EECE 350 – Computer Networks
Textbook

Computer Networking:
A Top Down Approach
 6th edition
 Jim Kurose, Keith Ross
 Addison-Wesley, 2012

Many slides are based on
material provided by the
authors. The authors
retain the copyright to
their material.
Online and computer tools


Moodle
Python
http://www.pythonlearn.com
http://www.learnstreet.com/lessons/python



OPNET
Wireshark
Other tools
Focus is on the Internet


Largest network of networks that connects
everything
Huge impact
 changed the way we consume and produce
information
 changed the way we relate to each other
 can you imagine life without it?
 Pope Francis on January 23, 2014
• “This is something truly good, a gift from God”
What is the Internet: “nuts and bolts” view
 millions
PC
server
wireless
laptop
smartphone
of connected
computing devices:
 hosts = end systems
 running network apps
 communication
wireless
links
wired
links
links
 fiber, copper, radio,
satellite
 transmission rate:
bits per second
 Packet
router
switches: forward
“packets” (chunks of data)
 routers and switches
mobile network
global ISP
home
network
regional ISP
institutional
network
ISP: Internet Service Provider
What is the Internet: “nuts and bolts” view
The Internet Protocol (IP) is a protocol, or set of rules, for routing and addressing packets of data
so that they can travel across networks and arrive at the correct destination.

Internet: “network of networks”
mobile network
 Interconnected ISPs

protocols control sending,
receiving of messages
 e.g., TCP, IP, HTTP, 802.11

global ISP
Internet standards
home
network
 RFC: Request for comments
 IETF: Internet Engineering Task
Force
 IEEE
An ISP (internet service
provider) is a company that
provides individuals and
organizations access to the
internet and other related
services.
An internet standard (STD) is
a specification that has been
approved by the Internet
Engineering Task Force
(IETF). Such standard helps
to promote a consistent and
universal use of the internet
worldwide.
institutional
network
regional ISP
What is the Internet: a service view

Infrastructure that provides
services to applications
 Web, VoIP, messaging,
email, games, e-commerce,
video, social networks, …

provides a platform for apps
through a programming
interface (API)
 hooks that allow sending
and receiving app programs
to “connect” to Internet
 provides service options,
analogous to postal service
mobile network
global ISP
home
network
institutional
network
regional ISP
What is a protocol?
human protocols:



“what’s the time?”
“I have a question”
introductions
… specific messages sent
… specific actions taken
when messages
received, or other
events
network protocols:


machines rather than
humans
all communication activity
in Internet governed by
protocols
protocols define format and
order of messages sent and
received among network
entities, and actions taken
on message transmission,
receipt, or other event
What is a protocol?
a human protocol and a computer network protocol:
Hi
connection
request
Hi
connection
response
Got the
time?
GET http://www.aub.edu.lb
12:30
<file>
time
Network Scale
Networks can be classified by their scale:
Scale
Type
Vicinity
PAN (Personal Area Network)
Building
LAN (Local Area Network)
City
MAN (Metropolitan Area Network)
Country
WAN (Wide Area Network)
Planet
The Internet (network of networks)
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Personal Area Network
Connect devices over the range of a person
Example of a Bluetooth (wireless) PAN:
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Local Area Networks


Connect devices in a home or office building
Called enterprise network in a company
Wireless LAN
with Wi-Fi (802.11)
Wired LAN with
“switched Ethernet”
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Metropolitan Area Networks
Connect devices over a metropolitan area
Example MAN based on cable TV:
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Wide Area Networks (1)


Connect devices over a country
Example WAN connecting three branch offices:
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Wide Area Networks (2)


An ISP network is also a WAN
Customers buy connectivity from the ISP to use it
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Internet Challenges

There are many challenges that make
designing the Internet harder than just
sending bits on a wire
 and that make the study of computer networks
more interesting…
Following 10 slides based on Dr. Scott
Shenker’s EE 122 course at Berkeley
Scale

Billions of Internet human users….

