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CPSC 441: Introduction 1-1 CPSC 441: Computer Communications Instructor: Carey Williamson Office: ICT 740 Email: [email protected] Class Location: ICT 122 Lectures: MWF 12:00 – 12:50 Notes derived from “Computer Networking: A Top Down Approach”, by Jim Kurose and Keith Ross, Addison-Wesley. Slides are adapted from the book’s companion Web site, with changes by Anirban Mahanti and Carey Williamson.

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Page 1: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-1

CPSC 441: Computer CommunicationsInstructor: Carey WilliamsonOffice: ICT 740Email: [email protected] Location: ICT 122Lectures: MWF 12:00 – 12:50Notes derived from “Computer Networking: A Top

Down Approach”, by Jim Kurose and Keith Ross,Addison-Wesley.

Slides are adapted from the book’s companion Web site,with changes by Anirban Mahanti and Carey Williamson.

Page 2: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-2

RoadmapWhat is a Computer Network?Applications of Networking Classification of Networks Layered ArchitectureNetwork CoreDelay & Loss in Packet-switched Networks Structure of the Internet Summary

Page 3: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-3

Computer Network?

“interconnected collection of autonomouscomputers connected by a communicationtechnology”

What is the Internet?“network of networks”“collection of networks interconnected byrouters”“a communication medium used by millions”Email, chat, Web “surfing”, streaming media

Internet Web¿

Page 4: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-4

The “nuts and bolts” view of the Internet millions of connected

computing devices calledhosts or end-systems

PCs, workstations, serversPDAs, phones, toasters

running network apps communication links

fiber, copper, radio, satellitelinks have differentcapacities (bandwidth)

routers: forward packets packet: piece of a message

(basic unit of transfer)

local ISP

companynetwork

regional ISP

router workstationserver

mobile

Page 5: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-5

RoadmapWhat is a Computer Network?Applications of Networking Classification of Networks Layered ArchitectureNetwork CoreDelay & Loss in Packet-switched Networks Structure of the Internet Summary

Page 6: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-6

Applications (1) end systems (hosts):

run application programse.g. Web, email, ftpat “edge of network”

client/server modelclient host requests, receivesservice from always-on servere.g. Web browser/server;email client/server

Client/server model haswell-defined roles for each.

Page 7: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-7

Applications (2) peer-to-peer model:

No fixed clients or serversEach host can act as both client and server at any time

Examples: Napster, Gnutella, KaZaA, BitTorrent

Page 8: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-8

Applications (3) File transfer Remote login (telnet, rlogin, ssh)World Wide Web (WWW) Instant Messaging (Internet chat, textmessaging on cellular phones)

Peer-to-Peer file sharing Internet Phone (Voice-Over-IP) Video-on-demandDistributed Games

Page 9: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-9

RoadmapWhat is a Computer Network?Applications of Networking Classification of Networks Layered ArchitectureNetwork CoreDelay & Loss in Packet-switched Networks Structure of the Internet Summary

Page 10: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-10

A Classification of Networks

Local Area Network (LAN)Metropolitan Area Network (MAN)Wide Area Network (WAN)

Wireless LAN (WLAN)Home Networks Personal Area Network (PAN) Body Area Network (BAN)

Page 11: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-11

Local Area Network (LAN)

company/univ local areanetwork (LAN) connectsend system to edge router

Ethernet:shared or dedicated linkconnects end systemand router (a few km)10 Mbps, 100Mbps,Gigabit Ethernet

widespread deployment:companies, univ, homeLANs

LANs: chapter 5

Page 12: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-12

Metropolitan Area Network (MAN)

home

cable headend

cable distributionnetwork (simplified)

Typically 500 to 5,000 homes

ACable TV Network is an example of a MAN

“City sized”: tens of kilometers

Page 13: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-13

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork (simplified)

Page 14: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-14

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork

server(s)

Page 15: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-15

Wide Area Network (WAN)

Spans a largegeographic area, e.g., acountry or a continent

A WAN consists ofseveral transmissionlines and routers

Internet is an exampleof a WAN

Page 16: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-16

Wireless Networks (WLANs) shared wireless access

network connects end systemto router

via base station or “access point” wireless LANs:

802.11b (WiFi): 11 Mbps wider-area wireless access

provided by telco operator3G ~ 384 kbps• Will it happen??

