chapter 1: introduction - university of minnesota
TRANSCRIPT
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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
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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
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Internet is the network!
• It’s big!
• It’s diverse!
• It’s complex!
• It’s everywhere (almost)!
• … and it keeps growing and changing!
CSci4211: Introduction 3
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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
CSci4211: Introduction 4
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Map of Internet
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csci4211 Introduction 6
Internet Usage Statistics
source: http://www.internetworldstats.com/stats.htm
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csci4211 Introduction 7
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• 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
CSci4211: Introduction 8
More gadgets are plugged in … New Era of Internet of Things (IoT)
IoT & Smart Cities
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1: Introduction9
Why VIA –Hardware structure
CPU CPU
MemoryController
PCI Bridge
Memory
PCI Bus
SCSI Ethernet FC
SANLAN
Disk
Disk
Disk
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1: Introduction10
A Case for Data and Control Flow between Host and NIC
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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 !
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CSci4211: Introduction 12
So what’s so special about the Internet?
But first, what is a Network?
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CSci4211: Introduction 13
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
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CSci4211: Introduction 14
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
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• 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
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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
institutionalnetwork
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CSci4211: Introduction 17
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 ?
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CSci4211: Introduction 18
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
…
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CSci4211: Introduction 19
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
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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
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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
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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
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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!)
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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”
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CSci4211: Introduction 25
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
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CSci4211: Introduction 26
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!
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CSci4211: Introduction 27
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!
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Analogy: railroad and train
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Analogy: Highway and cars
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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?
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Circuit Switching: FDM and TDM
FDM
frequency
time
TDM
frequency
time
4 users
Example:
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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
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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
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CSci4211: Introduction 34
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
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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
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CSci4211: Introduction 36
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
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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
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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
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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
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CSci4211: Introduction 40
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
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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)?
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CSci4211: Introduction 42
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!
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CSci4211: Introduction 43
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
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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
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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
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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
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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
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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
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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
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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
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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?
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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
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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
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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
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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
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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
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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
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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
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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
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CSci4211: Introduction 60
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
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CSci4211: Introduction 61
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
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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!!!)
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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
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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
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CSci4211: Introduction 65
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
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CSci4211: Introduction 66
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
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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
CSci4211: Introduction 67
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CSci4211: Introduction 68
Internet and ISO/OSI Reference Models
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CSci4211: Introduction 69
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
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CSci4211: Introduction 70
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
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CSci4211: Introduction 71
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?
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CSci4211: Introduction 72
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
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What real Internet looks like now?
CSci4211: Introduction 73
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CSci4211: Introduction 74
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
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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
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Tier-1 ISP: e.g., Sprint
…
to/from customers
peering
to/from backbone
…
.………
POP: point-of-presence
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Internet structure: network of networks
• “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
CSci4211: Introduction 77
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Internet structure: network of networks
• “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 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 are customers ofhigher tier ISPsconnecting them to rest of Internet
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Internet structure: network of networks
• a packet passes through many networks!
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
IXP
Tier-2 ISPTier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
localISPlocal
ISPlocalISP
localISP
localISP Tier 3
ISP
localISP
localISP
localISP
traceroute www.cnn.com
CSci4211: Introduction79
Routing & forwarding:how do packets gofrom A to B?
B
A
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Map of Internet
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Why it takes so long to download my friends’ pictures from web?
Or why $#@! can’t I access the Internet now?
Motivating Question 6
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CSci4211: Introduction 82
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
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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
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CSci4211: Introduction 84
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!
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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
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CSci4211: Introduction 86
Statistical Multiplexing and Queueing
A
B
C10 MbsEthernet
1.5 Mbs
45 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
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CSci4211: Introduction 87
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!
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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
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“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
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“Real” Internet delays and routes
Let’s Traceroute to www.bbc.com
CSci4211: Introduction90
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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
CSci4211: Introduction 91
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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
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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
CSci4211: Introduction 93
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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
CSci4211: Introduction 94
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CSci4211: Introduction 95
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
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CSci4211: Introduction 96
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
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CSci4211: Introduction 97
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
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CSci4211: Introduction 98
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
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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!
CSci4211: Introduction99
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CSci4211: Introduction 100
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
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CSci4211: Introduction 101
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
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CSci4211: Introduction 102
Supplementary Readings
• Physical Media
• Access Network Technologies
• History of Internet
• Internet “Governing” Bodies
• Network Security: Cyber Attacks
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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?
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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
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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)= =
CSci4211: Introduction 100
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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
CSci4211: Introduction 101
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CSci4211: Introduction 107
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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1: Introduction
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1: Introduction
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1: Introduction
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1: Introduction
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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
institutionalnetwork
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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
CSci4211: Introduction 116
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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
CSci4211: Introduction 105
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Access Network: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html118CSci4211: Introduction
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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
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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
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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
CSci4211: Introduction 109
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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)
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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
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• 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
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CSci4211: Introduction 125
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
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CSci4211: Introduction 126
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)
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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
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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
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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
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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
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CSci4211: Introduction 131
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, …
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CSci4211: Introduction 132
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)
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CSci4211: Introduction 133
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
– ….
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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!
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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
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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
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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
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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)