Download Lecture 1

Introduction to Communication
Networks –67594
Dr. Michael Schapira
Rothberg A413
[email protected]
Some of the slides were taken from Prof. Scott Shenker, UC Berkeley
Administrative
• Lectures on Sundays 10:00-11:45 (Michael)
• Tutorials on Tuesdays and Thursdays
14:00-15:45 (Nir)
• Tutorials are important!
– Repeating material that was taught in the lectures
– Introducing new subjects that complements the
subjects in class.
– Teaching mathematical material that will be needed
both for lectures and homeworks.
Homeworks and Grading
• Four assignments: 30%
– Mandatory
– Each assignments 7.5%
– Need average grade>54 to pass
• Final exam: 70%
– Depending on the number of assignments we end
up with
– Need grade>54 to pass
Course Books
• Computer Networking: A Top Down Approach
(5th or 6th Edition) – J. Kurose, K. Ross
• Computer Networks: A System approach (5th
Edition) – L. L. Peterson, B. S. Davie
• Network Algorithmics: An Interdisciplinary
Approach to Designing Fast Networked Devices.
G. Varghese, 2004.
What Will You Learn?
• Key concepts in networking
– Different ways to route?
– What is congestion control?
• Domain-specific knowledge: how the
Internet works
– What does an IP packet look like?
– How can a single typo bring down a third of
the Internet?
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Why is Networking Fascinating?
• The Internet has had a tremendous impact
• The Internet changed the networking
paradigm
• The design of the Internet presents
interesting intellectual challenges
• Many of these intellectual challenges
remain unsolved
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Impact
• Internet changed the way we gather
information
– Web, search engines
• Internet changed the way we relate to each
other
– Email, facebook, twitter
• Which would you choose?
– Computers without the Internet (standalone PCs)
– Internet without computers (or really old ones)
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Intellectual Challenges
• Connecting two computers is easy
– So why is designing the Internet hard?
• Internet must cope with unprecedented
scale, diversity and dynamic range
– More about this later in lecture….
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Unsolved challenges
• Security
– Security of infrastructure
– Security of users
• Availability
– Internet is very resilient
– But availability is not sufficient for critical
infrastructures
• Evolution
– It is too hard to change the Internet architecture
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Quote from John Day
(Internet Pioneer)
There is a tendency in our field to believe that
everything we currently use is a paragon of
engineering, rather than a snapshot of our
understanding at the time. We build great
myths of spin about how what we have
done is the only way to do it to the point that
our universities now teach the flaws to
students (and professors and textbook
authors) who don't know better.
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Computer Network’s Ultimate Goal
Transmitting data
between end-users
Computer Network: Bird’s Eye view
End Users
(hosts,
terminals,
stations)
“The Network”
Links
Nodes
(routers,
switches)
(optical fiber,
wireless)
Networks Come at Different Sizes
Internet: 2,038,600,000 Users (2010), 4.99M in Israel
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Small Home/Office Network
Diversity of End Users
•
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Cell phones
Supercomputer clusters
Tablets
Televisions
Gaming consoles
Web cams
Automobiles
Sensing devices
Picture frames
Security systems
Power grid
……
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Diversity of Network Devices
Links
Interfaces
Fibers
Ethernet card
Nodes
Large router
Wireless card
Coaxial Cable
Telephone
switch
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Diversity of (Application) Requirements
•
•
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•
•
Size of transfers
Bidirectionality (or not)
Latency sensitive (or not)
Tolerance of jitter (or not)
Tolerance of packet drop (or not)
Need for reliability (or not)
Multicast (or not)
…..
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Computer Network vs.
Distributed System
• Computer network is the infrastructure and
technologies needed to transmit data between
hosts
• Distributed system is the set of hosts using the
computer network
The network itself does not produce data
(except control data to manage the network)
Properties of Links
• Delay (sometimes also called latency)
– Propagation time for data sent along the link
– Corresponds to the “length” of the link
• Bandwidth (capacity)
– Amount of data sent (or received) per unit time
– Corresponds to the “width” of the link
• Bandwidth-delay product: (BDP)
– Amount of data that can be “in flight” at any time
– Propagation delay × bits/time = total bits in link
bandwidth
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delay x bandwidth
delay
Throughput
• Fraction of time link is busy transmitting
– Often denoted by ρ
– Also, called the utilization of the link
• Ratio of arrival rate to bandwidth
– Arrival: A bits/sec on average
– Bandwidth: B bits/sec
– Throughput = A/B
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Speed of Light
• Question: how long does it take light to
travel from Jerusalem to New York?
