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Communication Networks
P. Demeester
Chapter 1
Computer Networks
and the Internet
Computer networking A top-down approach featuring the
internet
4th Edition, 2008
Addison Wesley
James F. Kurose, Keith W. Ross
ISBN 0-321-49770-8
Introduction
Part of slides provided by J.F Kurose and K.W. Ross, All Rights Reserved
1-1
Chapter 1: Introduction
Our goal:
 Introduction to computer networks
 Limit to >80% of the technologies used today
 Technologies : TCP+UDP, IP, Ethernet
 Describe applications (file transfer, e-mail ,
Web access,…)
Introduction
1-2
Chapter 1 outline
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 in packet-switched networks]
1.7 Protocol layers, service models
[1.8 History]
Introduction
1-3
Move towards all-IP
 VoIP  Skype
 Video over IP  Belgacom TV
 Internet of Things  Smart environments
 Operators are transforming
their network towards all IP
Introduction
1-4
BT's 21st Century Network : current network
Today:
multiple services
on multiple platforms
PSTN
PSTN
DPCN
Copper
KStream
DSL
CWSS
Fibre
DWSS
ATM
Inter-node
transmission
provided by
SDH/PDH
platforms
Other
Networks
IP
Mesh -SDH
MSH -SDH
ASDH
End
User
~5k
nodes
~2k
nodes
PDH
~1k ~400
~100
nodesnodes nodes
~15
nodes
Introduction
1-5
BT's 21st Century Network : Future
access
aggregation
core
Class 5
Call Server
Copper
WWW
DSL
GFP on SDH
Fibre &
Copper
End
User
Agg Box
~5k
nodes
IP-MPLS-WDM
PSTN services migrate to IP
Content
ISP
Other
Networks
~100
nodes
Introduction
1-6
What’s the Internet: “nuts and bolts” view
ISP
•
•
•
•
How to address the terminals ?
How to find them ?
How to route the information ?
…
router
workstation
server
mobile
ISP : Internet Service Provider
Introduction
1-7
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. Gnutella, KaZaA
Introduction
1-11
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
1-12
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
Keep in mind:
 bandwidth (bits per
second) of access
network?
 shared or dedicated?
Introduction
1-13
Network taxonomy
Telecommunication
networks
Circuit-switched
networks
FDM
TDM
Packet-switched
networks
Networks
with VCs
Datagram
Networks
Internet provides both connection-oriented (TCP) and
connectionless services (UDP) to apps.
Introduction
1-14
Chapter 1 outline
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 in packet-switched networks
1.7 Protocol layers, service models
1.8 History
Introduction
1-15
Internet structure: network of networks
 roughly hierarchical
 at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity,
Sprint, AT&T), national/international coverage
 treat each other as equals
Tier-1
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
NAP
Tier-1 providers
also interconnect
at public network
access points
(NAPs)
Tier 1 ISP
Introduction
1-16
Tier-1 ISP: e.g., Sprint
Sprint US backbone network
Introduction
1-17
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
NAP
Tier 1 ISP
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at NAP
Tier-2 ISP
Tier-2 ISP
Introduction
1-18
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
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Introduction
1-19
Internet structure: network of networks
 a packet passes through many networks!
local
ISP
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
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Introduction
1-20
Routers in the Internet : Global
In 2005 the number of (pubic) routers is in the 100.000 - 200.000 range.
The average number of hops a packet has to cross is in the Introduction
order of 13.1-21
Routers in the Internet : Zoom-In
Introduction
1-22
Routers in the Internet : Global
NSFNET
1992
www.cybergeography.org/atlas/geographic.html
Introduction
1-23
Routers in the Internet : Global
2001
Introduction
1-24
Routers in the Internet : Global
2000
Introduction
1-25
Chapter 1 outline
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 in packet-switched networks
1.7 Protocol layers, service models
1.8 History
Introduction
1-26
Protocol “Layers”
Networks are complex!
 many “pieces”:
 hosts
 routers
 links of various
media
 applications
 protocols
 hardware,
software
Question:
Is there any hope of
organizing the structure
of a network?
Or at least in our discussion
of networks?
Introduction
1-27
An example : postal services
writer
“application” service :
friend reads book
friend
they agree and
make a “connection”
they use the unreliable
postal services “network”
Introduction
1-28
An example : web access
find info about Internet
on the Web !
IETF web
page
URL: www.ietf.org
client
RFC 1011 : “Internet Protocols”
server
TCP-Connection
router
Internet Protocol
56.6 kbit/s
modem connection
local telephone
switch
2 Mbit/s
ISDN link
140 Mbit/s
PDH link
100 Gbit/s WDM
transatlantic optical cable
Introduction
1-29
10 Mbit/s
local area network
What’s a protocol ?
human protocols:
 “what’s 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, reception
Introduction
1-30
What’s 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
Introduction
1-31
Why layering ?
Dealing with complex systems:
 explicit structure allows identification of functions,
relationship between complex system’s possible
 layered reference model
 modularization eases maintenance and updating of
system
 change of implementation of layer’s service is
transparent to rest of system
 e.g., change in postal service implementation
doesn’t affect rest of system (as long as
interfaces are the same)
Introduction
1-32
Internet protocol stack
 application: supporting network
applications

