1

Chapter 1: roadmapChapter roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links1.3 Network core

network structure, circuit switching, packet switching 1 4 Delay loss and throughput performance in1.4 Delay, loss and throughput performance in

packet-switched networks 1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

9/12/2017 Introduction (SSL) 1-1

2

What’s the Internet: “nuts and bolts” view

billions of connected computing devices:

Mobile networkGlobal ISP

PC

server computing devices: hosts = end systems running network apps

Home network

Global ISPserver

wirelesslaptopcellular Home network

Regional ISP

cellular Handheld

“things”

communication linksInstitutional network

wiredlinks

access points

communication links fiber, copper, coax,

radio, satellite

router

transmission rate

Routers and switches

9/12/2017 Introduction (SSL) 1-2

r ut rforward packets

3

What’s the Internet: architecture & protocolsprotocols Internet: “network of

networks”

Mobile network

Global ISPnetworks loosely hierarchical public Internet versus

i iHome network

private intranet protocols control sending,

receiving of msgs

Regional ISP

rece v ng of msgs e.g., TCP, IP, HTTP, BGP,

Ethernet, Skype Internet standards

Institutional network

Internet standards RFC: Request for comments IETF: Internet Engineering

k FWhat are required for

Task Force

9/12/2017 Introduction (SSL) 1-3

qglobal connectivity?

4

What’s the Internet: a service view communication

infrastructure enables distributed applications: Web, VoIP, email, games,

fil sh ie-commerce, file sharing communication services

provided to apps:p pp reliable data delivery

from source to d ti tidestination

“best effort” (unreliable) data deliverydata delivery

9/12/2017 Introduction (SSL) 1-4

5

What’s a protocol?What s a protocol?human protocols:

“ h t’ th ti ?”network protocols: hi th th “what’s the time?”

“I have a question” introductions

machines rather than humans

all communication introductions all communication activity in Internet governed by protocols

protocols define format, order of msgs sent and received among networkreceived among network

entities, and actions taken on msg transmission,

ipt tim t

9/12/2017 Introduction (SSL) 1-5

receipt, or timeout

6

What’s a protocol?What s a protocol?a human protocol and a computer network protocol:

HiTCP connection

HiGot the

TCP connectionrequest

TCP connectionresponseGot the

time?2:00

Get http://www.awl.com/kurose-ross

response

<file><file>time

9/12/2017 Introduction (SSL) 1-6

Q: Other human protocols?

7

From physical media to communication channels—basiccommunication channels basic concepts (not in textbook)

9/12/2017 Introduction (SSL) 1-7

8

Modulation and DemodulationModulat on and Demodulat on Common

l s: diexamples: radio, television channels forchannels for analog signals Bandwidth in hertz

Can also be used for digital signals (encoding binary d t )data)

9/12/2017 Introduction (SSL) 1-8

)2cos( 0 θπ +tfA

9

Shannon’s TheoremShannon s Theorem

C = B log2 (1 + S/N)

where C max capacity in bits/secwhere max capac ty n b ts/secB bandwidth in hertzS/N signal to noise ratio

9/12/2017 Introduction (SSL) 1-9

10

FDM vs. TDMFDM vs. TDM

Duration of frame (or superframe) is 125 µsec in digital telephone

t k

9/12/2017 Introduction (SSL) 1-10

networks

11

TDM in Telephone NetworksTDM n Telephone Networks

Why 125 μsec for Sampling rate for y μframe duration?

Sampling Theorem:An analog signal can be

p gvoice = 8000 samples/sec or one voice sample every 125An analog signal can be

reconstructed from samples taken at a

voice sample every 125 μsec

Digital voice channel rate equal to twice the signal bandwidth

Bandwidth for voice

(uncompressed),8 bits x 8000/sec =64 Kbps Bandwidth for voice

signals is 4 Khz; for hi fidelity music, 22.05 Kh h l

64 Kbps

Khz per channel

9/12/2017 Introduction (SSL) 1-11

12

Other Multiplexing TechniquesOther Mult plex ng Techn ques Space division

multiplex Wavelength division

multiplexmultiplex Same frequency used in

different cables Same frequency used in

multiplex Light pulses sent at

different wavelengths in optical fiber Same frequency used in

different (nonadjacent) cells

in optical fiber Code division multiplex

(in chapter 7 of text)A

e.g., CDMA for cell phonesd

FG

BF

GA

A

B

rA

A

FE

AD

BC

EADA

C

9/12/2017 Introduction (SSL) 1-12

A

13

Chapter 1: roadmapChapter roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links1.3 Network core