Potentially hundreds of billions of devices
Diversity of end systems












Cell phones / smartphones
Tablets
Laptops/PCs
Televisions
Gaming consoles
Sensing devices
Picture frames
Security systems
Supercomputer clusters
Automobiles
Power grid
……
Diversity of application
requirements








Size of transfers
Bi-directionality (or not)
Latency sensitive (or not)
Tolerance to jitter (or not)
Tolerance to packet drop (or not)
Need for reliability (or not)
Multipoint (or not)
…..
Dynamic Range

Round-trip times (latency) from 10 secs to
secs
 5 orders of magnitude

Data rates (bitrate) from kbps (kilo bits per
second) to 100 Gbps
 8 orders of magnitude



Queuing delays in the network vary from 0 to
several secs
Packet loss varies from 0 to more than 90%
…..
Ad hoc deployment

Can’t assume carefully managed deployment
 Network must work without planning!
Ad Hoc deployment. Principle. This type of
deployment allows you to distribute your
application on an iPhone or iPad device.
Networks contain many
components
Links
Interfaces
Fibers
Ethernet card
Switches/routers
Large router
Wireless card
Coaxial Cable
22
Small
switch
They can all fail….

Consider communication that uses 50
components
 Assume that each works correctly 99% of the time
 What is the likelihood that communication fails?

Answer: success requires that they all function, so
failure probability = 1 – (.99)50 ≈ 39.5%
 Approximately 40% probability of failure


Even if components are 99.9% reliable, failure
probability is still close to 5%
Must design the system to expect failure in
components but not fail as a system!
Physics challenge - Speed of
Light

Question: how long does it take light to travel
from Beirut to Paris?

Answer:
 Distance Beirut  Paris: 2,840 km
 Traveling at 300,000 km/s: 9.5 msec
Implications for Networking


Question: how many cycles does your PC
execute before it can possibly get a reply to a
message it sent to a web server in Paris?
Answer:
 Round trip takes  20 msec
(optimistic)
 PC runs at (say) 2.4 GHz
 2,400,000,000 cycles/s ✕ 0.02 sec = 48,000,000
cycles

A long time even on Local Area Networks
with a round trip time in microseconds
Summary

The Internet is a large complicated system
that must meet an unprecedented variety of
challenges







Scale
Diversity
Dynamic range
Ad hoc deployment
Failures
… Security
… Politics
How do we deal with
complexity

Divide the problem into smaller more
manageable parts
 Complex problem transformed into several less
complex problems
Design exercise

Design a postal system for people in Lebanon
to exchange (paper) messages
Layered network architecture
your application
different entities with different IP
addresses
within each entity we sub entities that
will be differentiated by mini IP
addresses.
-point to point connectivity
-hardware connectivity
Example of a layered network system
From Computer Networks: A Systems Approach, 5e. Copyright © 2010, Elsevier Inc.
Protocols - again
interface: virtual connection point

Protocols define the interfaces between the
layers in the same system and with the layers
of the peer system

Each protocol object has two different
interfaces
 service interface: operations on this protocol b/w layer and layer
 peer-to-peer interface: messages exchanged with
within same layer
peer

Protocols form the building blocks of a
network architecture
From Computer Networks: A Systems Approach, 5e. Copyright © 2010, Elsevier Inc.
Interfaces
Service and Peer Interfaces
From Computer Networks: A Systems Approach, 5e. Copyright © 2010, Elsevier Inc.
Protocol Layers
Protocol layering is the main structuring method used
to divide up network functionality.
•
Each protocol talks virtually
to its peer
but physically it is talking to the layer under it
•
•
•
Each layer directly
communicates only by
using the one below
Lower layer services are
accessed by an interface
At bottom, messages are
carried by the “medium”
how are you sending the
message, ie: how many volts in a
bit, or ethernet
lower layers are providing services to the upper layers, and upper layers are using the services of the lower layers
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
OSI Reference Model



