packet switching neil tang 10/6/2008

CS440 Computer Networks 1

Packet SwitchingPacket Switching

Neil TangNeil Tang10/6/200810/6/2008


OutlineOutline

Switching

Datagram (Connectionless)

Virtual Circuit (Connection-oriented)

Source Routing


Why SwitchingWhy Switching

The limitation of number of hosts

The limitation of the geographic area


SwitchingSwitching

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.


SwitchSwitch

Function: it receives incoming packets on one of its links and to transmit them on some other link.

Input and output ports

Inputports

T3T3

STS-1

T3T3STS-1

Switch

Outputports


Datagram Datagram Every packet contains the destination address. Every switch maintains a routing table.

0

132

0

1 3

2

013

2

Switch 3 Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host EHost F

Host G

Host H

Destination Port

A 3

B 0

C 3

D 3

E 2

F 1

G 0

H 0

Routing table in switch 2


DatagramDatagram A host can send a packet anywhere at any time. Any packet that turn

s 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.


DatagramDatagram

Strength: high resource utilization and high throughput.

Weakness: no QoS guarantees.


Virtual Circuit SwitchingVirtual Circuit Switching

Incoming Port Incoming VCI Outgoing Port Outgoing VCI

2 5 1 11

Switch 1


3 11 2 7

Switch 2


0 7 1 4

Switch 3

Routing Table



0

1

2

3

0

13

01

2

3

0

1

22

3

Host A Host B

Switch 3

Switch 2Switch 1

5

11

Data Transfer

0

1

2

3

0

1

2

3

0

1

2

3

0

1

2

3

Host A Host B

Switch 3

Switch 2Switch 1

7

11



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.


Virtual Circuit SwitchingVirtual 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.


Virtual Circuit SwitchingVirtual 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.



Strength: QoS (bandwidth, delay, jitter, packet loss) provisioning.

Weakness: High overhead, poor throughput.



Typical networks using VCS

Frame Relay

X.25

ATM


Source RoutingSource 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.

0

132

01 3

2

0

13

2

0

13

2

3 0 1 3 01

30 1

Switch 3

Host B

Switch 2

Host A

Switch 1



Methods to handle headers

Rotation

Stripping

Pointer

Header enteringswitch

Header leavingswitch

(a) (b) (c)

D C B A D C B A

D C BA D C B

Ptr D C B A

Ptr D C B A



Strength: Simple design for switches (no routing tables). Weakness: Bad scalability and high overhead (unbounded

packet header).

packet switching neil tang 10/6/2008

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