circuit switching, packet switching, network structure1 4 Delay loss and throughput in packet-switched1.4 Delay, loss and throughput in packet switched

networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

9/12/2017 Introduction (SSL) 1-13

14

A closer look at network structure: network edge:

hosts: clients and serversmobile network

t c nt an r r servers often in data

centersglobal ISP

access networks, physical media: wired, wireless

regional ISP

home network

communication links

network core: network core: interconnected routers network of networks institutional

Introduction (SSL)

network

1-149/12/2017

15

Access net: digital subscriber line (DSL)central office telephone

network

ISP

DSLAM

i d t t itt d

DSLmodem

splitter

voice, data transmittedat different frequencies over

dedicated line to central officeDSL access

multiplexer

use FDM in telephone line to central office DSLAM data over DSL line goes to Internet

i DSL li t t l h t voice over DSL line goes to telephone net asymmetric bandwidths/transmission rates (data

download much faster than upload)

Introduction (SSL)

p )

1-159/12/2017

16

Access net - hybrid fiber coax (HFC)

…cable headend

cable modemtermination system

data, TV transmitted at different

cablemodem

splitterCMTSfiber node

Data service

,frequencies over shared cable

distribution networkISP

Data service homes share coax cable to cable headend

(unlike DSL, which has dedicated access to central office)d t h ls h s t i t s d th h d b data channels have asymmetric rates and they are shared by homes - multiple access protocol required for uplink

Fiber t the h me (Veriz n G le) all ptical switches

Introduction (SSL)

Fiber to the home (Verizon, Google) – all optical switches

1-169/12/2017

17

Access net: home network

wirelessdevices

to/from headend or central office

often combined

cable or DSL modem

often combined in single box

router, firewall, NAT

wired Ethernet

wireless access point

Introduction (SSL) 1-179/12/2017

18

Enterprise access networks (Ethernet)

institutional link to ISP (Internet)

Ethernet switch

institutional mail,b

institutional router

today, end systems typically connect into Ethernet

switch web servers

switch 100Mbps, 1Gbps, 10Gbps transmission rates

A large enterprise network is connected to multiple ISPs

Introduction (SSL)

A large enterprise network is connected to multiple ISPs • multi-homing

1-189/12/2017

19

Wireless access networks shared wireless access network connects end system

to router via “access point” or “cell tower” via access point or cell tower

wireless LANs: within building (100 ft)

wide-area wireless access provided by telco (cellular)

p t s 10’s km ( h within building (100 ft) 802.11g/n/ac (WiFi)

operators, 10’s km (when no obstacle)

3G, 4G: LTE

I

Introduction (SSL)

to Internet to Internet

1-199/12/2017

20

Chapter 1: roadmapChapter roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links1.3 Network core

circuit switching, packet switching, network structure1 4 Delay loss and throughput in packet-switched1.4 Delay, loss and throughput in packet switched

networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

9/12/2017 Introduction (SSL) 1-20

21

The Network CoreThe Network Core mesh of interconnected

routersrouters the fundamental

question: how is data transferred through net? circuit switching:

dedicated circuit perdedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”

9/12/2017 Introduction (SSL) 1-21

22

Network Core: Circuit Switchingg

End-to-end resourcesEnd to end resources reserved for each “call”

E.g., link bandwidth FDM, TDM

d t d i it lik end-to-end circuit-like (guaranteed) performance

call setup requiredcall setup requ red resource piece idle if not

used by the call (no sharing)

9/12/2017 Introduction (SSL) 1-22

23

Numerical exampleNumer cal example

How long does it take to send a file ofHow long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? all links are 1.536 Mbps each link uses TDM with 24 slots/sec (i.e., one

sl t p i it)slot per circuit) 500 msec to establish end-to-end circuit

Let’s work it out!