• Answer:
– Distance Jerusalem  New York: 9,164 km
(great circle)
– Traveling 300,000 km/s: 30.55 msec
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Latency
• Propagation Delay
– Depends on distance, medium speed (e.g., 2/3 speed of
light)
• Transmission Time
– Depends on the bandwidth of the link and line-card
properties
• Queuing Delay
– Congestion within the network (competition)
– Will be discussed later…
• Either one-way, round-trip (depends on context)
Examples - Single Link
Bandwidth-Delay (BDP)
Transmission Times
Same city over slow link:
– Bwdth~100mbps
– Delay~.1msec
– BDP ~ 10000bits ~
1.25MBytes
Cross-country over fast link:
– Bwdth~10Gbps
– Delay~10msec
– BDP ~ 108bits ~
12.5GBytes
• 1500 byte packet over 14.4k
modem: ~1 sec
• 1500 byte packet over
10Gbps link: ~10-6sec
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Examples – End to End
• Question: how long does it take an Internet “packet” to
travel from Jerusalem to New York?
• Answer:
– For sure  30.55 msec
– Depends on:
• The route the packet takes (could be circuitous!)
• The propagation speed of the links the packet traverses
– E.g., in optical fiber light propagates at about 2/3 C
• The transmission rate (bandwidth) of the links (bits/sec)
– and thus the size of the packet
• Number of hops traversed (store-and-forward delay)
• The “competition” for bandwidth the packet encounters (congestion).
It may have to sit & wait in router queues.
– In practice this boils down to  70 msec
• Within Israel  10 msec
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Dynamic Range of All Properties…
• Round-trip times (delay/latency) from
10secs to seconds
– 5 orders of magnitude
• Data rates (bandwidth) from kbps to 100
Gbps
– 8 orders of magnitude
• Queueing delays in the network vary from 0
to seconds
• …..
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Latency and Implications to Networking
• Question: how many cycles does your PC execute before
it can possibly get a reply to a message it sent to a New
York web server?
• Answer:
– Round trip takes  140 msec
– PC runs at (say) 3 GHz
– 3,000,000,000 cycles/sec*0.14 sec = 420,000,000 cycles
= Forever!
– Communication feedback is always dated
– Communication fundamentally asynchronous
• Same even between machines that are directly connected
(via a local area network or LAN)?
– 0.2 ms = 200 sec = 600,000 cycles
– Still a loooong time … and asynchronous
Network Topologies
Objective: Connectivity between each pair of end-users
Network Topologies
Objective: Connectivity between each pair of end-users
Clique
• Pros: Each pair of hosts
has direct link. No
competition on
resources.
• Cons: EXPENSIVE!
(except in small
networks)
Network Topologies
Objective: Connectivity between each pair of end-users
Tree
Pros: Very cheap, only N
links required for N hosts.
Cons:
1. Failure-prone (what
happened if a single link
fails?)
2.Coordination/Congestion
resolution mechanisms are
needed.
Network Topologies
Objective: Connectivity between each pair of end-users
• In practice, most networks has some topology in
between, trading off resilience and cost.
• Some networks has a known topology
– E.g. Telephony, local area networks, data centers
• Other networks’ topology is not known
– Each node only knows its neighbors
– Learning the entire network is too time- and resourceconsuming
– All control is done in a distributed manner
Ad hoc Deployment
• Can’t assume carefully managed
deployment
– Network must work without planning
– Topologies are changed and are not regular
– “Network of Networks”
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More things to consider: Failures
• Consider communication that uses 50 components
– Assume each work correctly 99% of the time
– What is likelihood communication fails?
• Answer: success requires that they all function, so
failure probability = 1 - (.99)50 ≈ 39.5%
• Even if nodes are 99.9% reliable, failure probability is
still close to 5%...
• Must design the system to expect failure!
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More things to consider: Greed
• There are greedy people out there who
want to:
– Steal your financial information (bank, credit
card, etc.)
