1 Stan Kurkovsky Computer Networks Computer Networks Network Edge and Network Edge and Network Core Network Core Based on Computer Networking, 4 th Edition by Kurose and Ross Stan Kurkovsky What What’ s the Internet: s the Internet: “Nuts and Bolts Nuts and Bolts” View View • millions of connected millions of connected computing devices: computing devices: hosts hosts = end systems = end systems • running running network apps network apps • communication links communication links • fiber, copper, radio, fiber, copper, radio, satellite satellite • transmission rate = transmission rate = bandwidth bandwidth • routers: routers: forward packets forward packets (chunks of data) (chunks of data) Home network Institutional network Mobile network Global ISP Regional ISP router PC server wireless laptop cellular handheld wired links access points
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Stan Kurkovsky
Computer NetworksComputer Networks
Network Edge and Network Edge and Network CoreNetwork Core
Based on Computer Networking, 4th Edition by Kurose and Ross
Stan Kurkovsky
WhatWhat’’s the Internet: s the Internet: ““Nuts and BoltsNuts and Bolts”” ViewView
•• millions of connected millions of connected computing devices: computing devices: hosts hosts = end systems= end systems
•• running running network appsnetwork apps•• communication linkscommunication links
•• routers:routers: forward packets forward packets (chunks of data)(chunks of data)
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
router
PC
server
wirelesslaptopcellular handheld
wiredlinks
access points
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Stan Kurkovsky
““CoolCool”” Internet AppliancesInternet Appliances
Shaver with a LAN connectivity
IP picture framehttp://www.ceiva.com/
Web-enabled toaster +weather forecaster
Internet phones
Stan Kurkovsky
WhatWhat’’s the Internet: s the Internet: ““Nuts and BoltsNuts and Bolts”” ViewView
•• protocolsprotocols control sending, control sending, receiving of messagesreceiving of messages•• e.g., TCP, IP, HTTP, FTP, PPP, e.g., TCP, IP, HTTP, FTP, PPP,
Skype, EthernetSkype, Ethernet
•• Internet: Internet: ““network of networksnetwork of networks””•• loosely hierarchicalloosely hierarchical•• public Internet versus private public Internet versus private
intranetintranet
•• Internet standardsInternet standards•• RFC: Request for commentsRFC: Request for comments•• IETF: Internet Engineering Task IETF: Internet Engineering Task
ForceForce
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
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Stan Kurkovsky
WhatWhat’’s the Internet: a Service Views the Internet: a Service View
•• communication communication infrastructure infrastructure enables distributed applications:enables distributed applications:•• Web, email, voice over IP, Web, email, voice over IP,
•• communication services provided communication services provided to apps:to apps:•• reliable data delivery from source reliable data delivery from source
to destinationto destination•• ““best effortbest effort”” (unreliable) data (unreliable) data
deliverydelivery
Stan Kurkovsky
WhatWhat’’s a Protocol?s a Protocol?
human protocols:human protocols:•• ““whatwhat’’s the time?s the time?””•• ““I have a questionI have a question””•• introductionsintroductions
…… specific specific msgsmsgs sentsent…… specific actions taken when specific actions taken when msgsmsgs
received, or other eventsreceived, or other events
network protocols:network protocols:•• machines rather than humansmachines rather than humans•• all communication activity in Internet all communication activity in Internet
governed by protocolsgoverned by protocols
protocols define format, order of protocols define format, order of msgsmsgs sent and received among sent and received among network entities, and actions taken network entities, and actions taken on on msgmsg transmission, receipttransmission, receipt
A closer Look at Network StructureA closer Look at Network Structure
•• network edge:network edge:•• applications and hostsapplications and hosts
•• network core:network core:•• interconnected routersinterconnected routers•• network of networksnetwork of networks
•• access networks, physical access networks, physical media:media:•• wired and wireless wired and wireless
communication linkscommunication links
Stan Kurkovsky
The Network EdgeThe Network Edge
