CS440 Computer Networks 1 Packet Switching Neil Tang 10/6/2008

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. DestinationPort A3 B0 C3 D3 E2 F1 G0 H0 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 Incoming PortIncoming VCIOutgoing PortOutgoing VCI Switch 1 Incoming PortIncoming VCIOutgoing PortOutgoing VCI Switch 2 Incoming PortIncoming VCIOutgoing PortOutgoing VCI 0714 Switch 3 Routing Table

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).