Introduction - CSE Labs User Home Pages

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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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Introduction
1
Goal and Motivating Questions
Motivating Questions:
Our goal:
•
• get “feel” and
terminology
• more depth, detail •
later in course
•
• approach:
•
– use Internet as
example
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!
CSci4211:
Introduction
3
Inter-networking
 A network can be defined recursively as...
– two or more nodes
connected by a link, or

two or more networks
connected by two or
more nodes
 Internet: networks of networks

started as ARPAnet with only 4 nodes
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4
Map of Internet
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7
More gadgets are plugged in …
• servers, desktops, laptops, …
• smart mobile phones, iPads, e-readers, …
• now TVs, thermostats, smart meters,
etc., soon toasters, fridges, … 
Wireless technologies revolutionizing Internet!
 WiFi, bluetooth, 3/4G cellular networks, …
Low-tier
High-tier
mobile computing
location services
Local Area
Wide Area
High Mobility
CSci4211:
Low Mobility
Introduction
8
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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9
So what’s so special about the
Internet?
But first, what is a Network?
CSci4211:
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10
What is a Network?
There are many types of networks!
 Key Features of Networks


Providing certain services
•

Shared resources


used by many users, often concurrently
Basic building blocks
•
•

transport goods, mail, information or data
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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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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Nuts and Bolts Description
Network 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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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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Inter-networking
• A network can be defined recursively as...
– two or more nodes
connected by a link, or
– two or more networks
connected by two or more
nodes
• Internet: networks of networks
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15
Service Perspective
Basic Services Provided

Postal: deliver mail/package from people to people


Telephone: connect people for talking



First class, express mail, bulk rate, certified, registered, …
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 Networking
Network 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
•
How do we do it?
– Provide services for many people at same time
– Carry bits/information for many people at same time
– 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 hours!
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Analogy: railroad and train
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Analogy: Highway and cars
22
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Introduction
Circuit Switching
network 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?
23
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Introduction
Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
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time
Introduction
24
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 Switching
e.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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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 Switching
Each end-end “data stream”
divided into packets
• users A, B packets share
network resources
• each packet uses full link
bandwidth
• resources used as needed
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
CSci4211:
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!


Introduction
28
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
CSci4211:
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Packet Switching: Statistical Multiplexing
100 Mb/s
Ethernet
A
B
statistical multiplexing
C
1.5 Mb/s
queue of packets
waiting for output
link
D
E
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.
CSci4211:
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Packet-switching: store-and-forward
L
R
R
• 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
R
Example:
• L = 7.5 Mbits
• R = 1.5 Mbps
15 sec
• delay = ?
forward
• delay = 3L/R (assuming
zero propagation delay)
more on delay later …
31
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Introduction
Packet switching versus circuit switching
Packet switching allows more users to use network!
• 1 Mb/s link
• each user:
– 100 kb/s when “active”
N users
– active 10% of time
1 Mbps link
• circuit-switching:
– 10 users
• packet switching:
– with 35 users,
probability > 10 active
less than .0004
Q: how did we get value 0.0004?
M  n
M n