WAP/GPRS in EuropeWiMax available now

basestation

mobilehosts

router

To the wired network

Page 17: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-17

Home networksTypical home network components: ADSL or cable modem router/firewall/NAT Ethernet wireless access

point

wirelessaccesspoint

wirelesslaptops

router/firewall

cablemodem

to/fromcable

headend

Ethernet(switched)

Page 18: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-18

“internetworking”?

internetwork – interconnection ofnetworks – also called an “internet”

subnetwork – a constituent of an internet intermediate system – a device used toconnect two networks allowing hosts of thenetworks to correspond with each other

BridgeRouter

Internet is an example of an internetwork.

Page 19: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-19

RoadmapWhat is a Computer Network?Applications of Networking Classification of Networks Layered ArchitectureNetwork CoreDelay & Loss in Packet-switched Networks Structure of the Internet Summary

Page 20: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-20

Layered Architecture: Why?

Networks are complex with many piecesHosts, routers, links, applications,protocols, hardware, software

Can we organize it, somehow? Let’s consider a Web page request:

Browser requests Web page from serverServer should determine if access is privilegedReliable transfer page from server to clientPhysical transfer of “bits” from server to client

Page 21: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-21

Motivation Continued …

NetworkServices

ApplicationServices

CommunicationService

NetworkServices

ApplicationServices

CommunicationService

Web Server WebClient

Application logic

Reliable delivery

Transfer “bits”

Page 22: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-22

Motivation Continued …Dealing with complex systems: explicit structure allows identification, relationship

of complex system’s pieceslayered reference model for discussion

modularization eases maintenance, updating of systemchange of implementation of layer’s servicetransparent to rest of systeme.g., change in network technology doesn’t affectrest of system

layering considered harmful? (design vs implemention)

Page 23: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-23

Layers, Protocols, Interfaces

Web Server WebClient

Application logicprotocol

Reliable deliveryprotocol

Transfer “bits”protocol

NetworkServices

ApplicationServices

CommunicationService

NetworkServices

ApplicationServices

CommunicationService

LayerInterface

LayerInterface

Page 24: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-24

Layered Architecture

Networks organized as a stack of layers?The purpose of a layer is to offer services tothe layer above it using a well-definedinterface (programming language analogy:libraries hide details while providing a service)Reduces design complexity

Protocols: “horizontal” conversations at anylayer n (i.e., between peer layers)

Data Transfer: each layer passes data &control information to the layer below;eventually physical medium is reached.

Page 25: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-25

Layered Architecture (cont’d)

A set of layers & protocols is called aNetwork Architecture. Thesespecifications enable hardware/softwaredevelopers to build systems compliant witha particular architecture.

E.g., TCP/IP, OSI

Page 26: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-26

Layering: Design IssuesHow many layers? What do they each do?How to identify senders/receivers?

AddressingUnreliable physical communication medium?

Error detectionError controlMessage reordering

Sender can swamp the receiver?Flow control

Multiplexing/Demultiplexing

Page 27: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-27

Network Reference Models

Open Systems Interconnection (OSI)Model

Classic 7-layer model (covered in Wed tutorial) TCP/IP Model

Streamlined practical 4-layer protocol stack

Page 28: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-28

Reference Models (2)

Page 29: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-29

TCP/IP Model: History

Originally used in the ARPANETARPANET required networksusing leased telephone lines &radio/satellite networks tointeroperate

Goals of the model are:Seamless interoperabilityWide-ranging applicationsFault-tolerant to some extent

Application

Transport

Internet

Host-to-Network

Page 30: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-30

The Application Layer

Residence of network applications and theirapplication control logic

Examples include:HTTP (Hyper-Text Transfer Protocol)FTP (File Transfer Protocol)TelnetSMTP (Simple Mail Transfer Protocol)DNS (Domain Name Service)

Page 31: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-31

The Transport Layer

Concerned with end-to-end data transferbetween end systems (hosts)

Transmission unit is called segment TCP/IP networks such as the Internetprovides two types of services toapplications

“connection-oriented” service – TransmissionControl Protocol (TCP)“connectionless” service - User DatagramProtocol (UDP)

Page 32: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-32

TCP: Connection-oriented Service

Handshaking between client & serverprograms

Parameters for ensuing exchangeMaintain connection-state

Packet switches do not maintain anyconnection-state; state is at end systems

hence “connection-oriented”Similar to a phone conversation TCP is bundled with reliability, congestioncontrol, and flow control.

