代写代考 Chapter 1: introduction

Chapter 1: introduction
Chapter goal: Overview/roadmap:
▪Get “feel,” “big picture,” introduction to terminology
• more depth, detail later in course

▪ Approach:
• use Internet as example
▪ What is the Internet?
▪ What is a protocol?
▪ Network edge: hosts, access network, physical media
▪ Network core: packet/circuit switching, internet structure
▪ Performance: loss, delay, throughput ▪ Security
▪ Protocol layers, service models
Introduction: 1-1

The Internet: a “nuts and bolts” view
Billions of connected computing devices:
▪ hosts = end systems
▪ running network apps at Internet’s “edge”
Packet switches: forward packets (chunks of data)
▪ routers, switches Communication links
▪ fiber, copper, radio, satellite ▪ transmission rate: bandwidth
▪ collection of devices, routers, links: managed by an organization
mobile network
home network
enterprise network
national or global ISP
local or regional ISP
content provider network
datacenter network
Introduction: 1-2

“Fun” Internet-connected devices
Pacemaker & Monitor
sensorized, bed mattress
Amazon Echo
Internet refrigerator
Security Camera
Internet phones
IP picture frame
Slingbox: remote control cable TV
Tweet-a-watt: monitor energy use
Web-enabled toaster + weather forecaster
AR devices
Introduction: 1-3

The Internet: a “nuts and bolts” view
▪ Internet: “network of networks” • Interconnected ISPs
▪ protocols are everywhere
• control sending, receiving of
• e.g., HTTP (Web), streaming video, Skype, TCP, IP, WiFi, 4G, Ethernet
▪Internet standards
• RFC: Request for Comments
• IETF: Internet Engineering Task Force
mobile network 4G
national or global ISP
local or regional ISP
Streaming video
datacenter network
home network
content provider network
enterprise network
Introduction: 1-4

The Internet: a “service” view
▪ Infrastructure that provides services to applications:
• Web, streaming video, multimedia teleconferencing, email, games, e- commerce, social media, inter- connected appliances, …
▪ provides programming interface to distributed applications:
• “hooks” allowing sending/receiving apps to “connect” to, use Internet transport service
• provides service options, analogous to postal service
mobile network
national or global ISP
Streaming video
datacenter network
local or regional ISP
home network
enterprise network
content provider network
Introduction: 1-5

What’s a protocol?
Human protocols:
▪ “what’s the time?” ▪ “I have a question” ▪ introductions
… specific messages sent
… specific actions taken when message received, or other events
Network protocols:
▪ computers (devices) rather than humans
▪ all communication activity in Internet governed by protocols
Protocols define the format, order of messages sent and received among network entities, and actions taken
on msg transmission, receipt
Introduction: 1-6

What’s a protocol?
A human protocol and a computer network protocol:
Got the time?
TCP connection request
TCP connection response
GET http://gaia.cs.umass.edu/kurose_ross
Q: other human protocols?
Introduction: 1-7

Chapter 1: roadmap
▪ What is the Internet?
▪ What is a protocol?
▪ Network edge: hosts, access network, physical media
▪ Network core: packet/circuit switching, internet structure
▪ Performance: loss, delay, throughput
▪ Security
▪ Protocol layers, service models
Introduction: 1-8

A closer look at Internet structure
mobile network
Network edge:
▪hosts: clients and servers ▪servers often in data centers
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
datacenter network
Introduction: 1-9

A closer look at Internet structure
mobile network
Network edge:
▪hosts: clients and servers ▪servers often in data centers
Access networks, physical media:
▪wired, wireless communication links
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
datacenter network
Introduction: 1-10

A closer look at Internet structure
mobile network
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
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
datacenter network
Introduction: 1-11

Access networks and physical media
Q: How to connect end systems to edge router?
▪ residential access nets
▪ institutional access networks (school,
▪ mobile access networks (WiFi, 4G/5G)
What to look for:
▪ transmission rate (bits per second) of access network?
▪ shared or dedicated access among users?
mobile network
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
datacenter network
Introduction: 1-12

Access networks: cable-based access
cable headend
cable splitter
O VVVVVV N IIIIIIDDT DDDDDDAAR EEEEEETTO OOOOOOAAL
123456789 Channels
frequency division multiplexing (FDM): different channels transmitted in different frequency bands
Introduction: 1-13

Access networks: cable-based access
cable splitter
data, TV transmitted at different frequencies over shared cable
distribution network
cable headend
cable modem termination system
▪ HFC: hybrid fiber coax
• asymmetric: up to 40 Mbps – 1.2 Gbs downstream transmission rate, 30-100 Mbps
upstream transmission rate
▪ network of cable, fiber attaches homes to ISP router
• homes share access network to cable headend
Introduction: 1-14

Access networks: digital subscriber line (DSL)
DSL splitte modem
voice, data transmitted at different frequencies over dedicated line to central office
central office
DSL access multiplexer
telephone network
▪ 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
▪24-52 Mbps dedicated downstream transmission rate ▪3.5-16 Mbps dedicated upstream transmission rate
Introduction: 1-15

