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Packet Switching Neil Tang 10/6/2008

Packet Switching Neil Tang 10/6/2008. Outline. Switching Datagram (Connectionless) Virtual Circuit (Connection-oriented) Source Routing. Why Switching. The limitation of number of hosts The limitation of the geographic area. Switching. A switch provides a star topology.

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Packet Switching Neil Tang 10/6/2008

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  1. Packet SwitchingNeil Tang10/6/2008 CS440 Computer Networks

  2. Outline • Switching • Datagram (Connectionless) • Virtual Circuit (Connection-oriented) • Source Routing CS440 Computer Networks

  3. Why Switching • The limitation of number of hosts • The limitation of the geographic area CS440 Computer Networks

  4. Switching • A switch provides a star topology. • A large switching network can be built by interconnecting a number of switches and hosts using point-to-point links. CS440 Computer Networks

  5. Switch • Function: it receives incoming packets on one of its links and to transmit them on some other link. • Input and output ports CS440 Computer Networks

  6. Datagram • Every packet contains the destination address. • Every switch maintains a routing table. Routing table in switch 2 CS440 Computer Networks

  7. Datagram • A host can send a packet anywhere at any time. Any packet that turns up at a switch can be quickly forwarded. • When a host sends a packet, it has no way of knowing if the network is capable of delivering it or if the destination is working. • Each packet is forwarded independently and two consecutive packets of a flow may follow different paths (routing table change). • The network can tolerate a switch or a link failure by finding an alternative route around the failure and updating the related routing tables accordingly. CS440 Computer Networks

  8. Datagram • Strength: high resource utilization and high throughput. • Weakness: no QoS guarantees. CS440 Computer Networks

  9. Virtual Circuit Switching Routing Table Switch 1 Switch 2 Switch 3 CS440 Computer Networks

  10. Virtual Circuit Switching Data Transfer CS440 Computer Networks

  11. Virtual Circuit Switching Connection Setup and Teardown • The source node sends a setup message into the network. • The message will be forwarded to the destination node via other switches. Every time when the packet passes a switch, the switch will pick an available VCI for this connection. • The destination node sends back an ACK message which notifies each switch which VCI is used by its downstream node for this connection. • At the end of a connection, the source node sends a teardown message. The switch on the way removes the related entry in its routing table and forwards it. CS440 Computer Networks

  12. Virtual Circuit Switching • There is at least one RTT of delay before data transfer. • VCI is only unique on one link. • If a link or a switch fails, the old connection needs to be torn down to free up resources and a new one needs to be established. • Routing tables are computed by routing algorithms. CS440 Computer Networks

  13. Virtual Circuit Switching • Buffers are allocated to each virtual circuit when it is initialized. • The sliding window protocol is used between each source-destination pair for flow control. • The circuit is rejected by a given switch if not enough buffers are available at that switch. CS440 Computer Networks

  14. Virtual Circuit Switching • Strength: QoS (bandwidth, delay, jitter, packet loss) provisioning. • Weakness: High overhead, poor throughput. CS440 Computer Networks

  15. Virtual Circuit Switching Typical networks using VCS • Frame Relay • X.25 • ATM CS440 Computer Networks

  16. Source Routing Basic Idea: The whole route (a set of ports) is computed by the source node and is included in the header in the packets. CS440 Computer Networks

  17. Source Routing • Methods to handle headers • Rotation • Stripping • Pointer CS440 Computer Networks

  18. Source Routing • Strength: Simple design for switches (no routing tables). • Weakness: Bad scalability and high overhead (unbounded packet header). CS440 Computer Networks

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