 n  p 1  p 
n  N 1 

M
32
CSci4211:
Introduction
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
CSci4211:
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Packet switching vs. circuit switching
Is packet switching a “slam dunk winner?”
• 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)
Q: human analogies of reserved resources (circuit
switching) versus on-demand allocation (packet-switching)?
34
CSci4211:
Introduction
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
–
“dumb” network (core): simple packet-switched, store-
–
“smart” end systems/edges: servers, PCs, mobile devices, …;
–
applications and accommodates evolving technologies
forward, connectionless “datagram” service, with core
functions: global addressing, routing & forwarding
diverse and ever-emerging new applications!
CSci4211:
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Internet Hourglass Architecture
enabling diverse applications
& new types of end devices
bitTorrent, DHT, SIP, DASH, ….
accommodating evolving
& new technologies
network core
network edge/end hosts
p2p file sharing, skype, YouTube,
Netflix, Cloud Computing
WiFi, Bluetooth,
Docsis, gMPLS,
DWDM/fiber, …,
3G/4G cellular,
….
CSci4211:
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“Dumb” Networks &
“Smart” End Systems
• Five Layer Architecture:
– Lower three layers are implemented everywhere
– Top two layers are implemented only at hosts
Host A
Host B
Application
Application
Transport
Transport
Router
Network
Datalink
Physical
Network
Datalink
Physical
Physical medium
Network
Datalink
Physical
37
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Introduction
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
38
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Introduction
What’s the Internet: “nuts and bolts” view
• millions of connected
computing devices: hosts
= end systems
• running network apps
• communication links
router
server
workstation
mobile
local ISP
– fiber, copper, radio,
satellite
– transmission rate =
regional ISP
bandwidth
• routers: forward packets
(chunks of data)
company
network
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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
40
CSci4211:
Introduction
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
41
CSci4211:
Introduction
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 connectionoriented service
CSci4211:
TCP service [RFC 793]
• reliable, in-order bytestream data transfer
– loss: acknowledgements
and retransmissions
• flow control:
– sender won’t overwhelm
receiver
• congestion control:
– senders “slow down sending
rate” when network
congested
Introduction
42
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
CSci4211:
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
Introduction
43
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?
44
CSci4211:
Introduction
On the Internet Edge …
• Large # of (mobile &
stationary) users
• Large # of “dumb” or
web
smart devices &
appliances
• Some “always-on,” highspeed connection
smart pads &
• Others intermittent e-readers
connectivity with
varying bandwidth
music
video streaming streaming
& IPTV
others
Internet
• Diverse applications
and services
• Heterogeneous
technologies
social networks
games
dumb &
smart phones POTS
home users
sensors &
smart home
surveillance
VoIP banking & & security
e-commerce
45
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Introduction
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
46
CSci4211:
Introduction
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
47
CSci4211:
Introduction
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
48
CSci4211:
Introduction
Organization of air travel
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
• a series of steps
49
CSci4211:
Introduction
Layering of airline functionality
ticket (purchase)
ticket (complain)
ticket
baggage (check)
baggage (claim
baggage
gates (load)
gates (unload)
gate
runway (takeoff)
runway (land)
takeoff/landing
airplane routing
airplane routing
airplane routing
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
arrival
airport
Layers: each layer implements a service
– via its own internal-layer actions
– relying on services provided by layer below
50
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Introduction
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
51
CSci4211:
Introduction
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
application
transport
network
link
physical
– PPP, Ethernet
• physical: bits “on the wire”
52
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Introduction
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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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!!!)
55
CSci4211:
Introduction
Human protocol
Alice
Hi
Bob
• protocols define:
– Format.
– Order of msgs sent and
received among network
entities (two or more)
– Actions taken on msg
transmission, receipt
Hi
Got the
time?
2:00pm
Q: What are the purposes of first hi-hi exchange
Make sure 1.
Bob is awake
Bob can understand
English
2
Bob can speak
3 English
Bob is willing
4 to talk
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What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
request
Hi
TCP connection
response
Got the
time?
Get http://www.cnn.com
2:00
<file>
time
Q: Other human protocols? (e.g., in-class interaction)
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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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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
CSci4211:
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Encapsulation
source
message
segment
M
Ht
M
datagram Hn Ht
M
frame Hl Hn Ht
M
application
transport
network
link
physical
link
physical
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
Hn Ht
Hl Hn Ht
application
transport
network
link
physical
M
M
network
link
physical
Hn Ht
M
router
CSci4211:
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Internet and
ISO/OSI Reference Models
CSci4211:
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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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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 Introduction
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”
Flash
DASH
RealAudio
SOAP
applicatio
n
IPTV
P2P
…..…..
HTTP
DNS
VoIP
RealVideo
NFS/RPC
FTP, SCP
telnet, ssh
SMTP
ICMP,
OSPF, RIP,
BGP, …
MPLS/gMPLS
PPP
802.11 WiFi
DWDM
DSL or
DOCSIS
CSci4211:
