The%20Internet:%20Packet%20Switching%20and%20Other%20Big%20Ideas

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The InternetPacket Switching and Other Big Ideas

• Ian Foster

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The Internet
1969 4 nodes
2004 100s of millions
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The Internet

• Clearly a huge success in terms of not only
  impact but also scalability
• Some (not all) of the basic notions have scaled
  over eight orders of magnitude
• What were underlying big ideas? Lets say
• Packet switching
• End-to-end principle
• Internet community standards process
• Also other important algorithms, e.g.
• Routing, naming, multicast
• Common thread (fairly) robust emergent behaviors
  from simple local strategies

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Overview

• Birth of the Internet
• Packet switching
• Process and governance
• End-to-end principle
• E.g., congestion avoidance and control
• Decentralized, adaptive algorithms
• Routing
• Naming
• Multicast

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Simple Switching Network
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Problem Statement

• Many stations connected by point-to-point
  connections (with some redundancy)
• Enable any station to send messages to any
  other station, despite diverse failure modes
• And further
• Be efficient in use of network resources
• Support stations of diverse capabilities
• Support diverse applications behaviors,
  including many not yet known (!)

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Traditional ApproachCircuit Switching

• A dedicated communication path between the two
  stations
• Communication involves
• Circuit Establishment
• Point to Point from terminal node to network
• Internal Switching and multiplexing among
  switching nodes.
• Data Transfer
• Circuit Disconnect
• E.g., the telephone network

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Circuit Switching

• Once connection is established
• Network is transparent
• Nodes seems to be directly connected
• Fixed data rate with no delay
• However
• Can be inefficient resources are dedicated to
  connection even if no data is sent
• Delay prior to usage of connection

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Public Switching Telecommunication Network

• The generic component of the public switching
  telecommunication network is divided into
• Subscribers
• Local loop (connects subscribers to the network)
• Exchange (switching centers)
• (end office)
• Trunks (connection between exchanges)
• (carry multiple voice channels using FDM or
  STDM)

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History of the InternetApplication Pull

• Emergence of (timeshared) computers supporting
  interactive use
• J.C.R. Licklider of MIT, proposes a global
  network of computers
• L.C.R. Licklider W. Clark, "On-Line Man
  Computer Communication", August 1962.
• A globally interconnected set of computers
  through which everyone could quickly access data
  and programs from any site
• Moves to the Advanced Research Projects Agency
  (ARPA) late in 1962
• Lobbies to realize his vision

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Licklider As VisionaryMan-Computer Symbiosis
(1960)

• is an expected development in cooperative
  interaction between men and electronic computers.
  The main aims are
• to let computers facilitate formulative thinking
  as they now facilitate the solution of formulated
  problems, and
• to enable men and computers to cooperate in
  making decisions and controlling complex
  situations without inflexible dependence on
  predetermined programs.

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Technology Push

• 1962 Paul Baran
• Commissioned by the U.S. Air Force to study how
  it could maintain command and control over its
  missiles and bombers after a nuclear attack
• Invents packet switching ideas (but talks about a
  Distributed Adaptive Message Block Network)
• 1961-65 Leonard Kleinrock (MIT ? UCLA)
• Develops the theory of packet switching
• 1965 Donald Davies in the UK
• Independently invents packet switching, coins
  the term packet

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Baran (1964)

• There is an increasingly repeated statement made
  that one day we will require more capacity for
  data transmission than needed for analog voice
  transmission. If this statement is correct, then
  it would appear prudent to broaden our planning
  consideration to include new concepts for future
  data network directions. Otherwise, we may
  stumble into being boxed in with the
  uncomfortable restraints of communications links
  and switches originally designed for high quality
  analog transmission. New digital computer
  techniques using redundancy make cheap unreliable
  links potentially usable. A new switched network
  compatible with these links appears appropriate
  to meet the upcoming demand for digital service.
  This network is best designed for data
  transmission and for survivability at the outset.