9/12/2017 Introduction (SSL) 1-23

Note: 1.536 Mbps was realistic in the early days of ARPAnet

24

Packet Switching: Statistical Multiplexing

A C100 Mb/sEthernet statistical multiplexing

B1.5 Mb/s

queue of packetsq pwaiting for output

link

Sequence of A & B packets does not have fixed patternb d idth h d d d t ti ti l lti l i

D E

bandwidth shared on demand statistical multiplexing queueing delay, packet loss

9/12/2017 Introduction (SSL) 1-24

25

Network Core: Packet Switchinggeach end-end data stream

divided into packetsresource contention: tdivided into packets

packets of different users share network resources

aggregate resource demand can exceed amount available

each packet uses full link bandwidth

congestion: packets queue, wait for link uset d f d store and forward:

packets move one hop at a timeBandwidth division into “pieces” Each node receives the

complete packet before forwarding it

pDedicated allocationResource reservation

9/12/2017 Introduction (SSL) 1-25

g

26

Disadvantage of store-and-forwardD g f f

R R RL

takes L/R seconds to transmit (push out) a

Example: L = 7.5 Mbits

R R R

transmit (push out) a message of L bits into a link at R bps

L 7.5 Mbits R = 1.5 Mbps End-to-end delay more

than 15 seconds store and forward:

entire message must arrive at router before Solution: A

fil / larrive at router before it can be transmitted on next link

file/message larger than packet size is transmitted as multiple packets

9/12/2017 Introduction (SSL) 1-26

27

Circuitvs. Messagevs. Packet Switching

violates store-and-forward?

9/12/2017 Introduction (SSL) 1-27

28

Packet Switching versus Message Switchingg g g

Advantages of packet switching

Smaller end-to-end delay from pipelining

Advantages of packet switching

Less data loss from transmission errors

More header bits

Disadvantages of packet switching

More header bits Additional work to do segmentation

and reassembly

9/12/2017 Introduction (SSL) 1-28

y

29

Packet switching versus circuit switchingg g 1 Mb/s link each user: each user:

100 kb/s when “active” active 10% of time (a

“bursty” user)bursty user)

circuit-switching:10 N users

10 users packet switching:

with 35 users, l

N users1 Mbps link

probability > 10 active at same time is less than .0004

Q: how did we get value 0.0004?

9/12/2017 Introduction (SSL) 1-29

30

Packet switching versus circuit switching

great for bursty data

Is packet switching a “slam dunk winner?”

g y resource sharing simpler, no call setup

excessive congestion -> packet delay and loss protocols needed for reliable data transfer,

congestion controlcongestion control Q: How to provide circuit-like behavior?

bandwidth guarantees needed forbandwidth guarantees needed for interactive audio/video apps providing virtual links to enterprise network p g p

customers (under service contracts)9/12/2017 Introduction (SSL) 1-30

31

Network TaxonomyTelecommunication

networks

Circuit-switched Packet-switchedknetworks networks

k DFDM/WDM TDM Networkswith VCs*

DatagramNetworks

A h l b d i InternetAny technology can be used in link layer of Internet under IP

Internet won!

VC examples: ATM networks MPLS tunnels

9/12/2017 Introduction (SSL) 1-31

VC examples: ATM networks, MPLS tunnels

32

Internet structure: network of networksQuestion: given millions of access ISPs, how to connect them together?

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

access

accessnet

accessnet

accessnet

net

accessnet

accessnet

netaccess

netaccessnet

accessnet

9/12/2017 Introduction (SSL) 1-32

33

Internet structure: network of networksOption: connect each access ISP to every other access ISP?

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

connecting each access ISP t h th di tl d ’t

access

accessnet

accessnet

to each other directly doesn’t scale: O(N2) connections.

accessnet

net

accessnet

accessnet

netaccess

netaccessnet

accessnet

9/12/2017 Introduction (SSL) 1-33

34

Internet structure: network of networksOption: connect each access ISP to a global transit ISP:Option: connect each access ISP to a global transit ISP: 1. Financed by US government: ARPAnet, NSFnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

global

access

accessnet

accessnet

globalISP

accessnet

net

accessnet

accessnet

netaccess

netaccessnet

accessnet

9/12/2017 Introduction (SSL) 1-34

35

Internet structure: network of networksBut if one global ISP is viable business, there will be competitors ….

