Chapter5 Network Layer

8/22/2019 Chapter5 Network Layer

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NETWORK LAYER DESIGN ISSUES

Network Layer The network layer is concerned with getting

packets from the source to the destination

The Network layer is the lowest layer that deals

with end to end transmissionDesign Issues

These issues include the service provided to the

transport layer

Routing of the packets through the subnet

Congestion control

Connection of multiple networks together


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Chapter 5 : Network Layer


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Services to transport layer

Network layer runs in the IMPs andthe Transport layer runs in the

Hosts.

Services provided by the networklayer defines the servicesa provided

by the subnet


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Goals to design

The services should be independent from

the subnet technology.

The subnet should shielded of the subnets

present

Network addresses should use a uniform

numbering plan even across LANs and

WANs.


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Connection oriented

The freedom of services provided to the

Transport layer degenerates into two

factions

a) Connection oriented

b) connection less service.


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Differences

Issues Connection Connectionless

Initial setup Required Not possible

Destination only needed on every

address during setuppacket

Packet Guaranteed Not


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differences

Control Done by Done by

Error Network Lay Transport Layer

Flow Network Layer Network Layer

Option

Negation

Possible

YesNo

Identifiers Yes No


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MIXTURES OF SERVICES

Layers Connection oriented Connectionless

service service

76

5

4

3

2

1


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OSI network service Primitives

1. That apply at the boundary between the network

layer and transport layer.

2. Both connection-oriented and connection lessprimitives are provided

3. A connection is a pair of conceptual queues

between two NASP

4. One queue for traffic in each direction.

5. The OSI connection -oriented network service

6. Primitives are listed grouped into four categories


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Connection oriented

N-CONNECT .request ( callee,

aks_wanted,exp_wanted,qos, user_data )

N_CONNECT.indication ( )

N-CONNECT.responce ( responer,acks_wanted,exp_wanted,

qos,user_data )

N-CONNECT.conformation( )

N_DISCONNECT.request (originator ,reason,user_

data,responding_address )

N-DISCONNECT.indication( )


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Connection oriented

N-DATA.request (user_data) N-DATA.indication (user_data)

N_DATA-ACKNOWLEDGE.request()

N-DATA-ACKNOWLEDGE.indiction()

N-DATA.request (user_data) N-DATA.indication (user_data)

N_DATA-ACKNOWLEDGE.request()

N-DATA-ACKNOWLEDGE.indiction()

N-RESET.indication(originator,reason )

N-RESET.response()

N-REPORT.confirm()


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Connectionless oriented N-UNITDATA.request (source_

address,destination_add, qos,user_data )

N-UNITDATA.indication( )

N-FACILITY.request (qos)

N-FACILITY.indication(drstinati on_address,qos,reason )

N-REPORT.indication (destination_ address,qos,) callee: NASP to be called

caller:NASP used by calling

transport entity

Acks_wanted: Boolean flag


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terminology

callee: NASP to be called

caller:NASP used by calling

transport entity Acks_wanted: Boolean flag


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terminology

Exp_wanted :Boolean flag

Qos:Quality of service

User_data: Bytes transmitted but not examined


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terminology

Responder: NASP connected to

at the destination

Originator:Specification ofwho initiated the N-RESET

Reason: Specification of why the

event happened


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network layer

One of the functions of the network layer

is to provide a uniform naming for the

Transport layer to use


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network layer

The second orIDI ( initial Domain Identifier

) field

specifies the domain to which the number

in the

Dsp part belongs.


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NASP address

The full NASP address is variable

length

upto 40 decimal digits or 20 bytes

long.


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NSAPS

In the analogy ,the telephone

sockets are the NSAPs

and the telephone numbersare the NSAP addresses.


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INTERNAL ORGANIZATION

OF THE NETWORK LAYER

The OSI model does not provide

a specification of the key

algorithm, such as routing and

congestion control


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In subnet design (as opposed to

OSI), We will use the

subnet terms "IMPS"and "HOST"

instead of

"network layer" and "transport layer"


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The internal operation of the

subnet,a connection is

usually called a VIRTUAL CIRCUIT,


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the independent packets of the

connectionless

organization are DATAGRAMS,

in analogy with telegrams


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Virtual circuits are generally used

in subnets whose

primary service is connection

oriented


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In contrast with a datagram subnet

no routes are worked out in advance

,even if the sevice

is connection-oriented


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to forward packets for each

of the currently open Virtual

circuits passing through it.