Access networks: home networks
wireless devices
often combined in single box
WiFi wireless access point (54, 450 Mbps)
to/from headend or central office
cable or DSL modem
router, firewall, NAT wired Ethernet (1 Gbps)
Introduction: 1-16

Wireless access networks
Shared wireless access network connects end system to router ▪ via base station aka “access point”
Wireless local area networks (WLANs)
▪ typically within or around building (~100 ft)
▪ 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate
to Internet
Wide-area cellular access networks
▪ provided by mobile, cellular network operator (10’s km)
▪ 10’s Mbps
▪ 4G cellular networks (5G coming)
to Internet
Introduction: 1-17

Access networks: enterprise networks
Ethernet switch
Enterprise link to
ISP (Internet) institutional router
institutional mail, web servers
▪ companies, universities, etc.
▪ mix of wired, wireless link technologies, connecting a mix of switches
and routers (we’ll cover differences shortly)
▪ Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps ▪ WiFi: wireless access points at 11, 54, 450 Mbps
Introduction: 1-18

Host: sends packets of data
host 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
two packets, L bits each
R: link transmission rate = L (bits)
R (bits/sec)
21 • link transmission rate, aka link host
capacity, aka link bandwidth
packet = transmission
time needed to transmit L-bit packet into link
Introduction: 1-19

Links: physical media
▪ bit: propagates between transmitter/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 • Category6:10GbpsEthernet
Introduction: 1-20

Links: physical media
Coaxial cable:
▪ two concentric copper conductors
▪ bidirectional
▪ broadband:
• multiple frequency channels on cable • 100’sMbpsperchannel
Fiber optic cable:
▪ glass fiber carrying light pulses, each pulse a bit
▪ high-speed operation:
• high-speed point-to-point
transmission (10’s-100’s Gbps) ▪ low error rate:
• repeaters spaced far apart
• immune to electromagnetic noise
Introduction: 1-21

Links: physical media
Wireless radio
▪signal carried in electromagnetic spectrum
▪no physical “wire” ▪broadcast and “half-duplex”
(sender to receiver)
▪propagation environment effects:
• reflection
• obstruction by objects • interference
Radio link types:
▪terrestrial microwave • up to 45 Mbps channels
▪ Wireless LAN (WiFi) • Up to 100’s Mbps
▪ wide-area (e.g., cellular) • 4G cellular: ~ 10’s Mbps
▪ satellite
• up to 45 Mbps per channel • 270 msec end-end delay
• geosynchronous versus low- earth-orbit
Introduction: 1-22

The network core
▪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
mobile network
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
datacenter network
Introduction: 1-24

Packet-switching: store-and-forward
per packet
source 3 2 1 destination R bps R bps
▪ Transmission delay: takes L/R seconds to transmit (push out) L-bit packet into link at R bps
▪ Store and forward: entire packet must arrive at router before it can be transmitted on next link
▪ End-end delay: 2L/R (above), assuming zero propagation delay (more on delay shortly)
One-hop numerical example:
▪L = 10 Kbits
▪R = 100 Mbps
▪ one-hop transmission delay = 0.1 msec
Introduction: 1-25

Packet-switching: queueing delay, loss
R = 100 Mb/s A
R = 1.5 Mb/s queue of packets
waiting for output link
Packet queuing and loss: if arrival rate (in bps) to link exceeds transmission rate (bps) of link for a period of time:
▪packets will queue, waiting to be transmitted on output link
▪packets can be dropped (lost) if memory (buffer) in router fills up
Introduction: 1-26

Two key network-core functions
routing algorithm
locall forwarding table
header value
output link
0100 0101 0111 1001
Forwarding:
▪local action: move arriving packets from router’s input link to appropriate router output link
destination address in arriving packet’s header
▪ global action: determine source- destination paths taken by packets
▪ routing algorithms
Introduction: 1-27

Alternative to packet switching: circuit switching
end-end resources allocated to, reserved for “call” between source and destination
▪ in diagram, each link has four circuits. • call gets 2nd circuit in top link and 1st
circuit in right link. ▪dedicated resources: no sharing
• circuit-like (guaranteed) performance ▪ circuit segment idle if not used by call (no
▪ commonly used in traditional telephone networks
Introduction: 1-28

Circuit switching: FDM and TDM
Frequency Division Multiplexing (FDM)
▪ optical, electromagnetic frequencies divided into (narrow) frequency bands
▪ each call allocated its own band, can transmit at max rate of that narrow band
Time Division Multiplexing (TDM)
▪time divided into slots
▪ each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band, but only during its time slot(s)
Introduction: 1-29

