Chapter 1: Introduction - University of Minnesota

CSci4211: Introduction 1

Chapter 1: Introduction

What is a Network? What is Internet?Compared with postal service & telephone system

“Nuts and Bolts” description

Services provided

Packet Switching vs. Circuit Switching

Fundamental Issues in Computer Networking

Protocol and Layered Architecture

Internet Protocols, Architecture & History

Readings: Chapter 1, Lecture Notes

Goal and Motivating Questions

Our goal: • get “feel” and

terminology

• more depth, detail later in course

• approach:– use Internet as

example

Motivating Questions:

• What is internet? What’s so special about it?

• What’s a protocol?

• How do I build a network?

• How do I deal with the complexity?

• What does real Internet look like now?

• Why I download slowly?

CSci4211: Introduction

2

Internet is the network!

• It’s big!

• It’s diverse!

• It’s complex!

• It’s everywhere (almost)!

• … and it keeps growing and changing!


Inter-networking

– two or more nodes connected by a link, or

two or more networks connected by two or more nodes

A network can be defined recursively as...

Internet: networks of networks started as ARPAnet with only 4 nodes


Map of Internet

csci4211 Introduction 6

Internet Usage Statistics

source: http://www.internetworldstats.com/stats.htm

csci4211 Introduction 7

• servers, desktops, laptops, …

High-tier

Low-tier

High Mobility Low MobilityWide Area

Local Area

Wireless technologies revolutionizing Internet! WiFi, bluetooth, NFC, Zigbee, 3/4G (soon 5G) cellular networks

mobile computing

location services

• smart mobile phones, iPads, e-readers, … • now TVs, lightbulbs, thermostats, cars,

etc., soon fridges, … everything


More gadgets are plugged in … New Era of Internet of Things (IoT)

IoT & Smart Cities

1: Introduction9

Why VIA –Hardware structure

CPU CPU

MemoryController

PCI Bridge

Memory

PCI Bus

SCSI Ethernet FC

SANLAN

Disk

Disk

Disk

1: Introduction10

A Case for Data and Control Flow between Host and NIC

Internet:a huge transformative & disruptive force!

What has become of the Internet: •Information Service and E-Commerce Platform

– deliver all kinds of information, news, music, video, shopping – web, spotify, iTune, youtube, Netflix, Hulu, …

• Global Information Repository– store and search for all kinds of information– google, flickr, dropbox, icloud, …

•Cyberspace and Virtual Communities– keep in touch with friends and strangers – email, facebook, twitter, …

• Enormous Super-Computer– mobile, cloud computing and services

We’re increasingly depending on it !



So what’s so special about the Internet?

But first, what is a Network?


What is a Network? There are many types of networks!

Key Features of Networks

Providing certain services• transport goods, mail, information or data

Shared resources

used by many users, often concurrently

Basic building blocks • nodes (active entities): process and transfer goods/data

• links (passive medium): passive “carrier” of goods/data

Typically distributed & “multi-hop”: two “end points” cannot directly reach each other

need other nodes/entities to relay


What is a Network …

Compare Internet with

Postal Service and Telephone System

Services Provided

Various Key Pieces and Their Functions

How the pieces work together to provide services

• Internet: “network of networks”

– Interconnected ISPs

• protocols control sending, receiving of messages

– e.g., TCP, IP, HTTP, Skype, 802.11

• Internet standards– RFC: Request for comments

– IETF: Internet Engineering Task Force

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

mobile network

global ISP

regional ISP

home network

institutionalnetwork


What’s the Internet: a service view

• infrastructure that provides services to applications:

– Web, VoIP, email, games, e-commerce, social nets, …

• provides programming interface to apps

– hooks that allow sending and receiving app programs to “connect” to Internet

– provides service options, analogous to postal service

mobile network

global ISP

regional ISP

home network




Nuts and Bolts DescriptionNetwork is fundamentally distributed in nature: a collection of distinct entities: “nodes” and “links”

Postal: Mailboxes

Local/Branch Postal Offices, Regional, Central Postal Offices

Mail Sorting Machines

Postmen, Delivery Trucks/Trains/Planes, Roads, …

Telephone: Phones

Local Switching Office, Central Switching Offices, …

Telephone Switches

Wires

Internet ?


Internet: Building Blocks

• Nodes: PCs, special-purpose hardware, …– Hosts (or end systems): servers, PCs, laptops, mobile

devices, smart meters, ……– Switches: routers, switches, …

• Links: coax cable, optical fiber, wireless, …– point-to-point

– multiple access

…


Inter-networking

– two or more nodes connected by a link, or

– two or more networks connected by two or more nodes

• A network can be defined recursively as...

