CS 313 Introduction to Computer Networking & Telecommunication

1. CS 313 Introduction to Computer Networking & Telecommunication Chapter 5 Network Layer Chi-Cheng Lin, Winona State University

2. Topics • Design Issues • Routing Algorithms • Congestion Control • Internetworking

3. Network Layer Design Issues • Concern: How to get packets from source to destination • Issues • Store-and-forward packet switching • Services provided to transport layer • Implementation of connectionless service • Implementation of connection-oriented service • Comparison of virtual-circuit and datagram networks

4. Store-and-Forward Packet Switching • The environment of the network layer protocols ISP’s equipment

5. Services Provided to Transport Layer • Designing goals • Independent of subnet technology • Transport layer shielded from number, type, and topology of subnets • Uniform network address numbering • Even across LANs and WANs • Two Types of Services • Connectionless, e.g., IP • Connection-oriented, e.g., ATM

6. Implementation of Connectionless Service ISP’s equipment • Routing within a diagram subnet A’s table (initially) A’s table (later) C’s Table E’s Table

7. Implementation of Connection-Oriented Service ISP’s equipment • Routing within a virtual-circuit subnet. A’s table C’s Table E’s Table

8. Implementation of Connection-Oriented Service • Routing within a virtual-circuit subnet.

9. Comparison of Virtual-Circuit and Datagram Subnets

10. Routing Algorithms • Network layer software • Deciding which output line an incoming packet should be transmitted to • Datagram: made for each packet • VC: made for new VC setup

11. Routing Algorithms • Desirable Properties • Correctness • Simplicity • Robustness • Stability • Fairness • Optimality

12. Routing Algorithms - Optimality • What to optimize? • Minimizing mean packet delay • Maximizing total network throughput • Problem • The above two are in conflicts (see next slide) • Compromise • Minimizing number of hops a packet must take from source to destination

13. Fairness vs. Efficiency Network with a conflict between fairness and efficiency.

14. Classes of Routing Algorithm • Two major classes • Non-adaptive • Adaptive

15. Two Major Classes of Routing Algorithm • Adaptive • Algorithms differ in • where to get information, e.g., • Locally • From adjacent routers • From all routers • when to change routes, e.g., • Every T sec • When the load changes • When the topology changes • what metric used for optimization, e.g., • Distance • Number of hops • Estimated transit time

16. Routing Algorithms to be Studied • Static (i.e., nonadaptive) routing • Shortest path routing • Flooding • Dynamic (i.e., adaptive) routing • Distance vector routing • Link state routing

17. L Optimality Principle • If router J is on the optimal path from router I to router K, then the optimal path from J to K also falls along the same route. • Proof? K J I

18. Sink Tree • Direct consequence of optimality principle • Network graph • Node/vertice: router • Edge: link • The set of optimal routes from all sources to a given destination form a tree rooted at the destination • Might not be unique • Goal of routing algorithms • Discover and use the sink tree for all routers

19. Sink Tree • Example • Distance metric: number of hops Network Sink tree

20. Shortest Path Algorithm • Given a pair of routers, find the shortest path between them • Network graph • Node/vertice: router • Edge: link • Labels of edges • Function of factors • Weighting function changed for different criteria

21. Dijkstra's Algorithm • Each node labeled with • (distance from source, best known path) • Initially • distance: infinity • best known path: unknown • Label might change to reflect better paths • Node is either tentative or permanent • Initially tentative • As a path from source to that node discovered, label becomes permanent and never gets changed

22. First six steps to compute shortest path from A to D

23. Dijkstra's Algorithm • How do we find the path? • The algorithm given in the book works from destination to source • Why?

24. Flooding • Every incoming packet is sent out on every outgoing line except for the input line • Problem • Large number of packets are generated • Solutions • Hop counter • Avoiding duplicates • Selective flooding

25. Flooding - Conclusion • Optimal • Shortest path is always chosen  No other algorithm can produce a shorter delay • Robust • Easy to set up • Only need to know its neighbors • Not practical in most applications • Useful in some applications • Military application: robustness • Wireless networks • Metric of other routing algorithms

26. Distance Vector Routing • Table in each router • Giving • Best known distance to each destination • Which line to use to get there • Indexed by each router • Each entry contains two parts • Preferred outgoing line to use for that destination • Estimate of "distance" to go there • Best known distance to each destination • Updated by exchanging information with neighbors

27. Distance Vector Routing • Router • Knows "distance" to each neighbor • Sends list to each neighbor every T msec  Receives lists from neighbors every T msec • If neighbor X knows • Distance from X to I is XI, and • Distance from the router to X is m then delay from router to I via X is (XI + m) • Performs calculation for each neighbor to find the best • Old table not used in calculation

28. A network. Input from A, I, H, K, and the new routing table for J.

29. Distance Vector Routing • Count-to-Infinity Problem • Good news spread fast, bad news leisurely • Infinity has to be defined

30. Link State Routing • A router • Discovers its neighbors and learn their network addresses - HELLO • Measures the delay or cost to each neighbor - ECHO • Constructs a packet telling all it has just learned – link state packet • Sends this packet to all other routers – flooding w/ duplicate avoidance • Computes the shortest path to every other router – Dijkstra’s algorithm

31. Link State Routing • In effect • Complete topology and all delays are experimentally measured and distributed to every router • Construct network topology from link state packets • Dijkstra's algorithm is applied to find the shortest path

32. Building Link State Packets (a) A network. (b) The link state packets for this network.

33. Hierarchical Routing • Problem • Network size grows  Routing tables grow • Solution • Hierarchical routing • Idea • Routers are divided into "regions" • Router knows detail about routing within its region • Router knows nothing about internal structure of other region

34. Path length from 1A to 5C?