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Session 7.Packet Switching Networks

Dongsoo S. Kim (dskim@iupui.edu)Electrical and Computer EngineeringIndiana U. Purdue U. Indianapolis

7-1

p

OverviewGeneral Layer-3 (Network) ProtocolsPacket network topologyPacket network topologyIP networkATM networkDatagram and virtual circuitsRoutingShortest path algorithms

7-2

Shortest path algorithmsOptimal path algorithmsTraffic controlCongestion control

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Network Protocol StackEnd

systemEnd

system

Upper Upper

Transportlayer

Networklayer

Transportlayer

Networklayer

Networklayer

Networklayer

Upperlayers

Upperlayers

Network device(switch, router)

Network device(switch, router)

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Physicallayer

Physicallayer

Physicallayer

Physicallayer

Data Linklayer

Data Linklayer

DLlayer

DLlayer

Physicallayer

Physicallayer

DLlayer

DLlayer

Network Layer FunctionsTransport packet from sending to receiving hosts Network layer protocols in every host, routerTh i f iThree important functions:

Path determination: route taken by packets from source to destinationRouting algorithms

Central routing algorithmDistributive algorithm

Switching: move packets from router’s input to appropriate router outputSwitch architecture

Shared memory architecture/shared bus architecture

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Shared memory architecture/shared bus architectureMultistage switch architecturesInput buffer vs. output buffer

Call setup: some network architectures require router call setup along path before data flows

SignalingAdmission control

3

Network service modelQuestion: What service model of the channel for

transporting packets from sender to receiver?p g p

Service modelGuaranteed bandwidth?Preservation of inter-packet timing (no jitter)?Loss-free delivery?In-order delivery?Congestion feedback to

??

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Congestion feedback to sender?Cost? ? ?IP

Virtual CircuitsCircuitA source-to-destination path behaves much like telephone circuitcircuit

Performance-wiseNetwork actions along source-to-destination pathcall setup, teardown for each call before and after data flows

VirtualNo link is dedicated to the connection not as pure circuit switchingA channel may not dedicate to the connection not as TDM

Not a Datagram

7-6

Each packet carries VC identifier (not destination host OD)Every router on source-destination paths maintain state for each passing connectionLink, router resources (bandwidth, buffers) may be allocated to VC to get circuit-like performance

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Virtual Circuits: Signaling Protocols

Used to setup, maintain teardown VCUsed in ATM frame-relay X 25Used in ATM, frame relay, X.25

3 Accept call4. Call connected

5. Data flow begins6. Receive data

NetworkTransport

Transport

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1. Initiate call2. incoming call

3. Accept call

physicalData linkNetwork

physicalData linkNetwork

Datagram Networks: the Internet modelNo call setup at network layerRouters: no state about end-to-end connections

No network-level concept of connection

Packets typically routed using destination host IDPackets between same source-destination pair may take different paths

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1. Send data 2. Receive data

physicalData linkNetworkTransport

physicalData linkNetworkTransport

5

Network Layer Service Models

TimingOrderLossBandwidth

Guarantees?ServiceModel

NetworkArchitecture

YesYesYesGuaranteedRate

VBR

YesYesYesConstantRate

CBRATM

NoNoNoNoneBest EffortInternet

TimingOrderLossBandwidth

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NoYesNoNone UBR

NoYesNoGuaranteed minimum

ABR

Datagram or VC network: why?

InternetData exchange among computers

ATMDesigned for integration (B ISDN)elastic service

no strict timing requirement Intelligent end-systems (computers)

Can adapt, perform control, error recoverySimple inside network, complexity at edge

(B-ISDN)Telephony

strict timing, reliability requirementsneed for guaranteed service

Stream ServiceDelay is OKVariance of delay is critical

Data communiction

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complexity at edgeMany link types

Different characteristicsUniform service difficult

Bursty Need bandwidth as much as possible, but not always

Complication of network device

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What Are They?

Application

Network

Transport

or below

IPwares AAL

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Physical

Data LinkATM

Ether

DSL

Network Topology

XX ∩Internet

Gateway or Border Routerserver switches

X X

XX ∩

router

High speedbackboneDistributive

servers AutonomousSystem

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hub

Departmentserver

7

IP RouterGeneral computer system with two or more IP interfacesinterfacesEach IP packet contains the destination addressThe IP router decides the next router with the dest address

No previous knowledge on the dest addressNo touch on the IP packet (with a little exception?)

