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ECE453 – Introduction to Computer Networks. Lecture 9 - The Network Layer (I) - Routing. Recap. Framing Error detection and correction Reliable or unreliable data transfer Channel allocation protocol. network data link physical. network data link physical. logical end-end transport.

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ece453 introduction to computer networks

ECE453 – Introduction to Computer Networks

Lecture 9 - The Network Layer (I) - Routing

recap
Recap
  • Framing
  • Error detection and correction
  • Reliable or unreliable data transfer
  • Channel allocation protocol
network core and network edge

network

data link

physical

network

data link

physical

logical end-end transport

Network Core and Network Edge

Network

core

application

transport

network

data link

physical

Network

edge

application

transport

network

data link

physical

network core information transmission
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
design issues
Design Issues
  • Store-and-forward packet switching
  • Services to the transport layer
    • Connection-oriented vs. Connectionless
    • Quality of service (QoS)
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

Network Layer Services
the internet network layer

ICMP protocol

  • error reporting
  • router “signaling”
  • IP protocol
  • addressing conventions
  • datagram format
  • packet handling conventions
  • Routing protocols
  • path selection
  • RIP, OSPF, BGP

routing

table

The Internet Network Layer

Transport layer: TCP, UDP

Network

layer

Link layer

physical layer

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)

Different Strategies
graph abstraction for routing algorithms

5

3

5

2

2

1

3

1

2

1

A

D

B

E

F

C

Graph Abstraction for Routing Algorithms
  • Graph nodes  Routers
  • Graph edge  Physical links
  • Edge weight  Link cost
  • Link cost
    • Delay, power consumption, congestion level, $cost, etc.
  • Good path or optimal path
    • Minimum link cost
two fundamental routing algorithms
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
a link state routing algorithm
A Link State Routing Algorithm

Dijkstra’s 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 nodes
  • Iterative algorithm
dijkstra s algorithm an example

5

3

5

2

2

1

3

1

2

1

A

D

E

B

F

C

Dijkstra’s Algorithm: An Example

D(B),p(B)

2,A

2,A

2,A

D(D),p(D)

1,A

D(C),p(C)

5,A

4,D

3,E

3,E

D(E),p(E)

infinity

2,D

Step

0

1

2

3

4

5

start N

A

AD

ADE

ADEB

ADEBC

ADEBCF

D(F),p(F)

infinity

infinity

4,E

4,E

4,E

dijkstra s algorithm discussion

A

A

A

A

D

D

D

D

B

B

B

B

C

C

C

C

2+e

2+e

0

0

1

1

1+e

1+e

0

e

0

0

Dijkstra’s Algorithm: Discussion

Algorithm 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 traffic

1

1+e

0

2+e

0

0

0

0

e

0

1

1+e

1

1

e

… recompute

… recompute

routing

… recompute

initially

dv routing

cost to destination via

E

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

destination

distance from X to

Y, via Z as next hop

X

=

D (Y,Z)

Z

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

=

w

DV Routing
  • Each 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

Iterative routing

Distributed routing

dv example

2

1

7

X

Z

Y

DV: Example

Y

X

Z

dv link cost changes

1

4

1

50

X

Z

Y

DV: Link Cost Changes

Y

“good news travels fast”

Z

X

algorithm

terminates

dv link cost changes1

60

4

1

50

X

Z

Y

DV: Link Cost Changes

“bad news travels slow”

Count to infinity problem

algorithm

continues

on!

distance table an example

cost to destination via

E

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

1

7

2

8

1

destination

2

A

D

E

B

C

E

E

E

D (C,D)

D (A,D)

D (A,B)

D

B

D

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

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

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

=

=

=

w

w

w

=

=

=

8+6 = 14

2+2 = 4

2+3 = 5

Distance Table: An Example

loop!

loop!