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

ISP A

access

accessnet

accessnetISP B

ISP C

accessnet

net

accessnet

accessnet

netaccess

netaccessnet

accessnet

9/12/2017 Introduction (SSL) 1-35

36

Internet structure: network of networksBut if one global ISP is viable business there will beBut if one global ISP is viable business, there will be competitors …. Two ISPs are connected in a “provider-customer” or “peer-peer” relationship

Internet exchange pointaccess

netaccess

net

accessnet

accessnet

accessnet

Internet exchange point(hundreds of ISPs)

accessnet

accessnet

ISP AIXP

access

accessnet

accessnetISP B

ISP C

IXP

accessnet

net

accessnet

accessnet

private linknet

accessnetaccess

net

accessnet

9/12/2017 Introduction (SSL) 1-36

37

Internet structure: network of networks… and regional networks may arise to connect access nets to ISPs

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

ISP AIXP

access

accessnet

accessnetISP B

ISP C

IXP

accessnet

net

accessnet

accessnet

regional netnet

accessnetaccess

net

accessnet

9/12/2017 Introduction (SSL) 1-37

38

Internet structure: network of networks… and a content provider network (e.g., Akamai, Google, Microsoft) may run its own network to bring services, content close to end users

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

ISP AIXP

Content provider network

access

accessnet

accessnetISP B

ISP B

IXPp

accessnet

net

accessnet

accessnet

regional netnet

accessnetaccess

net

accessnet

9/12/2017 Introduction (SSL) 1-38

39

Internet structure: network of networks

Tier 1 ISP Tier 1 ISP Google

IXP IXP IXP

Regional ISP Regional ISP

at center: small # of well-connected large networks

accessISP

accessISP

accessISP

accessISP

accessISP

accessISP

accessISP

accessISP

g “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT),

national & international coverage content provider networks (e g Google): private network that

Introduction (SSL)

content provider networks (e.g., Google): private network that connects its data centers to Internet, often bypassing tier-1 and regional ISPs

1-399/12/2017

40

Chapter 1: roadmapChapter roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links1.3 Network core

circuit switching, packet switching, network structure1 4 Delay loss and throughput in packet-switched1.4 Delay, loss and throughput in packet switched

networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

9/12/2017 Introduction (SSL) 1-40

41

How do loss and delay occur?

packet arrival rate to link temporarily d l kexceeds output link capacity

packets queue, wait for turn

packet being transmitted (delay)

A

Bpackets queueing (delay)

9/12/2017 Introduction (SSL) 1-41

free (available) buffers: arriving packets dropped (loss) if no free buffers

42

Four sources of packet delayFour sources of packet delay

1. nodal processing: 2. queueing 1. nodal processing check bit errors determine output link

2. queueing time waiting at output

link for transmission depends on congestion

Atransmission

depends on congestion level of router

A

B

propagation

Bnodal

processing queueing

9/12/2017 Introduction (SSL) 1-42

43

Delay in packet-switched networks3. Transmission delay: R: link bandwidth (bps)

4. Propagation delay: d: length of physical link( p )

L: packet length (bits) time to send bits into

li k L/R

s: propagation speed in medium (~2x108 m/sec)

propagation delay = d/slink = L/R propagation delay = d/s

Note: s and R are very diff t titi !

A propagation

transmission different quantities!

Bnodal

9/12/2017 Introduction (SSL) 1-43

processing queueing

44

End-to-End DelayEnd to End Delay Nodal delay (from when last bit of packet arrives at this node

to when last bit arrives at next node)

dnodal = dproc + dqueue + dtrans + dprop

End to end delay over N identical nodes/links End-to-end delay over N identical nodes/links from client c to server s (from when last bit of packet leaves client to when last bit arrives at server)

d d Nddc-s = dprop + Ndnodal

Round trip time (RTT)pRTT = dc-s + ds-c + tserver

where tserver is server processing time

9/12/2017 Introduction (SSL) 1-44

45

“Real” Internet delays and routesy What do “real” Internet delay & loss look like? traceroute program: provides delay measurement traceroute program: provides delay measurement

from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path

towards destination router i will return packets to sender sender times interval between transmission and reply.

