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Network Layer Design Issues: Packet Switching and Routing Algorithms

This chapter explores the design considerations of the network layer, including store-and-forward packet switching, implementation of connectionless and connection-oriented services, and the comparison of virtual-circuit and datagram subnets. It also delves into various routing algorithms such as shortest path routing, flooding, distance vector routing, and link state routing. Additionally, it covers congestion control and quality of service techniques.

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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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