unit 3 dcn class lectures .ppt

8/16/2019 UNIT 3 dcn class lectures .ppt

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Unit 3 DATA COMMUNICATION NETWORKUnit 3 DATA COMMUNICATION NETWORK

The Network Layer: Design Issues.Routing Algorithms.Congestion control Algorithms.Subnet concept

irtual circuit an! Data gram Subnet "low control#Internetworking $ri!ges# Routers# %ateways an! !i&&erent le'el

switches.


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Computer Networks , Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, !"

Network Layer Design Issues

# $tore-and-forward %ac&et switchin'# $ervices %rovided to trans%ort layer # (m%lementation of connectionless service# (m%lementation of connection-oriented service# )om%arison of virtual-circuit and data'ram

networ&s


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$tore-and-Forward Pac&et $witchin'

($P*s e+ui%ment


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(m%lementation of )onnectionless $ervice

outin' within a data'ram networ&

($P*s e+ui%ment


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(m%lementation of)onnection- riented $ervice

outin' within a virtual-circuit networ&

($P*s e+ui%ment


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Comparison o& irtual(Circuitan! Datagram Networks


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Bridges, Routers, Gateways and different levelswitches. (Network Devices)

• Modem: a device that modulates a digital signal onto analog signal for transmission over

telephone lines.• Repeater: Re-generates the signal again.


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Switches/routers/gateways (cont.)

• Mainly from the point of software, i.e., containing functions of lower three layers

• There are some requirements for hardware such as speed, disk, memory, multipleinterfaces.

• Bridge : used to connect multiple similar !"s.• ircuit switch , used in telephone networks• !acket switch/router/gateway :

# $enerally consider them as the same meaning # !acket switch deals with a uniform routing procedure, within one homogenous

network, one pair of data link and physical layer # Router/gateway deals with routing in multiple heterogeneous networks, more

than one pair of data link and physical layers

# Gateway : used to connect multiple different !"s


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"etwork devices %ith ayer

Layers Network Devices

Application Layer Application gateway

Transport Layer Transport gateway

Network Layer Router and gateway

Data link layer Bridge and Switch

Physical Layer Repeater, Hu and !ode"#


11/101Computer Networks , Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, !"

Routing Algorithms

# %timality %rinci%le# $hortest %ath al'orithm

# Floodin'# Distance vector routin'# .in& state routin'# outin' in ad hoc networ&s



outin' Al'orithms / 0

# 1roadcast routin'# 2ulticast routin'

# Anycast routin'# outin' for mobile hosts# outin' in ad hoc networ&s



The %timality Princi%le

/a0 A networ&3 /b0 A sin& tree for router B.


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/a0 A networ&3/b0 (n%ut from A, I, H, K, and the new routing table for J.

Distance 4ector outin'



The )ount-to-(nfinity Problem

The count-to-infinity %roblem


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.earnin' about the 9ei'hbors /"0

9ine routers and a broadcast .A93



.earnin' about the 9ei'hbors / 0

A 'ra%h model of %revious slide3



1uildin' .in& $tate Pac&ets

/a0 A networ&3 /b0 The lin& state %ac&ets for this networ&3



Distributin' the .in& $tate Pac&ets

The %ac&et buffer for router B in %revious slide


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1roadcast outin'

everse %ath forwardin'3 /a0 A networ&3 /b0 A sin& tree3/c0 The tree built by reverse %ath forwardin'3



2ulticast outin' /"0

/a0 A networ&3 /b0 A s%annin' tree for the leftmost router3 /c0 A

multicast tree for 'rou% "3 /d0 A multicast tree for 'rou% 3



2ulticast outin' / 0

/a0 )ore-based tree for 'rou% "3

/b0 $endin' to 'rou% "3



Anycast outin'

/a0 Anycast routes to 'rou% "3

/b0 To%olo'y seen by the routin' %rotocol3


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CONGESTION CONTROLCONGESTION CONTROL

Congestion in a network may occur if the load on theCongestion in a network may occur if the load on thenetwork—the number of packets sent to the network— network—the number of packets sent to the network—

is greater than the capacity of the network—theis greater than the capacity of the network—thenumber of packets a network can handle.number of packets a network can handle.

Congestion control refers to the mechanisms andCongestion control refers to the mechanisms and

techniques to control the congestion and keep the loadtechniques to control the congestion and keep the loadbelow the capacity.below the capacity.