3 probes

3 probes

3 probes

9/12/2017 Introduction (SSL) 1-45

46

“Real” Internet delays and routesytraceroute: gaia.cs.umass.edu to www.eurecom.fr

Three delay measurements from

1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms

Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu

g ( )4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms

trans-oceaniclink( )

9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms

linkdifferent packets( )

14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms17 * * *18 * * * * means no response (probe lost, router not replying)

9/12/2017 Introduction (SSL) 1-46

19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 msp p p y g

47

Throughput - rate at which bits are transferred from source to destination (in bits/sec.)

Rs < Rc end-end throughput less than ___ ?

Rs bits/sec Rc bits/sec

Rs > Rc end-end throughput less than ___ ?

Rs bits/sec Rc bits/sec

link on end-end path that constrains end-end throughputbottleneck link

9/12/2017 Introduction (SSL) 1-47

link on end end path that constrains end end throughput

48

Throughput: Internet scenarioThroughput Internet scenario per-connection end-to-

end throughput is g papproximately

min(Rc, Rs, R/10) Actually sharing a

Rs

Rs

Rs Actually sharing a

bottleneck equally is ideal but unrealistic R

In practice, Rc or Rs is often the bottleneck

Rc

Rc

Rc

or the server is the bottleneck

10 connections (fairly) share

c

9/12/2017 Introduction (SSL) 1-48

10 connections (fairly) share backbone bottleneck link R bits/sec

49

Queueing delay (waiting time) R: link bandwidth (bps) L: packet length (bits)

R/L ( k / )

averagequeueing delay

service rate = R/L (pkts/sec) ave. service time = L/R (sec)

λ: packet arrival rate λ: packet arrival ratetraffic intensity =arrival rate/service rate = λL/R λL/Rarrival rate/service rate = λL/R λL/R ~ 0: average queueing delay small λL/R -> 1: delays become large

1λL/R

0

λL/R > 1: delays become large λL/R > 1: more “work” arriving than can be

served, average delay infinite!

9/12/2017 Introduction (SSL) 1-49

In reality, buffer overflow when λL/R -> 1

50

Packet loss buffer in router for each link has finite

capacitycapac ty lost packet may be retransmitted by previous

node, by source end system, or not at all, y y ,

Apacket being transmitted

buffer (waiting area)A

p g m(waiting area)

packet arriving tofull buffer is lost

B

9/12/2017 Introduction (SSL) 1-50

51

Little’s law and a useful queueing delay formula

motivation for packet switching to replace circuit switching

9/12/2017 Introduction (SSL) 1-51

52

Little’s LawL ttle s Law =

N1average delay delayiN i 1Average population = (average delay) x

(throughput rate)

=where N is number of departures

iN i 1

(throughput rate)

=h T is d ti f xp im t

throughput rate N/Twhere T is duration of experiment

average populationaverage population(to be defined)

9/12/2017 Introduction (SSL) 1-52

53

n(t)

syste

m n

mbe

r in

Time t

Num

Time t

1average population ( )n t dtττ= 0

where is duration of the experiment

average population ( )n t dtτ

τ

9/12/2017 Introduction (SSL) 1-53

54

random variable samples

xx x x1 2

i=1

samples , , ... ,1mean (average)

nn

i

x x x

x xn

= i=1

2 2 2

1

1second moment ( ) ( )n

ii

x x xn =

= ≥2

mean residual life2 2x xx

= ≥

Special case: is a constantx

2 2x

2 2

2

( )( )mean residual life2 2

x xx x

=

= =

9/12/2017 Introduction (SSL) 1-54

2 2x

55

random variable x

with discrete values x1, x2, … , xm

let pi = probability [x = xi] for i = 1, 2, …, m

by definition

mean ii

m pxx =

second moment2 2

m

iix x p=

i = 1

1 ii =

(Aside: For a continuous random variable, use integration instead of summation.)