•• end systems (hosts):end systems (hosts):•• run application programsrun application programs•• e.g. Web, emaile.g. Web, email•• at at ““edge of networkedge of network””
service from alwaysservice from always--on serveron server•• e.g. Web browser/server; email e.g. Web browser/server; email
client/serverclient/server
•• peerpeer--peer model:peer model:•• minimal (or no) use of dedicated minimal (or no) use of dedicated
serversservers•• e.g. Gnutella, e.g. Gnutella, KaZaAKaZaA, Skype, , Skype,
BitTorrentBitTorrent
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Stan Kurkovsky
Network Edge: Reliable Data Transfer ServiceNetwork Edge: Reliable Data Transfer Service
Goal:Goal: data transfer between end systemsdata transfer between end systems•• handshaking:handshaking: setup (prepare for) data transfer ahead of timesetup (prepare for) data transfer ahead of time
•• Hello, hello back human protocolHello, hello back human protocol•• set up set up ““statestate”” in two communicating hostsin two communicating hosts
•• TCP TCP -- Transmission Control Protocol Transmission Control Protocol •• InternetInternet’’s connections connection--oriented serviceoriented service
TCP serviceTCP service [RFC 793][RFC 793]•• reliable, inreliable, in--orderorder bytebyte--stream data transferstream data transfer
•• loss: acknowledgements and retransmissionsloss: acknowledgements and retransmissions
•• congestion control:congestion control:•• senders senders ““slow down sending rateslow down sending rate”” when network congestedwhen network congested
Stan Kurkovsky
Network Edge: Best Effort (Unreliable) Data Transfer ServiceNetwork Edge: Best Effort (Unreliable) Data Transfer Service
Goal:Goal: data transfer between end systemsdata transfer between end systems•• same as before!same as before!
•• UDPUDP -- User Datagram Protocol [RFC 768]: User Datagram Protocol [RFC 768]: •• connectionless connectionless •• unreliable data transferunreliable data transfer•• no flow controlno flow control•• no congestion controlno congestion control
AppApp’’s using UDP:s using UDP:•• streaming media, teleconferencing, DNS, Internet telephonystreaming media, teleconferencing, DNS, Internet telephony
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Stan Kurkovsky
Access Networks and Physical MediaAccess Networks and Physical Media
•• How to connect end systems to How to connect end systems to edge router?edge router?•• residential access netsresidential access nets•• institutional access networks institutional access networks
(school, company)(school, company)•• mobile access networksmobile access networks
•• Keep in mind:Keep in mind:•• bandwidth (bits per second) of bandwidth (bits per second) of
access network?access network?•• shared or dedicated?shared or dedicated?
Stan Kurkovsky
Residential Access: Point to Point AccessResidential Access: Point to Point Access
•• Dialup via modemDialup via modem•• up to 56Kbps direct access to router up to 56Kbps direct access to router
(often less)(often less)•• CanCan’’t surf and phone at same time: t surf and phone at same time:
cancan’’t be t be ““always onalways on””
•• DSL:DSL: digital subscriber linedigital subscriber line•• deployment: telephone company deployment: telephone company
(typically)(typically)•• up to 1 Mbps upstream (today up to 1 Mbps upstream (today
typically < 256 kbps)typically < 256 kbps)•• up to 8 Mbps downstream (today up to 8 Mbps downstream (today
typically < 1 Mbps)typically < 1 Mbps)•• dedicated physical line to telephone dedicated physical line to telephone
cable distributionnetwork (simplified) Typically 500 to 5,000 homes
Channels
VIDEO
VIDEO
VIDEO
VIDEO
VIDEO
VIDEO
DATA
DATA
CONTROL
1 2 3 4 5 6 7 8 9
FDM
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Stan Kurkovsky
Company Access: Local Area NetworksCompany Access: Local Area Networks
•• company/university company/university local area local area networknetwork (LAN) connects end (LAN) connects end system to edge routersystem to edge router
10Gbps Ethernet10Gbps Ethernet•• modern configuration: end modern configuration: end
systems connect into systems connect into EthernetEthernet switchswitch
•• LANs: chapter 5LANs: chapter 5
Stan Kurkovsky
Wireless Access NetworksWireless Access Networks
•• shared shared wirelesswireless access network access network connects end system to routerconnects end system to router•• via base station aka via base station aka ““access access
pointpoint””
