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CS412 Introduction to Computer Networking & Telecommunication

CS412 Introduction to Computer Networking & Telecommunication. Chapter 5 Network Layer. Topics. Design Issues Routing Algorithms Congestion Control Internetworking. Position of network layer. Network Layer Design Issues. Store-and-Forward Packet Switching

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CS412 Introduction to Computer Networking & Telecommunication

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  1. CS412 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. Position of network layer

  4. Network Layer Design Issues • Store-and-Forward Packet Switching • Services Provided to the Transport Layer • Implementation of Connectionless Service • Implementation of Connection-Oriented Service • Comparison of Virtual-Circuit and Datagram Subnets

  5. Store-and-Forward Packet Switching • The environment of the network layer protocols.

  6. Figure 19.3Network layer in an internetwork

  7. Figure 19.4Network layer at the source

  8. Figure 19.5Network layer at a router

  9. Figure 19.6Network layer at the destination

  10. 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 • Two Types of Services • Connectionless • Connection-oriented

  11. Implementation of Connectionless Service • Routing within a diagram subnet.

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

  13. Comparison of Virtual-Circuit and Datagram Subnets 5-4

  14. 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

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

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

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

  18. 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

  19. 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

  20. 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

  21. Sink Tree • Direct consequence of optimality principle • Subnet graph • Node: router • Arc: 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

  22. Sink Tree • Example • Distance metric: number of hops Subnet Sink tree

  23. Shortest Path Routing • Given a pair of routers, find the shortest path between them • Subnet graph • Node: router • Arc: link • Labels of arcs • Function of factors • Weighting function changed for different criteria

  24. 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

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

  26. 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

  27. Flooding - Conclusion • Optimal • Shortest path is always chosen  No other algorithm can produce a shorter delay • Robust • Not practical in most applications • Useful in some applications • Military application: robustness • Distributed database applications • Concurrent update • Wireless networks • Metric of other routing algorithms

  28. 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

  29. 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

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

  31. 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

  32. Link State Routing • In effect • Complete topology and all delays are experimentally measured and distributed to every router • Dijkstra's algorithm is applied to find the shortest path

  33. Link State Packets

  34. 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

  35. Path length from 1A to 5C?

  36. Congestion Control • Congestion • Too many packets present in the subnet • Effects • Performance degraded • Packet lost

  37. Congestion and Network Performance (Could be achieved by congestion control) (Without congestion control)

  38. Causes of Congestion • Causes • Too many packets need an output line  queuing • Problem: not enough memory  packets dropped • Solution(?): adding more memory • New problem: timeout and retransmit  worse • Slow processors • Low bandwidth lines • Congestion tends to feed upon itself and become worse

  39. Congestion Control v.s. Flow Control • Flow control relates to traffic between two machines, while congestion control is more global • Examples • No congestion, flow control needed • Flow control not needed, congestion • Confusion • Congestion control might send back "slow down" messages  Not an acknowledgement

  40. Congestion Control - General Principles • Two groups • Open loop • Closed loop – feedback loop • Explicit feedback • Implicit feedback • Source deduces existence of congestion by making local observation • E.g., time needed for acknowledgement to come back

  41. Congestion Control - General Principles • Closed Loop - Approach • Monitor the system to detect when and where congestion occurs • Pass this information to places where actions can be taken • Adjust system operation to correct the problem

  42. Congestion Control • Congestion = (Load > Resources) • Solutions • Increase resources • Decrease load

  43. Congestion Prevention Policies Policies that affect congestion. 5-26

  44. Congestion Control in VC Subnets • Admission control • Alternative routes • Resource reservation • Based on negotiated agreement

  45. Congestion Control in VC Subnets

  46. Choke Packets • Used in both VC and datagram subnets • Approach • Each router monitors output line utilization • Threshold for "warning state" • A receiving router • Checks packet to see if output line in warning state • If yes then send a "choke packet" back to source host original packet tagged and forwarded

  47. Choke Packet • Source, upon receiving a choke packet • Reduces traffic by a percentage after receiving choke packet • Choke packet referred to same destination is ignored for a fixed time interval • After time interval expired, listens • If choke packet received then goto the step of reducing traffic else increase traffic

  48. Choke Packet • Typically • First choke packet causes data rate reduced to 50%, then 25%, … • Traffic is increased in smaller increments • Why?

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