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Baran (1964)

• The requirements for a future all-digital-data
  distributed network which provides common user
  service for a wide range of users having
  different requirements is considered. The use of
  a standard format message block permits building
  relatively simple switching mechanisms using an
  adaptive store-and-forward routing policy to
  handle all forms of digital data including
  "real-time" voice. This network rapidly responds
  to changes in the network status. Recent history
  of measured network traffic is used to modify
  path selection. Simulation results are shown to
  indicate that highly efficient routing can be
  performed by local control without the necessity
  for any central--and therefore vulnerable--control
  point.

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Barans ProposalA Packet Switched Network

• Packet switching is the breaking down of data
  into datagrams or packets that are labeled to
  indicate the origin and the destination of the
  information and the forwarding of these packets
  from one computer to another computer until the
  information arrives at its final destination
  computer. This was crucial to the realization of
  a computer network. If packets are lost at any
  given point, the message can be resent by the
  originator.

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Packet Switching

• Basic idea
• Data to be transmitted is divided into small
  packets of information and labeled to identify
  the sender and recipient
• Sent over a network and then reassembled at their
  destination
• If any packet did not arrive or was not intact,
  original sender requested to resend the packet
• Note that this implies (relative to circuit
  switching)
• Less state at intermediate nodes
• More flexibility in end system behaviors
• More efficient use of networks
• More sophistication at end points

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The Importance ofTechnology Trends

• Packet switching was arguably a logical
  consequence of Moores law
• Computers became fast enough to enable smart
  terminals able to perform substantial processing

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Theoretical Underpinnings

• Packet switching was new andradical in the
  1960s. In order to planto spend millions of
  dollars andstake my reputation, I needed
  tounderstand that it would work.Without
  Kleinrocks work onNetworks and Queuing Theory,
  Icould never have taken such a radicalstep. All
  the communicationscommunity argued that it
  couldntwork. This book was critical to
  mystanding up to them and betting thatit would
  work.
• Larry Roberts

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1969 Press ReleaseUCLA to be the First Station
in Nationwide Computer Network

• "As of now, computer networks are still in
  their infancy," says Dr. Kleinrock. "But as they
  grow up and become more sophisticated, we will
  probably see the spread of 'computer utilities'
  which, like present electric and telephone
  utilities, will service individual homes and
  offices across the country.

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History of the Internet

• 1968 ARPA awarded the ARPANET contract to BBN.
  BBN had selected a Honeywell minicomputer as the
  base on which they would build the switch. The
  physical network was constructed in 1969, linking
  four nodes University of California at Los
  Angeles, SRI (in Stanford), University of
  California at Santa Barbara, and University of
  Utah. The network was wired together via 50 Kbps
  circuits.
• Backbones 50Kbps ARPANET - Hosts 4
• 1972 First e-mail program created by Ray
  Tomlinson of BBN.ARPANET used the Network
  Control Protocol or NCP to transfer data. This
  allowed communications between hosts running on
  the same network.
• Backbones 50Kbps ARPANET - Hosts 23
• 1973 Development began on the protocol to be
  called TCP/IP, by a group headed by Vint Cerf
  from Stanford and Bob Kahn from ARPA. This new
  protocol was to allow diverse computer networks
  to interconnect and communicate with each other.
• Backbones 50Kbps ARPANET - Hosts 23

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History of the Internet

• 1974 First Use of term Internet by Vint Cerf and
  Bob Kahn in paper on Transmission Control
  Protocol.
• Backbones 50Kbps ARPANET - Hosts 23
• 1976 Dr. Robert M. Metcalfe develops Ethernet,
  which allowed coaxial cable to move data
  extremely fast. This was a crucial component to
  the development of LANs.The packet satellite
  project went into practical use. SATNET, Atlantic
  packet Satellite network, was born.UUCP
  (Unix-to-Unix CoPy) developed at ATT Bell Labs
  and distributed with UNIX one year later.DOD
  began to experiment with the TCP/IP protocol and
  soon decided to require it for use on ARPANET
• Backbones 50Kbps ARPANET, plus satellite and
  radio connections - Hosts 111

