1. CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 21 Introduction to Computer Networks

2. CMPE 150- Introduction to Computer Networks 2 Announcements Homework 4 up. Due on 11.23.05. Lab this week: –“The Internet Behind the Web” video.

3. CMPE 150- Introduction to Computer Networks 3 Today Finish DLL!

4. CMPE 150- Introduction to Computer Networks 4 Last Class… Network Layer. –Focus on packet switching networks. –Main functions. –Different network layer implementations. Datagrams versus virtual circuits.

5. CMPE 150- Introduction to Computer Networks 5 Virtual-Circuit versus Datagram Subnets 5-4

6. CMPE 150- Introduction to Computer Networks 6 Routing

7. CMPE 150- Introduction to Computer Networks 7 Routing One of the main functions of network layer. Routing versus forwarding? Datagram versus VC networks?

8. CMPE 150- Introduction to Computer Networks 8 Routing Algorithm Computes routing tables. Properties: –Correctness. –Robustness. –Stability. –Optimality. Try to optimize a certain metric.

9. CMPE 150- Introduction to Computer Networks 9 Optimality Principle General statement about optimal routes (topology, routing algorithm independent). If router J is on optimal path between I and K, then the optimal path from J to K also falls along the same route. –Proof by contradiction. Corollary: –Set of optimal routes from all sources to destination form a tree rooted at destination. –Sink tree.

10. CMPE 150- Introduction to Computer Networks 10 Types of Routing Algorithms Non-adaptive versus adaptive.

11. CMPE 150- Introduction to Computer Networks 11 Adaptive and Non-adaptive Routing Non-adaptive routing: –Fixed routing, static routing. –Do not take current state of the network (e.g., load, topology). –Routes are computed in advance, off-line, and downloaded to routers when booted. Adaptive routing: –Routes change dynamically as function of current state of network. –Algorithms vary on how they get routing information, metrics used, and when they change routes.

12. CMPE 150- Introduction to Computer Networks 12 Static Algorithms  (Non-Adaptive) 1.Shortest-path routing. 2.Flooding.

13. CMPE 150- Introduction to Computer Networks 13 Shortest-Path Routing Problem: Given a graph, where nodes represent routers and edges, links, find shortest path between a given pair of nodes. What is shortest in shortest path? –Depends on the routing metric in use. –Example: number of hops (static), geographic distance (static), delay, bandwidth (raw versus available), combination of a subset of these. Dijkstra’s shortest-path algorithm (19590.

14. CMPE 150- Introduction to Computer Networks 14 Dijkstra’s Shortest-Path Algorithm Initially, links are assigned costs. As the algorithm executes, nodes are labeled with its distance to source along best known path. Initially, no routes known, so all nodes are labeled with infinity. Labels change as the algorithm proceeds. Labels can be temporary or permanent. –Initially all labels are tentative. –A label becomes permanent if it represents the shortest path from the source to the node.

15. CMPE 150- Introduction to Computer Networks 15 Shortest Path Routing Find shortest-path from A to D: Start Label each adjacent node with distance to A. B is made permanent.

16. CMPE 150- Introduction to Computer Networks 16 Flooding Every incoming packet forwarded on every outgoing link except the one it arrived on. Problem: duplicates. Constraining the flood: –Hop count. –Keep track of packets that have been flooded. Robust, shortest delay (picks shortest path as one of the paths).

17. CMPE 150- Introduction to Computer Networks 17 Flooding: Example Stallings Figure 12.4 (hop-count=3)

18. CMPE 150- Introduction to Computer Networks 18 Dynamic Routing Algorithms ( Adaptive Routing) –Distance vector routing. –Link state routing.

19. CMPE 150- Introduction to Computer Networks 19 Distance Vector Routing Aka, Bellman-Ford (1957), Ford-Fulkerson (1962). Original ARPANET routing; also used by Internet’s RIP. Each router keeps routing table (or routing vector) with best known distance to each destination and corresponding outgoing interface. Routing tables are updated by exchanging routing information with neighbors.

20. CMPE 150- Introduction to Computer Networks 20 Distance Vector (Cont’d) Routing table at each router: –One entry per participating router. –Each entry contains outgoing interface and distance to corresponding destination. –Metric: number of hops, delay, queue length. –Each router knows distance to its neighbors. Old ARPANET algorithm: DV where cost metric is outgoing link queue length.

21. CMPE 150- Introduction to Computer Networks 21 Distance Vector Routing (a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.

22. CMPE 150- Introduction to Computer Networks 22 Routing Updates Every T interval, routers exchange routing updates. Routing update from router X consists of a vector with all destinations and the corresponding distance from X to them. When router Y receives an update from X, it can estimate its distance to router Z through X as D yz = D yx + D xz. Router Y receives update from all its neighbors and builds a new RT.

23. CMPE 150- Introduction to Computer Networks 23 Distance Vector: Example 1 4 6 2 3 5 1 Node Distance Next 2 3 3 2 1 9 9 5 1 2 10 - 2 2 2 3 5 3 4 1 4 5 6 3 6 8 3 T=T 0 T=T 1 375 2 3 4 0 4 2 302 220 311 533 Node Distance Next 10 - 2 2 2 3 3 4 4 1 4 5 2 4 6 4 4 T=T 2 7