• Internet: networks of networks

1: Introduction20

Physical Media

• physical link:transmitted data bit propagates across link

• guided media:– signals propagate in

solid media: copper, fiber

• unguided media:– signals propagate

freelye.g., radio

Twisted Pair (TP)

• two insulated copper wires– Category 3: traditional

phone wires, 10 Mbps ethernet

– Category 5 TP: 100Mbps ethernet

1: Introduction21

Physical Media: coax, fiber

Coaxial cable:• wire (signal carrier)

within a wire (shield)– baseband: single channel

on cable

– broadband: multiple channel on cable

• bidirectional

• common use in 10Mbs Ethernet

Fiber optic cable: glass fiber carrying

light pulses

high-speed operation: 100Mbps Ethernet

high-speed point-to-point transmission (e.g., 5 Gps)

low error rate

1: Introduction22

Physical media: radio

• signal carried in electromagnetic spectrum

• no physical “wire”

• bidirectional

• propagation environment effects:– reflection

– obstruction by objects

– interference

Radio link types: microwave

e.g. up to 45 Mbps channels

LAN (e.g., waveLAN) 2Mbps, 11Mbps

wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps

satellite up to 50Mbps channel (or

multiple smaller channels)

270 Msec end-end delay

geosynchronous versus LEOS


Service Perspective

Basic Services Provided Postal: deliver mail/package from people to people

First class, express mail, bulk rate, certified, registered, …

Telephone: connect people for talking You may get a busy dial tone Once connected, consistently good quality, unless using cell phones

Internet: transfer information between people/machines Reliable connection-oriented or unreliably connectionless services! You never get a busy dial tone, but things can be very slow! You can’t ask for express delivery (not at the moment at least!)


Fundamental Issues in NetworkingNetwork is a shared resource

– Provide services for many people at same time– Carry bits/information for many people at same time

•Switching and Multiplexing – How to share resources among multiple users, and

transfer data from one node to another node

•Naming and Addressing– How to find name/address of the party (or parties) you

would like to communicate with– Address: byte-string that identifies a node

• unicast, multicast and broadcast addresses

•Routing and (end-to-end) Forwarding: – Routing: process of determining how to send packets

towards the destination based on its address• find out neighbors, build “maps” (routing tables), …

– transfer data from source to destination “hop-by-hop”


What’s so special about the Internet?

• Internet is based on the notion of “packet switching”

– enables statistical multiplexing– better utilization of network resources for transfer of

“bursty” data traffic


Switching & Multiplexing

• Network is a shared resource– Provide services for many people at same time– Carry bits/information for many people at same time

• How do we do it? – Switching: how to deliver information from point A to

point B?– Multiplexing: how to share resources among many users

Think about postal service and telephone system!

Switching and multiplexing are closely related!


Switching Strategies• Circuit switching

– set up a dedicated route (“circuit”) first – carry all bits of a “conversation” on one circuit

• original telephone network• Analogy: railroads and trains/subways

• Packet switching– divide information into small chunks (“packets”)– each packet delivered independently – “store-and-forward” packets

• Internet(also Postal Service, but they don’t tear your mail into pieces first!)

• Analogy: highways and cars

• Pros and Cons? - think taking subways vs. driving cars, during off-peak vs. rush hours!

Analogy: railroad and train


http://www.answers.com/main/Record2?a=NR&url=http://commons.wikimedia.org/wiki/Image:Railway%2520turnout%2520labelled.jpg

http://www.answers.com/main/Record2?a=NR&url=http://commons.wikimedia.org/wiki/Image:Railway%2520turnout%2520labelled.jpg

Analogy: Highway and cars


29

Circuit Switchingnetwork resources

(e.g., bandwidth) divided into “pieces”

• pieces allocated to calls

• resource piece idle if not used by owning call (no sharing)

dividing link bandwidth into “pieces”

frequency division

time division

code division

Trivia Q:You must have heard of the term

“CDMA” (think the company Qualcom, for which it is most associated with), what does “CD” in CDMA stands for?


30

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:


31

Numerical example

• How 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

– 500 msec to establish end-to-end circuit

Let’s work it out!

10.5 seconds


Networks with Circuit Switchinge.g., conventional (fixed-line) telephone networks

End-end resources reserved for “call”

• link bandwidth, switch capacity

• dedicated resources: no sharing

• circuit-like (guaranteed) performance

• call setup required



Circuit Switched Networks• All resources (e.g. communication links) needed by

a call dedicated to that call for its duration– Example: telephone network

– Call blocking when all resources are used

Packet SwitchingEach end-end “data stream”

divided into packets

• users A, B packets sharenetwork resources

• each packet uses full link bandwidth

• resources used as needed

resource contention:

aggregate resource demand can exceed amount available

congestion: packets queue, wait for link use

store and forward: packets move one hop at a time Node receives complete

packet before forwarding

Packets may suffer delay or losses!

