the routing processor. Since each router executes its own routing functions, the control plane is decentralised. Ways to classify Routing Protocols 1. Dynamic versus static protocols Dynamic algorithms change the routing paths as the network state changes, that is change in network traffic load, topology change etc. With Static routing algorithms, the routes do not change or changes very slowly over time. 2. Linkstate versus distance vector routing protocol Link state routing algorithms: These algorithms have complete information on the state of the network, that is, they have complete knowledge on complete network topology and all link costs. NB: in routing every link as a weight we call link cost. The cost depend on how good or bad the link is. 9
- 10. Link state routing In link state routing, using complete topology of the network,each node develops a graph and then the algorithm executes to find the best path (least cost paths). Example of link state routing algorithm is Dijkstra’s Algorithm Dijkstra’s Algorithm computes the least cost path in a graph from one node to all other nodes in the network. 10
- 11. Dijkstra’s Algorithm The algorithm is used for solving the shortest path problem in a graph or network How it works Step1 Mark all nodes unvisited. Create a set of all the unvisited nodes called the unvisited set. Step2 Assign to every node a tentative distance value: set it to zero for our initial node and to infinity for all other nodes. Set the initial node as current. 11
- 12. Dijkstra’s Algorithm Step3 For the current node, consider all of its unvisited neighbours and calculate their tentative distances through the current node. Compare the newly calculated tentative distance to the current assigned value and assign the smaller one. For example, if the current node A is marked with a distance of 6, and the edge connecting it with a neighbour B has length 2, then the distance to B through A will be 6 + 2 = 8. If B was previously marked with a distance greater than 8 then change it to 8. Otherwise, the current value will be kept. Step4 When we are done considering all of the unvisited neighbours of the current node, mark the current node as visited and remove it from the unvisited set. A visited node will never be checked again. 12
- 13. Dijkstra’s Algorithm Step5 If the destination node has been marked visited (when planning a route between two specific nodes) or if the smallest tentative distance among the nodes in the unvisited set is infinity (when planning a complete traversal; occurs when there is no connection between the initial node and remaining unvisited nodes), then stop. The algorithm has finished. Step6 Otherwise, select the unvisited node that is marked with the smallest tentative distance, set it as the new "current node", and go back to step 3. 13
- 25. Distance Vector Routing Protocol Distance routing protocol is a decentralised routing algorithm in which each node constructs a one dimensional array (vector or table) containing distances to its neighbour. Through broadcasting each node communicates its vector or table to others. These tables are broadcasted periodically. The nodes compares their stored tables with received table with received information from neighbours and make appropriate update. The Initial assumption of the algorithm is that each node knows the cost of the link to each of its directly connected neighbours. When a link goes down it is assigned infinity cost meaning that it is disconnected. 25
- 26. Distance Vector Routing Protocol The minimum distance between any two nodes is computed using Bellman’s Ford Algorithm dx (y) = minv {c(x,v) + dv (y)}, where dx (y) is the minimum distance from node x to node y 26
- 27. Example Consider the network shown in the figure below with four nodes. Cost links are shown in the diagram. Give the distance-vector routing tables for node C in the following two consecutive steps. ● Step 0: C knows the distances to its immediate neighbours and ● Step 1: information from step 0 is exchanged as per the distance-vector algorithm. 27
- 28. Example 28 B C D E C 1 0 2 5 B C D E C 1 0 2 3