•• wireless LANs:wireless LANs:•• 802.11b/g (802.11b/g (WiFiWiFi): 11 or 54 Mbps): 11 or 54 Mbps
•• widerwider--area wireless accessarea wireless access•• provided by provided by telcotelco operatoroperator•• ~1Mbps over cellular system ~1Mbps over cellular system
(EVDO, HSDPA)(EVDO, HSDPA)•• next up (?): next up (?): WiMAXWiMAX (10(10’’s Mbps) s Mbps)
over wide areaover wide area
basestation
mobilehosts
router
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Stan Kurkovsky
Home NetworksHome Networks
Typical home network components: Typical home network components: •• DSL or cable modemDSL or cable modem•• router/firewall/NATrouter/firewall/NAT•• EthernetEthernet•• wireless access pointwireless access point
wirelessaccess point
wirelesslaptops
router/firewall
cablemodem
to/fromcable
headend
Ethernet
Stan Kurkovsky
Physical MediaPhysical Media
•• Bit:Bit: propagates between propagates between transmitter/receiver pairstransmitter/receiver pairs
•• physical link:physical link: what lies between what lies between transmitter & receivertransmitter & receiver
•• guided media:guided media:•• signals propagate in solid media: signals propagate in solid media:
•• single channel on cablesingle channel on cable•• legacy Ethernetlegacy Ethernet
•• broadband:broadband:•• multiple channels on cablemultiple channels on cable•• HFCHFC
•• Fiber optic cable:Fiber optic cable: glass fiber glass fiber carrying light pulses, each pulse a carrying light pulses, each pulse a bitbit
•• highhigh--speed operation:speed operation:•• highhigh--speed pointspeed point--toto--point point
transmission (e.g., 10transmission (e.g., 10’’ss--100100’’s s GpsGps))
•• low error rate: repeaters spaced low error rate: repeaters spaced far apart; far apart;
•• immune to electromagnetic noiseimmune to electromagnetic noise
Stan Kurkovsky
Physical MediaPhysical Media
•• Radio:Radio: signal carried in electromagnetic spectrumsignal carried in electromagnetic spectrum•• no physical no physical ““wirewire””•• bidirectionalbidirectional•• propagation environment effects:propagation environment effects:
•• reflection reflection •• obstruction by objectsobstruction by objects•• interferenceinterference
•• Radio link types:Radio link types:•• terrestrial microwaveterrestrial microwave
•• e.g. up to 45 Mbps channelse.g. up to 45 Mbps channels
•• LAN (e.g., LAN (e.g., WiFiWiFi))•• 2Mbps, 11Mbps, 54 Mbps2Mbps, 11Mbps, 54 Mbps
•• widewide--area (e.g., cellular)area (e.g., cellular)•• e.g. 3G: hundreds of kbpse.g. 3G: hundreds of kbps
•• satellitesatellite•• Kbps to 45Mbps channel (or multiple smaller channels)Kbps to 45Mbps channel (or multiple smaller channels)•• 270 270 msecmsec endend--end delayend delay•• geosynchronous versus low altitudegeosynchronous versus low altitude
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Stan Kurkovsky
The Network CoreThe Network Core
•• mesh of interconnected routersmesh of interconnected routers•• thethe fundamental question:fundamental question: how is how is
data transferred through net?data transferred through net?
•• circuit switching:circuit switching: dedicated circuit dedicated circuit per call: telephone netper call: telephone net
•• packetpacket--switching:switching: data sent thru data sent thru net in discrete net in discrete ““chunkschunks””
EndEnd--end resources reserved for end resources reserved for ““callcall””
•• link bandwidth, switch capacitylink bandwidth, switch capacity•• dedicated resources: no sharingdedicated resources: no sharing•• circuitcircuit--like (guaranteed) like (guaranteed)
•• network resources (e.g., network resources (e.g., bandwidth) bandwidth) divided into divided into ““piecespieces””•• pieces allocated to callspieces allocated to calls•• resource piece resource piece idleidle if not used by if not used by
owning call owning call (no sharing)(no sharing)•• dividing link bandwidth into dividing link bandwidth into
““piecespieces””•• frequency divisionfrequency division•• time divisiontime division
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Stan Kurkovsky
Circuit Switching: FDM and TDMCircuit Switching: FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 usersExample:
Stan Kurkovsky
Numerical ExamplesNumerical Examples
•• How long does it take to send a file of 640,000 bits from host How long does it take to send a file of 640,000 bits from host A to host B over a circuitA to host B over a circuit--switched network?switched network?