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"igure Traffic descriptors


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Con#estion Control

When one part of the subnet (e.g. one or morerouters in an area) becomes overloaded,congestion results.Because routers are receiving packets fasterthan they can for ard them, one of t o thingsmust happen!

"he subnet must prevent additional packets fromentering the congested region until those already

present can be processed. "he congested routers can discard #ueued packets tomake room for those that are arriving.


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CONGESTION CONTROLCONGESTION CONTROL

Congestion control refers to techniques and mechanismsCongestion control refers to techniques and mechanismsthat can either prevent congestion, before it happens, orthat can either prevent congestion, before it happens, orremove congestion, after it has happened.remove congestion, after it has happened.

In general, we can divide congestion control mechanisms In general, we can divide congestion control mechanismsinto two broad categories:into two broad categories:

..open!loop congestion control "prevention#open!loop congestion control "prevention#

$.$. closed!loop congestion control "removal#.closed!loop congestion control "removal#.

l l f l


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General !rinci#les of ongestion ontrolclosed!loop congestion control closed!loop congestion control

$. %onitor the syste& . # detect when and where congestion occurs.

B. !ass infor&ation to where action can 'e taken.. $d ust syste& o#eration to correct the #ro'le&.


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W i Bi


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Warning Bit$ special bit in the packet header is set by the routerto arn the source hen congestion is detected.

"he bit is copied and piggy%backed on the $&' andsent to the sender.

"he sender monitors the number of $&' packets itreceives ith the arning bit set and ad usts its

transmission rate accordingly.


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"igure Choke packet

H %b %H &h k k


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Hop%by%Hop &hoke acketsver lon' distances or at hi'h s%eeds cho&e %ac&ets are not

very effective3 A more efficient method is to send to cho&e %ac&ets ho%-by-ho%3This re+uires each ho% to reduce its transmission even beforethe cho&e %ac&et arrive at the source3

$oad Sheddin#


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$oad Sheddin#

When bu ers become full, routers simply discardpackets.Which packet is chosen to be the victim depends on theapplication and on the error strategy used in the datalink layer.

-or a le transfer, for, e.g. cannot discard older packetssince this ill cause a gap in the received data.-or real%time voice or video it is probably better tothro a ay old data and keep ne packets.

/et the application to mark packets ith discardpriority.

0 d 1 l 2i d (012)


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0andom 1arly 2iscard (012)This is a %roactive a%%roach in which the router discards oneor more %ac&ets before the buffer becomes com%letely full3

Each time a %ac&et arrives, the ED al'orithm com%utes theavera'e +ueue len'th, avg 3(f a g is lower than some lower threshold, con'estion isassumed to be minimal or non-e:istent and the %ac&et is+ueued3(f a g is 'reater than some u%%er threshold, con'estion isassumed to be serious and the %ac&et is discarded3(f a g is between the two thresholds, this mi'ht indicate theonset of con'estion3 The %robability of con'estion is thencalculated3


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QUALITY OF SERVICEQUALITY OF SERVICE

&uality of service "&o'# is an internetworking issue&uality of service "&o'# is an internetworking issuethat can be defined as something a flow seeks tothat can be defined as something a flow seeks toattain.attain.


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TECHNIQUES TO IMPROVE QoSTECHNIQUES TO IMPROVE QoS

Scheduling+raffic Sha#ingResource Reservation$d&ission ontrol


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"igure *riority queuing


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"igure +eighted fair queuing


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"he "oken Bucket $lgorithm

(a) Before. (b) $fter.

/(-


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$ea + "uc et s /o en "uc etLB discards packets@ "B does not. "B

discards tokens.With "B, a packet can only betransmitted if there are enough tokensto cover its length in bytes.LB sends packets at an average rate.

"B allo s for large bursts to be sentfaster by speeding up the output.

"B allo s saving up tokens(permissions) to send large bursts. LBdoes not allo saving.


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Hyung Min Lee

Su'netting and Su#ernetting About subnetting & Supernetting

• &nsu'netting , a network is divided into several smaller su'network with eachsu'network(or su'net) having its own su'network address.

• &nsu#ernetting , an organi*ation can com'ine several class + addresses to create a largerrange of addresses(supernetwork).

5.1 SUBNETTING• +lass !, , + in & addressing are designed with two levels of hierarchy.(netis and

hostid)

• The organi*ation has two-level hierarchical addressing, 'ut it cannot have more than one physical network. The host cannot 'e organi*ed into groups, and all of the hosts are atthe same level. The organi*ation has one network with many hosts.

• /ne solution to this pro'lem is su'netting, the further division of a network into smallernetworks called su'networks.