Introduction
2.5G/3G/4G
(GPRS,UMTS,
WiMAX, LTE,
…) Cellular
Radio Networks
65
What real Internet looks like now?
CSci4211:
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66
Internet Structure
Internet: “networks of networks”!
International
lines
IXPs
or private peering
National or
tier-1 ISP
National or
tier-1 ISP
Regional or
local ISP
company
university
Internet
eXcange
Points
Regional
ISPs
local ISPs
company
LANs
Home users
access via WiFi
hotspots
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
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
CSci4211:
IXP
Tier-1 providers
also interconnect
at Internet
Exchange Point
Tier 1 ISP
Introduction
68
Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone
peering
…
…
.
…
…
…
to/from customers
CSci4211:
Introduction
69
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-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
 tier-2 ISP is
customer of
tier-1 provider
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
CSci4211:
IXP
Tier 1 ISP
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at IXP
Tier-2 ISP
Tier-2 ISP
Introduction
70
Internet structure: network of networks
• “Tier-3” ISPs and local ISPs
– last hop (“access”) network (closest to end systems)
local
ISP
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
CSci4211:
IXP
Tier 1 ISP
Tier-2 ISP
local
ISP
Introduction
Tier-2 ISP
local
ISP
71
Internet structure: network of networks
• a packet passes through many networks!
traceroute www.cnn.com
A
local
ISP
Routing &
forwarding:
how do
packets go
from A to B?
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
CSci4211:
IXP
Tier 1 ISP
Tier-2 ISP
local
ISP
Introduction
Tier-2 ISP
local
ISP
B
72
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
CSci4211:
Introduction
74
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
CSci4211:
Introduction
75
Four sources of packet delay
1. nodal processing:
2. queueing
• check bit errors
• determine output link
•
•
time waiting at output link
for transmission
depends on congestion level
of router
transmission
A
propagation
B
nodal
processing
queueing
CSci4211:
Introduction
76
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
transmission
A
propagation
B
nodal
processing
queueing
CSci4211:
Note: s and R are very
different quantitites!
Introduction
77
Nodal delay
dnodal  dproc  dqueue  dtrans  dprop
• 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
CSci4211:
Introduction
78
Statistical Multiplexing and Queueing
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
45 Mbs
D
CSci4211:
E
Introduction
79
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!
CSci4211:
Introduction
80
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
CSci4211:
Introduction
81
“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
CSci4211:
Introduction
82
“Real” Internet delays and routes
Let’s Traceroute to www.bbc.com
CSci4211:
Introduction
83
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
link
capacity
that
can carry
server,
with
server
sends
bits pipe
Rs bits/sec
fluid
at rate
file of
F bits
(fluid)
into
pipe
Rs bits/sec)
to send to client
CSci4211:
link that
capacity
pipe
can carry
Rfluid
c bits/sec
at rate
Rc bits/sec)
Introduction
84
Throughput (cont’d)
• Rs < Rc What is average end-end throughput?
Rs bits/sec
 Rs
Rc bits/sec
> Rc What is average end-end throughput?
Rs bits/sec
Rc bits/sec
bottleneck link
link on end-end path that constrains end-end throughput
CSci4211:
Introduction
85
Throughput: Internet scenario
• per-connection
end-end
throughput:
min(Rc,Rs,R/10)
• in practice: Rc or
Rs is often
bottleneck
CSci4211:
Rs
Rs
Rs
R
Rc
Rc
Rc
10 connections (fairly) share
backbone bottleneck link R bits/sec
Introduction
86
What’s the Internet: Recap
• protocols control sending,
receiving of messages
router
server
– e.g., TCP, IP, HTTP, FTP, PPP
• Internet: “network of
networks”
workstation
mobile
local ISP
– loosely hierarchical
– public Internet versus
private intranet
regional ISP
• Internet standards
– RFC: Request for comments
– IETF: Internet Engineering
Task Force
– IEEE
CSci4211:
company
network
Introduction
87
Fundamental Issues in Networking
Network 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
CSci4211:
Introduction
88
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
CSci4211:
Introduction
89
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/nodes
Goal: to fill the gap between what applications
expect and what underlying technology provides
CSci4211:
Introduction
90
The Internet Network layer
Transport layer: TCP, UDP
Network
layer
IP protocol
•addressing conventions
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF, BGP
routing
table
ICMP protocol
•error reporting
•router “signaling”
Data Link layer (Ethernet, WiFi, PPP, …)
Physical Layer (fiber optics, radio, …)
CSci4211:
Introduction
91
Introduction: Summary
Answers 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?
CSci4211:
You now have:
• context, overview,
“feel” of
networking
• more depth, detail
Introduction
to follow!
92
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
CSci4211:
Introduction
93
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
CSci4211:
Introduction
94
Supplementary Readings
•
•
•
•
•
How big is Internet, Who pays for it?
Access Network Technologies
NAPs, Private Peering and ISPs
Internet “Governing” Bodies
History of Internet
CSci4211:
Introduction
95
Access networks and physical media
Q: How to connect end
systems to edge router?
• residential access nets
• institutional access
networks (school,
company)
• mobile access networks
CSci4211:
Introduction
96
Physical Media
• physical link:
transmitted data bit
propagates across link
• guided media:
– signals propagate in solid
media: copper, fiber
Twisted Pair (TP)
• two insulated copper
wires
– Category 3: traditional
phone wires, 10 Mbps
Ethernet
– Category 5 TP: 100Mbps
Ethernet
• unguided media:
– signals propagate freely
e.g., radio
CSci4211:
Introduction
97
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
CSci4211:
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
Introduction
98
Physical media: radio
• signal carried in
electromagnetic
spectrum
• no physical “wire”
• bidirectional
• propagation
environment effects:
– reflection
– obstruction by objects
– interference
CSci4211:
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
Introduction
99
Access networks
Q: How to connection 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?
CSci4211:
Introduction
100
Residential access: Dial-up Modem
central
office
home
PC