•• All links are 1.536 MbpsAll links are 1.536 Mbps•• Each link uses FDM with 24 Each link uses FDM with 24
channels/frequencieschannels/frequencies•• 500 500 msecmsec to establish endto establish end--toto--end end
circuitcircuit
•• All links are 1.536 MbpsAll links are 1.536 Mbps•• Each link uses TDM with 24 Each link uses TDM with 24
slots/secslots/sec•• 500 500 msecmsec to establish endto establish end--toto--end end
Sequence of A & B packets does not have fixed pattern, shared onSequence of A & B packets does not have fixed pattern, shared on demand demand statistical multiplexingstatistical multiplexing
TDM: each host gets same slot in revolving TDM frameTDM: each host gets same slot in revolving TDM frame
•• Takes L/R seconds to transmit (push out) packet of L bits on to Takes L/R seconds to transmit (push out) packet of L bits on to link or R link or R bpsbps
•• Entire packet must arrive at router before it can be transmitteEntire packet must arrive at router before it can be transmitted on next d on next link: link: store and forwardstore and forward
•• delay = 3L/R (assuming zero propagation delay)delay = 3L/R (assuming zero propagation delay)
•• Example:Example:•• L = 7.5 L = 7.5 MbitsMbits•• R = 1.5 MbpsR = 1.5 Mbps•• Transmission delay = 15 secTransmission delay = 15 sec
R R RL
Stan Kurkovsky
Packet Switching versus Circuit SwitchingPacket Switching versus Circuit Switching
•• Packet switching allows more users to use network!Packet switching allows more users to use network!
•• Great for Great for burstybursty datadata•• resource sharingresource sharing•• simpler, no call setupsimpler, no call setup
•• Excessive congestion:Excessive congestion: packet delay and losspacket delay and loss•• protocols needed for reliable data transfer, congestion controlprotocols needed for reliable data transfer, congestion control
•• Q: How to provide circuitQ: How to provide circuit--like behavior?like behavior?•• bandwidth guarantees needed for audio/video appsbandwidth guarantees needed for audio/video apps•• still an unsolved problemstill an unsolved problem
•• Q: What are human analogies?Q: What are human analogies?•• reserved resources (circuit switching) reserved resources (circuit switching) •• onon--demand allocation (packetdemand allocation (packet--switching)switching)
N users1 Mbps link
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Stan Kurkovsky
Internet Structure: Network of NetworksInternet Structure: Network of Networks
•• roughly hierarchicalroughly hierarchical•• at center: at center: ““tiertier--11”” ISPs ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), (e.g., Verizon, Sprint, AT&T, Cable and Wireless),
national/international coveragenational/international coverage•• treat each other as equalstreat each other as equals
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1 providers interconnect (peer) privately
Stan Kurkovsky
TierTier--1 ISP: e.g., Sprint1 ISP: e.g., Sprint
…
to/from customers
peering
to/from backbone
….
………
POP: point-of-presence
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Stan Kurkovsky
Internet Structure: Network of NetworksInternet Structure: Network of Networks
•• ““TierTier--22”” ISPs: smaller (often regional) ISPsISPs: smaller (often regional) ISPs•• Connect to one or more tierConnect to one or more tier--1 ISPs, possibly other tier1 ISPs, possibly other tier--2 ISPs2 ISPs
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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 InternetTier-2 ISP is customer oftier-1 provider
Tier-2 ISPs also peer privately with each other.
Stan Kurkovsky
Internet Structure: Network of NetworksInternet Structure: Network of Networks
•• ““TierTier--33”” ISPs and local ISPs ISPs and local ISPs •• last hop (last hop (““accessaccess””) network (closest to end systems)) network (closest to end systems)
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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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Stan Kurkovsky
Internet Internet StructureStructure: : NetworkNetwork of of NetworksNetworks
•• a packet passes through many networksa packet passes through many networks