Routing table

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Routing tableList of destination address and port numberTable lookup for each packet, and forward to the port number

Virtual Circuit SwitchingEstablish a connection (fixed path) before communication

Set parameters in the switches along the pathBandwidthLoss rateDelay and delay variants

A physical link can be shared by multiple connectionsAdmission control - whether allow the establishment or not based on the available resources in the network

All packets for a connection follow the same path

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Each packet contains a VCI but not a full destination addressThe overhead caused by the full source and destination address Table lookup with the VCI to find the next-hop port and VCIThe VCI in the packet is translated to the next VCI

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VC Switching - Illustration

Node 1A

BVCNdVCNd

OutgoingIncomingNode 3

1 8Node 1A

5A331A23335A231A

VCNdVCNdOutgoingIncoming

2332VCNdVCNd

OutgoingIncoming

314424162176162444317621

738B135B5B138B73

VCNdVCNdOutgoingIncoming

Node 4

Node 62

7

5

4

3

7-15C

D

6C34346C

VCNdVCNdOutgoingIncoming 4355

322355432332

542D2D54

VCNdVCNdOutgoingIncoming

Node 2 Node 5

2

5

Routing - OverviewFind the best path. But, what is the best?

Shortest path - minimum number of hopsMinimum end-to-end delayMaximum available bandwidth

ClassificationStatic or Dynamic (Adaptive)

Static routing - pre-assigned path, manually configured routing tableGood for small and fixed network topologyDisadvantage for network failure

Dynamic (adaptive) routing - each router learns the network topology by exchanging information with its neighbors

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exchanging information with its neighborsComplication in each router

Centralized or Distributed routingCentralized routing - a central computer calculates all possible paths and dictates routers in the network

Flood of routing informationDistributed routing - each router calculates paths with available information

Inconsistency, loop

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Principle of OptimalityPrinciple

If a node b is on the optimal path from node a to node c,th th ti l th f b t l f ll th ththen the optimal path from b to c also falls the same path

ExpansionSink-tree

a b c

optimal

Then, optimal

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Hierarchical RoutingSize of routing table

US telephone - 10 billion entriesUS telephone 10 billion entriesIP - 32 bit address (4 billion entries)IPv6 - 128 bit address (256x1036 entries)

Common prefixProcess only the prefix if they are not in my network

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317 - 274 - 4493

Area code

Central office identifier

Port # in CO

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Distance Vector RoutingDistributed Bellman-Ford algorithmVariations

Original ARPANET routing algorithmInternet RIPNovell’s IPXAppleTalk and Cisco routers

Structure of Routing Table F

A

E

B

C

DRoute Info

Delay NextHopE, 3C, 4A, 8To

8

4

3

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530488

E2326FE02314E

3047

591216

-9DC12CC5BA0A 3

Distance Vector Routing - continuedHOWTO

Metric - Number of hops Time delay or something elseMetric Number of hops, Time delay, or something elseEach router know the metrics to the next routerEach router sends its table to each neighbor router periodicallyEach router receives the table of each neighbor and updates its routing table

Question: What if a link fails?Hey baby, don’t worry.

C

BA D

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y y, yReally?Counting-to-infinity

Question: Where is the bandwidth consideration?

E

11

Link State RoutingDijkstra’s algorithm for finding the shortest pathEnhancingEnhancing

Measure the delay or other cost as bandwidthPropagate a new information quickly

ExampleOSPF

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Flooding Forward an incoming packet to all ports except the one the packet was received fromApplication

When no routing table is availableFor broadcasting

ProblemsSwamp the network. Packets are multiplicated exponentiallyCirculating packets

SolutionUse TTL fields

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Use TTL fieldsEach router adds an ID to each packet. If the router sees the packet again, then discard it.Each router records an packet ID before flooding. It the router sees the packet again, then discard it.

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Hot potato routingDeflecting routingHowtoHowto

In normal situation, a router forwards packets to a default portIf the default port is busy or congested, forward any other port

ProblemsOut-of order deliveryNo path via the alternative port

Advantage

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AdvantageVery simple. No routing table is requiredNo buffer required

Source routingHowto

A source host knows the complete route to the destinationA source host knows the complete route to the destinationEach packet contains the route information in the headerEach route strips off the label indicating the node and forwards the packet to the next node

CharacteristicsIn-order guaranteedOverhead of packets

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pOverhead in the source host

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Optimal routing Based on the queuing modelDelay or cost on a link between node i and j = DDelay or cost on a link between node i and j = Dij

Minimize the total cost

FFC

FFCDT ij

ijij

ij +−

== ∑∑subject to

),(

Minimize

βα

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CFF≤

and t,requiremen thesatisfying flowdity multicommo a is j

Useful for network planning

ATM NetworksPrinciples

Connection-oriented packet switching (virtual circuit switching)Integration voice data multimediaIntegration - voice, data, multimedia Statistical multiplexingShort fixed-length packets (cells): 53 bytes

5 bytes ATM header and 48 bytes Payload Why 48 bytes?

