Computer Networks and the Internet
Learning Outcomes
159.334
At the end of this session, the students should be able to:
Explain what the Internet is all about
Explain what is a protocol
Describe what comprises the network edge
Describe what comprises the network core
Explain connection-oriented service
Explain connectionless service
Compare circuit-switched network against
packet-switched network
Answer the short exercises given in the session
Introduction
What’s the Internet?
UNIX-based
workstations
laptop
Digital cameras
Automobile
Email server
Web-page server
WebTV
PDAs with wireless
Internet connections
toaster
Household appliances
HOSTS or END SYSTEMS
What’s the Internet?
Nuts and bolts of the Internet
• Hardware components
• Software
Networking Infrastructure
• provides services to distributed
applications
• infrastructure where new applications
are being constantly invented and
deployed
What’s the Internet?
“Nuts and Bolts” View
 global network of networks
 Interconnects hundreds of millions of computing
devices
 provides:
• Global communication
• Storage
• Computation infrastructure
End-to-End System:
End System
“edge”
Core
End System
“edge”
SETI@HOME—MASSIVELY
DISTRIBUTED COMPUTING FOR SETI
What is SETI@home?
SETI@home is a
scientific experiment that
uses Internet-connected
computers in the Search
for Extraterrestrial
Intelligence (SETI). You
can participate by
running a free program
that downloads and
analyzes radio telescope
data.
http://setiathome.ssl.berkeley.edu/
The Network Structure
• network edge:
applications and
hosts
• network core:
– routers
– network of
networks
• access networks,
physical media:
communication
links
What’s the Internet? “Nuts and Bolts” View
End-to-End System
End System=HOST
End System
Core
End System
Access networks
Physical media
Where much internet
architecture
complexity is placed
Communication links
Communication
Transport data
links
Switches
Characterized
of bandwidth
Made up in
of terms
different
types of physical media:
Link transmission
Coaxial cablerate
Measured
in bits/second
Copper
wire
Fiber optics
Radio Spectrum
Connect end
systems to the
network core
Where is the Network Core?
?
NETWORK CORE
Sender
End System
Router
End System
X
End System Receiver
End System
path or route
What’s in the Links?
End System
Router
Internet uses packet switching to allow
Takes a chunk of information arriving on one of
for multiple communicating end systems
its incoming communication links and forwards
tothat
share
a path,
or partson
ofone
a path,
the
chunk
of information
of its at
outgoing
same
time
communication
links
packet
What’s a protocol?
a human protocol and a computer network
protocol:
TCP connection request
Hi
Hi
TCP connection reply
Got the time?
Get http://www.massey.ac.nz/
2:00
<file>
time
What’s a protocol?
Human Protocols:
Network Protocols:
• Something we execute all
the time
• Offer a greeting
• Wait for a response
• Analyze the response
• Act accordingly
• Similar to human protocol,
except that entities are
machines rather than
humans
• all communication activities
in the Internet are governed
by protocols
In order for protocols to work, both entities must observe the same protocol.
• There is a set of conventional actions taken when messages are sent and
received.
Networking – understanding the what, why and how of networking
protocols
What’s a protocol?
A protocol defines:
ofthat
messages
* Allformat
activitiesand
in theorder
Internet
involves 2 orsent
moreand
communicating
entities are
governed entities
by a protocol.
received among
network
and
taken
There
areactions
protocols
in: on the transmission
and/or receipt of a message, or other
• Routers
Protocols determine a packet’s path from source to
event
destination
• NIC
hardware-implemented protocols control the flow of
Communicating
Entities:
the bits on the “wire”
• End
Systems Software components
Hardware,
congestion-control protocols control the rate at which
packets are transmitted between sender and receiver
Different protocols are used to accomplish different
communication tasks:
What’s the Internet? “nuts & bolts” view
Protocols
- control the sending and receiving of information within the Internet
- run by End Systems, routers, etc.;
TCP
IP
Two of the most important protocols in the Internet
(principal protocols)
TCP – Transmission Control Protocol
IP – Internet Protocol – specifies the format of the packets that are sent
and received among routers and end systems
INTERNET STANDARDS
Made possible through standards developed by (IETF) Internet
Engineering Task Force
RFCs (Request for Comments)
define protocols such as TCP, IP, HTTP, SMTP
What’s the Internet? A Service View
• Provides a communication infrastructure that allows distributed
applications running on its end systems to exchange data with each
other.
Remote login
email
Web surfing
Instant messaging
Internet telephony
“the Web” – distributed application that use the communication services
provided by the Internet
• Communication services provided to distributed applications:
Connection-Oriented Reliable Service
Guarantees that data is delivered orderly and completely
(sender to receiver)
Connectionless Unreliable Service
Delivery is not guaranteed
Question
Why would we opt for a
connectionless unreliable
service when there is a
connection-oriented reliable
service that is available?
Hold on to that thought for a while…
?
A closer look at the Network Edge
What happens in the network edge?
The sending End System
doesn’t know how messages
are actually sent.
It only needs to know what services
are provided, and so the “nuts and
bolts” of the Internet serves as a
“black box” that transfers messages
between distributed communicating
components.
There is some level of
abstraction that hides the nittygritty part of the communication
process between two end
Client/Server Model - Most prevalent structure for Internet applications; although not
systems
all applications are purely client, or purely of server type (e.g. P2P file sharing)
A closer look at the Network Edge
What happens in the network edge?
End Systems (Hosts):
• run application program
• e.g., WWW, email
• at “edge of network”
 Client/Server Model
• client host requests, receives
service from server
• e.g. WWW client (browser)/server;
email client/server
Client/Server Model - Most prevalent structure for Internet applications; although not
all applications are purely client, or purely of server type (e.g. P2P file sharing)
The Network Edge
“Connection” between two End
Systems: (e.g. Web application or
Internet phone application)
– Nothing more than allocated
buffers and state variables in
the End-Systems
Internet provides two type of services
