Transcript ppt

NET0183 Networks and Communications
Lecture 10
Basic Internet Measurements
8/25/2009
NET0183 Networks and Communications
by Dr Andy Brooks
1
Internet Protocol address (IP address)
• A computer on the Internet has a unique 32-bit
identifier called its IP address.
• IP addresses are described for convenience in
dotted decimal notation where each 8-bits are
given in decimal (0 to 255).
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NET0183 Networks and Communications
by Dr Andy Brooks
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Windows: ipconfig
An address mask or subnet mask has one bits to mark the network prefix and zero bits
to mark the host portion. Here the network prefix is 130.208.225.
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NET0183 Networks and Communications
by Dr Andy Brooks
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Windows: Control Panel -> Network connections
-> Local Area Network->
Dynamic Host Configuration Protocol (DHCP)
Andy asks:
Under DHCP does a computer’s IP address change from one session to the next?
Is the default gateway the computer which connects the local network to the Internet?
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NET0183 Networks and Communications
by Dr Andy Brooks
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There are five kinds of geographical
location which characterize a website.
1.
2.
3.
4.
5.
Where are the website´s users located?
Where is the website´s legal owner located?
Where is the website´s maintainer located?
Where is the hardware located?
To where does the website content refer?
A website about Denmark could be hosted on a server in Iceland which is maintained
by someone in Ireland for a website owner in Norway. Since the site is in Scottish
Gaelic, all the users are located in Scotland.
Location can be imprecise. For example, the location of a web server might be
known only at a city level rather than a building and street level.
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NET0183 Networks and Communications
by Dr Andy Brooks
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Where is the hardware located?
May not be correct.
May not be accurate.
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Who registered the domain name?
http://www.domaintools.com/
May not be correct.
May not be accurate.
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by Dr Andy Brooks
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What do users in Iceland do?
http://www.alexa.com/
May not be correct.
May not be accurate.
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NET0183 Networks and Communications
by Dr Andy Brooks
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latency/biðtími
Measuring time distance
• It is possible to measure the time it takes to transmit
and receive data between two nodes.
– latency (or delay)
• If latency is large, then this implies a larger physical
distance between the two nodes.
• Different hardware and different network
configurations have an influence on latency.
• The relationship between physical distance and time
distance is not always linear, however.
• Latency can increase in the presence of network
congestion in the same way average traffic speed
reduces when a road network becomes congested.
8/25/2009
NET0183 Networks and Communications
by Dr Andy Brooks
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ping
<networking, tool> (ping, originally contrived to match submariners' term for the
sound of a returned sonar pulse) A program written in 1983 by Mike Muuss
(who also wrote TTCP) used to test reachability of destinations by sending them
one, or repeated, ICMP echo requests and waiting for replies. Since ping works
at the IP level its server-side is often implemented entirely within the operating
system kernel and is thus the lowest level test of whether a remote host is alive.
Ping will often respond even when higher level, TCP-based services cannot.
Sadly, Mike Muuss was killed in a road accident on 2000-11-20.
The term is also used as a verb: "Ping host X to see if it is up."
The Unix command "ping" can be used to do this and to measure round-trip
delays.
ICMP Internet Control Message Protocol
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by Dr Andy Brooks
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Windows: ping www.hi.is
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Windows: ping www.bbc.co.uk
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Western Europe
Andy asks:
Is the hardware for www.hi.is in Reykjavik?
Is the hardware for www.bbc.co.uk in London?
46/6 = 7.6666
Is London 7.666 times further away than
Reykjavik is from Akureyri?
http://www.distance-calculator.co.uk/
“The approximate distance as the crow flies
from London to Akureyri is 1168.16 miles.”
“Distance between Reykjavik and Akureyri is
154 miles or 247.79 Kilometers.” (by road?)
1168/154 = 7.5844
Andy comments: When making calculations
like this we need to be clear about how the
physical distance measure is calculated.
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NET0183 Networks and Communications
by Dr Andy Brooks
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Measuring time distance and routes
• The utility traceroute works in a way similar to ping but
provides more details.
• The route taken over intermediate nodes is listed with
an indication of the round-trip times for each segment
of the journey.
– Nodes are identified by domain name or IP address.
– Nodes with long or unusual domain names may be special
routing computers which normal users are unaware of.
• The political and economic structures of the Internet
can be exposed using traceroute.
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NET0183 Networks and Communications
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Windows: tracert www.hi.is
On each line are three measurements of round-trip time.
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Windows: tracert www.bbc.co.uk
Andy asks:
Are the round-trip times, accumulating times?
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Security and confidentiality concerns
• Some networks block the use of ping and traceroute
for security reasons.
– The ping of death involved sending a malformed ping
which crashed the target computer.
– A ping flood involved sending so many ping requests that
the target computer could do nothing else but deal with
ping traffic. (A denial-of-service.)
• If country x and y are in conflict. Should network traffic
to someone in country x be routed through country y?
– Should government traffic to Norway from Iceland be
routed through the UK? Only if it is encrypted.
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by Dr Andy Brooks
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