Asynchronous – no global clock pulse or no synchronous frameSwitched Virtual Circuit (SVC)

Connection-establishing on demand by ATM signalingPermanent Virtual Path, Circuit, or Channel (PVP, PVC)

Man all set p the connection fo leased lines

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Manually set up the connection for leased linesTraffic Descriptor and Quality-of-Service

Peak cell rateLong-term average cell rateMaximum length of bursty cellsCell delay and cell loss rateCell delay jitter

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Virtual Path and Virtual ChannelHierarchical switching to release the load in switches

A physical link contains 4096 (or 256) VPC’s (VPI)A VPC contains 65536 VCC’s (VCI)A VPC contains 65536 VCC’s (VCI)

VP segments

ATMSW ATM

SW

ATMDCC

ATMSW

ATMSW

b

a a

VP Segment of a

7-27VP Segment of b

ATMDCC

ATMSW

b

b

b

ATM SW can do either VP or VC switching,but ATM DCC does VP switching only

Traffic Management and QOSProvide quality-of-service to individual packet thru the series of switches or routersWhat to Provide

End-to-end delaySum of individual delays

Jitter experience Variation of packet delayDifference of minimum delay and maximum delay

Packet loss rate

The link doesn’t have a control, but the node does.

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Caused by faults in equipments, or traffic congestion

Bandwidth

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QueuesFIFO Queue

A single common buffergThe order of arrival is the order of departureEqual opportunity to every packet – no control of QOSFairness? – a low-volume traffic vs. a high-volume traffic

Priority QueueA separate buffer to each priority classTransmit packets in a class if all higher queues are emptyFairness in a priority class?

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Fairness in a priority class?

Packetdistributor

Fist class queue

Economy class queue

When high-priority queue empty

Fair QueueTo provide equal opportunity to access to transmission bandwidth

Each user has its own logical queueIdeally, the network bandwidth is divided equally among the queuesPractically, it is impossible to divide the bandwidth equally

Fluid-flow system:both packets served at rate 1/2

Packet-by-packet system:queue 1 served first at rate 1;then queue 2 served at rate 1.Packet from

queue 2 waiting

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1

t1 2

Both packetscomplete serviceat t=2

0

1

t1 2

Packet from queue 2being served

Packet fromqueue 1 beingserved

queue 2 waiting

0

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Approximation of Fair QueueDifficulties

Large frame sizeLarge frame sizeVariable frame size

ApproximationWhen a packet arrives at a user queue, the ideal completion time is derived and this number is used as a finish tagAfter completing a packet transmission, pick the next packet to be transmitted with the smallest finish tag

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Fair Queue, GeneralizationPacket flow 1

Packet flo 2

Approximated bit-levelround robin service

The actual duration of a given round is proportional to the actual number of buffers (nA).Let R(t) is the number of rounds at time t then

C bits/second

Transmission link

Packet flow 2

Packet flow n

Let R(t) is the number of rounds at time t, then

where C is the capacity

)()(

tnC

dttdR

A=

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Computing Finishing TagNotation:

F(i k t) = finish time of kth packet that arrives at time t to flowF(i,k,t) = finish time of kth packet that arrives at time t to flow i . P(i,k,t) = size of kth packet that arrives at time t to flow i. R(t) = round number at time t

Fair Queueing:F(i,k,t) = max{F(i,k-1,t), R(t)} + P(i,k,t)

W i ht d F i Q iWeighted Fair Queueing:F(i,k,t) = max{F(i,k-1,t), R(t)} + P(i,k,t)/wi

Buffer 1at t=0

Buffer 2at t=0

1

Fluid-flow system:both packets served at rate 1/2

Packet from buffer 2 served at rate 1

2

2t

30

1

Packet-by-packet fair queueing:buffer 2 served at rate

Packet from buffer 2 waiting. Finish tag for buffer 2 is 2.

1

t1 2

1Packet from buffer 1 served at rate 1.Finish tag for buffer 1 is 1.

