1. Network Technologies essentials Week 5: Routing Compilation made by Tim Moors, UNSW Australia Original slides by David Wetherall, University of Washington ISBN-10: 0133766624 © 2014 For the book A. Tanenbaum and D. Wetherall: Computer Networks, 5 th ed, Prentice-Hall, 2011A. Tanenbaum and D. Wetherall: Computer Networks, 5 th ed, Prentice-Hall, 2011 Tim added green graffiti

2. Computer Networks2 Shortest Paths (4) Optimality property: – Subpaths of shortest paths are also shortest paths ABCE is a shortest path  So are ABC, AB, BCE, BC, CE AB C D E F G H 2 1 10 2 2 4 2 4 4 3 3 3 5-2.7

3. Computer Networks3 Dijkstra’s Algorithm Algorithm : Mark all nodes tentative, set distances from source to 0 (zero) for source, and ∞ (infinity) for all other nodes While tentative nodes remain: – Extract N, a node with lowest distance – Add link to N to the shortest path tree – Relax the distances of neighbors of N by lowering any better distance estimates 5-3.4

4. Dijkstra’s Algorithm (2) Initialization Computer Networks4 AB C D E F G H 2 1 10 2 2 4 2 4 4 3 3 3 0 ∞ ∞ ∞ ∞ ∞ ∞ We’ll compute shortest paths from A ∞ 5-3.5

5. Computer Networks5 Distance Vector Algorithm Each node maintains a vector of distances (and next hops) to all destinations 1.Initialize vector with 0 (zero) cost to self, ∞ (infinity) to other destinations 2.Periodically send vector to neighbors 3.Update vector for each destination by selecting the shortest distance heard, after adding cost of neighbor link – Use the best neighbor for forwarding 5-4.5

6. DV Example (4) Second exchange for all nodes to find best 2-hop routes Computer Networks6 A learns Cost Next 0-- 3B 9D 6B To A says B says C says D says A03∞7 B3063 C∞602 D7320 = learned better route B C D A 2 6 7 3 3 B learns Cost Next 3A 0-- 5D 3D C learns Cost Next 9B 5D 0-- 2D D learns Cost Next 6B 3B 2C 0-- 5-4.9

7. Forwarding with ECMP (2) Computer Networks7 AB C D E F G H 2 2 10 1 1 4 2 4 4 3 3 3 Multipath routes from F/E to C/HE’s Forwarding Choices Flow Possible next hops Example choice F  HC, DD F  CC, DD E  HC, DC E  CC, DC Use both paths to get to one destination 5-7.9

8. Hierarchical Routing (4) Penalty is longer paths Computer Networks8 1C is best route to region 5, except for destination 5C 5-9.10

9. Subnets Internally split up one IP prefix Computer Networks9 64K addresses One prefix sent to rest of Internet 16K 32K 8K CompanyRest of Internet 5-10.7

10. Computer Networks10 Routing Policies – Transit One party (customer) gets TRANSIT service from another party (ISP) – ISP accepts traffic from customer to deliver to the rest of Internet – ISP accepts traffic from the rest of the Internet to delivery to customer – Customer pays ISP for the privilege Customer 1 ISP Customer 2 Rest of Internet Non- customer 5-11.7

11. Computer Networks11 Routing Policies – Peer Both party (ISPs in example) get PEER service from each other – Each ISP accepts traffic from the other ISP only for their customers – ISPs do not carry traffic to the rest of the Internet for each other – ISPs don’t pay each other Customer A1 ISP A Customer A2 Customer B1 ISP B Customer B2 5-11.8

12. BGP Example AS2 buys TRANSIT service from AS1 and PEER service from AS3 Computer Networks12 5-12.8 ← B, AS3 → A, AS2 ← C, AS4 → B, AS3 ↑ C, AS4 ↓ A, AS1,AS2 ↓ B, AS1, AS3 ↑ A, AS2 ↓ B, AS1, AS3 ↓ C, AS1, AS4 ↑ B, AS3 ↓ A, AS1, AS2 ↓ C, AS1, AS4

13. END © 2013 D. Wetherall Slide material from: TANENBAUM, ANDREW S.; WETHERALL, DAVID J., COMPUTER NETWORKS, 5th Edition, © 2011. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Computer Networks13