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If packet flowing over a given

virtual circuit always

take through the subnet, each

IMP must remember where


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OSI Network service primitives

user 1 user2

oprior to establishing a connection

network layer

NSAP


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OSI networklayer serviceprimitives

After establishing a connection

user 1 user2

queue


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After threee packets have been sent and

recieved

OSI networklayer serviceprimitives

user 1

packet

oo o

user 2


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Network addresses

The format of OSI network addresses(NSAPs)

AFI IDI DSP

idp

IDP:initialdomain part

AFI:Authority and format indicator

IDI:initialDomain identifier

DSP:Domain specific part


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subnet

a

b c

d

e f

h

h h

h

hh


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Eight virtual circuits through the

subnet originating at A originating at b

0 - abcd 0 - bcd

1 -aefd 1 -bae 2 -abfd 2 -bf

3 -aec

4 -aecdfb


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changes as the packet

progresses line

packet

4 3 1

2

H TO A A TO E E TO C

C TO D D TO F F TO B

0 0


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AND DATAGRAMS WITHIN

THE SUBNETS

BOTH HAVE THEIR SUPPORTERS

AND DETRACTORS

THE MAIN OBJECTIVES ARE IMP

MEMORY

SPACE AND BANDWIDTH


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DIFFERENCES

VIRTUAL CIRCUITS ALLOW PACKETS

TO

CONTAIN CIRCUIT NUMBERS INSTEAD

OF

FULL DESTINATION ADDRESSES


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COMPARISION OF DATAGRAM AND

VIRTUALCIRCUIT SUBNETS

Datagram circuit setup not possible

Virtual circuit it is required

Data gram addressing is from the sourceto the destination

In VC each packet contains a short

VC number


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comparision

Datagram each packet is routed

independently

In VC route is choosen when the

VC is setup

In Datagram congestion control

is difficultIn VC it is easy if enough buffers

are allocated


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Routing

In real function Network layer is

routing packets from the source tothe destination

Te algorithms that choose the route

and Data structures that they use

are a major area of Network layer

design

R i l i h


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Routing algorithm

The routing algorithm is that part of the

network layer software

responsible for desiding which

output line an incoming packet

is to be transmitted on


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routing Algorithms

The routing algorithms must be

able to cope with the changes in

the topology and traffic withoutrequiring all jobs in all hosts to

be abortedand the network to

be rebooted every time some IMPcrashes


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Congestion

When too many packets are

present in the subnet, performance

degrades ,

this situation is called

congestion


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congestion

d -------------------capacity

congestion

desirede

liv

ered

packets sent

perfect


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congestion

Congestion tends to feed upon

itself and become worse

If an IMP has no free buffer,it must ignore newly arriving

packets.

Diff b t ti


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Differences between congestion

control and flow control

congestion control is concerned

about the subnet is able to carry the

offered traffic.

Flow control , in contrast ,relates

to the point-to -point traffic between agiven sender and a given reciever.


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Internetworking

Just as Congestion control is

closely related to the primary

function of the network layer,

routing, so is Networking.


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InterNetworking

Routing aside, another problem

with Internetworking is that not

all networks use the same protocols.

Different protocols imply different packet

formats,headers,flow- control,development

rules,and more.


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Internetworking

As a consequence , When

packets move from network

to network, conversions

are necessary.


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Routing Algorithm Definition

1. Routing is the decision which path to take

2. A routing algorithm decides which output link an

incoming packet should be transmitted on

3. A routing table contains the mappings from thenetworks and host addresses to output ports on the router

4. The routing algorithm builds this table


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The Optimality Principle

1. Optimal routes without regard to network, topology and

traffic

2. Optimal routes form a tree rooted at the destination

3. The tree is called the sink tree

4. The goal of all routing algorithms is to discover and use

the sink trees for all routers.


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Routing Algorithms Overview

Nonadaptive Algorithms

do not base their routing decisions on

mesurements or estimates of the current

traffic and topology.

Adaptive Algorithms

attempt to change their routing decisions to

reflect changes in topology and the currenttraffic.


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Nonadaptive Algorithms

Non-adaptive routing algorithms do not base their

routing decisions on the current state of the

network, This Procedure is sometimes called static

routing.

Examples:1. Flooding

2. Shortest Path Routing


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Flooding Algorithm

Every incoming packet is sent out on every

outgoing line except the one it arrived on,

No routing table, no lookup!