FTP, SMTP, HTTP
 transport: host-host data transfer
 TCP, UDP
 network: routing of datagrams from
source to destination

IP, routing protocols
 data link: data transfer between
neighboring network elements

5. Application Layer
4. Transport Layer
3. Network Layer
2. Data Link Layer
1. Physical Layer
PPP, Ethernet
 physical: bits “on the wire”
Introduction
1-33
Layering: logical communication
Each layer:
 distributed
 “entities”
implement
layer functions
in the node
 entities
perform
actions,
exchange
messages with
peers
application
transport
network
data link
physical
application
transport
network
data link
physical
network
data link data link
physical physical
application
transport
network
data link
physical
application
transport
network
data link
physical
Introduction
1-34
Layering: logical communication
E.g.: transport
 take data from
application layer
 add addressing,
reliability check,
info to form
“segment”
 send segment to
peer
 wait for peer to
ack receip
data
application
transport
network
data link
physical
application
transport
network
data link
physical
network
data link data link
physical physical
application
transport
network
data link
physical
data
application
transport
network
data link
physical
Introduction
1-35
Layering: “physical” communication
data
application
transport
network
data link
physical
application
transport
network
data link
physical
network
data link data link
physical physical
application
transport
network
data link
physical
data
application
transport
network
data link
physical
Introduction
1-36
Application Layer : File Transfer Protocol
FTP
server
OPEN
connection
CLOSE
connection
TCP control connection
OK
Login, password
Request directory
Request file transfer
TCP transfer connection
directory
TCP transfer connection
file transfer
Introduction
1-37
Application/Transport Layer
Login, password
OK
application layer : FTP
read directory
directory
“peer to peer” communication
Request file transfer
FTP-server
client layer
SAP
file transfer
SAP
server layer
transport layer : TCP
SAP : Service Access point
Remark : client - server
Introduction
1-38
Transmission Control Protocol
TCP connection
 connection oriented








