How many hops?
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Transcript How many hops?
Introduction and Overview of Network
and Telecommunications (contd.)
Review: Video: Warriors of the Net
• https://www.youtube.com/watch?v=PBWhzz_
Gn10
Some Key Points
• Data is transmitted as packets.
• Packets are handled by equipments at different
layers.
– Each layer has its own protocol to follow.
– At each layer, information, or header, is added (or
removed) based on the protocol before sending the
packet to the lower (or higher) layer.
Why using “protocol layers”?
Organization of air travel
ticket (purchase)
ticket (complain)
Ticket
baggage (check)
baggage (claim)
Baggage
gates (load)
gates (unload)
Gate
runway takeoff
runway landing
Takeoff/Landing
airplane routing
airplane routing
Airplane routing
airplane routing
Layers: each layer implements a service by
• Perform certain actions within the layer
• Using the services of the layer directly below it
Protocols and Layers
• Protocols and layering is the main structure
method used to divide up network
functionality
– Each instance of a protocol talks virtually to its
peer using the protocol
– Each instance of a protocol uses only the services
of the lower layer
Internet protocol stack
• Set of protocols in use in called a protocol stack
e.g., web browser
e.g., HTTP
e.g., web server
Internet protocol stack
•
Application: supporting network
applications
– FTP, SMTP, HTTP
•
Transport: process-process data
transfer
– TCP, UDP
•
Internet: routing of datagrams
from source to destination
– IP
•
Link: data transfer between
neighboring network elements
– Ethernet, 802.11 (WiFi), PPP
•
Physical: bits “on the wire”
TCP vs. UDP: Two Communication Paradigms
Stream
Message
Connection-oriented
Connectionless
1-1 communication
1-1, 1-many, many-many
Sequence of bytes
Sequence of messages
Arbitrary length transfer Limited to 64K bytes
Widely used
Multimedia applications
Based on TCP
(Guaranteed)
Built on UDP (Not
Guaranteed)
Message (Connectionless) Transport In
the Internet
• UDP (message service) does not make any
guarantees. A packet may be
– Lost
– Duplicated
– Delayed
– Delivered out-of-order
Why using message service?
• No connection establishment
– Get straight to the point
• No connection state
– A server can support more active clients
simultaneously
• Fine application-level control over what data is
sent, and when
Some Examples
Application
TCP or UDP?
E-mail
TCP
Remote terminal access
TCP
Web
TCP
File transfer
TCP
Streaming multimedia
UDP, TCP
Internet telephony
UDP, TCP
Name translation
Typically UDP
Remote file server
Typically UDP
TCP today:
• >95% Internet traffic
• ~75% on-demand and live streaming
Encapsulation
• Encapsulation is the mechanism used to effect
protocol layering
– Lower layer wraps higher layer content, adding its
own information (header) to make a new message
for delivery
– Like sending a letter in an envelope; postal service
doesn’t look inside
<HTML><BODY>Hello World!</BODY></HTML>
HTTP message
802.11
TCP
HTTP message
IP
TCP
HTTP message
IP
TCP
HTTP message
Wire
802.11
IP
TCP
HTTP message
IP
TCP
HTTP message
TCP
HTTP message
HTTP message
<HTML><BODY>HTML Example</BODY></HTML>
Another Illustration
• Think of these nesting dolls:
Why Using Packets?
Connections between Computers
n(n-1)/2
Overhead Telephone and Telegraph Wires in Broadway, 1890
From Book of Old New York. Henry Collins Brown. 1913
Switch Model: Reducing the Mess
• Analogy
– Airport hubs
Tie-1 ISP: e.g., Sprint
Question
• Why you fly, you can check a map to
determine what cities are on the route.
• However, there is no map of the internet, so
how do you know what route a packet is
taking?
An Analogy
State College
Local host: Alice
Philly
NYC
Boston
Remote host: Jack
Traceroute
• Widely used command-line tool to let hosts
peek inside the network
– On all OSes (tracert on Windows)
– Provides delay measurement from source to
router along end-end Internet path towards
destination.
Traceroute
• Probe successive hops to find network path
– sends 3 packets that will reach each router on path
towards destination
– Each router returns packets to sender
– sender times interval between transmission and reply.
3 probes
3 probes
3 probes
Local host: Alice
Remote host: Jack
Traceroute
• Technically, a hop is defined as the action that
occurs when a packet “jumps” from one router to
the next.
Remote host: Jack
Local host: Alice
1 hop
2 hop
3 hop
N-1 hop
N hop
Traceroute Question 1
How many computers (incl. local host, remote host, and
intermediate routers) does this route have?
Traceroute Question 2
How many hops does this route have?
Traceroute Question 3
Remote host: Jack
Local host: Alice
1 hop
2 hop
3 hop
N-1 hop
N hop
For a route with N hops, how many computer does it have?
Traceroute Question 4
Remote host: Jack
Local host: Alice
1 hop
2 hop
3 hop
N-1 hop
N hop
Local host sends 3 packets to each computer on the route
(incl. routers and remote host). If the route has N hops, how
many packets does local host send in total?