Bandwidth division into “pieces”

Dedicated allocation

Resource reservation

35CSci4211: Introduction


Statistical Multiplexing

• Time division, but on demand rather than fixed

• Reschedule link on a per-packet basis

• Packets from different sources interleaved on the link

• Buffer packets that are contending for the link

• Buffer buildup is called congestion

• This is packet switching, used in computer networks

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing.

TDM: each host gets same slot in revolving TDM frame.

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link


Packet-switching: store-and-forward

• Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps

• Entire packet must arrive at router before it can be transmitted on next link: store and forward

• delay = 3L/R (assuming zero propagation delay)

Example:• L = 7.5 Mbits• R = 1.5 Mbps• delay = ?

R R R

L

more on delay later …

15 sec


38

Packet switching versus circuit switching

• 1 Mb/s link

• each user: – 100 kb/s when “active”

– active 10% of time

• circuit-switching: – 10 users

• packet switching: – with 35 users,

probability > 10 active less than .0004

Packet switching allows more users to use network!

N users

1 Mbps link

Q: how did we get value 0.0004?

M

Nn

nMn ppn

M

1

1


39


Circuit Switching vs Packet SwitchingItem Circuit-switched Packet-switched

Dedicated “copper” path Yes No

Bandwidth available Fixed Dynamic

Potentially wasted bandwidth Yes No (not really!)

Store-and-forward transmission No Yes

Each packet/bit always follows the same route

Yes Not necessarily

Call setup Required Not Needed

When can congestion occur At setup time On every packet

Effect of congestion Call blocking Queuing delay

Packet switching vs. circuit switching

• Great for bursty data– resource sharing

– simpler, no call setup

• Excessive congestion: packet delay and loss– protocols needed for reliable data transfer, congestion

control

• Q: How to provide circuit-like behavior?– bandwidth guarantees needed for audio/video apps

– still an unsolved problem (chapter 7)

Is packet switching a “slam dunk winner?”

Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?


41


What’s so special about the Internet?• Internet is based on the notion of “packet switching”

– enables statistical multiplexing– better utilization of network resources for transfer of

“bursty” data traffic

• Internet’s key organizational/architectural principle: “smart” end systems + “dumb” networks– architecture: functional division & function placement

– hourglass Internet architecture: enables diverse applications and accommodates evolving technologies

– “dumb” network (core): simple packet-switched, store-forward, connectionless “datagram” service, with core functions: global addressing, routing & forwarding

– “smart” end systems/edges: servers, PCs, mobile devices, …; diverse and ever-emerging new applications!


Internet Hourglass Architecture

WiFi, Bluetooth,

Docsis, gMPLS,

DWDM/fiber, …,

3G/4G cellular,

….

p2p file sharing, skype, YouTube,

Netflix, Cloud Computing

bitTorrent, DHT, SIP, DASH, ….

enabling diverse applications & new types of end devices

accommodating evolving & new technologies

net

wo

rk c

ore

net

wo

rk e

dge/

end

ho

sts

44

“Dumb” Networks & “Smart” End Systems

• Five Layer Architecture:– Lower three layers are implemented everywhere

– Top two layers are implemented only at hosts

Network

Datalink

Physical

Network

Datalink

Physical

Network

Datalink

Physical

Physical medium

Application

Transport

Host A

Application

Transport

Host B

Router


An Overview of Network Structure:a “horizontal view”

• network edge:applications and hosts

• network core:– routers

– network of networks

• access networks, physical media:communication links


45

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

• millions of connected computing devices: hosts = end systems

• running network apps

• communication links– fiber, copper, radio,

satellite

– transmission rate = bandwidth

• routers: forward packets (chunks of data)

local ISP

companynetwork

regional ISP

router workstation

servermobile


The network edge:• end systems (hosts):

– run application programs

– e.g. Web, email

– at “edge of network”

• client/server model– client host requests, receives

service from always-on server

– e.g. Web browser/server; email client/server

• peer-peer model:– minimal (or no) use of

dedicated servers

– e.g. Skype, BitTorrent, KaZaA


47

The network edge:• end systems (hosts):

– run application programs

– e.g. Web, email

– at “edge of network”

• client/server model– client host requests, receives

service from always-on server

– e.g. Web browser/server; email client/server

– Cloud & Mobile Computing

• peer-peer model:– minimal (or no) use of

dedicated servers

– e.g. Skype, BitTorrent, KaZaA cloud computing


48

Network edge: connection-oriented service

Goal: data transfer between end systems

• handshaking: setup (prepare for) data transfer ahead of time– Hello, hello back human

protocol

– set up “state” in two communicating hosts

• TCP - Transmission Control Protocol – Internet’s connection-

oriented service

TCP service [RFC 793]

• reliable, in-order byte-stream data transfer– loss: acknowledgements

and retransmissions

• flow control:– sender won’t overwhelm

receiver

• congestion control:– senders “slow down sending

rate” when network congested


Network edge: connectionless service

Goal: data transfer between end systems– same as before!