0 3

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Weighted Fair Queueing

Buffer 1 at t=0, weight is 1

Fluid-flow system:packet from buffer 1

Packet-by-packet weighted fair queueing:buffer 2 served first at rate 1;

weight is 1

Buffer 2 at t=0,Weight is 3 1

t1 2

packet from buffer 1served at rate 1/4;

Packet from buffer 1 served at rate 1Packet from buffer 2

served at rate 3/4 0

Packet frombuffer 1 waiting buffer 2 served first at rate 1;

then buffer 1 served at rate 11

t1 2

Packet from buffer 1 served at rate 1

Packet frombuffer 2 served at rate 1

buffer 1 waiting

0

Congestion ControlToo many packets try to access the same link (buffer)

The buffer builds up and eventually becomes fullp yPacket lossRetransmission will make it worse

StrategiesOpen-loop control

Prevent congestion from occurring by making sure that the traffic flow generated by the source will not degrade the performance of the network below the specified QoS

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Admission controlClosed-loop control

React to congestion when it is already happening or is about to happen by regulating the traffic flowFeedback information

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Open-Loop ControlsRegulate the traffic flow. Once it is accepted, its traffic flow will not overload the networkStrategies

Admission controlSpecify QoS parameters (or traffic descriptor) before establishing a connection as peak rate, average rate, maximum burst size, maximum delay, loss probability, and delay varianceConnection AC (CAC) systems should know the traffic flow of each source

PolicingOnce a connection is accepted the system must monitors the traffic flow

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Once a connection is accepted, the system must monitors the traffic flow to check whether it violates the initial contract or notLeaky bucket algorithm

Traffic ShapingReform a traffic flow to another flow

Leaky BucketCharacteristics

Irregular incoming packetWater poured

Constant rate outgoing packetThe overflowed packet is discarded

Capacity of the bucketDepends on the traffic patternSmoothed traffic needs small bucketBursty traffic requires a large bucket

Leaky bucket

Water pouredirregularly

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Drain rateBandwidth allocation to the specific connection

Water drains ata constant rate

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Leaky Bucket AlgorithmArrival of a packet at time ta I = nomial inter-arrival time

L+I = size of the bucketX’ = X - (ta - LCT)

X’ = 0

Nonconformingpacket

Yes

NoYes

X’ < 0?

X’ > L?

L+I = size of the bucket

The bucket will drain at a continuous rate of 1 unit per packet time, if it is not empty.

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X = X’ + ILCT = ta

conforming packet

p

X = value of the leaky bucket counterX’ = auxiliary variableLCT = last conformance time

No

Behavior of Leaky Bucket

Packetarrival

Nonconforming

I

L+I

Bucketcontent

Time

arrival

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I

Time* * * * * * * **

21

Dual Leaky BucketPolice the sustainable rate as well as the peak rate

Tagged or dropped

Untagged traffic

Incomingtraffic Leaky bucket 1

PCR and CDVT

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Untagged traffic

Leaky bucket 2SCR and MBS

Tagged or dropped

PCR: Peak Cell RateCDVT: Cell Delay Variation ToleranceSCR: Sustainable Cell RateMBS: Medium Burst Size

Traffic ShapingFor smoothing abrupt traffic flow Many possible waysy p y

Eg) application periodically generate 10 Kbit data every second 10 Kbps

Time0 1 2 3(a)

i

50 Kbps(b)

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Time0 1 2 3

Time0 1 2 3

100 Kbps

(c)

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Leaky Bucket Traffic ShaperDifference in implementation

Leaky bucket policing – countersLeaky bucket policing countersLeaky bucket traffic shaper - buffer

Incoming traffic Shaped traffic

Size N

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Packet

Token Bucket Traffic ShaperToo restricted leaky bucket traffic shaper (constant output rate)Flexibility for variable-rate traffic Tokens arrivey

Size K

Tokens arriveperiodically

Token

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Incoming traffic

Shaped trafficSize N

Server

Packet

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Closed-Loop ControlUse of feedback informationTCP congestion controlTCP congestion control

Implicit feedback – timeout

ATM congestion controlExplicit feedback – OAM message

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TCP Congestion ControlA sender maintains the “congestion window”The maximum amount of data that the sender can transmit is the minimum of an advertised window and the congestion windowDynamically adjust the congestion windows

Congestion15

20Congestionavoidance

Congestion occurs

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Congestionwindow

10

5

0Round-trip times

Slowstart

Threshold