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# Routing

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### Routing

1. 1. Routing
2. 2. Routing Routing is the process of selecting paths in a network along which to send network traffic Process of relaying packets among multiple interconnected networks
3. 3. Least Cost Routing Based on efficiency Which of the available paths is cheapest A value is assigned to each link; length of the particular route is equal to the total no of the values of the component links. Shortest means the route requiring the smallest no of relays
4. 4. Non adaptive and Adaptive routing Non Adaptive  Once the pathway to destination has been selected, the router sends all packets for that destination along that route Adaptive  Router may select a new route for each packet
5. 5. Routing Algorithms Shortest Path Routing Flooding Flow Based Routing Distance Vector Routing Link State Routing Hierarchical Routing Broadcast Routing Multicast routing
6. 6. Shortest Path Routing A graph of subnet is built. Each node represents a router Each arc of graph represents a communication link. To choose a route algorithm just finds the shortest path between them on the graph
7. 7. Flooding Every incoming packet is sent out on every outgoing line except the one it arrived on. So it generates vast no. of duplicate packets. A hop counter is maintained in header of each packet Hop counter is decremented at each hop Packet is discarded when the counter reaches zero Ideally hop counter is initialized to the length of the path from source to destination Selective flooding- incoming packets are sent on lines that are going approximately in the right direction
8. 8. Flow Based Routing Used when data flow between each pair of nodes is relatively stable and predictable Considers the flow in the network; mean consider the amount of traffic in the network before deciding on which outgoing line to place the packet. The key here is to be able to characterize the nature of the traffic flows over time. The basic idea behind the algorithm, is that any given line, if the capacity and average flow are known, it is possible to compute the mean packet delay on that line from queuing theory. The routing problem thus reduces to finding the routing algorithm that produces the minimum average delay for the subnet.
9. 9. Distance Vector Routing Each router maintains a table giving the best known distance to each destination and which line to use to get there These tables are updated by exchanging info from neighbors.
10. 10. Distance Vector Routing-Mechanism Each router maintains a table indexed by and containing one entry for each router in the subnet. The entry has two parts  Preferred outgoing line  Estimate of time or distance to that destination The router is assumed to know the “distance” to each of its neighbors.  if metrics is hops , distance is one hops  If metrics is queue length, the route simply examines each queue  If metrics is delay, the router can measure it directly with the special echo packets that the receiver just timestamps and send back as fast it can
11. 11. TO A I H K Line A 0 24 20 21 8 A B 12 36 31 28 20 A New estimated C 25 18 19 36 28 I delay from J D 40 27 8 24 20 H E 14 7 30 22 17 I A B C D F 23 20 19 40 30 I G 18 31 6 31 18 H H 17 20 0 19 12 H I 21 0 14 22 10 I F GE H J 9 11 7 10 0 - K 24 22 22 0 6 K L 29 33 9 9 15 K L K I J J computes its new route J to H=12 msec to router G H to G =6 msec JA Delay=8 J to A=8 msec Total=18 msec JI Delay=10 A to g=18 msec J to K =6 msec Total=26 msec K To G=31 msec JH Delay=12 Total=37 msec JK Delay=6 J to I=10 msec I to G=31 msec Best : 18 i.e via H Total=41 msec
12. 12. Link State Routing Each router must  Discover its neighbour and learn their network address by sending Hello packet  Measure the delay or cost to each of the neighbours by sending echo packet  Construct a packet telling all it has learned  Send this packet to all other routers  Compute the shortest path to every other router.  In effect, the complete topology and all delays are experimentally measured and distributed to every router.
13. 13. Hierarchical Routing As network grow, routing tables increase in size consuming router memory It is not feasible for every router to have an entry for every other router Routing is done hierarchically Routers are divided into regions Each router knows all details of router within its own region. In very large networks  regions -> clusters  Clusters->zones  Zones->groups
14. 14. Hierarchical Routing Region1 Region 2 Region 4 Region 3
15. 15. Routing for mobile hosts When a new user enters an area it must register itself with the foreign agent here Each foreign agent broadcasts a packet announcing its existence and address A newly arrived mobile host broadcasts a packet inquiring any foreign agents Mobile hosts registers with the foreign agent giving its home address, DL layer Address and security info Foreign agent contacts mobile host’s home address Home agent checks info sent by foreign agent, if ok, informs foreign agent to proceed Foreign agent the makes an entry in tables and informs mobile hosts that it is registered
16. 16. Broadcast Routing Send messages to many or all hosts simultaneously Each packet contains a list of destinations Router examines this list to determine set of output lines that will be needed It generates a new copy of packet for each output line to be used and include in each packet only those destination that are to use the line Destination set is partitioned amount output lines Each packet will carry only one destination
17. 17. Multicast Routing Sending messages to well defined network groups With a multicast design, applications can send one copy of each packet and address it to the group of computers that want to receive it. This technique addresses packets to a group of receivers rather than to a single receiver It depends on the network to forward the packets to only the networks that need to receive them.