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Network Layer Design Issues: Packet Switching and Routing Algorithms

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  1. The Network Layer Chapter 5

  2. Network Layer Design Isues • 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

  3. Store-and-Forward Packet Switching fig 5-1 The environment of the network layer protocols.

  4. Implementation of Connectionless Service Routing within a datagram subnet.

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

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

  7. Routing Algorithms • The Optimality Principle • Shortest Path Routing • Flooding • Distance Vector Routing • Link State Routing • Hierarchical Routing • Broadcast Routing • Multicast Routing • Routing for Mobile Hosts • Routing in Ad Hoc Networks

  8. Routing Algorithms (2)‏ Conflict between fairness and optimality.

  9. The Optimality Principle (a) A subnet. (b) A sink tree for router B. If J is on the path from I to K then the path from J to K follows the same route.

  10. Shortest Path Routing The first 5 steps used in computing the shortest path from A to D. The arrows indicate the working node.

  11. Flooding 5-8 top Dijkstra's algorithm to compute the shortest path through a graph.

  12. Flooding (2)‏ 5-8 bottom Dijkstra's algorithm to compute the shortest path through a graph.

  13. Distance Vector Routing (a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.

  14. Distance Vector Routing (2)‏ The count-to-infinity problem.

  15. Link State Routing Each router must do the following: • Discover its neighbors, learn their network address. • Measure the delay or cost to each of its neighbors. • Construct a packet telling all it has just learned. • Send this packet to all other routers. • Compute the shortest path to every other router.

  16. Learning about the Neighbors (a) Nine routers and a LAN. (b) A graph model of (a).

  17. Measuring Line Cost A subnet in which the East and West parts are connected by two lines.

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

  19. Distributing the Link State Packets The packet buffer for router B in the previous slide (Fig. 5-13).

  20. Hierarchical Routing Hierarchical routing.

  21. Congestion Control Algorithms • General Principles of Congestion Control • Congestion Prevention Policies • Congestion Control in Virtual-Circuit Subnets • Congestion Control in Datagram Subnets • Load Shedding • Jitter Control

  22. Congestion When too much traffic is offered, congestion sets in and performance degrades sharply.

  23. General Principles of Congestion Control • Monitor the system . • detect when and where congestion occurs. • Pass information to where action can be taken. • Adjust system operation to correct the problem.

  24. Classification of Congestion Control Open Loop Congestion Prevention Closed Loop Explicit Feedback Implicit Feedback

  25. Congestion Control in Virtual-Circuit Subnets • A congested subnet. (b) A redrawn subnet, eliminates congestion and a virtual circuit from A to B. • The point: can do admission control; congestion prevention

  26. Hop-by-Hop Choke Packets (a) A choke packet that affects only the source. (b) A choke packet that affects each hop it passes through. The point: have to respond to conditions with feedback mechanism

  27. Explicit Feedback • Congestion warning bit – fed forward to recipient who fed it back to the sender (DECnet) • Choke packets • What should the sender do when it receives congestion feedback? A thorny question!

  28. Implicit Feedback • Routers drop packets • Packets are delivered slowly • Sender responds to missing acks or delayed acks by slowing sending rate • Again, what is a good response? • For performance of this flow? • For overall performance of the network?

  29. Quality of Service • Requirements • Techniques for Achieving Good Quality of Service • Integrated Services • Differentiated Services • Label Switching and MPLS

  30. Requirements 5-30 How stringent the quality-of-service requirements are.

  31. Jitter Control (a) High jitter. (b) Low jitter.

  32. Buffering to reduce jitter Smoothing the playback stream by buffering packets.

  33. The Leaky Bucket Algorithm (a) A leaky bucket with water. (b) a leaky bucket with packets.

  34. Token Bucket

  35. The Leaky and Token Bucket Algorithms (a) Input to a leaky bucket. (b) Output from a leaky bucket. Output from a token bucket with capacities of (c) 250 KB, (d) 500 KB, (e) 750 KB, (f) Output from a 500KB token bucket feeding a 10-MB/sec leaky bucket.

  36. The Token Bucket Algorithm 5-34 (a) Before. (b) After.

  37. Admission Control 5-34 An example of flow specification.

  38. Integrated Services Convergence of data and audio networks From the IETF perspective From the telephony perspective Economic and technical issues Reserved resources in packet-oriented network Per-flow state in routers Advance setup Scalability

  39. Differentiated Services • Tag packets at network ingress with class of service • Example: expedited forwarding – some packets get better service than others • “Network Neutrality”

  40. Expedited Forwarding Expedited packets experience a traffic-free network.

  41. Label Switching and MPLS Transmitting a TCP segment using IP, MPLS, and PPP.

  42. The Network Layer in the Internet • The IP Protocol • IP Addresses • Internet Control Protocols • OSPF – The Interior Gateway Routing Protocol • BGP – The Exterior Gateway Routing Protocol • Internet Multicasting • Mobile IP • IPv6

  43. Design Principles for Internet • Make sure it works. • Keep it simple. • Make clear choices. • Exploit modularity. • Expect heterogeneity. • Avoid static options and parameters. • Look for a good design; it need not be perfect. • Be strict when sending and tolerant when receiving. • Think about scalability. • Consider performance and cost.

  44. The IP Protocol The IPv4 (Internet Protocol) header.

  45. The IP Protocol (2)‏ 5-54 Some of the IP options.

  46. IP Addresses IP address formats.

  47. IP Addresses (2)‏ Special IP addresses.

  48. Subnets A campus network consisting of LANs for various departments.

  49. Subnets (2)‏ A class B network subnetted into 64 subnets.

  50. CDR – Classless InterDomain Routing 5-59 A set of IP address assignments.

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