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Hyung Min Lee

Figure 5 1 network with two levels of hierarchy "not subnetted#


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Subnetting an! Supernetting


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Hyung Min Lee


Three evels of /ierarchy

• !dding su'networks creates an intermediate level of hierarchy in the &addressing system. #

Netid It !efines the site. # Su'netid It !efines the ph"si#al subnet$or% # ostid It !efines the #onne#tion of the host to the net$or% .


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Hyung Min Lee

0igure 1-5 ddresses in a network with and without subnetting


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Hyung Min Lee

0igure 1-4 /ierarchy concept in a telephone number


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Hyung Min Lee

0igure 1-1 0asking


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Hyung Min Lee

0igure 1-7 pplying bit!wise!and operation to achieve masking


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Hyung Min Lee

0igure 1-9 'pecial addresses in subnetting



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Hyung Min Lee

g p g

5. E/A'0 ES 2F SUBNETTING

( - ale / lass $)!n organi*ation with a class ! address at least 3,888 su'networks. 0ind the su'net mask and configuration of each su'network.

(Solution)

3. There is a need for at least 3,888 su'networks.(we need at least 3,882 su'networks toallow for the all-3s and all-8s su'netids)

2. This means that the minimum number of bits to 'e allocated for su'netting should 'e1,3 * 4 13,, 1, ).

5. 34 'its are left to define the hostids. u'net mask (33888888 ; 3


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Hyung Min Lee

0igure 1-= 0asks in e2ample

0igure 1-< 3ange of addresses in e2ample


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Hyung Min Lee

g g p


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Hyung Min Lee

0igure 1-38 'ubnetworks in e2ample



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Hyung Min Lee

( - ale 0 / lass B)

!n organi*ation with a class address needs at least $ subnetworks . 0ind the su'net maskand configuration of each su'network.

(Solution)

3. There is a need for at least 4 subnetworks .(we need at least 34 su'etworks to allowfor the all-3s and all-8s su'netids)

2. This means that the minimum number of bits to 'e allocated for su'netting should 'e63 * . 16 6 ).

5. 32 'its are left to define the hostids. u'net mask (33338888 ; 248).


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0igure 1-37 'ubnetworks in e2ample 7


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Hyung Min Lee

0igure 1-37 ubnetworks in e2ample 7


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Hyung Min Lee

5.6 8A9IAB E ENGT: SUBNETTING

• The &nternet allows a site to use varia'le-length su'netting.( - ale)• !n organi*ation with a class + address and needs to have 1 su'nets with the

following num'er of hosts: 78, 78, 78, 58, 58.(Solution)3. The site cannot use a su'net &ask with only 0 'its in the su'net section

'ecause this allows only 2 su'networks each 72 hosts(217 4 # 2 ; 72),(2 2 @1@25 )

0. Nor can the site use a su'net &ask with 1 'its in the su'net section 'ecausethis allows 3 su'networks each with 58 hosts(217 = # 2 # 58).

5. (?aria'le length su'netting ) The router uses 2 different masks, one appliedafter the other.

4. &t first uses the masks with 27 3s(33333333 33333333 33333333 33888888 or211.211.211.3


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Hyung Min Lee

0igure 1-39 8ariable!length subnetting


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Hyung Min Lee

0igure 1-28 Two ways of defining a supernet

238 ; 3824


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Hyung Min Lee

0igure 1-23 92ample of supernetting


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0igure 1-22 CI 3


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Hyung Min Lee

Internetworking


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Hyung Min Lee

Internetworking

# How networ&s differ # How networ&s can be connected# Tunnelin'# (nternetwor& routin'# Pac&et fra'mentation

How 9etwor&s Differ


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Hyung Min Lee

How 9etwor&s Differ

$ome of the many ways networ&s can differ

How 9etwor&s )an 1e )onnected


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Hyung Min Lee

) )

/a0 A %ac&et crossin' different networ&s3/b0 9etwor& and lin& layer %rotocol %rocessin'3

Tunnelin' /"0


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Hyung Min Lee

Tunnelin / 0

Tunnelin' a %ac&et from Paris to .ondon3

Tunnelin' / 0


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Hyung Min Lee

Tunnelin / 0

Tunnelin' a car from France to En'land


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Pac&et Fra'mentation / 0


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Hyung Min Lee

Pac&et Fra mentation / 0

/a0Trans%arent fra'mentation3/b09ontrans%arent fra'mentation


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unit 3 dcn class lectures .ppt

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