– Use your computer for attacks
• There is a thriving underground economy
for compromised computers and financial
information
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More things to consider: Malice
• There are malicious people out there who
want to:
– Bring your system down and/or steal
confidential data
• When attacker is a nation-state, attacks are
far harder to stop
– Many defensive techniques involve stopping
attacks that have been seen before
– But33 nation-states can use new attack vectors
Summary
• Modern network are very diverse
• Some of the design choices directly depends on
the settings
– E.g., the topology
• Some of the design “choices” are consequences
physics
– E.g., asynchronous transmissions
• Some of the design choices depends on user
behavior
– E.g. Security issue
Example: Telephone Network
• Alexander Graham Bell
– 1876: Demonstrates the telephone at US
Centenary Exhibition in Philadelphia
Telephone network uses circuit switching
• Establish: source creates circuit to destination
– Nodes along the path store connection info
– And reserve resources for the connection
– If circuit not available: “Busy signal”
• Transfer: source sends data over the circuit
– No destination address, since nodes know path
– Continual stream of data
• Teardown: source tears down circuit when done
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The switch in “circuit switching”
incoming links
Node
outgoing links
How does the node connect the incoming
link to the outgoing link?
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Circuit Switching With Human Operator
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“Modern” switches
• Almon Brown Strowger (1839 - 1902)
– 1889: Invents the “girl-less, cuss-less” telephone
system -- the mechanical switching system
Timing in Circuit Switching
Host 1
Switch 1
Switch 2
Host 2
time
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Timing in Circuit Switching
Host 1
Circuit
Establishment
Switch 1
Switch 2
Host 2
propagation delay
between Host 1
and Switch1
time
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Timing in Circuit Switching
Host 1
Switch 1
Switch 2
Host 2
Transmission delay
Circuit
Establishment
propagation delay
between Host 1
and Switch1
time
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Timing in Circuit Switching
Host 1
Switch 1
Switch 2
Host 2
Transmission delay
Circuit
Establishment
propagation delay
between Host 1
and Switch1
time
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Timing in Circuit Switching
Host 1
Switch 1
Switch 2
Host 2
Transmission delay
Circuit
Establishment
propagation delay
between Host 1
and Switch1
propagation delay
between Host 1
and Host 2
time
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Timing in Circuit Switching
Host 1
Switch 1
Switch 2
Host 2
Transmission delay
propagation delay
between Host 1
and Switch1
Circuit
Establishment
propagation delay
between Host 1
and Host 2
Transfer
Information
time
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Timing in Circuit Switching
Host 1
Switch 1
Switch 2
Host 2
Transmission delay
propagation delay
between Host 1
and Switch1
Circuit
Establishment
propagation delay
between Host 1
and Host 2
Transfer
Information
time
Circuit
Teardown
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Sharing a link
incoming links
Node
outgoing links
How do the black and orange circuits
share the outgoing link?
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Circuit Switching: Multiplexing a Link
– Each circuit allocated
certain time slots
time
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• Frequency-division
– Each circuit allocated
certain frequencies
frequency
• Time-division
time
Time-Division Multiplexing/Demultiplexing
Frames
Slots = 0 1 2 3 4 5
0 1 2 3 4 5
• Time divided into frames; frames into slots
• Relative slot position inside a frame determines to which
conversation data belongs
– E.g., slot 0 belongs to blue conversation
• Requires synchronization between sender and receiver
• Need to dynamically bind a slot to a conversation
• If a conversation does not use its circuit capacity is lost!
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Strengths of phone system
• Predictable performance
– Known delays
– No drops
• Easy to reason about
• Supports a crucial service
What about weaknesses?
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Weakness #1: Not resilient to failure
• Any failure along the path prevents
transmission
• Entire transmission has to be restarted
• “All or nothing” delivery model
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Weakness #2: Wastes bandwidth
• Consider a network application with:
– Peak bandwidth P
– Average bandwidth A
• How much does the network have to reserve for the
application to work?
– The peak bandwidth
• What is the resulting level of utilization (average
throughput)?
– Ratio
of A/P
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Smooth vs Bursty Applications
• Some applications have relatively small P/A ratios
– Voice might have a ratio of 3:1 or so
• Data applications tend to be rather bursty
– Ratios of 100 or greater are common
• Circuit switching too inefficient for bursty apps
• Generally:
– Don’t care about factors of two in performance
– But when it gets to several orders of magnitude….
Weakness #3: Designed Tied to App
• Design revolves around the requirements of
voice
• Not general feature of circuit switching
– But definitely part of the telephone network design
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Weakness #4: Setup Time
• Every connection requires round-trip time to
set up
– Slows down short transfers
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How to overcome these problems?
