Computer Network (http://voip.csie.
org/CN2009/)
Instructor
Ai-Chun Pang , acpang@csie.ntu.edu.tw
Office Number: 417
Textbook
Computer Networking: A Top Down Approach Featuring the
Internet, Fourth edition, Jim Kurose and Keith Ross, AddisonWesley.
Requirements
Mid-term exam
Final exam
Programming Assignment x 3
30%
30%
40%
TA
, r97922031@ntu.edu.tw
, r97944023@ntu.edu.tw
TA Hour: Wednesday 10:00am~12:00pm, Office Number: 442
Introduction
1-1
Chapter 1: Introduction
Our goal:
Overview:
get feel and
whats the Internet
terminology
more depth, detail
later in course
approach:
use Internet as
example
whats a protocol?
network edge
network core
access net, physical media
Internet/ISP structure
performance: loss, delay and
throughput
protocol layers, service models
Introduction
1-2
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay, loss and throughput in packetswitched networks
1.7 Protocol layers, service models
1.8 History
Introduction
1-3
Whats the Internet: nuts and bolts view
millions of
connected
computing devices:
hosts = end
systems
running network
apps
communication
links
fiber, copper,
radio, satellite
transmission rate =
PC
Mobile network
server
wireless
laptop
cellular
handheld
Global ISP
Home network
Regional ISP
access
points
wired
links
Institutional network
bandwidth
routers: forward
packets (chunks of
data)
router
Introduction
1-4
Cool internet appliances
Web-enabled toaster +
weather forecaster
IP picture frame
http://www.ceiva.com/
Worlds smallest web server
http://www-ccs.cs.umass.edu/~shri/iPic.html
Internet phones
Introduction
1-5
Whats the Internet: nuts and bolts view
protocols control sending,
Mobile network
receiving of msgs
e.g., TCP, IP, HTTP, FTP, PPP
Internet: network of
networks
Global ISP
loosely hierarchical
public Internet versus
private intranet
Internet standards
RFC: Request for comments
IETF: Internet Engineering
Task Force
Home network
Regional ISP
Institutional network
Introduction
1-6
Whats the Internet: a service view
communication
infrastructure enables
distributed applications:
Web, email, games, ecommerce, file sharing
communication services
provided to apps:
Connectionless unreliable
connection-oriented
reliable
Introduction
1-7
Whats a protocol?
human protocols:
whats the time?
I have a question
introductions
specific msgs sent
specific actions taken
when msgs received,
or other events
network protocols:
machines rather than
humans
all communication
activity in Internet
governed by protocols
protocols define format,
order of msgs sent and
received among network
entities, and actions
taken on msg
transmission, receipt
Introduction
1-8
Whats a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
req
Hi
TCP connection
response
Got the
time?
Get http://www.awl.com/kurose-ross
2:00
<file>
time
Q: Other human protocols?
Introduction
1-9
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay, loss and throughput in packetswitched networks
1.7 Protocol layers, service models
1.8 History
Introduction
1-10
A closer look at network structure:
network edge:
applications and
hosts
network core:
routers
network of
networks
access networks,
physical media:
communication links
Introduction
1-11
The network edge:
end systems (hosts):
run application programs
e.g. Web, email
at edge of network
peer-peer
client/server model
client host requests, receives
service from always-on server
client/server
e.g. Web browser/server;
email client/server
peer-peer model:
minimal (or no) use of
dedicated servers
e.g. Skype, BitTorrent
Introduction
1-12
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
Internets connectionoriented service
TCP service [RFC 793]
reliable, in-order bytestream data transfer
flow control:
loss: acknowledgements
and retransmissions
sender wont overwhelm
receiver
congestion control:
senders slow down sending
rate when network
congested
Introduction
1-13
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
Apps using TCP:
HTTP (Web), FTP (file
transfer), Telnet
(remote login), SMTP
(email)
Apps using UDP:
streaming media,
teleconferencing, DNS,
Internet telephony
Introduction
1-14
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay, loss and throughput in packetswitched networks
1.7 Protocol layers, service models
1.8 History
Introduction
1-15
The Network Core
mesh of interconnected
routers
the fundamental
question: how is data
transferred through net?
circuit switching:
dedicated circuit per
call: telephone net
packet-switching: data
sent thru net in
discrete chunks
Introduction
1-16
Network Core: Circuit Switching
End-end resources
reserved for call
link bandwidth, switch
capacity
dedicated resources:
no sharing
circuit-like
(guaranteed)
performance
call setup required
Introduction
1-17
Network Core: Circuit Switching
network resources
(e.g., bandwidth)
divided into pieces
pieces allocated to calls
resource piece
dividing link bandwidth
into pieces
frequency division
time division
idle if
not used by owning call
(no sharing)
Introduction
1-18
Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
time
Introduction
1-19
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
500 msec to establish end-to-end circuit
Work it out!
Introduction
1-20
Network Core: Packet Switching
each end-end data stream
divided into packets
user A, B packets share
network resources
each packet uses full link
bandwidth
resources used as needed
Bandwidth division into pieces
Dedicated allocation
Resource reservation
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
Introduction
1-21
Packet Switching: Statistical Multiplexing
10 Mb/s
Ethernet
A
B
statistical multiplexing
1.5 Mb/s
queue of packets
waiting for output
link
Sequence of A & B packets does not have fixed
pattern statistical multiplexing.
Introduction
1-22
Packet switching versus circuit switching
Packet switching allows more users to use network!
1 Mb/s link
each user:
100 kb/s when active
active 10% of time
circuit-switching:
10 users
N users
1 Mbps link
packet switching:
with 35 users,
probability > 10 active
less than .0004
Introduction
1-23
Packet switching versus 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
Introduction
1-24
Packet-switching: store-and-forward
L
R
Takes L/R seconds to
transmit (push out)
packet of L bits on to
link (R bps)
Entire packet must
arrive at router before
it can be transmitted
on next link: store and
Example:
L = 7.5 Mbits
R = 1.5 Mbps
delay = 15 sec
forward
delay = 3L/R
Introduction
1-25
Packet-switched networks: forwarding
Goal: move packets through routers from source to
destination
well study several path selection (i.e. routing) algorithms
(chapter 4)
datagram network:
destination address in packet determines next hop
routes may change during session
analogy: driving, asking directions
virtual circuit network:
each packet carries tag (virtual circuit ID), tag
determines next hop
fixed path determined at call setup time, remains fixed
thru call
routers maintain per-call state
Introduction
1-26
Network Taxonomy
Telecommunication
networks
Circuit-switched
networks
FDM
TDM
Packet-switched
networks
Networks
with VCs
Datagram
Networks
Datagram network is not either connection-oriented
or connectionless.
Internet provides both connection-oriented (TCP) and
connectionless services (UDP) to apps.
Introduction
1-27
