Lecture 8

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Lecture 8

Switching and Forwarding

Adhi Harmoko S, M.Komp


• Switch

Scalable Networks

• forwards packets from input port to output port

• port selected based on address in packet header

T3

T3

STS-1

Input

ports

Switch

T3

T3

STS-1

Output

ports

• Advantages

• cover large geographic area (tolerate latency)

• support large numbers of hosts (scalable bandwidth)


• No connection setup phase

Datagram Switching

• Each packet forwarded independently

• Sometimes called connectionless model

Host D

• Analogy: postal system

• Each switch maintains Host C

2

2

3

a forwarding (routing)

table 0

3

0

Switch 1

1

Host E

Switch 2

1

Host F

Host A

Host G 0 Switch 3

1 3

Host B

2

Host H


Datagram Model

• There is no round trip time delay waiting for connection

setup; a host can send data as soon as it is ready

• Source host has no way of knowing if the network is

capable of delivering a packet or if the destination host is

even up.

• Since packets are treated independently, it is possible to

route around link and node failures

• Since every packet must carry the full address of the

destination, the overhead per packet is higher than for

the connection-oriented model


Virtual Circuit Switching

• Explicit connection setup (and tear-down) phase

• Subsequence packets follow same circuit

• Sometimes called connection-oriented model

0

• Analogy:

phone call

• Each switch

maintains a

VC table

3

0

2

1

Switch 1

3

0

2

1

Switch 2

0

3

2

1

Switch 3

Host A

3

1

2

Host B


Message from Host A to Host B

3

0

2

1

Switch 1

11

3

0

2

1

Switch 2

3

0

2

1

5

3

0

Switch 3

1

Host A

2

Host B


Message from Host A to Host B

• Virtual circuit numbers are unique per link - I.e., link local

3

0

1

11

3

0

1

3

0

1

2

Switch 1

2

Switch 2

2

7

3

0

Switch 3

1

Host A

2

Host B


Virtual Circuit Model

• Typically wait full RTT for connection setup before

sending first data packet.

• While the connection request contains the full address for

destination, each data packet contains only a small

identifier, making the per-packet header overhead small.

• If a switch or a link in a connection fails, the connection is

broken and a new one needs to be established.

• Connection setup provides an opportunity to reserve

resources.


Source Routing

• Each packet carries the routing information

• Source host knows the exact route

• Node rotate the address

3

0

2

Switch 1

1

3 0 1 1 3 0

3

2

0

Switch 2

1

3

0

2

1

Host A

0 1

3

0 Switch 3

1 3

2

Host B


Maintaining the source route

• a) rotation of route

• We will know the reverse path

• b) Stripping

• Packets become smaller

• c) Pointer

• Each router updates the pointer

Header entering

switch

D C B A D C B A

Ptr D C

B A

Header leaving

switch

A D C B D C B Ptr D C B A

(a) (b) (c)

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