Routing A broad Perspective

1. Routing A broad Perspective Presenters Vasudha Chandrasekaran (Vas) Prajakta Vaidya (Priya)

2. Routing is… • What fans do at football games • What pigs do for truffles • Send corps of infantrymen into full scale, disorganized retreat. Uh! So what is Routeing? • Something that Brits use, to distinguish between what happens in Networks from what happened to them in New Orleans in 1814. • What ISO and CCITT use in their documents • And Yes, it is…

3. Routeing is… • The process of determining, selecting the best outgoing path that a packet has to take in a Internetwork. But, with due apologies to British Standards Institute, lets stick to Routing!

4. OSI Routing Scheme • A set of Routing Protocols • Allows end systems and intermediate systems to collect and distribute information necessary to determine routes • Routing Information base • Table containing all routing relevant connectivity • Routing Algorithms • Uses information in routing information base to derive routes between end systems.

5. OSI Routing Architecture

6. For the Next 10 min…. • Design Goals of Routing Algorithms • Algorithm Types • Routing Metrics • Dynamic Routing Algorithms

7. Design Goals • Optimality • Simplicity and low Overhead • Robustness and stability • Rapid Convergence • Flexibility

8. Algorithm types • Static Vs Dynamic • Single Path Vs Multiple Path • Host Intelligent Vs Router Intelligent • Intradomain Vs Interdomain

9. Routing Metrics • Path Length • Reliability • Delay • Bandwidth • Load • Communication cost

10. Static Algorithms • Shortest Path Algorithm • Flooding • Flow based Routing Dynamic Algorithms • Distance Vector Routing • Link State Routing • Hierarchical Routing

11. Distance Vector Routing • Bellman Ford/ Ford Fulkerson Algorithm - Original ARPANET routing algorithm • AppleTalk and CISCO routers use improves distance vector protocols • Each router knows the id of every other router in the network. • Each router maintains a vector with an entry for every destination that contains: • The cost to reach the destination from this router. • The direct link that is on that least cost path. • Each router, periodically sends its vector to his direct neighbors. • Upon receiving a vector a router updates the localvector basedon the direct link’s cost and the received vector.

12. Link State Routing • Discover the neighbors and learn the network addresses • Measure the delay or cost to each of the neighbors • Construct a packet telling all it has just learnt • Send this packet to all other routers • Compute the shortest path to every other router

13. Link State Routing contd.. • Each router knows the id of every other router in the network. • Each router maintains a topology map of the whole network. • Each router, periodically floods its link state updates (with its direct connectivity information). • Upon receiving a vector, a router updates the local topology map and re-calculates shortest paths.

14. Increasing Network Size • Router memory requirements increase. • More CPU time for look up and more bandwidth needed to send status reports. • Routing table size increases with increase in network size.

15. Hierarchical Routing • Routers divided into Regions. • Regions > Clusters > Zones > Groups > • Internal structure of a region known only to routers within that region. • Different networks do not need to know the topological structure of other ones.

16. Example of Hierarchical Routing

17. Penalty for Hierarchical Routing • Path length may increase. But this increase is sufficiently small and usually acceptable. • The optimum number of levels for an N router subnet is ln N, with a total of e ln N entries per router.

18. Internet Control Protocols • Routing Information Protocol • Open Shortest Path First • Interior Gateway Routing Protocol • Exterior Gateway protocol • Border Gateway Protocol

19. Routing Information Protocol • The protocol is limited to networks whose longest path is 15 hops. • Inappropriate for larger networks. • Uses fixed metrics to compare alternative routes. • Not appropriate for routing based on dynamic parameters. • Cannot operate within a hierarchical structure.

20. Open Shortest Path First • OSPF is a link-state routing protocol that calls for the sending of link-state advertisements to all other routers within the same hierarchical area. • As OSPF routers accumulate link-state information, they use the SPF algorithm to calculate the shortest path to each node using Dijkstra’s Shortest Path Algorithm . • Can operate in a hierarchy. • Supports multi-path routing and type of service based routing.

21. Interior Gateway Routing Protocol • A distance-vector routing protocol. • Replaced RIP • Network Size • Single Metric • IGRP uses five criteria to determine the best path: the link's speed, delay, packet size, loading and reliability. • The weighting factors for each of these metrics can be fixed up by Network Administrators. • Permits multi-path routing.

22. Exterior Gateway Protocol • First exterior routing protocol to gain widespread accepted on the internet. • Dynamic routing protocol with a very simple design used for communication of reachability information. • It does not use metrics and therefore cannot make intelligent routing decisions

23. Border Gateway Protocol • Internet standard for inter-domain exterior routing. • BGP is used to exchange routing information between Internet service providers (ISP). • External / Interior BGP • Extremely robust and scalable • Supports classless inter-domain routing. • BGP-4, the latest version, lets administrators configure cost metrics based on policy statements. • Routers using BGP, exchange routing information using TCP connection.

24. BGP contd. • BGP communicates with autonomous (local) networks using Internal BGP (IBGP) since it doesn't work well with IGP. The routers inside the autonomous network thus maintain two routing tables: one for the interior gateway protocol and one for IBGP. • Policies are manually configured into each BGP router. • Never put Iraq on a route starting at Pentagon. • Traffic starting and ending at IBM should not transit Microsoft.

25. Questions ?