to End-System Applications:
1. Connection-oriented service – (TCP)
App’s using TCP:
HTTP (WWW), FTP (file transfer), Telnet
(remote login), SMTP (email)
2. Connectionless service – (UDP)
App’s using UDP:
streaming media, teleconferencing,
Internet telephony
“Connection”
Network edge: connection-oriented service
performs handshaking
* Goal: data transfer between
Q
end system.
• handshaking: setup
(prepare for) data
transfer ahead of time
– Hello, hello back human
protocol
– set up “state” in two
communicating hosts
Transmission Control
Protocol (TCP)
• Internet’s connection-oriented
service
TCP service [RFC 793]
Provides:
• Reliable data transfer:
– loss: handled using
acknowledgements and
retransmissions
• Flow Control:
– Ensures that the sender
won’t overwhelm receiver
• Congestion Control:
– Instructs senders to “slow
down sending rate” when
network is congested
– Prevents gridlock
Network Edge: TCP Service
3-way Handshake
Control packet
CONNECTION ESTABLISHED
DATA
acknowledgement
request
CLIENT
*
SERVER
Reliable data transfer is achieved through acknowledgements
and retransmissions
Data is delivered without error and in proper order
Network Edge: TCP Service
Handshaking Procedure:
Case: Retransmission of Request
Control packet
Client is packet
waitingwas
for Acknowledgement
Client assumes
lost, decides to retransmit
DATA
acknowledgement
CLIENT
*
SERVER
Reliable data transfer is achieved through acknowledgements
and retransmissions
Data is delivered without error and in proper order
Network Edge: TCP Service
Problem occurs when one communicating End-System transmits faster than the
other End-System
This End-System does not
receive an acknowledgement
yet, and so it issues another
packet
Control packet
CLIENT
CLIENT
CLIENT
CLIENT
SERVER
Flow control
forces the sending End System not to send too many packets too fast
for the receiver
TCP/IP provides the Flow control service
Network Edge: TCP Service
Problem: Gridlock sets-in when there is packet loss due to router congestion
CLIENT
The sending system’s message is lost due to
congestion, and is alerted when it stops
receiving acknowledgements of packets sent
SERVER
Congestion control
forces the End Systems to decrease the rate at which packets are sent during
periods of congestion
Network edge: connectionless service
No handshaking procedure; End-Systems just simply send the packet
Goal: data transfer between end
systems
– same as before!
• UDP - User Datagram Protocol
– [RFC 768]: Internet’s connectionless
service
– unreliable data transfer
– no flow control
– no congestion control
Something to ponder on
?
Transmission rate of the link (Bandwidth) – how many
bits per second a network can transport
Propagation delay (Latency) – how many seconds it
takes for the first bit to get from the client to the server
Besides bandwidth and latency, what
other parameter is needed to give a
good characterization of the quality
of service offered by a network used
for digitized voice traffic?
Answer
A uniform delivery time is needed for voice, so the
amount of jitter in the network is important. This could
be expressed as the standard deviation of the delivery
time. Having short delay but large variability is actually
worse than a somewhat longer delay and low
variability.
The Network Core
• mesh of interconnected routers
The Network Core
• the fundamental question:
how is data transferred
through net?
Approaches to building a Network Core:
– circuit switching:
dedicated circuit per
call: telephone net
– packet-switching:
data sent through net
in discrete “chunks”
The Network CORE
Circuit Switching vs. Packet Switching
A Restaurant Analogy
What resources must be reserved?
Circuit-switched Networks
Resources
reserved
for reservation
the duration of the
Restaurantare
which
requires
communication session
• With a reservation, you can order right away when you get there
• guaranteed seats
Packet-switched Networks
Messages
the resources
on demand;
thus, may have
Restaurantuse
which
does not require
any reservation
to wait (queue) for access to a communication link
• you may have to wait on a queue to be served
• no sure seats
Network Core: Circuit Switching
End-end resources
reserved for “call”
– Reserved link bandwidth,
switch capacity
– Switches on the path
between sender and receiver
maintain connection state for
the duration of the session
– Resources are dedicated;
thus, no sharing
– Advantage: circuit-like
(guaranteed) performance
– call set-up required
(unless infinite resources are
available)
“Circuit”
Network Core: Circuit Switching
How is it implemented?
By dividing the link bandwidth
into “pieces”
 frequency division
 time division
Inefficiency: Resource piece is idle if not used by owning call
(no sharing)
Circuit Switching: FDM and TDM
Example:
FDM
4 users (or 4 circuits)
4KHz
frequency
time
Network dedicates a frequency band to each connection for the session
TDM
Frame
Slot
Used solely
by one
End System
frequency
time
Network dedicates one time slot in every frame of the connection
Question
How long does it take to send a file of
640,000 bits from Host A to Host B over a
circuit-switched network? Assume that
all links in the network use TDM with 24
slots and have a bit rate of 1.536 Mbps.
Also suppose that it takes 500 msec. to
establish an end-to-end circuit before
Host A can begin to transmit the file.
Further assume that propagation delay is
negligible.
?
Question
Answer
How long does it take to send a file of 640,000 bits from Host A to Host B over a circuit-switched
network? Assume that all links in the network use TDM with 24 slots and have a bit rate of 1.536
Mbps. Also suppose that it takes 500 msec. to establish an end-to-end circuit before Host A can
begin to transmit the file.
GIVEN:
Size of file to send: 640,000 bits
SOLUTION:
Establishment time + transmission time
Each circuit has a transmission rate of (1.536 Mbps)/24 slots= 64kbps
(or 64,000 bps).
So, it takes (640,000 bits)/(64,000 bps)= 10 sec. to transmit the file.
Considering the circuit establishment time, we add 0.5 sec; therefore,
It takes 10.5 sec. to transmit the file.
The transmission time would be 10 sec. if the end-to-end circuit passed
through 1 link or 100 links. (but the actual end-to-end delay also
includes a propagation delay)
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
Q
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
 transmit over link
 wait turn at next link
We stopped here last time 
Network Core: Packet Switching
Statistical multiplexing - on-demand sharing of resources
*
Q
10 Mbs
Ethernet
A
B
Sender:
Nodes A and B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for transmission
at the output link
45 Mbs
Receiver: Node E
D