• UDP - User Datagram Protocol [RFC 768]: – connectionless

– unreliable data transfer

– no flow control

– no congestion control

App’s using TCP:• HTTP (Web), FTP (file

transfer), Telnet (remote login), SMTP (email), Flash videos, DASH stream videos

App’s using UDP:• streaming media,

teleconferencing, DNS, Internet telephony


The Network Core

• mesh of interconnected routers shared by many users

• the fundamental questions:– how network is shared

– how to find the other party (person, website, …) you want

– how is data transferred through net?


51

On the Internet Edge …

Internet home users

banking &

e-commercedumb &

smart phonesPOTS

VoIP

music

streaminggames

surveillance

& security

video streaming

& IPTVweb

• Large # of (mobile & stationary) users

• Large # of “dumb” or smart devices & appliances

• Some “always-on,” high-speed connection

• Others intermittent connectivity with varying bandwidth

• Diverse applications and services

• Heterogeneous technologies

smart pads &

e-readers

social networks

sensors &

smart home

others


52

Within the Internet “Cloud” Network Core:•big ISPs (& cellular providers) with large geographical span

•As well as medium & smaller ISPs

And the “other end/edge”: •big content providers with huge data centers

High bandwidth, dense and rich topology

Enormous computing & storage capacities to support cloud, mobile computing/services


53

Well, Internet is too complex for me to learn.

How can they even build it?

And what’s a protocol & why do we need protocols?

Motivating Questions 3-5


54

Network Architecture(or organizational principles)

Networks are complex!

• many “pieces”:– hosts

– routers

– links of various media

– hardware, software

– applications

– protocols

– …..

Question:Is there any hope of organizing structure or principle of network?

Or at least our discussion of networks?

Network architecture:“blue prints” (or principles) regarding

functional division and function placement


55

Organization of air travel

• a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing


56

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departure

airportarrival

airport

intermediate air-traffic

control centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Layering of airline functionality

Layers: each layer implements a service– via its own internal-layer actions

– relying on services provided by layer below


57

Why Layering?

Dealing with complex systems:• explicit structure allows identification,

relationship of complex system’s pieces– layered reference model for discussion

• modularization eases maintenance, updating of system– change of implementation of layer’s service transparent

to rest of system

– e.g., change in gate procedure doesn’t affect rest of system


58

Internet Protocol Stack• application: supporting network

applications– FTP, SMTP, HTTP, DASH, …

• transport: process-process data transfer– TCP, UDP

• network: routing of datagrams from source to destination– IP, routing protocols

• link: data transfer between neighboring network elements– PPP, Ethernet

• physical: bits “on the wire”

application

transport

network

link

physical


59


Layered Architecture

• Layering simplifies the architecture of complex system

• Layer N relies on services from layer N-1 to provide a service to layer N+1

• Interfaces define the services offered

• Service required from a lower layer is independent of its implementation

– Layer N change doesn’t affect other layers

– Information/complexity hiding

– Similar to object oriented methodology


Protocols and Services• Protocols are used to implement services

– Peering entities in layer N provide service by communicating with each other using the service provided by layer N-1

• Logical vs physical communication

What’s a protocol?human protocols:

• “what’s the time?”

• “I have a question”

• introductions

network protocols:

• machines rather than humans

• all communication activity in Internet governed by protocols (why this concept is so important!!!)


62

Make sure Bob is awake

Bob can speak English

Bob can understand English

Bob is willing to talk

1.

3

2

4

Human protocol

• protocols define:– Format.

– Order of msgs sent and received among network entities (two or more)

– Actions taken on msg transmission, receipt

Hi

Hi

Got thetime?

Alice

Bob

Q: What are the purposes of first hi-hi exchange

2:00pm


63

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

Q: Other human protocols? (e.g., in-class interaction)

Hi

Hi

Got thetime?

2:00

TCP connectionrequest

TCP connectionresponse

Get http://www.cnn.com

<file>time



Protocols• Protocol: rules by which network elements communicate

• Protocols define the agreement between peering entities– The format and the meaning of messages exchanged

• Protocols in everyday life– Examples: traffic control, open round-table discussion etc


Protocol Packets• Protocol data units (PDUs):

– packets exchanged between peer entities• Service data units (SDUs):

– packets handed to a layer by an upper layer• Data at one layer is encapsulated in packet at a lower layer

– Envelope within envelope: PDU = SDU + (optional) header or trailer

source

applicationtransportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

applicationtransportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

Encapsulationmessage M

Ht M

Hn

frame



Internet and ISO/OSI Reference Models


ISO/OSI Reference Model• Application layer

• Examples: smtp, http, ftp, dash, etc

– process-to-process communication

– all layers exist to support this layer

• Presentation layer (OSI only)– conversion of data to common format

• Example: “little endian” vs. “big endian” byte orders

– multimedia streaming presentation (e.g., mpeg-dash)