ISO: International Standards Organization
OSI: Open Systems Interconnection
A principled 7-layer model to connect different systems
– Provides functions needed by users
merged
– Converts different representations
it's any application u are
using , ie whatsapp,
facebook....
how to present the data
– Manages synchronization, check-pointing
will give each application that's using the
internet an address( port numbers) ie, my
email client to his email client(specific)
– Provides end-to-end delivery
dividing apps
– Sends packets over multiple links
– Sends frames of information
point to point, from one hardware address to another one
– Sends bits as signals
how are you actually sending,ie signals....
it is end to end, but also host to host
kind of protocol; its responsibilty is to
send packets to the multiple links, it
should be able to know the best ,
fastest route; it is able to route
packets from any source to any
destination.
ISO/OSI Seven-Layer Reference Model
this is the router, it has 3 layers
From Computer Networks: A Systems Approach, 5e. Copyright © 2010, Elsevier Inc.
Description of Layers in ISO/OSI
we go bottom up

Physical Layer (Layer 1)
 Handles the transmission of raw bits over a communication link
(medium)

Data Link Layer (Layer 2)
mac address
 Collects a stream of bits into a larger aggregate called a frame
 Hardware/software
• Network adapter and device driver implement the protocol in this layer
 Frames are delivered over a single link

Network Layer (Layer 3)
those 3 layers are available
in any networking device
 Handles “routing” among nodes within a network
 Unit of data exchanged between nodes in this layer is called a packet
(datagram on the Internet)
The lower three layers are implemented on all network nodes
From Computer Networks: A Systems Approach, 5e. Copyright © 2010, Elsevier Inc.
Description of Layers

Transport Layer (Layer 4)
 Implements process-to-process end-to-end communication
“channel”
 Unit of data exchange in this layer is called a message or
segment

Session Layer (Layer 5)
http...
 Concerned with synchronization and check-pointing

Presentation Layer (Layer 6)
 Concerned about the format of data exchanged between peers

Application Layer (Layer 7)
 Standardize application messages
The transport layer and the higher layers run only on hosts and not
on the intermediate nodes
Internet network architecture

Application (Layer 7): network applications
 HTTP, SMTP


No Layer 6, no Layer 5
Transport (Layer 4): process-process data
transfer
 TCP, UDP

Network (Layer 3): routing of packets or
datagrams from source to destination
 IP, routing protocols

Link (Layer 2): data transfer between
neighboring network elements
 Ethernet, 802.11 (Wi-Fi), PPP

Physical (Layer 1): bits on medium
Application
Transport
Network
Link
Physical
mac:hardware address
IP: virtual address of the machine
port nb differentiates b/w the different addresses
Encapsulation
source
message
segment
HTt
H
M
M
datagram H
HN
n HT M
frame HL HN HT M
application
transport
network
link
physical
i write the message. it is given to the servers, the transport
layer will encapsulate it and add a header( sar esma segment l
message)
it adds a port number, with a source and a destination.
it goes to the network layer, the segment becomes a
datagram. we add another header( IP address of source
(ana), and destination(final).
now in the link layer, it becomes a frame,
so it's gonna add both mac addresses of source and
destination
we check if It has seen this IP before( within my network),
then it will directly allow it, else it will check for a default
gateway(router), then we take it to the link layer,and it will add
its mac address and the other mac address
M
HT
M
H’N HT
M
H’L H’N HT
M
destination
H’N HT
M
application
transport
network
link
physical
H’L H’N HT
M
layers 1 and 2 are point to point,
layers 3, 4 , 5 are end to end
network
link
physical
HN HT
M
Layer-3
router
to the
switch if
there was
one
the router will take it in, will decapsulate it, and we give it to the network
layer that will check the IP address.
Again: OSI reference model
always 7 layers for this
reference model
Network standardization
 Standards define what is needed for
interoperability
 Some of the many standards bodies:
Body
Area
Examples
ITU
Telecommunications
G.992, ADSL
H.264, MPEG4
IEEE
Communications
802.3, Ethernet
802.11, Wi-Fi
IETF
Internet
RFC 2616, HTTP/1.1
RFC 1034/1035, DNS
W3C
Web
HTML5 standard
CSS standard
 Others?
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
k is always 1000 except for file
sizes, because of the storage of bits
in memory is to the power of 2
ONLY for file sizes
Metric units
b:bit
-1Kb=1024 bits=2^10 bits
-1Mb= 2^20 bits
The main prefixes we use:
Prefix Exp.
1B(1 byte)= 8 bits
prefix exp.
K(ilo) or k 103
m(illi) 10-3
M(ega) 106
μ(micro) 10-6
G(iga) 109
n(ano) 10-9
 Use powers of 10 in networks
• Example: 1 Mbps = 1,000,000 bits per second
• Frame of size 4 kbit = 4000 bits
• Frame of size 1 kB = 1000 bytes = 8000 bits
 “B” is for bytes, “b” is for bits
 Do not confuse with memory/file sizes
• KB there is 1024 bytes
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Switching in networks