Packet switching versus circuit switching
packet switching allows more users to use network!
▪ 1 Gb/s link
▪ each user:
• 100 Mb/s when “active” • active 10% of time
▪circuit-switching: 10 users
▪packet switching: with 35 users, probability > 10 active at same time is less than .0004 *
1 Gbps link
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive
Q: how did we get value 0.0004? Q: what happens if > 35 users ?
Introduction: 1-30

Packet switching versus circuit switching
Is packet switching a “slam dunk winner”?
▪ great for “bursty” data – sometimes has data to send, but at other times not • resource sharing
• simpler, no call setup
▪ excessive congestion possible: packet delay and loss due to buffer overflow • protocols needed for reliable data transfer, congestion control
▪ Q: How to provide circuit-like behavior?
• bandwidth guarantees traditionally used for audio/video applications
Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet switching)?
Introduction: 1-31

Internet structure: a “network of networks”
▪ Hosts connect to Internet via access Internet Service Providers (ISPs)
• residential, enterprise (company, university, commercial) ISPs ▪ Access ISPs in turn must be interconnected
• so that any two hosts can send packets to each other ▪ Resulting network of networks is very complex
• evolution was driven by economics and national policies
▪ Let’s take a stepwise approach to describe current Internet structure
Introduction: 1-32

Internet structure: a “network of networks”
Question: given millions of access ISPs, how to connect them together?
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
Introduction: 1-33

Internet structure: a “network of networks”
Question: given millions of access ISPs, how to connect them together?
access net
access net
access net
access net
access net
access net
access net
access net
access net
connecting each access ISP to each other directly doesn’t scale: O(N2) connections.
access net
access net
access net
access net
access net
access access net
Introduction: 1-34

Internet structure: a “network of networks”
Option: connect each access ISP to one global transit ISP? Customer and provider ISPs have economic agreement.
access net
access net
access net
access net
access net
access net
access net
access net
access net
global ISP
access net
access net
access net
access net
access net
access net
access net
Introduction: 1-35

Internet structure: a “network of networks”
But if one global ISP is viable business, there will be competitors ….
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
Introduction: 1-36

Internet structure: a “network of networks”
But if one global ISP is viable business, there will be competitors …. who will
want to be connected
access net
access net
Internet exchange point
access net
access net
access net
access net
access net
access net
peering link
access net
access net
access net
access net
access net
access net
access net
access net
Introduction: 1-37

Internet structure: a “network of networks”
… and regional networks may arise to connect access nets to ISPs
access net
access net
access net
access net
access net
access net
access net
access net
regional ISP
access net
access net
access net
access net
access net
access access net
access net
Introduction: 1-38

Internet structure: a “network of networks”
… and content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users
access net
access net
access net
access net
access net
access net
access net
access net
Content provider network
regional ISP
access net
access net
access net
access net
access net
access access net
access net
Introduction: 1-39

Internet structure: a “network of networks”
Tier 1 ISP Tier 1 ISP
Regional ISP
access access access
ISP ISP ISP ISP
Regional ISP
access access access ISP ISP ISP
access ISP
At “center”: small # of well-connected large networks
▪ “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage
▪ content provider networks (e.g., Google, Facebook): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs
Introduction: 1-40

Tier-1 ISP Network map: Sprint (2019)
POP: point-of-presence
to/from other Sprint PoPS
links to peering networks
links to/from Sprint customer networks
Introduction: 1-41

How do packet loss and delay occur?
packets queue in router buffers
▪ packets queue, wait for turn
▪ arrival rate to link (temporarily) exceeds output link capacity: packet loss
packet being transmitted (transmission delay)
packets in buffers (queueing delay)
free (available) buffers: arriving packets dropped (loss) if no free buffers
Introduction: 1-43

Packet delay: four sources
transmission
propagation
processing queueing
dnodal = dproc + dqueue + dtrans + dprop
dproc: nodal processing ▪ check bit errors
▪ determine output link ▪ typically < msec dqueue: queueing delay ▪ time waiting at output link for transmission ▪ depends on congestion level of router Introduction: 1-44 Packet delay: four sources propagation processing queueing transmission dnodal = dproc + dqueue + dtrans + dprop dtrans: transmission delay: ▪ L: packet length (bits) ▪ R: link transmission rate (bps) ▪dtrans = L/R d and d trans dprop: propagation delay: ▪ d: length of physical link ▪ s: propagation speed (~2x108 m/sec) ▪dprop = d/s * Check out the online interactive exercises: http://gaia.cs.umass.edu/kurose_ross very different Introduction: 1-45 Caravan analogy 100 km (aka 10-bit packet) (aka router) ▪ cars “propagate” at 100 km/hr ▪ toll booth takes 12 sec to service car (bit transmission time) ▪ car ~ bit; caravan ~ packet ▪ Q: How long until caravan is lined up before 2nd toll booth? ten-car caravan toll booth toll booth ▪ time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec ▪ time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr) = 1 hr ▪ A: 62 minutes Introduction: 1-46 Caravan ana 程序代写 CS代考加微信: powcoder QQ: 1823890830 Email: powcoder@163.com

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