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History of the Internet

• 1979 USENET (the decentralized news group
  network) created based on UUCP. BITNET
  introduced the "store and forward" network, used
  for email and listservs.
• Backbones 50Kbps ARPANET, plus satellite and
  radio connections - Hosts 111
• 1981 NSF created backbone called CSNET 56 Kbps
  network for institutions without access to
  ARPANET.
• Backbones 50Kbps ARPANET, 56Kbps CSNET, plus
  satellite and radio connections - Hosts 213
• 1983 Internet Activities Board (IAB)
  created.On January 1st, every machine connected
  to ARPANET had to use TCP/IP. TCP/IP became the
  core Internet protocol and replaced NCP
  entirely.University of Wisconsin created Domain
  Name System (DNS), which translated domain names
  into corresponding IP numbers. No need to
  remember numbers!
• Backbones 50Kbps ARPANET, 56Kbps CSNET, plus
  satellite and radio connections - Hosts 562

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Process and Governance

• A key to the rapid growth of the Internet has
  been the free and open access to the basic
  documents, especially the specifications of the
  protocols
• Request for Comments (RFC) documents
• Rough Consensus and Running Code
• Frequent face-to-face meetings
• Heavy use of email
• Emphasis on implementation experiences

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RFCs For Example

• RFC 791
• INTERNET PROTOCOL
• DARPA INTERNET PROGRAM
• PROTOCOL SPECIFICATION
• September 1981
• This document specifies the DoD Standard Internet
  Protocol. This
• document is based on six earlier editions of the
  ARPA Internet Protocol
• Specification, and the present text draws heavily
  from them. There have
• been many contributors to this work both in terms
  of concepts and in
• terms of text. This edition revises aspects of
  addressing, error
• handling, option codes, and the security,
  precedence, compartments, and
• handling restriction features of the internet
  protocol.

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And

• Network Working Group
  D. Waitzman
• Request for Comments 1149
  BBN STC
• 
  1 April 1990
• A Standard for the Transmission of IP
  Datagrams on Avian Carriers
• Status of this Memo
• This memo describes an experimental method for
  the encapsulation of
• IP datagrams in avian carriers. This
  specification is primarily
• useful in Metropolitan Area Networks. This is
  an experimental, not
• recommended standard. Distribution of this
  memo is unlimited.
• Overview and Rationale
• Avian carriers can provide high delay, low
  throughput, and low
• altitude service. The connection topology is
  limited to a single
• point-to-point path for each carrier, used
  with standard carriers,
• but many carriers can be used without
  significant interference with
• each other, outside of early spring. This is
  because of the 3D ether
• space available to the carriers, in contrast
  to the 1D ether used by

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Overview

• Birth of the Internet
• Packet switching
• Process and governance
• End-to-end principle
• E.g., congestion avoidance and control
• Decentralized, adaptive algorithms
• Routing
• Naming
• Multicast

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GeneralizingThe End-to-End Principle

• Reliable systems tend to require end-to-end
  processing to operate correctly, in addition to
  any processing in intermediate systems
• End-to-end processing alone suffices to make the
  system operate intermediate processing stages
  are largely redundant
• Thus, intermediate processing can be made
  simpler, relying on end-to-end processing to make
  the system work
• This leads to the model of a dumb network with
  smart terminals, a completely different model to
  the previous paradigm of the smart network with
  dumb terminals

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End-to-End Principle AppliedEnd-to-End Transport

• Dumb network each node repeatedly
• Receives a packet, with destination info
• Forwards it towards destination, if it can, with
  time to live decremented
• Note Must maintain routing information
• Smart terminals are responsible for
• Generating packets
• Receiving packets
• Detecting and dealing with out of order and
  missing packets

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Datagram Lifetime

• Datagrams could loop indefinitely
• Consumes resources
• Transport protocol may need upper bound on
  datagram life
• Datagram marked with lifetime
• Time To Live field in IP
• Once lifetime expires, datagram discarded (not
  forwarded)
• Hop count
• Decrement time to live on passing through each
  router
• Time count
• Need to know how long since last router

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IP Fragmentation

• IP re-assembles at destination only
• Uses fields in header
• Data Unit Identifier (ID)
• Identifies end system originated datagram
• Source and destination address
• Protocol layer generating data (e.g., TCP)
• Identification supplied by that layer
• Data length
• Length of user data in octets
• Offset
• Position of fragment of user data in original
  datagram
• In multiples of 64 bits (8 octets)
• More flag
• Indicates that this is not the last fragment

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IPv4 Header
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Dealing with Failure

• Re-assembly may fail if some fragments get lost
• Need to detect failure
• Re-assembly time out
• Assigned to first fragment to arrive
• If timeout expires before all fragments arrive,
  discard partial data
• Use packet lifetime (time to live in IP)
• If time to live runs out, kill partial data

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Virtual Circuit vs. Datagram

• Datagram
• Each packet is treated independently.
• Each packet has a full address of the destination
• Routing decision is taken for each packet at each
  node
• Different packets of one message may take
  different routes
• Virtual Circuit
• A connection is setup prior to data transfer
• Each packet contains a VC identifier
• Routing decision is made once for all packets
• All packets follow the same route

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Virtual Circuit vs. Datagram

• Virtual Circuit
• Transmission order preserved
• Error control is provided
• One routing decision per connection
• Receiver prepared for transmission
• Delays in making a connection
• Poor adaptation to node failure
• Poor spreading of load

• Datagram
• Call setup time is avoided
• Fast adaptation to congestion control
• Fast adaptation to node failure
• Transmission order is not preserved
• High load due to route processing (decision per
  packet)
• Receiver has no preparation for incoming
  transmissions

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Why Packetize

• For end-to-end route compromising of many links,
  packetizing allows for parts of message to be
  received, processed, and forwarded while others
  are still being prepared
• Amount of retransmitted data due to errors is
  reduced
• Memory capacity of internal network nodes can be
  reduced
• Transmission time can be reduced

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Effect of Packet Size on Transmission Time
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Circuit Switching vs. Packet Switching
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SummaryBig Ideas Underlying the Internet

• Packet switching
• Flexible, robust, efficient (in the network)
• Enabled by smart terminals
• End-to-end arguments in system design
• E.g., reliable in-order delivery via TCP
• Rough consensus and running code
• As a means of creating and evolving a complex
  artifact

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Further Reading

• Introduction to distributed communication network
• http//www.rand.org/publications/RM/RM3420
• A digital communications network for computers
• http//portal.acm.org/citation.cfm?id800001.81166
  9
• The Evolution of Packet Switching
• http//www.packet.cc/files/ev-packet-sw.html
• End-to-end arguments in system design
• http//citeseer.nj.nec.com/saltzer84endtoend.html

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Assignment

• 1) Compare the delay in sending an x-bit message
  over a k-hop path in a circuit-switched network
  and in a (lightly loaded) packet-switched
  network. The circuit setup time is s sec, the
  propagation delay is d sec per hop, the packet
  size is p bits, and the data rate is b bps. Under
  what conditions does the packet network have a
  lower delay?
• 2) Suppose that x bits of user data are to be
  transmitted over a k-hop path in a
  packet-switched network as a series of packets,
  each containing p data bits and h header bits,
  with x gtgt ph. The bit rate of the lines is b bps
  and the propagation delay is negligible. What
  value of p minimizes the total delay?
• 3) Calculate the total time required to transfer
  a 1.5-MB file in the following cases, assuming a
  round trip time (RTT) of 80 ms, a packet size of
  1 KB and an initial 2 x RTT of "hand-shaking"
  before data is sent.
• The bandwidth is 10 Mbps, and data packets can be
  sent continuously.
• The bandwidth is 10 Mbps, but after we finish
  sending each data packet we must wait one RTT
  before sending the next.
• The link allows infinitely fast transmit, but
  limits bandwidth such that only 20 packets can be
  sent per RTT.
• Zero transmit time as in (c), but during the
  first RTT we can send one packet, during the
  second RTT we can send two packets, during the
  third we can send four 2(3-1), and so on.