Page 33: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-33

UDP: Connectionless Service

No handshakingSend whenever and however you wantA “best effort” service

No reliabilityNo congestion & flow control services

Useful for network applications thatprefer quick delivery of most packetsrather than guaranteed (slow) delivery ofall packets (e.g., VOIP, video streaming)

Page 34: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-34

The Internet Layer

End systems inject datagrams in thenetworks

A transmission path is determined for eachpacket (routing)

A “best effort” serviceDatagrams might be lostDatagrams might be arrive out of order

Analogy: Postal system

Page 35: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-35

The Host-to-Network Layer

Somehow, host has to connect to thenetwork and be able to send IP Datagrams

How?

Page 36: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-36

Internet protocol stack application: supporting network

applicationsFTP, SMTP, STTP

transport: host-host data transferTCP, UDP

network: routing of datagrams fromsource to destination

IP, routing protocols link: data transfer between

neighboring network elementsPPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

Page 37: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-37

Layering: logical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical application

transportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

Each layer: distributed “entities”

implementlayer functionsat each node

entitiesperformactions,exchangemessages withpeers

Page 38: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-38

Layering: logical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical application

transportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data take data from app generate “segment”

according totransport protocol

add addressing,reliability check infoto form “datagram”

send datagram topeer

wait for peer to ackreceipt

data

transport

transport

ack

Page 39: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-39

Layering: physical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

Page 40: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-40

Protocol layering and data

Each layer takes data from above adds header information to create new data unit passes new data unit to layer below

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destinationMMMM

Ht

HtHn

HtHnHl

MMMM

Ht

HtHn

HtHnHl

messagesegmentdatagramframe

Page 41: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-41

RoadmapWhat is a Computer Network?Applications of Networking Classification of Networks Layered ArchitectureNetwork CoreDelay & Loss in Packet-switched Networks Structure of the Internet Summary

Page 42: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-42

The Network Core

mesh of interconnectedrouters

the fundamentalquestion: how is datatransferred through net?

circuit-switching:dedicated circuit percall: telephone netpacket-switching: datasent thru net indiscrete “chunks”

Page 43: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-43

Network Core: Circuit-Switching

End-to-end resourcesreserved for “call”

Link bandwidth, switchcapacity

Dedicated resourceswith no sharing

Guaranteed transmissioncapacity

Call setup required “Blocking” may occur

Page 44: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-44

Network Core: Circuit-Switching

Capacity of medium exceeds the capacityrequired for transmission of a single signal

How can we improve “efficiency”? Let’smultiplex.

Divide link bandwidth into “pieces”:frequency division - FDMAtime division – TDMAcode division - CDMA (cellular networks)wavelength division - WDM (optical)

Page 45: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-45

Circuit-Switching: FDMA and TDMA

FDMA

frequency

timeTDMA

frequency

time

4 usersExample:

Page 46: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-46

Network Core: Packet-Switching “store-and-forward” transmission source breaks long messages into smaller“packets”

packets share network resources each packet briefly uses full link bandwidth resource contention

aggregate resource demand can exceed amountavailablecongestion: packets queue, wait for link useanalogy: Calgary commute at rush hour

Page 47: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-47

Packet-Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixedpattern statistical multiplexing.

In TDM each host gets same slot in revolving TDMframe.

A

B

C10 MbsEthernet

1.5 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

Page 48: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-48

Packet-switching versus circuit-switching

Great for bursty dataresource sharing

Excessive congestion: packet delay and lossprotocols needed for reliable data transfer,congestion control

Q: How to provide circuit-like behavior?bandwidth guarantees needed for audio/videoappsstill an ongoing research problem

Is packet switching a “slam dunk” winner?