Problem: Vast number of duplicated packets

Reducing Flooding Algorithms Duplicate


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Solution 1: Hop counter

Have a hop counter in the packet header

Routers decrement each arriving packets hop

Counter

Routers discard a packet with hop count=0

Ideally, the hop counter should be initialized to the

length of the path from the source to the destination.

Hop count defines the Horizon of the packet, which

is how far a node can see.

Solution 2: Sequence Numbers Require the first routerhop to put a sequence number in each packet it

receives from its hosts

Each router maintains a table listing the sequence-

Reducing Flooding Algorithms Duplicate

Packets


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Flooding Uses

1. Used in small networks or limited horizon order

100s of nodes

2. Military Applications

1. Large number of routers is desirable2. If one router is taken out (by a bomb?) flooding will

still get packets to their destinations

3. Distributed Databases

1. Simultaneous updates of multiple databases can be donewith a single packet transmission

2. Gnutella search is an example, uses horizon


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Shortest Path Routing

This routing is conceptually simple.

It is a widely used technique to build a

graph of the subnet.

Nodes: IMPs

Arcs: Communication lines

Dijkstras algorithm are well-known.


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A Graph of the Subnet Undirected

Weighted

Geographic Distance

Mean queuing and transmission delay

for a standard test packet as determinedby hourly or daily test runs.

A

B C

E F D

G H

1

22

6 4

2

7

3

3

22


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Dijkstras Algorithm

Each node is labeled (in parentheses) with

its distance from the source node along

the best known path.

A

B C

E F D

G H

1

22

6 4

2

7

3

3

22


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Dijkstras Algorithm (contd)

We want to find the shortest pathfrom A to D.

Initially, no paths are known, so all

nodes are labeled with infinity.

A

B(-) C(-)

E(-) F(-) D(-)

G(-) H(-)


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We start out by marking node A (theworking node) as permanent.

We examine each of the nodes

adjacent A, relabeling each one withthe distance to A.

A

B(2, A) C(-)

E(-) F(-) D(-)

G(6, A) H(-)

2

6


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We make B with the smallest label

permanent.

B becomes the new working node.

A

B(2, A) C(-)

E(-) F(-) D(-)

G(6, A) H(-)


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We examine each of the nodes adjacent

B, relabeling each one with the distance to

B.

A

B(2, A) C(9 B)

E(4, B) F(-) D(-)

G(6, A) H(-)

2

7


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We make E with the smallest label

permanent.

E becomes the new working node.

A

B(2, A) C(9, B)

E(4, B) F(-) D(-)

G(6, A) H(-)


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E(4, B)



E, relabeling each one with the distance to

E.

1 2A

B(2, A) C(9, B)

F(6 E) D(-)

G(5, E) H(-)


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We make G with the smallest label

permanent.

G becomes the new working node.

A

B(2, A) C(9, B)

E(4, B) F(6, E) D(-)

G(5, E) H(-)


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E(4, B)



G, relabeling each one with the distance to

G.

4

A

B(2, A) C(9, B)

F(6, E) D(-)

G(5, E) H(9 G)


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We make F with the smallest label

permanent.

F becomes the new working node.

A

B(2, A) C(9, B)

E(4, B) F(6, E) D(-)

G(5, E) H(9, G)


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E(4, B)



F, relabeling each one with the distance to

F.

2A

B(2, A) C(9, B)

F(6, E) D(-)

G(5, E) H(8, F)

3


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We make H with the smallest label

permanent.

H becomes the new working node.

A

B(2, A) C(9, B)

E(4, B) F(6, E) D(-)

G(5, E) H(8, F)


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E(4, B)



H, relabeling each one with the distance to

H.

2

A

B(2, A) C(9, B)

F(6, E) D(10, F)

G(5, E) H(8, F)


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We make C with the smallest label

permanent.

C becomes the new working node.

E(4, B)A

B(2, A) C(9, B)

F(6, E) D(10, F)

G(5, E) H(8, F)


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C, relabeling each one with the distance to

C.

E(4, B)A

B(2, A) C(9, B)

F(6, E) D(10, F)

G(5, E) H(8, F)

3


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We make D with the smallest label

permanent.

D becomes the new working node.

E(4, B)A

B(2, A) C(9, B)

F(6, E) D(10, F)

G(5, E) H(8, F)


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The shortest path (Dijkstras) algorithm

from Source A to Destination D.

E(4, B)A

B(2, A) C(9, B)

F(6, E) D(10, H)

G(5 E) H(8 F)

Chapter5 Network Layer

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