E
sequence of A & B packets has no fixed timing pattern
 bandwidth shared on demand: statistical multiplexing.

Compare this to TDM: each host gets same slot in revolving
TDM frame.
Network Core: Packet Switching
Consider a message that is 7.5 x 106 bits long. Suppose that between
source and destination, there are 2 packet switches and 3 links, and that
each link has a transmission rate of 1.5 Mbps. Assuming that there is no
congestion in the network and negligible propagation delay, how much time
is required to move the message from source to destination with packet
switching?
Transmission delay
(7.5 Mbps/1.5 Mbps) * 3 =
15 sec.
Packet Switching:
Store and Forward Behaviour
Example:
store and forward behaviour:
 break message into
smaller chunks:
“packets”
 Store-and-forward:
switch waits until chunk
has completely arrived,
then forwards/routes
Pattern that can be deduced from the
packet flow depicted in the Figure:
Time of arrival = packet_num + 2
Packet Switching vs. Circuit Switching
Suppose that users share a 1 Mbps link, where each user
alternates between generating data at a constant rate of 100
kbps, and periods of inactivity. Also assume that each user is
active only 10% of the time.
Compare the performance of Circuit Switching against Packet
Switching.
Packet Switching vs. Circuit Switching
Packet switching allows more users to use network!
Example: 1 Mbit link shared by all users
• each user:
– Generates 100Kbps when “active”
(at constant rate)
– active 10% of time
1 Mbps link
N users
• circuit-switching:
– 10 users can only be supported
– 1,000,000 bits/sec divided by 100,000 bits/sec.
• packet switching:
– with 35 users, probability > 10 are active is less
than .0004
probability <= 10 users are active is 0.9996
Implies that 10 users
can be using the
circuit without
competing, just like
circuit-switching
(bandwidth is equally
distributed)
Packet switching allows for more than 3 times the number of users as
compared to circuit-switching
Question (Transmission delay)
A factor in the delay of a store-andforward packet-switching system is how
long it takes to store and forward a
packet through a switch. If switching time
is 10 µsec, is this likely to be a major
factor in the response of a client-server
system where the client is in Adelaide,
Australia and the server is in Auckland,
New Zealand? Assume the propagation
speed in copper and fiber to be 2/3 the
speed of light in vacuum.
Speed of light = 3 x 108 meters/sec.
?
Question (Transmission delay)
Answer
A factor in the delay of a store-and-forward packet-switching system is how long it
takes to store and forward a packet through a switch. If switching time is 10 µsec, is
this likely to be a major factor in the response of a client-server system where the
client is in Adelaide and the server is in Auckland? Assume the propagation speed in
copper and fiber to be 2/3 the speed of light in vacuum.
No. The speed of propagation is 200,000 km/sec or
200 meters/µsec. In 10 µsec the signal travels 2
km. Thus, each switch adds the equivalent of 2 km
of extra cable. If the client and server are separated
by 5000 km, traversing even 50 switches adds only
100 km to the total path, which is only 2%. Thus,
switching delay is not a major factor under these
circumstances.
Demo
• Total delay across a link = Transmission
delay + Propagation delay
Network Core: Packet Switching
• Advantages: Great for bursty data
– resource sharing
– no call set-up
• Drawbacks:
Excessive congestion, packet delay and loss
– protocols needed for reliable data transfer,
congestion control
• Issue: How to provide circuit-like behaviour?
– bandwidth guarantees needed for audio/video apps
– this is still an unsolved problem!
Access networks and physical media
Q: How to connect EndSystems 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?
Dial-up Modem
central
office
home
PC
home
dial-up
modem
telephone
network
Internet
ISP
modem
(e.g., AOL)

