ECE453 – Introduction to Computer Networks

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ECE453 – Introduction to Computer Networks

Lecture 9 - The Network Layer (I) - Routing

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RecapFramingError detection and correctionReliable or unreliable data transferChannel allocation protocol

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Network Core and Network Edge

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

networkdata linkphysical

networkdata linkphysical

logical end-end transportNetworkedge

Network core

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Network Core – Information Transmission

Circuit switching Telephone system

Message switching Mail delivery The message travels as a complete unit. At any

one time, it completely exists in one place.

Packet switching The Internet

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Design Issues

Store-and-forward packet switchingServices to the transport layer Connection-oriented vs.

Connectionless Quality of service (QoS)

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Network Layer Services

Connectionless Best-effort No guarantee The Internet No advance setup

is needed Datagram subnet

Connection-oriented Guaranteed delivery ATM A path from the

source router to the destination router is established before any data packets can be sent

Virtual circuit

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The Internet Network Layer

routingtable

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

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Routing - In the Middle of Intersection …

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Different Strategies

Nonadaptive algorithms (or static routing)

Adaptive algorithms (or dynamic routing) Global algorithm

(have a global knowledge – a map)

Decentralized algorithm (get information only from neighbor)

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Graph Abstraction for Routing Algorithms

Graph nodes RoutersGraph edge Physical linksEdge weight Link costLink cost Delay, power consumption, congestion

level, $cost, etc.

Good path or optimal path Minimum link cost

A

ED

CB

F

2

2

13

1

1

2

53

5

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Two Fundamental Routing Algorithms

Link state routing A global algorithm

Distance vector routing (or Bellman-Ford routing, Ford-Fulkerson routing) A decentralized algorithm The original ARPANET routing algorithm,

replaced by LS routing in 1979

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A Link State Routing Algorithm

Net topology, link costs known to all nodes accomplished via “link state broadcast” all nodes have the same info

computes least cost paths from the source to all other nodesIterative algorithm

Dijkstra’s algorithm

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Dijkstra’s Algorithm: An Example

Step012345

start NA

ADADE

ADEBADEBC

ADEBCF

D(B),p(B)2,A2,A2,A

D(C),p(C)5,A4,D3,E3,E

D(D),p(D)1,A

D(E),p(E)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

A

ED

CB

F

2

2

13

1

1

2

53

5

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Dijkstra’s Algorithm: DiscussionAlgorithm complexity: n nodes each iteration: need to check all nodes, not in N n*(n+1)/2 comparisons: O(n**2) more efficient implementations possible:

O(nlogn)

Oscillations possible: e.g., link cost = amount of carried trafficA

D

C

B1 1+e

e0

e

1 1

0 0

A

D

C

B2+e 0

001+e1

A

D

C

B0 2+e

1+e10 0

A

D

C

B2+e 0

e01+e1

initially… recompute

routing… recompute … recompute

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DV RoutingEach router maintains a distance table Initialization

0 for itself Infinity for non-neighbor Link cost for neighbor

Message exchange between neighbors

When a neighbor first comes up When information changes

(e.g. change in link cost)

Distance vector calculation Minimize cost to each

destination

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion

D (Y,Z)X

distance from X toY, via Z as next hop

c(X,Z) + min {D (Y,w)}Z

w

=

=

Iterative routingDistributed routing

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DV: Example

X Z12

7

Y

X

Y

Z

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DV: Link Cost Changes

X Z14

50

Y1

algorithmterminates

“good news travels fast”

X

Y

Z

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DV: Link Cost Changes

X Z14

50

Y60

algorithmcontinues

on!

“bad news travels slow”

Count to infinity problem

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Distance Table: An Example

A

E D

CB7

8

1

2

1

2

D (C,D)E

c(E,D) + min {D (C,w)}D

w== 2+2 = 4

D (A,D)E

c(E,D) + min {D (A,w)}D

w== 2+3 = 5

D (A,B)E

c(E,B) + min {D (A,w)}B

w== 8+6 = 14

loop!

loop!

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion