Computer Communication & Networks

1. Computer Communication & Networks Lecture 22 Network Layer: Delivery, Forwarding, Routing (contd.) http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp Waleed Ejaz waleed.ejaz@uettaxila.edu.pk

2. Network Layer

3. Network Layer Topics to Cover Logical Addressing Internet Protocol Address Mapping Delivery, Forwarding, Routing

4. Two-node Instability: Counting to Infinity Problem

5. 1 1 1 1 2 4 3 • N1 N2 N3 N4 • Initial (2,3) (3,2) (4,1) (4,0) • (2,3) (3,2) (-1,) (4,0) • (2,3) (-1,) (-1,) (4,0) • (-1,) (-1,) (-1,) (4,0) Split Horizon and Split Horizon with Poisoned Reverse • Reverse Route • a route pointing to the node where it has arrived • it creates potential cycle Subnet N R2 R1 Reverse Route • Split Horizon • min cost to a given destination is not sent to a neighbor if the neighbor is the next node along the shortest path • a route is not broadcast on the interface through which the node has learnt it • Split Horizon with Poisoned Reverse • send infinity

6. u v destinationhops u 1 v 2 w 2 x 3 y 3 z 2 w x z y C A D B RIP ( Routing Information Protocol) • Uses the distance-vector algorithm

7. Routing Information Protocol (RIP) • Runs on top of UDP, port number 520 • Metric: number of hops • Max limited to 15 • suitable for small networks (local area environments) • value of 16 is reserved to represent infinity • small number limits the count-to-infinity problem

8. RIP Operation • Router sends update message to neighbors every 30 sec • A router expects to receive an update message from each of its neighbors within 180 seconds in the worst case • If router does not receive update message from neighbor X within this limit, it assumes the link to X has failed and sets the corresponding minimum cost to 16 (infinity) • Uses split horizon with poisoned reverse • Convergence speeded up by triggered updates • neighbors notified immediately of changes in distance vector table

9. Example of a Domain using RIP

10. Open Shortest Path First • Fixes some of the deficiencies in RIP • Enables each router to learn complete network topology • Each router monitors the link state to each neighbor and floods the link-state information to other routers • Each router builds an identical link-state database • Allows router to build shortest path tree with router as root • OSPF typically converges faster than RIP when there is a failure in the network

11. Path Vector Routing

12. EGP: Exterior Gateway Protocol • designed for tree-structured Internet • concerned with reachability, not optimal routes • BGP – Border Gateway Protocol

13. Exterior Gateway Protocols • Within each AS, there is a consistent set of routes connecting the constituent networks • EGP enables two AS’s to exchange routing information about: • The networks that are contained within each AS • The AS’s that can be reached through each AS • EGP path selection guided by policy rather than path optimality • Trust, peering arrangements, etc

14. R2 R3 AS2 R1 R4 N1 AS1 AS3 EGP Example Only EGP routers are shown • R4 advertises that network N1 can be reached through AS3 • R3 examines announcement & applies policy to decide whether it will forward packets to N1 through R4 • If yes, routing table updated in R3 to indicate R4 as next hop to N1 • IGP propagates N1 reachability information through AS2 N1 reachable through AS3

15. R2 R3 AS2 R1 R4 N1 AS1 AS3 EGP Example • EGP routers within an AS, e.g. R3 and R2, are kept consistent • Suppose AS2 willing to handle transit packets from AS1 to N1 • R2 advertises to AS1 the reachability of N1 through AS2 • R1 applies its policy to decide whether to send to N1 via AS2 N1 reachable through AS2

16. EGP Requirements • Scalability to global Internet • Provide connectivity at global scale • Link-state does not scale • Fully distributed • EGP path selection guided by policy rather than path optimality • Trust, peering arrangements, etc • EGP should allow flexibility in choice of paths

17. Internet inter-AS routing: BGP • BGP provides each AS a means to: • Obtain subnet reachability information from neighboring ASs. • Propagate the reachability information to all routers internal to the AS. • Determine “good” routes to subnets based on reachability information and policy. • Allows a subnet to advertise its existence to rest of the Internet: “I am here”

18. Initial routing tables in path vector routing

19. Stabilized tables for three autonomous systems

20. BGP Policy • Examples of policy: • Never use AS X • Never use AS X to get to a destination in AS Y • Never use AS X and AS Y in the same path • Import policies to accept, deny, or set preferences on route advertisements from neighbors • Export policies to determine which routes should be advertised to which neighbors • A route is advertised only if AS is willing to carry traffic on that route

21. Why different Intra- and Inter-AS routing ? Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • Intra-AS: single admin, so no policy decisions needed Scale: • hierarchical routing saves table size, reduced update traffic Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance