Computer Networking

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Computer Networking
Yishay Mansour (mansour at cs.tau.ac.il)
Teaching Assistant Hillel Avni
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Course Information

• Lectures Sunday 4 7
  Orenstein 111
• Exercises Wednesday 11 12, 12 1 Super
  Center 315 ????? ??

Web site http//www.cs.tau.ac.il/research/hille
l.avni/courses/comnet10.html
Books

• A Top-down Approach to Computer Networking /
  Kurouse-Ross

• An Engineering Approach to Computer Networking /
  Keshav
• Computer Networks / Tanenbaum
• Data Networks / Bertsekas and Gallager

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Practical Information
Homework assignment Mandatory Both
theoretical and programming
Grades Final Exam 60 theory exercises
20 Programming exercises 20
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Motivation

• Todays economy
• manufacturing, distributing, and retailing goods
• but also creating and disseminating information
• publishing
• banking
• film making.
• part of the information economy
• Future economy is likely to be dominated by
  information!

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Information?

• A representation of knowledge
• Examples
• books
• bills
• CDs DVDs
• Can be represented in two ways
• analog (atoms)
• digital (bits)
• the Digital Revolution
• convert information as atoms to information as
  bits
• use networks to move bits around instead of atoms

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The Challenges

• represent all types of information as bits.
• move the bits
• In large quantities,
• everywhere,
• cheaply,
• Securely,
• with quality of service,
• .

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Todays Networks are complex!

• hosts
• routers
• links of various media
• applications
• protocols
• hardware, software

Tomorrows will be even more!
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Internet Physical Infrastructure

• Residential access
• Cable
• Fiber
• DSL
• Wireless

• The Internet is a network of networks
• Each individually administrated network is called
  an Autonomous System (AS)

• Campus access, e.g.,
• Ethernet
• Wireless

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This courses Challenge

• To discuss this complexity in an organized way,
  that will make todays computer networks (and
  their limitations) more comprehensive.
• identification, and understanding relationship of
  complex systems pieces.
• Problems that are beyond a specific technology

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Early communications systems

• I.e. telephone
• point-to-point links
• directly connect together the users wishing to
  communicate
• use dedicated communication circuit
• if distance between users increases beyond the
  length of the cable, the connection is formed by
  a number of sections connected end-to-end in
  series.

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Data Networks

• set of interconnected nodes exchange information
• sharing of the transmission circuits
  "switching".
• many links allow more than one path between every
  2 nodes.
• network must select an appropriate path for each
  required connection.

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Qwest backbone
http//www.qwest.com/largebusiness/enterprisesolut
ions/networkMaps/preloader.swf
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Networking Issues - Telephone

• Addressing - identify the end user
• phone number 1-201-222-2673 country code city
  code exchange number
• Routing - How to get from source to destination.
• Telephone circuit switching Based on the phone
  number.
• Information Units - How is information sent
• telephone Samples _at_ Fixed sampling rate. not self
  descriptive! have to know where and when a sample
  came

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Networking Issues - Internet

• Addressing - identify the end user
• IP addresses 132.66.48.37, Refer to a host
  interface network number host number
• Routing- How to get from source to destination
• Packet switching move packets (chunks) of data
  among routers from source to destination
  independently.
• Information Units - How is information sent.
• Self-descriptive data packet data metadata
  (header).

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• Telephone networks support a single, end-to-end
  quality of service but is expensive to boot

Internet supports no quality of service but is
flexible and cheap
Future networks will have to support a wide range
of service qualities at a reasonable cost
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History 1961-1972 Early packet-switching
principles

• 1961 Kleinrock - queuing theory shows
  effectiveness of packet-switching
• 1964 Baran - packet-switching in military
  networks
• 1967 ARPAnet conceived by Advanced Research
  Projects Agency
• 1969 first ARPAnet node operational

• 1972 ARPAnet demonstrated publicly
• NCP (Network Control Protocol) first host-host
  protocol
• first e-mail program
• ARPAnet has 15 nodes

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History 1972-1980 Internetworking, new and
proprietary nets

• 1970 ALOHAnet satellite network in Hawaii
• 1973 Metcalfes PhD thesis proposes Ethernet
• 1974 Cerf and Kahn - architecture for
  interconnecting networks
• late70s proprietary architectures DECnet, SNA,
  XNA
• late 70s switching fixed length packets (ATM
  precursor)
• 1979 ARPAnet has 200 nodes

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Cerf and Kahns internetworking principles

• minimalism, autonomy - no internal changes
  required to interconnect networks
• best effort service model
• stateless routers
• decentralized control

Defines todays Internet architecture
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History 1980-1990 new protocols, proliferation
of networks

• 1983 deployment of TCP/IP
• 1982 SMTP e-mail protocol defined
• 1983 DNS defined for name-to-IP-address
  translation
• 1985 FTP protocol defined
• 1988 TCP congestion control

• new national networks CSnet, BITnet, NSFnet,
  Minitel
• 100,000 hosts connected to confederation of
  networks

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History 1990 - commercialization and WWW

• early 1990s ARPAnet decommissioned
• 1991 NSF lifts restrictions on commercial use of
  NSFnet (decommissioned, 1995)
• early 1990s WWW
• hypertext Bush 1945, Nelson 1960s
• HTML, http Berners-Lee
• 1994 Mosaic, later Netscape
• late 1990s commercialization of WWW

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Demand and Supply

• Huge growth in users
• The introduction of the web
• Faster home access
• Better user experience.
• Infrastructure
• Significant portion of telecommunication.
• New evolving industries
• Although, sometimes temporary setbacks

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Internet Users
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Penetration around the Globe (2009)
http//www.internetworldstats.com/stats.htm
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Users around the Globe (2002/5/9)
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Technology Modem speed
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Todays options

• Modem 56 K
• ISDN 64K 128K
• Frame Relay 56K
• Today High Speed Connections
• Cable, ADSL, Satellite.
• All are available at
• 5Mb (2005)
• 30 Mb (2009)

OBSOLETE
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Coming soon (1999)
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Today (2005)
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Why do we need Standards

• Networks (and other media) support communication
  between different entities
• Need agreement to ensure correct, efficient and
  meaningful communication

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Various Organizations Issue Standards

• IEEE (Institute for Electrical and Electronic
  Engineers)
• IETF (Internet Engineering Task Force)
• ITU (International Telecommunications Union)
• ISO (International Organization for
  Standardization)
• W3C (World Wide Web Consortium)

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Protocol Layers

• A way for organizing structure of network

• Or at least our discussion of networks

• The idea a series of steps

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Protocol Layering

• Necessary because communication is complex
• Intended primarily for protocol designers
• Divides the problem into intellectually
  manageable pieces
• Provides a conceptual framework that can help us
  understand protocols
• Think of layering as a guideline, not a rigid
  specification
• Understand that optimizations may violate strict
  layering
• Should be invisible to users

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Mail system functionality
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How do we Communicate?
Hollywood, California

• Send a mail from Alice to Bob
• Alice in Champaign, Bob in Hollywood
• Example
• US Postal Service

Bob
Champaign, Illinois
Alice
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What does Alice do?
Alice 200 Cornfield Rd. Champaign, IL 61820
Bob 100 Santa Monica Blvd. Hollywood, CA 90028

• Bobs address (to a mailbox)
• Bobs name in case people share mailbox
• Postage have to pay!
• Alices own name and address
• in case Bob wants to return a message
• In case the mail has to be returned.

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What does Bob do?
Alice 200 Cornfield Rd. Champaign, IL 61820
Bob 100 Santa Monica Blvd. Hollywood, CA 90028

• Install a mailbox
• Receive the mail
• Get rid of envelope
• Read the message

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Layers
Person delivery of parcel Post office counter
handling Ground transfer loading on trucks
Airport transfer loading on airplane Airplane
routing from source to destination
Peer entities

• each layer implements a service
• via its own internal-layer actions
• relying on services provided by layer below

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Advantages of Layering

• explicit structure allows identification
  relationship of complex systems pieces
• layered reference model for discussion
• modularization eases maintenance updating of
  system
• change of implementation of layers service
  transparent to rest of system

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Protocols

• A protocol is a set of rules and formats that
  govern the communication between communicating
  peer
• set of valid messages - syntax
• meaning of each message - semantics
• Necessary for any function that requires
  cooperation between peers

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Protocols

• A protocol provides a service
• For example the post office protocol for
  reliable parcel transfer service
• Peer entities use a protocol to provide a service
  to a higher-level peer entity
• for example, truck drivers use a protocol to
  present post offices with the abstraction of an
  unreliable parcel transfer service

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Protocol Layers

• A network that provides many services needs many
  protocols
• Some services are independent, But others depend
  on each other
• A Protocol may use another protocol as a step in
  its execution
• for example, ground transfer is one step in the
  execution of the example reliable parcel transfer
  protocol
• This form of dependency is called layering
• Post office handling is layered above parcel
  ground transfer protocol.

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Open protocols and systems

• A set of protocols is open if
• protocol details are publicly available
• changes are managed by an organization whose
  membership and transactions are open to the
  public
• A system that implements open protocols is called
  an open system
• International Organization for Standards (ISO)
  prescribes a standard to connect open systems
• open system interconnect (OSI)
• Has greatly influenced thinking on protocol stacks

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ISO OSI reference model

• Reference model
• formally defines what is meant by a layer, a
  service etc.
• Service architecture
• describes the services provided by each layer and
  the service access point
• Protocol architecture
• set of protocols that implement the service
  architecture
• compliant service architectures may still use
  non-compliant protocol architectures

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The seven Layers
There are only 5 !!
Application
Intermediate system
End system
End system
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The seven Layers - protocol stack
data
TH
Network
Data Link
DHdataDT
Physical
bits

• Session and presentation layers are not so
  important, and are often ignored

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?????? ??????
????? X ?????? ????? ??? ?????? ????? X ???? ???
?????
Destination
Source
Application
Application
Identical message
Transport
Transport
Identical message
Network
Network
Identical message
Data-Link
Data-Link
Network
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Postal network

• Application people using the postal system
• Session and presentation chief clerk sends some
  priority mail, and some by regular mail
  translator translates letters going abroad.
• Transport layer mail clerk sends a message,
  retransmits if not acked
• Network layer postal system computes a route and
  forwards the letters
• datalink layer letters carried by planes,
  trains, automobiles
• physical layer the letter itself

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Internet protocol stack

• application supporting network applications
• ftp, smtp, http
• transport host-host data transfer
• tcp, udp
• network routing of datagrams from source to
  destination
• ip, routing protocols
• link data transfer between neighboring network
  elements
• ppp, ethernet
• physical bits on the wire

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Protocol layering and data
source
destination
message
application transport network Link physical
segment
datagram
frame
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Physical layer L1

• Moves bits between physically connected
  end-systems
• Standard prescribes
• coding scheme to represent a bit
• shapes and sizes of connectors
• bit-level synchronization
• Internet
• technology to move bits on a wire, wireless link,
  satellite channel etc.

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Datalink layer L2

• (Reliable) communication over a single link.
• Introduces the notion of a frame
• set of bits that belong together
• Idle markers tell us that a link is not carrying
  a frame
• Begin and end markers delimit a frame
• Internet
• a variety of datalink layer protocols
• most common is Ethernet
• others are FDDI, SONET, HDLC

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Datalink layer (contd.)

• Ethernet (broadcast link)
• end-system must receive only bits meant for it
• need datalink-layer address
• also need to decide who gets to speak next
• these functions are provided by Medium ACcess
  sublayer (MAC)

• Datalink layer protocols are the first layer of
  software
• Very dependent on underlying physical link
  properties
• Usually bundle both physical and datalink in
  hardware.

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Network layer L3

• Carries data from source to destination.
• Logically concatenates a set of links to form the
  abstraction of an end-to-end link
• Allows an end-system to communicate with any
  other end-system by computing a route between
  them
• Hides idiosyncrasies of datalink layer
• Provides unique network-wide addresses
• Found both in end-systems and in intermediate
  systems

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Network layer types

• In datagram networks
• provides both routing and data forwarding
• In connection-oriented network
• separate data plane and control plane
• data plane only forwards and schedules data
  (touches every byte)
• control plane responsible for routing,
  call-establishment, call-teardown (doesnt touch
  data bytes)

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Network layer (contd.)

• Internet
• network layer is provided by Internet Protocol
  (IP)
• found in all end-systems and intermediate systems
• provides abstraction of end-to-end link
• segmentation and reassembly
• packet-forwarding, routing, scheduling
• unique IP addresses
• can be layered over anything, but only
  best-effort service

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Network layer (contd.)

• At end-systems
• primarily hides details of datalink layer
• segments and reassemble
• detects errors

• At intermediate systems
• participates in routing protocol to create
  routing tables
• responsible for forwarding packets
• schedules the transmission order of packets
• chooses which packets to drop

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Transport layer L4

• Reliable end-to-end communication.
• creates the abstraction of an error-controlled,
  flow-controlled and multiplexed end-to-end link
• (Network layer provides only a raw end-to-end
  service)
• Some transport layers provide fewer services
• e.g. simple error detection, no flow control, and
  no retransmission

• Internet
• TCP provides error control, flow control,
  multiplexing
• UDP provides only multiplexing

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Transport layer (contd.)

• Error control
• GOAL message will reach destination despite
  packet loss, corruption and duplication
• ACTIONS retransmit lost packets detect,
  discard, and retransmit corrupted packets detect
  and discard duplicated packets
• Flow control
• match transmission rate to rate currently
  sustainable on the path to destination, and at
  the destination itself
• Multiplexes multiple applications to the same
  end-to-end connection
• adds an application-specific identifier (port
  number) so that receiving end-system can hand in
  incoming packet to the correct application

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Session layer

• Not common
• Provides full-duplex service, expedited data
  delivery, and session synchronization
• Internet
• doesnt have a standard session layer

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Session layer (cont.)

• Duplex
• if transport layer is simplex, concatenates two
  transport endpoints together
• Expedited data delivery
• allows some messages to skip ahead in end-system
  queues, by using a separate low-delay transport
  layer endpoint
• Synchronization
• allows users to place marks in data stream and to
  roll back to a prespecified mark

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Presentation layer

• Usually ad hoc
• Touches the application data
• (Unlike other layers which deal with headers)
• Hides data representation differences between
  applications
• characters (ASCII, unicode, EBCDIC.)
• Can also encrypt data
• Internet
• no standard presentation layer
• only defines network byte order for 2- and 4-byte
  integers

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Application layer

• The set of applications that use the network
• Doesnt provide services to any other layer

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?????? ??????
Destination
Source
VoIP
Application
Email(smtp)
ftp
UDP
Transport
TCP
Network (IPv4)
Network
Ethernet
Data-Link
WiFi
Modem
Network
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?????? ??????
Destination
Source
app3
app1
app2
UDP
TCP
Network (IPv4)
Ethernet
WiFi
Modem
Network
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Discussion

• Layers break a complex problem into smaller,
  simpler pieces.
• Why seven layers?
• Need a top and a bottom ? 2
• Need to hide physical link so need datalink ? 3
• Need both end-to-end and hop-by-hop actions so
  need at least the network and transport layers ?
  5

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Course outline