• Session layer (OSI only)– session setup (and authentication)

– recovery from failure (broken session)

• Internet applications perform presentation/session layer functions, e.g., “little” & “big” endian conversions


ISO/OSI Reference Model (cont’d)• Transport layer: end-to-end data delivery, e.g.,

– connection-oriented (TCP) or connection-less (UDP) services

– error control, flow/congestion control, …

• Network layer: examples: IP, X.25– (global) naming and addressing, routing (build routing tables)

– forwarding packets hop-by-hop across networks

– avoidance of congested/failed links, traffic engineering, …

• Data link layer: data transfer between “neighboring” elements

– Examples: Ethernet, 802.11 WiFi, PPP

– framing and error/flow control

– media access control

• Physical layer (EE stuff)– encoding/decoding information (bits) into physical media

– modulating & transmitting raw bits (0/1) over wire


Comments on Layering• Layering simplifies the architecture of complex system

• Advantages– modularization eases maintenance and updating

– hide lower layer complexity/implementation details from higher layers

• Layering considered harmful?– Q: which layer should implement what functionality?

• e.g., reliability, hop-by-hop basis or end-to-end basis?

• Possible Drawbacks?– possible duplication of functionality between layers

• error recovery at link layer and transport layer

– Other possible drawbacks?


Internet Protocol “Zoo”appli

cati

o

n

SMTP telnet, ssh

NFS/RPC

FTP, SCP

DNSHTTP

RealAudioRealVideo

802.11 WiFi

Flash DASH

SOAP

…..…..

VoIP

IPTV

2.5G/3G/4G

(GPRS,UMTS,

WiMAX, LTE,

…) Cellular

Radio Networks

DWDM

MPLS/gMPLS

DSL or

DOCSIS

PPP

ICMP,

OSPF, RIP,

BGP, …

P2P

What real Internet looks like now?



Internet Structure

LANs

International

lines

Regional or

local ISP local ISPscompany university

National or

tier-1 ISP

National or

tier-1 ISP

IXPsor private peering

Regional

ISPs

company

access via WiFi

hotspots

Internet: “networks of networks”!

Home users

Internet

eXcange

Points

Home users

Internet structure: network of networks

• Roughly hierarchical

• At center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, L3, Cable and Wireless), national/international coverage– treat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

IXP

Tier-1 providers also interconnect at Internet Exchange Point


Tier-1 ISP: e.g., Sprint

…

to/from customers

peering

to/from backbone

…

.………

POP: point-of-presence



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

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

IXP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at IXP



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

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

IXP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier-3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

CSci4211: Introduction78


• a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

IXP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

traceroute www.cnn.com


Routing & forwarding:how do packets gofrom A to B?

B

A

Map of Internet

Why it takes so long to download my friends’ pictures from web?

Or why $#@! can’t I access the Internet now?

Motivating Question 6



Fundamental Problems in Networking …

Or what can go wrong?• Bit-level errors: due to electrical interferences

• “Frame-level” errors: media access delay or frame collision due to contention/collision/interference

• Packet-level errors: packet delay or loss due to network congestion/buffer overflow

• Out of order delivery: packets may takes different paths

• Link/node failures: cable is cut or system crash

Four sources of packet delay

1. nodal processing:• check bit errors

• determine output link

A

B

propagation

transmission

nodalprocessing queueing

2. queueing• time waiting at output link

for transmission

• depends on congestion level of router



Delay in packet-switched networks

3. 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 very different quantitites!

Nodal delay

• dproc = processing delay– typically a few microsecs or less

• dqueue = queuing delay– depends on congestion

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

• dprop = propagation delay– a few microsecs to hundreds of msecs

proptransqueueprocnodal ddddd



Statistical Multiplexing and Queueing

A

B

C10 MbsEthernet

1.5 Mbs

45 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link


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!

Queueing delay and Packet loss

• Queue (aka buffer) preceding link in buffer has finite capacity

• When packet arrives to full queue, packet is dropped (aka lost)

• lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all


“Real” Internet delays and routes

• What do “real” Internet delay & loss look like?