uses existing telephony infrastructure
 home directly-connected to central office
up to 56Kbps direct access to router (often less)

can’t surf, phone at same time: not “always on”

Introduction 1-49
Central Office
Example: A central office in Dakota, U.S.A.
http://www.flickr.com/photos/afiler/3825218687/sizes/m/
Digital Subscriber Line (DSL)
Existing phone line:
0-4KHz phone; 4-50KHz
upstream data; 50KHz1MHz downstream data
home
phone
Internet
DSLAM
telephone
network
splitter
DSL
modem
home
PC
central
office

uses existing telephone infrastructure
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

Works only within 5 to 10 miles from the CO.



Introduction 1-51
For more info: http://www.systemtek.co.uk/modules.php?name=Content&pa=showpage&pid=18
Residential access: cable modems


uses cable TV infrastructure, rather than
telephone infrastructure
HFC: hybrid fiber coax
 asymmetric:
 up to 30Mbps downstream,
 2 Mbps upstream

network of cable, fiber attaches homes to ISP
router
 homes share access to router
 unlike DSL, which has dedicated access
Introduction 1-52
Residential access: cable internet access
Shared broadcast medium
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
Introduction 1-53
Cable Network Architecture: Overview
Typically 500 to 5,000 homes
cable headend
cable distribution
network (simplified)
home
Homes can be up to 100 miles from the cable headend
Introduction 1-54
Cable Network Architecture: Overview
server(s)
cable headend
cable distribution
network
home
Introduction 1-55
Cable Network Architecture: Overview
cable headend
cable distribution
network (simplified)
home
Introduction 1-56
Cable Network Architecture: Overview
FDM:
V
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D
E
O
V
I
D
E
O
V
I
D
E
O
V
I
D
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O
V
I
D
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O
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I
D
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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
home
Introduction 1-57
Fiber to the Home
Optical network terminator
ONT
optical
fibers
Internet
Optical line
terminator
OLT
central office
Shared optical
fiber
ONT
optical
splitter
ONT


optical links from central office to the home
two competing optical technologies:
 Passive Optical network (PON)
 Active Optical Network (AON) – switched ethernet

much higher Internet rates (download [10,20Mbps],
upload [2,10Mbps]); fiber also carries television and
phone services
Introduction 1-58
Ethernet Internet access
100 Mbps
Ethernet
switch
institutional
router
to institution’s
ISP
100 Mbps
1 Gbps
100 Mbps

server
typically used in companies, universities, etc
– (Users:10 Mbps, 100Mbps), (Servers:1Gbps,
10Gbps Ethernet)
– today, end systems typically connect into
Ethernet switch
Introduction 1-59
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
 Tens of meters from access point

router
base
station
wider-area wireless access
 provided by telco operator
 ~1Mbps over cellular system (3G –
packet-switched wide-area wireless
internet access)
 Tens of kilometers from access point
 next up (?): WiMAX – IEEE 802.16
(10’s Mbps) over wide area
mobile
hosts
Introduction 1-60
Home networks
Typical home network components:
 DSL or cable modem
 router/firewall/NAT
 Ethernet
 wireless access point
to/from
cable
headend
cable
modem
router/
firewall
Ethernet
wireless
laptops
wireless
access
point
Introduction 1-61
Physical Media



bit: propagates between
transmitter/rcvr pairs
physical link: what lies
between transmitter &
receiver
guided media:
 signals propagate in solid
media: copper, fiber, coax

Twisted Pair (TP)
 two insulated copper
wires
 Category 3: traditional
phone wires, 10 Mbps
Ethernet
 Category 5:
100Mbps Ethernet
unguided media:
 signals propagate freely, e.g.,
radio
Introduction 1-62
Physical Media: coax, fiber
Coaxial cable:



two concentric copper
conductors
bidirectional
baseband:
 single channel on cable
 legacy Ethernet

broadband:
 multiple channels on
cable
 HFC
Fiber optic cable:


glass fiber carrying light pulses,
each pulse a bit
high-speed operation:
 high-speed point-to-point
transmission (e.g., 10’s100’s Gpbs)

low error rate: repeaters spaced
far apart ; immune to
electromagnetic noise
Introduction 1-63
Physical media: radio




signal carried in
electromagnetic
spectrum
no physical “wire”
bidirectional
propagation environment
effects:
 reflection
 obstruction by objects
 interference
Radio link types:

terrestrial microwave
 e.g. up to 45 Mbps channels

LAN (e.g., WiFi)
 11Mbps, 54 Mbps

wide-area (e.g., cellular)
 3G cellular: ~ 1 Mbps

satellite
 Kbps to 45Mbps channel (or
multiple smaller channels)
 280 msec end-end delay
 geosynchronous versus low
altitude (Low-Earth-Orbiting
satellites) – future Internet
access
Introduction 1-64
How do loss and delay occur?
packets queue in router buffers


packet arrival rate to link exceeds output link capacity
packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Introduction 1-67
Loss in Packet-Switched Networks
- Length of Queue is finite
- Packets are lost when
queue is full
Queue
• Incoming packet is dropped
Queue
• packet in queue is dropped
Lost packet
- Retransmitted by application or transport layer protocol
Four sources of packet delay
transmission
A
propagation
B
nodal
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 1-69
Four sources of packet delay
transmission
A
propagation
B
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
dtrans: transmission delay:
 L: packet length (bits)
 R: link bandwidth (bps)
 dtrans = L/R
dtrans and dprop
very different
dprop: propagation delay:
 d: length of physical link
 s: propagation speed of
medium (~2x108 m/sec)
 dprop = d/s
Introduction 1-70
Queueing delay (revisited)
• R=link bandwidth (bits/sec)
• L=packet length (bits)
• a=average packet arrival
rate (packets/sec)
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!
This estimates the extent of queuing delay.
Design your system so that traffic intensity is not
greater than 1. Let’s look at a demo!
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
link that
capacity
pipe
can carry
Rfluid
c bits/sec
at rate
(Rc bits/sec)
Introduction 1-72
Throughput (more)
 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
bottleneck link
link on end-end path that constrains end-end throughput
Introduction 1-73
Throughput: Internet scenario
 per-connection
end-end
throughput:
min(Rc,Rs,R/10)
 in practice: Rc or
Rs is often
bottleneck
e.g.
10 clients downloading with 10 servers
Rc=1 Mbps, Rs=2Mbps, R=5Mbps
Rs
Rs
Rs
R
Rc
Rc
Rc
10 connections (fairly) share
backbone bottleneck link R bits/sec
Introduction 1-74
Delay and Routes in the Internet
TraceRoute(diagnostic program) -defined in RFC 1393
SOURCE HOST
Program
DESTINATION HOST
Program
SOURCE:
• records time elapsed (time received- time packet sent)
• determines the round-trip delays to all intervening routers
delays
If there areRound-trip
(N-1) routers,
then include:
SOURCE sends N special packets
• Router
processing
• Each packet
is addressed
to thedelay
ultimate destination
Queuing
delay (varies with time)
• marked 1•to
N
• Transmission delay
When DESTINATION
host receives
the Nthmarked
packet:i:
the ith router
receives
the
ith packet
• Propagation
delay
• DESTINATION
theofpacket,
then
router
destroys
the
packet
• records
name destroys
& address
router (or
destination HOST) that
••Sends
returns
message
back to name
the source
message
containing
and address of router back to the source
returnsathe
the
message
• reconstructs the route taken by the packets (source-to-destination)
www.TraceRoute.org
Route trace: From MIT to Massey University
Trace Route from MIT
Three delay measurements
Trace:3x
IMPORTANT: This tool works by sending a series of UDP packets with different port numbers and TTL (Time To Live). If you are running firewall software, your
software may interpret the incoming packets as a hostile "port scan" originating from this server (jis.mit.edu). Rest assured, your system is not being
attacked.
1
2
3
4
W92-RTR-1-W92SRV21.MIT.EDU
(18.7.21.1)
0.425 ms
0.287 ms
0.259 ms
EXTERNAL-RTR-1-BACKBONE.MIT.EDU (18.168.0.18) 21.179 ms 244.069 ms 223.625 ms
leg-208-30-223-5-CHE.sprinthome.com
(208.30.223.5) 0.589 ms
0.459 ms
0.542 ms
144.232.21.50
(144.232.21.50) 2.951 ms
3.146 ms
2.966 ms
5 sl-bb21-chi-6-2.sprintlink.net (144.232.19.205) 21.073 ms 48.427 ms 20.784 ms
6 sl-bb24-chi-9-0.sprintlink.net (144.232.26.77) 141.917 ms 229.305 ms 219.150 ms
7 sl-bb21-sj-8-0.sprintlink.net (144.232.20.161) 68.260 ms 68.102 ms 68.044 ms
8 sl-bb22-sj-15-0.sprintlink.net (144.232.3.162) 68.016 ms 68.036 ms 68.608 ms
9 144.232.20.47 (144.232.20.47)
73.346 ms 73.617 ms 73.508 ms
10 sl-newzeal-1-0.sprintlink.net (144.223.243.18) 70.804 ms 71.082 ms 70.787 ms
11 p5-2.sjbr1.global-gateway.net.nz (203.96.120.213) 71.132 ms 70.990 ms 70.903 ms
12 203.96.120.118 (203.96.120.118)
195.054 ms 195.579 ms 196.648 ms
13 203.96.120.201 (203.96.120.201)
198.228 ms 211.397 ms 197.358 ms
Trans-oceanic link
14 massey-uni-ak-int.tkbr4.global-gateway.net.nz (202.49.163.230) 202.604 ms 218.925 ms 199.836 ms
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
***
***
***
***
***
***
* means no response (probe lost, router not replying)
***
***
***
***
***
***
***
***
***
***
6 columns: n, name of router, address of router, trip delay1,trip delay2,trip delay3
* - indicates packet loss
www.TraceRoute.org
Route trace: From MIT to Massey University
Trace Route from MIT
IMPORTANT: This tool works by sending a series of UDP packets with different port numbers and TTL (Time To Live). If you are running firewall software, your
software may interpret the incoming packets as a hostile "port scan" originating from this server (jis.mit.edu). Rest assured, your system is not being
attacked.
1
2
3
4
W92-RTR-1-W92SRV21.MIT.EDU
(18.7.21.1)
0.425 ms
0.287 ms
0.259 ms
EXTERNAL-RTR-1-BACKBONE.MIT.EDU (18.168.0.18) 21.179 ms 244.069 ms 223.625 ms
leg-208-30-223-5-CHE.sprinthome.com
(208.30.223.5) 0.589 ms
0.459 ms
0.542 ms
144.232.21.50
(144.232.21.50) 2.951 ms
3.146 ms
2.966 ms
5 sl-bb21-chi-6-2.sprintlink.net (144.232.19.205) 21.073 ms 48.427 ms 20.784 ms
6 sl-bb24-chi-9-0.sprintlink.net (144.232.26.77) 141.917 ms 229.305 ms 219.150 ms
7 sl-bb21-sj-8-0.sprintlink.net (144.232.20.161) 68.260 ms 68.102 ms 68.044 ms
8 sl-bb22-sj-15-0.sprintlink.net (144.232.3.162) 68.016 ms 68.036 ms 68.608 ms
9 144.232.20.47 (144.232.20.47)
73.346 ms 73.617 ms 73.508 ms
10 sl-newzeal-1-0.sprintlink.net (144.223.243.18) 70.804 ms 71.082 ms 70.787 ms
11 p5-2.sjbr1.global-gateway.net.nz (203.96.120.213) 71.132 ms 70.990 ms 70.903 ms
12 203.96.120.118 (203.96.120.118)
195.054 ms 195.579 ms 196.648 ms
13 203.96.120.201 (203.96.120.201)
198.228 ms 211.397 ms 197.358 ms
The round-trip delay
decreased between
the two routers!
14 massey-uni-ak-int.tkbr4.global-gateway.net.nz (202.49.163.230) 202.604 ms 218.925 ms 199.836 ms
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
***
***
***
***
***
***
***
***
***
***
***
***
***
***
***
***
Can you explain why the delays
sometimes decrease from one
router to the next?
6 columns: n, name of router, address of router, trip delay1,trip delay2,trip delay3
* - indicates packet loss
Tracert (from xtra to mit)
C:\>tracert web.mit.edu
Tracing route to web.mit.edu [18.7.22.69]
over a maximum of 30 hops:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
1 ms 1 ms 1 ms 192.168.1.1
2 ms 2 ms 2 ms 192.168.8.1
56 ms 59 ms 55 ms 219-89-32-1.dialup.xtra.co.nz [219.89.32.1]
*
53 ms 54 ms 222.152.127.169
*
66 ms * 202.50.236.105
*
*
* Request timed out.
482 ms *
* so-0-2-0.labr3.global-gateway.net.nz [202.50.232.26]
*
*
* Request timed out.
*
341 ms 290 ms g11-2-107.core01.lax05.atlas.cogentco.com [154.54.11.145]
243 ms 213 ms * t3-4.mpd01.lax01.atlas.cogentco.com [154.54.6.189]
217 ms 280 ms * g9-0-0.core01.lax01.atlas.cogentco.com [154.54.2.117]
*
344 ms 325 ms p2-0.core01.dfw01.atlas.cogentco.com [154.54.5.93]
*
*
282 ms p15-0.core02.dfw01.atlas.cogentco.com [66.28.4.26]
250 ms *
* p15-0.core01.mci01.atlas.cogentco.com [66.28.4.38]
*
*
* Request timed out.
*
367 ms * p15-0.core01.ord01.atlas.cogentco.com [66.28.4.61]
*
386 ms 434 ms p14-0.core01.alb02.atlas.cogentco.com [154.54.1.57]
*
345 ms 448 ms p6-0.core01.bos01.atlas.cogentco.com [154.54.7.42]
*
282 ms 285 ms g8.ba21.b002250-1.bos01.atlas.cogentco.com [66.250.14.210]
*
*
408 ms MIT.demarc.cogentco.com [38.112.2.214]
342 ms *
* W92-RTR-1-BACKBONE.MIT.EDU [18.168.0.25]
*
344 ms * WEB.MIT.EDU [18.7.22.69]
*
342 ms 380 ms WEB.MIT.EDU [18.7.22.69] Tracert (also known as traceroute)
Trace complete.
C:\>
is a Windows based tool that
allows you to help test your
network infrastructure.
Tracert (from Massey to MIT)
D:\Massey Papers\159334\Codes\Game Protocol v3.6>tracert web.mit.edu
Tracing route to web.mit.edu [18.7.22.69]
over a maximum of 30 hops:
1
<1 ms
<1 ms
<1 ms it023453-vlan205.massey.ac.nz [130.123.246.129]
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
<1 ms <1 ms <1 ms it028100-vlan801.massey.ac.nz [10.100.254.3]
1 ms <1 ms <1 ms 210.7.32.1
1 ms <1 ms <1 ms 210.7.36.67
142 ms 142 ms 142 ms 210.7.47.22
142 ms 142 ms 142 ms abilene-1-lo-jmb-706.sttlwa.pacificwave.net [207.231.240.8]
179 ms 187 ms 180 ms dnvrng-sttlng.abilene.ucaid.edu [198.32.8.50]
189 ms 189 ms 202 ms kscyng-dnvrng.abilene.ucaid.edu [198.32.8.14]
201 ms 214 ms 201 ms iplsng-kscyng.abilene.ucaid.edu [198.32.8.80]
202 ms 215 ms 202 ms chinng-iplsng.abilene.ucaid.edu [198.32.8.76]
202 ms 206 ms 207 ms ge-0-0-0.10.rtr.chic.net.internet2.edu [64.57.28.1]
219 ms 230 ms 220 ms so-3-0-0.0.rtr.wash.net.internet2.edu [64.57.28.13]
225 ms 224 ms 224 ms ge-1-0-0.418.rtr.chic.net.internet2.edu [64.57.28.10]
229 ms 229 ms 229 ms nox300gw1-Vl-110-NoX-ABILENE.nox.org [192.5.89.221]
229 ms 229 ms 229 ms nox230gw1-Vl-802-NoX.nox.org [192.5.89.254]
481 ms 230 ms 230 ms nox230gw1-PEER-NoX-MIT-192-5-89-90.nox.org [192.5.89.90]
230 ms 230 ms 230 ms W92-RTR-1-BACKBONE.MIT.EDU [18.168.0.25]
230 ms 230 ms 230 ms WEB.MIT.EDU [18.7.22.69]
Trace complete.
D:\Massey Papers\159334\Codes\Game Protocol v3.6>
Roadmap
1.1 What is the Internet?
1.2 Network edge
end systems, access networks, links
1.3 Network core
circuit switching, packet switching, network
structure
1.4 Delay, loss and throughput in packetswitched networks
1.5 Protocol layers, service models
Introduction 1-80
Protocol “Layers”
Networks are complex,
with many “pieces”:
– hosts
– routers
– links of various
media
– applications
– protocols
– hardware, software
Question:
Is there any hope of
organizing structure of
network?
Or at least our discussion
of networks?
Introduction 1-81
An analogy: Organization of air travel
ticket (purchase)
ticket (complain)
Ticketed passengers
baggage (check)
baggage (claim)
Baggage-checked,
gatesTicketed
(load) passengers
gates (unload)
runway
landing
Baggage-checked,
Ticketed,
passed through the gate
passengers
runway
takeoff
airplane routing
airplane routing
Passenger in-flight
airplane routing
a
series of steps
Introduction 1-82
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
Introduction 1-83
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., In the air travel analogy, a change in
gate procedure doesn’t affect rest of system