9/12/2017 Introduction (SSL) 1-55

use integration instead of summation.)

56

Single-Server Queue

λ μqueue server

average service time in secondsx average service time, in secondsservice rate, in packets/second ( = 1/ )arrival rate, in packets/second

xxμ μ

λ , putilization of serverρ

Conservation of flow (assuming no loss)Conservation of flow (assuming no loss)

xλμλρρμλ

===

9/12/2017 Introduction (SSL) 1-56

μ

57

M/G/1 queueSingle server

does not idle when there is work, no overhead, i.e.,it performs 1 second of work per second

FIFO service

Arrivals according to a Poisson process atArrivals according to a Poisson process at rate λ packets/second

Service times of arrivals are x1 x2 xiService times of arrivals are x1, x2, …, xi … which are independent, identicallydistributed (with a general distribution)g

Average service time is , average wait is W, average delay is T = W +

xxg y

9/12/2017 Introduction (SSL) 1-57

58

Let be the unfinished work at time t( )U t( )( )U t

2 2x w3 3x w

21

12

x 22

12

x 23

12

x

1 2 3 1 2 3 4 5arrivals and departures

time0

9/12/2017 Introduction (SSL) 1-58

arrivals and departures

59

Derivation of WTime average of unfinished work over duration

( )1U U t dtτ=

τ

0

2

1

( )

1 n n

U

x x w

U t dtτ +

=xi and wi are

1 121

i i ii i

x x wτ = == +

xi and wi are independent

21 21 where 2 i i i ii i i x w x wn x x wτ ×= = + ×

For Poisson arrivals, the average wait is equal to from the Poisson arrivals see time U f m n rr maverage (PASTA) Theorem

U

59Introduction (SSL)9/12/2017

60

Derivation of W (cont.) The average wait is

2 221

2 2 2x xW x xW xW Wλ λλ λ ρ = + = + = +

2 2 2

2

(1 ) xW λρ = Pollaczek-Khinchin (P-K)

l f l2

(1 )2

W ρ

λ

− =

mean value formula2

2(1 )

xW λρ

=−

9/12/2017 Introduction (SSL) 1-60

61

M/G/1 queue

Markovian T

PoissonGeneral

0 1 0 ρ

xAverage delay is

0 1.0 ρ2

2(1 )

xT x W x λρ

= + = +−( )ρ

Also called Pollaczek-Khinchin (P-K) mean value formula

9/12/2017 Introduction (SSL) 1-61

( ) f

62

Special Cases T decreases as λ i1. Service times have an

exponential distribution(M/M/1) W th h

λλ→→

1010

increases

(M/M/1). We then have

μλ

μλρ

μμ

==

→

101010

2 22( )x x=2 2λ λ

T W

T2 2(2)( ) ( )

2(1 ) 1 1x x xW λ λ ρρ ρ ρ

= = =− − −

1 1

T W xx x x xxρ ρ ρ

= ++ −= + = x μ

1 11

1 1x

ρ ρρ

ρ ρ λ

− −

= = ρ →0 1 0

x 0.1 x 10μ

9/12/2017 Introduction (SSL) 1-62

1 1ρ ρ λ− − ρ →0 1.0

63

2. Service times are constant (deterministic)

M/D/12 2( )x x=2( )

2(1 ) 2(1 )x xW λ ρ

ρ ρ= =− −

( )

( 2 2 )2(1 ) 2(1 )

x xT xρ ρ ρ+ −= + =

2(1 ) 2(1 )ρ ρ

2(1 ) 2(1 )