Network core consists of a
mesh of interconnected nodes
(switches, routers, …)
Two main switching
techniques
 Circuit Switching (telephone
networks)
 Packet Switching (Internet)
Circuit switching
End-end resources allocated
to, reserved for “call”
between source &
destination:


In diagram, each link has four
sub-links.
 call gets 2nd sub-link in top
link and 1st sub-link in right
link.
 Resources also allocated in
switches
dedicated resources: no sharing
 circuit-like (guaranteed)
performance
properties
1- no sharing
2- Need a set up
Circuit switching (2)



circuit sub-link is idle if not
used by call (no sharing)
Call setup often required
Commonly used in traditional
telephone networks
Circuit switching: FDM and TDM
Example:
Frequency Division Multiplexing
4 users
Every user is given a certain frequency that is allowed to send on it the whole time
total
bandwidth
frequency
Time Division Multiplexing
time
Every user is allowed to send on all frequencies but on a certain time
frequency
time
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 Mbps
 Each link uses TDM with 20 time-slots
 It takes 500 msec to establish the end-to-end circuit
Solution



Initial time to setup circuit = Tsetup = 500 msec.
Ttransmission =
time needed to
transmit N bits
over circuit
=
N (bits)
R (bits per second)
What is R in this case?
 All links are 1 Mbps, but there are 20 time-slots
 Each call or circuit gets 106/20 = 50 000 bps

Ttransmission = 640 000 / 50 000 = 12.8 sec
Total time to send = Tsetup + Ttransmission = 13.3 sec

When does the file arrive completely at destination?

the propagation delay is directly dependent on the distance between the two points, and the sped of the
signal within the medium.
Propagation delay
A
propagation
B
time needed for
dprop=signal to travel in = Distance from A to B (m)
medium
Signal speed in medium (m/sec)
from A to B

Signal speed is usually c = speed of light, down to 0.6c
 Example: what is the propagation delay in a 1000 km fiber,
with a signal speed of 0.67c?
 dprop = 106 m /(0.67✕3✕108 m/sec) = 4.975 msec
Space-time graph
A
B
going down is the actual time
Assumes setup messages
are very small (have zero
transmission time)
propagate
Also assumes that
propagation delays are the
same in the two directions
sender's perspective ma fiya propagation time
Example
2^20x8 bits


Sending 1 MByte file over 1 Mbps link versus 1 Gbps link, both with 100 ms
Round Trip Time (RTT) set up time
Where did RTT come from?
work done in a period of time

Compare effective “Throughput” from sender’s perspective:
Transfer Size / (total) Transfer Time

Transfer Time = Tsetup + Ttransmission
(setup message size is very small)
= RTT + TransferSize/BitRate
 1 Mbps case:
 Transfer Time = 100 msec + 220 x 8 / 106 = 0.1 + 8.39 = 8.49 s
 Throughput = 220 x 8 / 8.49 s = 0.99 Mbps … 99% of link bit rate
 1 Gbps case:
 Transfer Time = 100 msec + 220 x 8 / 109 = 108.4 ms
 Throughput = 220 x 8 / 0.1084 s = 77.4 Mbps … only 8% of link bit rate
Packet switching