layering considered harmful?
Introduction 1-84
Tasks of Layers
Each layer may perform one or more of the following tasks:
Error Control
Flow control
Segmentation and Reassembly
Multiplexing
Connection Set-up
Potential Drawbacks of Layering:
Duplication of services
Possible violation of layer dependency (conflicting
information dependency among layers)
Communication in a Layered Architecture
Concept of Protocol Layering
Let’s consider 2 Network Entities (e.g. End Systems, Packet Switches)
Sending
side
Receiving
side
Layer 4
M
M
Layer 3
H3 M1
H3 M2
H3 M1
H3 M2
Layer 2
H2 H3 M1
H2 H3 M2
H2 H3 M1
H2 H3 M2
Layer 1
H1 H2 H3 M1
H1 H2 H3 M2
H1 H2 H3 M1 H1 H2 H3 M2
SOURCE
DESTINATION
What happens when the SOURCE wants to send a message to the
Comprised
of 4 Layers; where each layer n is governed by a protocol.
DESTINATION?
Layers communicate by exchanging layer-n messages called (n-PDUs) Protocol
Data Units.
The contents, format, and
procedure for exchanging PDUs
are defined by Layer-n Protocol
source
message
segment
M
Ht
M
datagram Hn Ht
M
frame Hl Hn Ht
M
Encapsulation
application
transport
network
link
physical
link
physical
switch
destination
M
Ht
M
Hn Ht
M
Hl H n Ht
M
application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
M
router
Introduction 1-88
Internet protocol stack
• application: supporting network
applications
Mostly software implemented
application
– ftp, smtp, http
transport
• transport: host-host data transfer
Guaranteed delivery of application layer messages
– tcp, udp
network
• network: routing of datagrams from
source to destination Defines fields in IP datagrams (destination address),
how end systems and routers act link
on them
– ip, routing protocols
(Hardware+Software)
physical
• link: data transfer between
Moves
packets
from
one node
or packet
Ethernet
& ATM
cards(host
implement
both
neighbouring network
elements
switch) to the next
linknode
and Physical Layers
– ppp, ethernet
Move individual
• physical:
bitsbits
“onwithin
theframe
wire”from one node to the next
Introduction 1-91
Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone
peering
…
…
.
…
…
…
to/from customers
Introduction 1-93
Internet structure: network of networks


roughly hierarchical
at center: small # of well-connected large networks
 “tier-1” commercial ISPs (e.g., Verizon, Sprint, AT&T, Qwest,
Level3), national & international coverage
 large content distributors (Google, Akamai, Microsoft)
 treat each other as equals (no charges)
IXP
Tier-1 ISPs &
Content
Distributors,
interconnect
(peer) privately
… or at Internet
Exchange Points
IXPs
Large Content
Distributor
(e.g., Akamai)
IXP
Tier 1 ISP
Tier 1 ISP
Large Content
Distributor
(e.g., Google)
Tier 1 ISP
http://www.akamai.com/html/industry/index.html
Introduction 1-94
End of Session 