(virtual connection in software of terminals : state)
3-way handshake protocol
point to point, full duplex
exchange of segments (=unit of data)
(during a file transfer the file will be cut in pieces : segments)
reliable transport (=acknowledgement, retransmission, timers, …)
flow control (sender won’t overwhelm receiver)
congestion control
(senders “slow down sending rate” when network congested)
segments are sent over a network of routers in the IP layer
typically used for http, ftp, smtp, pop, …
Introduction
1-39
TCP connection setup
Setup TCP
connection
Server side
SYN : SYNchronization
ACK : ACKnowledgment
C : Client side
S : Server side
progressing time
3-way handshake
Client side
If set-up segment is lost
==> time-outs
Introduction
1-40
TCP connection for file transfer
0001-1000
TCP for file transfer
1001-2000
2001-3000
3001-4000
4001-5000
time-out
Introduction
1-41
User Datagram Protocol
UDP
audio
server
 very simple “protocol”
 alternative to complex TCP
 unreliable and connectionless
 no flow control or congestion control
 unidirectional
 no extra delays due to acknowledgments
 typically used for real-time and control applications
Introduction
1-42
UDP for real time audio transfer
0001-1000
1001-2000
2001-3000
UDP real time audio
• send as quick as possible
• no extra delay due to acknowledgment
• no retransmissions
3001-4000
4001-5000
Introduction
1-43
Transport/Network Layer
transport layer : TCP
TCP connection
client layer
IP datagrams
router
server layer
network layer : IP
SERVICE : Transfer TCP segment from source to destination
SOLUTION : Encapsulate TCP segment in IP datagram (destination indicated !).
IP network layer will transfer IP datagram over links and routers to destination.
Introduction
1-44
Encapsulation example: FT /TCP/IP
01101011101011010101
10101101010101110101
Data file
00110101011100010101
01101011100001010100
11111101010100000110
TCP segment
01101011101011010101 FTP
TCP
payload
IP datagram
TCP
header
01101011101011010101 FTP TCP, S-D
IP
payload
IP
header
FTP
IP
TCP
APPLICATION TRANSPORT NETWORK
LAYER
LAYER
LAYER
server
Introduction
1-45
Network Layer : Internet Protocol
C
Router
Link
W
A
Y
Routing
Table
Y:to B
Z:to D
Buffer
Y:to Y
B
Y:to C
Z:to E
Store
and
Forward
E
X
computer
host
terminal
Source:X
Destination:Y
Content:TCP
D
Z
Introduction
1-46
Network Layer : Internet Protocol
 Unidirectional
 Datagram (“packet”) based store and forward
 Connectionless
 Flexibility is provided by network elements (routers)
 Best effort : no guarantee on delay, delivery, …
(no Quality of Service : QoS)
 IP layer is the server layer of the TCP layer
and the client layer of the Data Link Layer
Introduction
1-47
Network/Data Link/Physical Layer
network layer : IP
client layer
point-to-point
server layer
shared
client layer
data link layer
twisted pair
fiber
Physical layer
server Introduction
layer
1-48
Data Link Layer
point-to-point example :
 PPP protocol (Point-to-Point Protocol)
 establish a connection between two routers or a router and
a terminal
 can use e.g. an SDH VC-4 150 Mbit/s connection between 2
routers or a telephone ISDN connection of 64 kbit/s
between a terminal at home and the access router of an
ISP.
shared medium example :
 Ethernet
 MAC : CSMA/CD
Les 1-2
Introduction
1-49
Summary
FTP
HTTP
DNS
TCP
...
SNMP
UDP
IP
CSMA/CD
PPP
...
coax
fiber
...
application
application
transport
transport
network
network
data link
data link data link
data link
data link
physical
physical physical
physical physical
physical
terminal
router
switch
network
Introduction
terminal1-50
Summary
FTP
20,21
HTTP
80
DNS
53
TCP
6
...
UDP
17
IP
157.193.
122.1
SNMP
161,162
Identification :
Application : port number
FTP port 20 (data), 21 (control)
TCP or UDP : protocol number
TCP : 6
UDP : 17
Host : IP address (e.g. 157.193.122.1)
Note : some terminology :
Application : message
Transport : segment
Network : datagram (or packet)
Data Link : frame
Introduction
1-51
Table of contents
1.1 What is the Internet?
 1.5 Internet structure and ISPs
 1.7 Protocol layers, service models
 Summary
 Table of contents

3
15
26
50
52
Introduction
1-52