Packet-switching: hosts break
application data into packets
 Host sends packets into network
 Packets forwarded from one
router to the next, across links,
on path from source to
destination
 Each packet transmitted at full
link capacity
Sending packets of data
host sending function:
 takes application message
 breaks into smaller
chunks, forms packets, of
length L bits
 transmits packet into
network at full link
transmission rate Rf
 full link bitrate
packet
transmission
time
=
two packets,
L bits each
2 1
Rf: Full link bitrate
host
time needed to
transmit L-bit
packet over link
=
L (bits)
Rf (bits/sec)
Packet switching: Store-and-Forward
L bits
per packet
source


3 2 1
Rf1 bps
takes L /Rf seconds to
transmit (push out) L-bit
packet onto link at Rf bps
Store-and-Forward: entire
packet must arrive at
intermediate node (router)
before it can be transmitted
on next link
Rf2 bps
destination
For N links and N – 1 routers:
- The packet will be
transmitted N times
- Total transmission time is
L /Rf1+ L /Rf2+…+ L /RfN
Packet switching: processing and queuing delays
B

R = 1.5 Mbps
D
queue of packets
waiting for output link
E
processing



C
R = 100 Mbps
A
packets need to be “looked at”, checked, and processed
the time this takes is referred to as Processing Delay
queuing


if arrival rate to link exceeds its transmission rate for a period
of time, packets will “queue”, wait to be transmitted on link
this is referred to as Queuing Delay
Packet switching: queuing delay and loss
A
B
C
R = 100 Mb/s
R = 1.5 Mb/s
D
queue of packets
waiting for output link
queuing and loss:


when packets queue, they are stored in a memory (buffer)
buffer space is limited, and packets can be dropped (lost) if
buffer memory fills up
E
Four sources of packet delay
transmission
A
B
propagation
nodal
processing
queueing
delay = dproc + dqueue + dtransmission + dpropagation
dproc: processing time
 check packet for errors
 determine output link
dqueue: queueing delay
 time waiting in output
queue for transmission
 depends on congestion
level of node/router
Four sources of packet delay
transmission
A
B
propagation
nodal
processing
queueing
dtotal = dproc + dqueue + dtransmission + dpropagation
dtransmission: transmission delay
 L: packet length (bits)
 Rf: link bitrate (bps)
 dtransmission = L/Rf
dpropagation: propagation delay
 d: length of physical link in m
 s: propagation speed in medium
(~2 to 3x108 m/sec)
 dpropagation = d/s
Note on jitter

Variation in delay between packets is called jitter
 important for audio/video applications
 how do we deal with jitter?
Packets
4
3
Interpacket gap
2
1
4
Network
3
2
1
Packet switching versus circuit switching
packet switching allows more users to use network!
example:
 1 Mbps link
 each user:
• 100 kbps when “active”
N
users
1 Mbps link
 circuit-switching:
 Can support 1000 kbps / 100 kbps = 10 users
with dedicated resources

But each user is active 15% of the time
 packet
switching:
 with 26 users, probability > 10 active
at same time is 0.00074
Q: how did we get 0.00074?
Q: what happens if > 10 users?
This corresponds to just one
minute in a 24 hour period.
Psending[Users_, n_, p_] := Binomial[Users, n] p^n (1 - p)^(Users - n)
TotalUsers = 26;
ProbOneUserSending = 0.15;
ProbMoreThan10 =
Sum[Psending[TotalUsers, i, ProbOneUserSending], {i, 11, TotalUsers}];
MinutesPerDay = ProbMoreThan10*24*60
Out = 1.06803
Packet switching versus circuit switching
is packet switching always “better?”



great for bursty data
 resource sharing
 simpler
excessive congestion possible: packet delay and loss
 protocols needed for reliable data transfer, congestion
control – tend to complicate things
Q: How to provide circuit-like behavior?
 bandwidth guarantees needed for audio/video apps
 challenging problem