Page 49: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-49

Packet-switching: store-and-forward

Takes L/R seconds totransmit (push out)packet of L bits on tolink or R bps

Entire packet mustarrive at router beforeit can be transmittedon next link: store andforward

delay = 3L/R

Example: L = 7.5 Mbits R = 1.5 Mbps delay = 15 sec

R R RL

Page 50: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-50

Packet-Switching: Message Segmenting

Now break up the messageinto 5000 packets

Each packet 1,500 bits 1 msec to transmit

packet on one link pipelining: each link

works in parallel Delay reduced from 15

sec to 5.002 sec

Page 51: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-51

Packet-switched networks: forwarding

datagram network:destination address in packet determines next hoproutes may change during session (flexible?)no “per flow” state, hence more scalable

virtual circuit network:each packet carries tag (virtual circuit ID), tagdetermines next hopfixed path determined at call setup timepath is not a dedicated path as in circuit switched (i.e.,store & forward of packets)routers maintain per-call state

datagram networks need per packet routing.

Page 52: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-52

Network TaxonomyTelecommunication

networks

Circuit-switchednetworks

FDM TDM

Packet-switchednetworks

Networkswith VCs

DatagramNetworks

Page 53: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-53

RoadmapWhat is a Computer Network?Applications of Networking Classification of Networks Layered ArchitectureNetwork CoreDelay & Loss in Packet-switched Networks Structure of the Internet Summary

Page 54: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-54

How do loss and delay occur?packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn if queue is full, arriving packets dropped (Drop-Tail)

A

B

packet being transmitted (delay)

packets queueing (delay)free (available) buffers: arriving packetsdropped (loss) if no free buffers

Page 55: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-55

Four sources of packet delay

1. Processing delay:check bit errorsdetermine output link

A

B

propagation

transmission

nodalprocessing queueing

2. Queueing delay:time waiting at outputlink for transmissiondepends on congestionlevel of router

Page 56: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-56

Delay in packet-switched networks3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into

link = L/R

4. Propagation delay: d = length of physical link s = propagation speed in

medium (~2x108 m/sec) propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Note: s and R are verydifferent quantities!

Page 57: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-57

Nodal processing delay

dproc = processing delaytypically a few microsecs or less

dqueue = queuing delaydepends on congestion

dtrans = transmission delay= L/R, significant for low-speed links

dprop = propagation delaya few microsecs to hundreds of msecs

d nodal=d procdqueued transd prop

Page 58: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-58

Queueing delay (revisited)

R=link bandwidth (bps) L=packet length (bits) a=average packet

arrival rate

traffic intensity = La/R

La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be

serviced, average delay infinite!

Page 59: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-59

“Real” Internet delays and routes

What do “real” Internet delay & loss look like? Traceroute program: provides delay

measurement from source to router along end-endInternet path towards destination. For all i:

sends three packets that will reach router i on pathtowards destinationrouter i will return packets to sendersender times interval between transmission and reply.

3 probes

3 probes

3 probes

Page 60: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-60

RoadmapWhat is a Computer Network?Applications of Networking Classification of Networks Layered ArchitectureNetwork CoreDelay & Loss in Packet-switched Networks Structure of the Internet Summary

Page 61: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-61

Internet structure: network of networks

roughly hierarchical at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity,

Sprint, AT&T), national/international coveragetreat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1providersinterconnect(peer)privately

NAP

Tier-1 providersalso interconnectat public networkaccess points(NAPs)

Page 62: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-62

Tier-1 ISP: e.g., SprintSprint US backbone network

Page 63: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-63

Internet structure: network of networks

“Tier-2” ISPs: smaller (often regional) ISPsConnect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP paystier-1 ISP forconnectivity torest of Internet tier-2 ISP iscustomer oftier-1 provider

Tier-2 ISPsalso peerprivately witheach other,interconnectat NAP

Page 64: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-64

Internet structure: network of networks

“Tier-3” ISPs and local ISPslast hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier-3 ISPs arecustomers ofhigher tierISPsconnectingthem to restof Internet

Page 65: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-65

Internet structure: network of networks

a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Page 66: CPSC 441: Computer Communicationsledvina/DHT/ucalgary/introduction.pdfinternetwork – interconnection of networks – also called an “internet” subnetwork – a constituent of

CPSC 441: Introduction 1-66

Introduction: SummaryCovered a “ton” of material! Internet overview what’s a protocol? network edge, core, access

networkpacket-switching versuscircuit-switching

Internet/ISP structure performance: loss, delay layering and service models Internet history (tutorial)

You now have: context, overview,

“feel” of networking more depth, detail to

follow!