• Paul Baran, “On Distributed Communications” (1964)
– Distributed control
– Message blocks (packets)
– Store-and-forward delivery
• Analyzed by Kleinrock
– Statical multiplexing
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Taxonomy of Communication Networks
• Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
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Taxonomy of Communication Networks
• Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Broadcast
Communication
Network
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Broadcast Communication Networks
• Information transmitted by any node is
received by every other node in the network
– Usually only in LANs (Local Area Networks)
• E.g., WiFi, Ethernet (classical, but not current)
• E.g., lecture!
• What problems does this raise?
• Problem #1: limited range
• Problem #2: coordinating access to the shared
communication medium
– Multiple Access Problem
• Problem #3: privacy of communication
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Taxonomy of Communication Networks
• Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched
Communication
Network
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Broadcast
Communication
Network
Taxonomy of Communication Networks
• Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
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Broadcast
Communication
Network
Taxonomy of Communication Networks
• Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Switched
Communication
Network
Circuit-Switched
Communication
Network
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Broadcast
Communication
Network
Packet-Switched
Communication
Network (Datagram
Network)
Packets / Datagrams
• Payload (Body)
– Data being transferred
• Header
– Instructions to the network for how to handle
packet
Header
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Payload
Datagram Packet Switching
• Each packet is independently switched
– Each packet header contains full destination address
• No resources are pre-allocated (reserved) in
advance
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Timing of Datagram Packet Switching
Host 1
Node 1
Packet 1
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Node 2
propagation
delay between
Host 1 and
Node 1
Host 2
Timing of Datagram Packet Switching
Host 1
transmission
time of Packet 1
at Host 1
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Node 1
Packet 1
Node 2
propagation
delay between
Host 1 and
Node 1
Host 2
Timing of Datagram Packet Switching
Host 1
transmission
time of Packet 1
at Host 1
Node 1
Packet 1
Host 2
Node 2
propagation
delay between
Host 1 and
Node 1
Packet 1
processing
delay of
Packet 1 at
Node 2
Packet 1
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Timing of Datagram Packet Switching
Host 1
transmission
time of Packet 1
at Host 1
Node 1
Packet 1
Host 2
Node 2
propagation
delay between
Host 1 and
Node 1
Packet 2
Packet 1
Packet 3
processing
delay of
Packet 1 at
Node 2
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
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Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 4
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Node 7
Host E
Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 4
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Node 7
Host E
Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 4
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Node 7
Host E
Bursts
• Packets arrive at the nodes in different
patterns:
time
time
– Bursty/smooth sources
– Multiplexing different flows creates bursts
Buffers to mitigate bursts
• Due to bursts, sometime transient arrival rate is
larger than transmission rate
– Even if long-term arrival rate is smaller than
transmission rate
• One possibility: drop packets
• Better approach: add buffers to save excess
packets (until the bursts ends)
– Even when buffers, sometimes packets are lost
– Even with infinite buffers, sometimes packets are
lost
Basic Queueing Theory Terminology
• Arrival process: how packets arrive
– Average rate A
– Peak rate P
• Service process: transmission times
– Average transmission time
– For networks, function of packet size
• W: average time packets wait in the queue/buffer
– W for “waiting time”
• L: average number of packets waiting in the queue
– L for “length of queue”
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Statistical Multiplexing
Three Flows with Bursty Arrivals
Data Rate 1
Time
Data Rate 2
Capacity
Time
Data Rate 3
Time
When Each Flow Gets 1/3rd of Capacity
Data Rate 1
Frequent Overloading
Time
Data Rate 2
Time
Data Rate 3
Time
When Flows Share Total Capacity
Time
No Overloading
Time
Statistical multiplexing relies on the assumption
that not all flows burst at the same time.
Very similar to insurance,
and has same failure case
Time
Another Take on “Stat Mux”
• Assume time divided into frames
– Frames divided into slots
Frame
• Flows generate packets during each frame
Slots
– Peak number of packets/frame P
– Average number of packets/frame A
• Single flow: must allocate P slots to avoid drops
– But P might be much bigger than A
– Very wasteful!
• Use the
“Law of Large Numbers”….
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Law of Large Numbers (~1713)
• Consider any probability distribution
– Can be highly variable, such as varying from 0 to P
• Take N samples from probability distribution
– In this case, one set of packets from each flow
• Thm: the sum of the samples is very close to N×A
– And gets percentage-wise closer as N increases
• Sharing between many flows (high aggregation),
means that you only need to allocate slightly more
than average A slots per frame.
– Sharing smooths out variations
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So, if you were building a network….
• Which would you choose?
– Circuit switched?
– Packet-switched?
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