• 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


“Real” Internet delays and routes

Let’s Traceroute to www.bbc.com


Throughput

• throughput: rate (bits/time unit) at which bits transferred between sender/receiver– instantaneous: rate at given point in time

– average: rate over longer period of time

server, withfile of F bits

to send to client

link capacityRs bits/sec

link capacityRc bits/sec

pipe that can carryfluid at rateRs bits/sec)

pipe that can carryfluid at rateRc bits/sec)

server sends bits (fluid) into pipe


Throughput (cont’d)

• Rs < Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

link on end-end path that constrains end-end throughput

bottleneck link


Throughput: Internet scenario

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

Rs

Rs

Rs

Rc

Rc

Rc

R

• per-connection end-end throughput: min(Rc,Rs,R/10)

• in practice: Rc or Rs is often bottleneck


What’s the Internet: Recap

• protocols control sending, receiving of messages– e.g., TCP, IP, HTTP, FTP, PPP

• Internet: “network of networks”– loosely hierarchical

– public Internet versus private intranet

• Internet standards– RFC: Request for comments

– IETF: Internet Engineering Task Force

– IEEE

local ISP

companynetwork

regional ISP

router workstation

servermobile



Fundamental Issues in NetworkingNetwork is a shared resource

– Provide services for many people at same time– Carry bits/information for many people at same time

• Switching and Multiplexing – How to share resources among multiple users, and

transfer data from one node to another node

• Naming and Addressing– How to find name/address of the party (or parties) you

would like to communicate with– Address: byte-string that identifies a node

• unicast, multicast and broadcast addresses

• Routing and Switching/Forwarding: – process of determining how to send packets towards the

destination based on its address: finding out neighbors, building routing tables

– transferring data from source to destination


Fundamental Problems in Networking …

Or what can go wrong?• Bit-level errors: due to electrical interferences

• “Frame-level” errors: media access delay or frame collision due to contention/collision/interference

• Packet-level errors: packet delay or loss due to network congestion/buffer overflow

• Out of order delivery: packets may takes different paths

• Link/node failures: cable is cut or system crash


Fundamental Problems in Networking

What can be done?• Add redundancy to detect and correct erroneous

packets• Acknowledge received packets and retransmit lost

packets• Assign sequence numbers and reorder packets at

the receiver• Sense link/node failures and route around failed

links/nodesGoal: to fill the gap between what applications

expect and what underlying technology provides


The Internet Network layer

routing

table

Routing protocols

•path selection

•RIP, OSPF, BGP

IP protocol

•addressing conventions

•packet handling conventions

ICMP protocol

•error reporting

•router “signaling”

Transport layer: TCP, UDP

Data Link layer (Ethernet, WiFi, PPP, …)

Physical Layer (fiber optics, radio, …)

Network

layer

Introduction: SummaryAnswers to 6 motivating questions

• What is internet? What so special about it?

• What internet looks like now?

• How I deal with the complexity?

• What’s a protocol?

• How I build a network?

• Why do I suffer delays?

You now have:• context, overview,

“feel” of networking

• more depth, detail to follow!



Internet Summary• Computer networks/Internet use packet switching

• Layered architecture for handling complexity & attaining maintainability– Key notions: protocols, services and interfaces

• Internet is based on TCP/IP protocol suite– Networks of networks!

– Shared, distributed and complex system in global scale

– No centralized authority

• Fundamental issues in networking– addressing/naming

– routing/forwarding

– error/flow/congestion control, media access control


Readings for Next Week

• Read Chapter 1

• Review these lecture notes– Read the supplementary notes that follow these one if

you have time

• Read Chapter 2: sections 2.1 –2.6– Learn how web works

– Learn how email works

– Understand what Domain Name System does for us

– P2P File Sharing

– Glance through Chapter 7: sections 7.1-7.2


Supplementary Readings

• Physical Media

• Access Network Technologies

• History of Internet

• Internet “Governing” Bodies

• Network Security: Cyber Attacks

Access networks and physical mediaQ: How to connect end

systems to edge router?

• residential access nets

• institutional access networks (school, company)

• mobile access networks

keep in mind:

• bandwidth (bits per second) of access network?

• shared or dedicated?


Physical media

• bit: propagates betweentransmitter/receiver pairs

• physical link: what lies between transmitter & receiver

• guided media: – signals propagate in solid

media: copper, fiber, coax

• unguided media:– signals propagate freely,

e.g., radio

twisted pair (TP)

• two insulated copper wires• Category 5: 100 Mbps, 1

Gbps Ethernet

• Category 6: 10Gbps


Host: sends packets of datahost sending function:

• takes application message

• breaks into smaller chunks, known as packets, of length L bits

• transmits packet into access network at transmission rate R

• link transmission rate, aka link capacity, aka link bandwidth

R: link transmission ratehost

12

two packets,

L bits each

packettransmission

delay

time needed totransmit L-bit

packet into link

L (bits)

R (bits/sec)= =


Physical media: coax, fiber

coaxial cable:• two concentric copper

conductors

• bidirectional

• broadband:• multiple channels on cable

• HFC

fiber optic cable: glass fiber carrying light

pulses, each pulse a bit

high-speed operation:• high-speed point-to-point

transmission (e.g., 10’s-100’s Gbps transmission rate)

low error rate: • repeaters spaced far apart

• immune to electromagnetic noise



Physical media: radio

• signal carried in electromagnetic spectrum

• no physical “wire”