(2 ) 1

ρ ρ

ρ ρ

− −T decreases as i

λ(2 ) 1

2(1 )T ρ ρρ λ

−= −increases

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64

Two Servers and Two Queues:60 jobs/sec

100 jobs/sec

60 jobs/secj

100 jobs/sec

Single Higher Speed Server:120 jobs/sec

200 jobs/sec

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65

Chapter 1: roadmapChapter roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links1.3 Network core

circuit switching, packet switching, network structure1 4 Delay loss and throughput in packet-switched1.4 Delay, loss and throughput in packet switched

networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

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66

Layered architecturey as reference model for protocol design by

community effort• decompose a large system into smaller pieces

which can be designed and implemented by different people/teamsdifferent people/teams

modularity eases maintenance and evolution of system• allows changes in implementation method so

long as API remains the same, e.g., different Ethernet technologiesEthernet technologies

strict layering often violated for efficient protocol implementationp p• cross-layer design

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67

Each protocol involves two or more peers two kinds of specificationsp

• service interface: operations a local user can perform on a protocol entity and get

Hi h l l Hi h l lserviceHost 1 Host 2

p y gresults

• protocol spec: format and meaning of messages

High-level entity

High-level entity

serviceinterface

g gexchanged by protocol entities (also called peers) to provide protocol service

Protocol entity

Protocol entity

protocolspec

The term “protocol” generally refers to protocol spec

spec

spec

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68

Internet protocol stack application: protocols that support

network applications SMTP, HTTP, DNS

transport: process-process data transfer

application

transporttransfer TCP, UDP

network: routing of datagrams from

transport

networksource to destination IP, routing protocols

link: data transfer between

link

link: data transfer between neighboring network elements PPP, Ethernet, 802.11 (WiFi)

physical

physical: how to send and receive bits9/12/2017 Introduction (SSL) 1-68

69

ISO/OSI reference model presentation: allow applications to

interpret meaning of data, e.g., i i d li iencryption, compression, data

description (e.g. machine-specific convention)

applicationpresentation

) session: delimiting and synchronization

of data exchange (e.g., checkpointing and recovery)

sessiontransport

and recovery) Internet stack “missing” these layers!

these services if needed must be

networklink

these services, if needed, must be implemented in application (or application protocol)

physical

very wise decision for ARPAnet!

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70

Internet Architecture Internet Engineering

Task Force (IETF)li i l

FTP HTTP DNS TFTP

application protocols support applications

hourglass shape (only IP

TCP UDP

IPg p ( yin network layer) best effort service

=> any delivery

IP

NET1 NET2 NETn. . .=> any delivery service can be used by IP

limitation of hourglass limitation of hourglass

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71

Encapsulation Host 2Host 1

Protocol peers provide a data delivery

User

D t

User

D ty

service How do protocol peers

in different machinesUpper layer

Data

Upper layer

Data

in different machines exchange protocol messages between

layer

Lower

Datalayer

Lower

HU Data HU

themselves? In protocol header

encapsulated and

layerlayer

encapsulated and de-encapsulated HL HU Data

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Logical communication between peersEE.g.: transport accept data

fromapplicationtransport

data

transportfrom application

add addressing, l l

pnetwork

linkphysical

k

transport

kreliability check info to form a message

applicationtransportnetwork

networklink

physical

ack

dataf m m g send message

to peer via a d li i

networklink

physical applicationtransport

applicationtransport

data

transportdelivery service wait for peer’s

reply (ack)

transportnetwork

linkphysical

transportnetwork

linkphysical

transport

reply (ack)

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73

Physical path of datay p fEach layer takes data (service data unit) from above adds header to create its own protocol data unit adds header to create its own protocol data unit passes protocol data unit to layer below

li timessage M l

network network

applicationtransportnetwork

messagesegmentdatagram

M

MH 4MH 4H 3

applicationtransportnetwork

linkphysical

linkphysical

linkphysicalsource destination

frame MH 4H 3H 2 T2bits

linkphysical

router routerprotocol data

...

sourcehost

destinationhost

router routerprotocol data units

Note: In the past, a switch implements only two layers (physical and link). Nowadays many switches function as routers (3 layers)

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Note: layer 2½ may exist (i.e. virtual circuits)

74

Chapter 1: roadmapChapter roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links1.3 Network core

circuit switching, packet switching, network structure1 4 Delay loss and throughput in packet-switched1.4 Delay, loss and throughput in packet switched

networks1.5 Protocol layers, service models1.6 Networks under attack – please read on your own

1 H l d1.7 History – please read on your own

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