Traceroute Question 5
Remote host: Jack
Local host: Alice
1 hop
2 hop
3 hop
N-1 hop
N hop
For a route with N hops, how many packets are forwarded by
the router that is closest to the local host?
Traceroute Example 1
C:\> tracert allspice.lcs.mit.edu
1
2.
3
4
20ms
100ms
300ms
339ms
25ms 15ms
120ms 140ms
280ms 260ms
279ms 279ms
3 delay measurements
philadelphia4.t3.ans.net (140.222.27.221)
New-York2.t3.ans.net (140.223.37.9)
Cambridge1.bbnplanet.net (40.1.122)
Allspice.lcs.mit.edu (18.26.0.115)
Traceroute Question 6
C:\> tracert allspice.lcs.mit.edu
1
2.
3
4
20ms
100ms
300ms
339ms
25ms 15ms
120ms 140ms
280ms 260ms
279ms 279ms
philadelphia4.t3.ans.net (140.222.27.221)
New-York2.t3.ans.net (140.223.37.9)
Cambridge1.bbnplanet.net (40.1.122)
Allspice.lcs.mit.edu (18.26.0.115)
How many computers are there in this route? How many
hops?
Traceroute Question 7
C:\> tracert allspice.lcs.mit.edu
1
2.
3
4
20ms
100ms
300ms
339ms
25ms 15ms
120ms 140ms
280ms 260ms
279ms 279ms
philadelphia4.t3.ans.net (140.222.27.221)
New-York2.t3.ans.net (140.223.37.9)
Cambridge1.bbnplanet.net (40.1.122)
Allspice.lcs.mit.edu (18.26.0.115)
How many packets are sent by local host?
How many packets are forwarded by New York (NewYork2.t3.ans.net )?
Traceroute Question 8
C:\> tracert allspice.lcs.mit.edu
1
2.
3
4
20ms
100ms
300ms
339ms
25ms 15ms
120ms 140ms
280ms 260ms
279ms 279ms
philadelphia4.t3.ans.net (140.222.27.221)
New-York2.t3.ans.net (140.223.37.9)
Cambridge1.bbnplanet.net (40.1.122)
Allspice.lcs.mit.edu (18.26.0.115)
What’s the average round trip time form local host to
Cambridge1.bbnplanet.net?
Traceroute Example 2
C:\> tracert www.oxford.ac.uk
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
85 ms 1 ms 1 ms 192.168.1.1
9 ms 11 ms 9 ms c-71-58-208-1.hsd1.pa.comcast.net [71.58.208.1]
11 ms 10 ms 10 ms te-0-7-0-6-sur02.statecollege.pa.pitt.comcast.net [162.151.68.245]
comcast
17 ms 15 ms 15 ms be-3-ar03.lowerpaxton.pa.pitt.comcast.net [69.139.194.81]
21 ms 21 ms 27 ms he-0-15-0-0-ar01.pittsburgh.pa.pitt.comcast.net[69.139.168.185]
45 ms 31 ms 32 ms be-7016-cr01.newyork.ny.ibone.comcast.net [68.86.91.29]
29 ms 28 ms 29 ms ae12.edge1.NewYork2.level3.net [4.68.127.1]
145 ms 302 ms 306 ms ae-4-4.car1.Manchesteruk1.Level3.net [4.69.133.101]
116 ms 210 ms 207 ms ae-4-4.car1.Manchesteruk1.Level3.net [4.69.133.101] Level3: Trans-atlantic link
107 ms 103 ms 106 ms 195.50.119.98
107 ms 107 ms 107 ms ae29.erdiss-sbr1.ja.net [146.97.33.41]
114 ms 106 ms 106 ms ae31.londpg-sbr1.ja.net [146.97.33.21]
109 ms 109 ms 109 ms ae21.read-rbr3.ja.net [146.97.37.206]
Janet: UK’s research network
106 ms 117 ms 105 ms ae1.read-rbr2.ja.net [193.63.108.129]
108 ms 107 ms 107 ms ae2.oxfo-rbr2.ja.net [193.63.108.134]
108 ms 107 ms 107 ms Oxford-University-2.ja.net [193.63.109.114]
109 ms 110 ms 108 ms csurb.backbone.ox.ac.uk [192.76.21.21]
281 ms 110 ms 110 ms boucs.backbone.ox.ac.uk [192.76.22.200]
oxford
106 ms 107 ms 107 ms boucs-lompi1.sdc.ox.ac.uk [192.76.32.62]
109 ms 109 ms 108 ms aurochs-web-154.nsms.ox.ac.uk [129.67.242.154]
Trace complete.
Try it yourself
• Step 1: Start Run cmd
• Step 2: Trace route to
– Somewhere in the U.S.
– Somewhere outside U.S.
• Discuss the results with classmates next to you
– How may hops are there in the path?
– What’s the average delay from you computer to the
remote host?
– How many networks does the path traverse?