telephone
network
home
dial-up
modem
Internet
ISP
modem
(e.g., AOL)
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”
CSci4211:
Introduction
101
Residential access: Digital Subscriber Line (DSL)
Existing phone line:
0-4KHz phone; 4-50KHz
upstream data; 50KHz-1MHz
downstream data
home
phone
Internet
DSLAM
telephone
network
splitter
DSL
modem
home
PC
central
office
Also uses existing telephone infrastruture
 up to 1 Mbps upstream (today typically < 256 kbps)
 up to 8 Mbps downstream (today typically < 1 Mbps)
 dedicated physical line to telephone central office

CSci4211:
Introduction
102
Residential access: cable modems
• HFC: hybrid fiber coax
– asymmetric: up to 30Mbps downstream, 2 Mbps
upstream
• Network of cable and fiber attaches homes to
ISP router
– homes share access to router
• Deployment: available via cable TV companies
– Comcast triple play: Internet, vioce and TV
CSci4211:
Introduction
103
Residential access: cable modems
CSci4211:
Introduction
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
104
Physical Media used in HFC
Coaxial cable:
Fiber optic cable:
• two concentric copper
conductors
• bidirectional
• baseband:
 glass fiber carrying light
• broadband:
 low error rate: repeaters
– single channel on cable
– legacy Ethernet
– multiple channels on
cable
– HFC
CSci4211:
pulses, each pulse a bit
 high-speed operation:

high-speed point-to-point
transmission (e.g., 10’s100’s Gps)
spaced far apart ; immune
to electromagnetic noise
Introduction
105
Cable Network Architecture: Overview
Typically 500 to 5,000 homes
cable headend
cable distribution
network (simplified)
CSci4211:
home
Introduction
106
Cable Network Architecture: Overview
cable headend
cable distribution
network (simplified)
CSci4211:
home
Introduction
107
Cable Network Architecture: Overview
FDM:
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
Channels
cable headend
cable distribution
network
CSci4211:
home
Introduction
108
Ethernet Internet access
100 Mbps
Institutional
router
Ethernet
switch
To Institution’s
ISP
100 Mbps
1 Gbps
100 Mbps
server
• Typically used in companies, universities, etc
 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet
 Today, end systems typically connect into Ethernet switch
CSci4211:
Introduction
109
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
router
base
station
– 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
CSci4211:
Introduction
mobile
hosts
110
Case Study: Home networks
Typical home network components:
• DSL or cable modem
• router/firewall/NAT
• Ethernet
• wireless access point
to/from
cable
headend
cable
modem
wireless
laptops
router/
firewall
Ethernet
CSci4211:
Introduction
wireless
access
point
111
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
CSci4211:
Introduction
112
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
•
•
•
•
1830: telegraph
1876: circuit-switching (telephone)
1896: radio
TV (1940?) , and later cable TV (1970s)
Electrical:
CSci4211:
Introduction
113
Internet History
1961-1972: Early packet-switching principles
• 1961: Kleinrock - queueing
theory shows
effectiveness of packetswitching
• 1964: Baran - packetswitching in military nets
• 1967: ARPAnet conceived
by Advanced Research
Projects Agency
• 1969: first ARPAnet node
operational
CSci4211:
• 1972:
– ARPAnet public demonstration
– NCP (Network Control Protocol)
first host-host protocol
– first e-mail program
– ARPAnet has 15 nodes
Introduction
114
Internet History
1972-1980: Internetworking, new and proprietary nets
• 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
CSci4211:
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
Introduction
115
Internet History
1980-1990: new protocols, a proliferation of networks
• 1983: deployment of
TCP/IP
• 1982: smtp e-mail
protocol defined
• 1983: DNS defined
for name-to-IPaddress translation
• 1985: ftp protocol
defined
• 1988: TCP congestion
control
CSci4211:
• new national networks:
Csnet, BITnet,
NSFnet, Minitel
• 100,000 hosts
connected to
confederation of
networks
Introduction
116
Internet History
1990, 2000’s: commercialization, the Web, new apps
• 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
CSci4211:
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!)
Introduction
117
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, …
CSci4211:
Introduction
118
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)
CSci4211:
Introduction
119
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
– ….
CSci4211:
Introduction
120
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