• bidirectional

• propagation environment effects:– reflection

– obstruction by objects

– interference

Radio link types:• microwave

– e.g. up to 45 Mbps channels

• LAN (e.g., waveLAN)– 2Mbps, 11Mbps

• wide-area (e.g., cellular)– e.g. CDPD, 10’s Kbps

• satellite– up to 50Mbps channel (or

multiple smaller channels)

– 270 Msec end-end delay

– geosynchronous versus LEOS

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1: Introduction

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A closer look at network structure:

• network edge:• hosts: clients and servers

• servers often in data centers

access networks, physical media: wired, wireless communication links

network core: • interconnected routers

• network of networks

mobile network

global ISP

regional ISP

home network



telephone

network Internet

home

dial-up

modem

ISP

modem

(e.g., AOL)

home

PC

central

office

Uses existing telephony infrastructure

Home is connected to central office

up to 56Kbps direct access to router (often less)

Can’t surf and phone at same time: not “always on”

Residential access: Dial-up Modem


ISP

Access network: digital subscriber line (DSL)

central office telephonenetwork

DSLAM

voice, data transmittedat different frequencies over

dedicated line to central office

use existing telephone line to central office DSLAM

• data over DSL phone line goes to Internet

• voice over DSL phone line goes to telephone net

< 2.5 Mbps upstream transmission rate (typically < 1 Mbps)

< 24 Mbps downstream transmission rate (typically < 10 Mbps)

DSLmodem

splitter

DSL access multiplexer


Access Network: cable modems

Diagram: http://www.cabledatacomnews.com/cmic/diagram.html118CSci4211: Introduction

Access network: cable network

cablemodem

splitter

…

cable headend

Channels

V

I

D

E

O

V

I

D

E

O

V

I

D

E

O

V

I

D

E

O

V

I

D

E

O

V

I

D

E

O

D

A

T

A

D

A

T

A

C

O

N

T

R

O

L

1 2 3 4 5 6 7 8 9

frequency division multiplexing: different channels transmittedin different frequency bands


ISPdata, TV transmitted at different

frequencies over shared cable distribution network

cablemodem

splitter

…

cable headend

CMTScable modem

termination system

HFC: hybrid fiber coax

• asymmetric: up to 30Mbps downstream transmission rate, 2 Mbps upstream transmission rate

network of cable, fiber attaches homes to ISP router

• homes share access network to cable headend

• unlike DSL, which has dedicated access to central office

Access network: cable network


Access network: home network

to/from headend or central office

cable or DSL modem

router, firewall, NAT

wired Ethernet (1 Gbps)

wireless access point (54 Mbps)

wireless

devices

often combined in single box


Enterprise access networks (Ethernet)

• typically used in companies, universities, etc. 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates

today, end systems typically connect into Ethernet switch

Ethernet switch

institutional mail,web servers

institutional router

institutional link to ISP (Internet)


Wireless access networks• shared wireless access network connects end system

to router– via base station aka “access point”

wireless LANs: within building (100 ft.)

802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate

wide-area wireless access provided by telco (cellular)

operator, 10’s km

between 1 and 10 Mbps

3G, 4G: LTE

to Internet

to Internet


• mesh of interconnected routers

• packet-switching: hosts break application-layer messages into packets– forward packets from one

router to the next, across links on path from source to destination

– each packet transmitted at full link capacity

The network core



Origin of Internet?

Started by U.S. research/military organizations:

• Three Major Actors:

– DARPA: Defense Advanced Research Projects Agency

• funds technology with military goals

– DoD: U.S. Department of Defense

• early adaptor of Internet technology for production use

– NSF: National Science Foundation

• funds university research


Pre-Internet Modes of Human Telecommunications

The Dark Age before the Internet: before 1960

Non-electrical (source: wikipedia)• Prehistoric: Fires, Beacons, Smoke signals, drums, Horns

• 6th century BCE: (snail) mail (e.g., delivered by human couriers on horse)

• 5th century BCE: Pigeon post

• 4th century BCE: Hydraulic semaphores, heliographs (shield signals)

• 15th century CE: Maritime flag semaphores

• 1672: First experimental acoustic (mechanical) telephone

• 1790: Semaphore lines (optical telegraphs)

• 1867: Signal lamps; 1877: Acoustic phonograph

Electrical:

• 1830: telegraph• 1876: circuit-switching (telephone)• 1896: radio• TV (1940?) , and later cable TV (1970s)

Internet History

• 1961: Kleinrock - queueing theory shows effectiveness of packet-switching

• 1964: Baran - packet-switching in military nets

• 1967: ARPAnet conceived by Advanced Research Projects Agency

• 1969: first ARPAnet node operational

• 1972:

– ARPAnet public demonstration

– NCP (Network Control Protocol) first host-host protocol

– first e-mail program

– ARPAnet has 15 nodes

1961-1972: Early packet-switching principles


Internet History

• 1970: ALOHAnet satellite network in Hawaii

• 1974: Cerf and Kahn -architecture for interconnecting networks

• 1976: Ethernet at Xerox PARC

• ate70’s: proprietary architectures: DECnet, SNA, XNA

• late 70’s: switching fixed length packets (ATM precursor)

• 1979: ARPAnet has 200 nodes

Cerf and Kahn’s internetworking principles:– minimalism, autonomy - no

internal changes required to interconnect networks

– best effort service model– stateless routers– decentralized control

define today’s Internet architecture

1972-1980: Internetworking, new and proprietary nets


Internet History

• 1983: deployment of TCP/IP

• 1982: smtp e-mail protocol defined

• 1983: DNS defined for name-to-IP-address translation

• 1985: ftp protocol defined

• 1988: TCP congestion control

• new national networks: Csnet, BITnet, NSFnet, Minitel

• 100,000 hosts connected to confederation of networks

1980-1990: new protocols, a proliferation of networks


Internet History

• Early 1990’s: ARPAnet decommissioned

• 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)

• early 1990s: Web

– hypertext [Bush 1945, Nelson 1960’s]

– HTML, HTTP: Berners-Lee

– 1994: Mosaic, later Netscape

– late 1990’s: commercialization of the Web

Late 1990’s – 2000’s:• more killer apps: instant

messaging, P2P file sharing

• network security to forefront

• est. 50 million host, 100 million+ users

• backbone links running at Gbps

• Napster, BitTorrent, …

• Myspace, Facebook, twitter,..

• YouTube, Netflix, Hulu, …

Now to the future:

• … (your invention here!)

1990, 2000’s: commercialization, the Web, new apps



Who Runs the Internet“nobody” really!

• standards: Internet Engineering Task Force (IETF)

• names/numbers: The Internet Corporation for Assigned Names and Numbers (ICANN)

• DNS root server operators, domain name registrars

• networks: ISPs (Internet Service Providers), IXPs (Internet Exchange Points), ……

• fibers: telephone companies (mostly)

• content: companies, universities, governments, individuals, …;

• content distribution networks, …


Internet “Governing” Bodies• Internet Society (ISOC): membership organization

– raise funds for IAB, IETF& IESG, elect IAB

• Internet Engineering Task Force (IETF):– a body of several thousands or more volunteers

– organized in working groups (WGs) – meet three times a year + email

• Internet Architecture Board– architectural oversight, elected by ISOC

• Steering Group (IESG): approves standards, – Internet standards, subset of RFC

• RFC: “Request For Comments”, since 1969– most are not standards, also

• experimental, informational and historic(al)


Internet Names and Addresses• Internet Corporation for Assigned Names and

Numbers (ICAAN):– coordinate IPv4 & IPv6 address spaces, keep track of numbers

(e.g., protocol identifiers), delegates Internet address assignment to regional Internet registries

– manage top-level domain names & operations of root name servers

– designate authority for each top-level domain; create new TLDs

• Regional Internet Registries: AfriNIC, APNIC, ARIN, LACMIC, RIPE NCC:

– manage the allocation and registration of Internet number resources

– e.g., hand out blocks of addresses to ISPs; assign AS numbers

– maintain WHOIS registries

– ….

Network security

• field of network security:– how bad guys can attack computer networks

– how we can defend networks against attacks

– how to design architectures that are immune to attacks

• Internet not originally designed with (much) security in mind

– original vision: “a group of mutually trusting users attached to a transparent network”

– Internet protocol designers playing “catch-up”

– security considerations in all layers!

134

Bad guys: put malware into hosts via Internet

• malware can get in host from:

– virus: self-replicating infection by receiving/executing object (e.g., e-mail attachment)

– worm: self-replicating infection by passively receiving object that gets itself executed

• spyware malware can record keystrokes, web sites visited, upload info to collection site

• infected host can be enrolled in botnet,used for spam. DDoS attacks

135

target

Denial of Service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic1. select target

2. break into hosts around

the network (see botnet)

3. send packets to target from

compromised hosts

Bad guys: attack server, network infrastructure

136

Bad guys can sniff packets

packet “sniffing”: broadcast media (shared Ethernet, wireless)

promiscuous network interface reads/records all packets (e.g., including passwords!) passing by

A

B

C

src:B dest:A payload

wireshark software used for end-of-chapter labs is a

(free) packet-sniffer

137

Bad guys can use fake addresses

IP spoofing: send packet with false source address

A

B

C

src:B dest:A payload

138

… lots more on security (throughout, Chapter 8)

Documents

Chapter 1: Introduction - University of Minnesota