CSci4211: Introduction 1 Chapter 1: Introduction What is a Network? What is Internet? Compared with postal service & telephone system Services provided “Nuts and Bolts” description 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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CSci4211: Introduction
1
Chapter 1: Introduction What is a Network? What is Internet?
Compared with postal service & telephone system Services provided “Nuts and Bolts” description
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!
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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
– 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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Service PerspectiveBasic 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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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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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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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
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
• 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 share network 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 allocationResource reservation
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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
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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 RL
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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Circuit Switching vs Packet Switching
Item 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)?
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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!
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
• 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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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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Internet Protocol “Zoo”ap
plic
atio
n
SMTP telnet, ssh
NFS/RPC
FTP, SCP
DNSHTTP
RealAudio RealVideo
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?
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Internet Structure
LANs
International lines
Regional or local
ISPlocal ISPs
company 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 eXcangePoints
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
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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
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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
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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
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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
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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/sA
B
propagation
transmission
nodalprocessing queueing
Note: s and R are very different quantitites!
Nodal delay
• dproc = processing delay– typically a few microsecs or less
• 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
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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
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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 capacity
Rs bits/sec
link capacity
Rc bits/sec pipe that can
carryfluid at rate
Rs bits/sec)
pipe that can carry
fluid at rate
Rc bits/sec)
server sends bits
(fluid) into pipe
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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
low error rate: repeaters spaced far apart ; immune to electromagnetic noiseCSci4211:
Introduction105
Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5,000 homes
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork (simplified)
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork
Channels
VIDEO
VIDEO
VIDEO
VIDEO
VIDEO
VIDEO
DATA
DATA
CONTROL
1 2 3 4 5 6 7 8 9
FDM:
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100 Mbps
100 Mbps
100 Mbps1 Gbps
server
Ethernetswitch
Institutionalrouter
To Institution’sISP
Ethernet Internet access
• Typically used in companies, universities, etc
10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet Today, end systems typically connect into
Ethernet switchCSci4211: Introduction
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Wireless access networks• shared wireless access network connects end system to router– via base station aka “access point”
• wireless LANs:– 802.11b/g (WiFi): 11 or 54 Mbps
• wider-area wireless access– provided by telco operator– ~1Mbps over cellular system (3G)– next up (?): WiMAX (10’s Mbps) over
wide area and LTE (100’s Mbps)
• satellite– Kbps to 45Mbps channel (or multiple
smaller channels)– 270 msec end-end delay– geosynchronous versus low altitude
basestation
mobilehosts
router
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Case Study: Home networks
Typical home network components: • DSL or cable modem• router/firewall/NAT• Ethernet• wireless access point
wirelessaccess point
wirelesslaptops
router/firewall
cablemodem
to/fromcable
headend
Ethernet
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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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Pre-Internet Modes of Human Telecommunications
The Dark Age before the Internet: before 1960Non-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 phonographElectrical:• 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
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Internet History
• 1970: ALOHAnet satellite network in Hawaii
• 1974: Cerf and Kahn - architecture for interconnecting networks
• 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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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