Part I Introduction

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					1: Introduction
   what’s the Internet
   what’s a protocol?
   network edge and core
   access net, physical media
   Internet/ISP structure
   performance: loss, delay
   protocol layers, service models
   history

                                      Introduction   1-1
What’s the Internet: “nuts and bolts” view
  millions of connected                router
     computing devices: hosts,
     end-systems                          server
        PCs workstations, servers   local ISP
        PDAs phones, toasters
     running network apps
    communication links                           regional ISP
        fiber, copper, radio,
        transmission rate =
    routers: forward packets        company
     (chunks of data)                network

                                                   Introduction   1-2
“Cool” internet appliances

     IP picture frame

                                              Web-enabled toaster+weather forecaster

World’s smallest web server

                                                                   Introduction   1-3
What’s the Internet: “nuts and bolts” view
   protocols control sending,             router     workstation
    receiving of msgs                        server
       e.g., TCP, IP, HTTP, FTP, PPP                    mobile
   Internet: “network of               local ISP
       loosely hierarchical
       public Internet versus                        regional ISP
        private intranet
 Internet standards
    RFC: Request for comments
    IETF: Internet Engineering
     Task Force                         company

                                                      Introduction   1-4
What’s the Internet: a service view
  communication
   infrastructure enables
   distributed applications:
       Web, email, games, e-
        commerce, database.,
        voting, file (MP3) sharing
  communication services
   provided to apps:
       connectionless
       connection-oriented

  cyberspace [Gibson]:
    “a consensual hallucination experienced daily by
      billions of operators, in every nation, ...."
                                               Introduction   1-5
What’s a protocol?
human protocols:           network protocols:
 “what’s the time?”        machines rather than
 “I have a question”        humans
 introductions             all communication
                             activity in Internet
… specific msgs sent         governed by protocols
… specific actions taken   protocols define format,
  when msgs received,        order of msgs sent and
  or other events           received among network
                              entities, and actions
                                  taken on msg
                              transmission, receipt
                                           Introduction   1-6
What’s a protocol?
a human protocol and a computer network protocol:

       Hi                      TCP connection
                               TCP connection
     Got the                   response
      time?                    Get

                                                    Introduction    1-7
A closer look at network structure:

 network edge:
  applications and
 network core:
   routers
   network of
 access networks,
  physical media:
  communication links
                              Introduction   1-8
 The network edge:
 end systems (hosts):
     run application programs
     e.g. Web, email
     at “edge of network”
 client/server model
     client host requests, receives
      service from always-on server
     e.g. Web browser/server;
      email client/server
 peer-peer model:
      minimal (or no) use of
      dedicated servers
     e.g. Gnutella, KaZaA
                                       Introduction   1-9
Network edge: connection-oriented service

 Goal: data transfer              TCP service [RFC 793]
   between end systems               reliable, in-order byte-
  handshaking: setup                 stream data transfer
   (prepare for) data                    loss: acknowledgements
   transfer ahead of time                 and retransmissions
       Hello, hello back human      flow control:
        protocol                         sender won’t overwhelm
       set up “state” in two             receiver
        communicating hosts
                                     congestion control:
  TCP - Transmission                    senders “slow down sending
   Control Protocol                       rate” when network
       Internet’s connection-            congested
        oriented service
                                                        Introduction   1-10
Network edge: connectionless service

 Goal: data transfer          App’s using TCP:
   between end systems         HTTP (Web), FTP (file
       same as before!         transfer), Telnet
  UDP - User Datagram          (remote login), SMTP
   Protocol [RFC 768]:          (email)
   connectionless service
                              App’s using UDP:
     unreliable data
                               streaming media,
                                teleconferencing, DNS,
     no flow control
                                Internet telephony
     no congestion control

                                           Introduction   1-11
The Network Core
 mesh of interconnected
 the fundamental
  question: how is data
  transferred through net?
    circuit switching:
     dedicated circuit per
     call: telephone net
    packet-switching: data
     sent thru net in
     discrete “chunks”

                              Introduction   1-12
Network Core: Circuit Switching

End-end resources
  reserved for “call”
 link bandwidth, switch
 dedicated resources:
  no sharing
 circuit-like
 call setup required

                             Introduction   1-13
Network Core: Circuit Switching
network resources              dividing link bandwidth
  (e.g., bandwidth)             into “pieces”
  divided into “pieces”           frequency division
 pieces allocated to calls       time division
 resource piece   idle if
  not used by owning call
  (no sharing)

                                               Introduction   1-14
Packet Switching: Statistical Multiplexing
         10 Mb/s
A        Ethernet     statistical multiplexing   C

                           1.5 Mb/s
           queue of packets
           waiting for output

                           D                     E

    Sequence of A & B packets does not have fixed
      pattern  statistical multiplexing.
    In TDM each host gets same slot in revolving TDM
                                                     Introduction   1-15
Packet switching versus circuit switching
Packet switching allows more users to use network!
 1 Mbit link
 each user:
    100 kbps when “active”
    active 10% of time

                               N users
 circuit-switching:                         1 Mbps link
    10 users

 packet switching:
    with 35 users,
     probability > 10 active
     less than .0004

                                            Introduction   1-16
Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”

 Great for bursty data
    resource sharing
    simpler, no call setup
 Excessive congestion: packet delay and loss
    protocols needed for reliable data transfer,
     congestion control
 Q: How to provide circuit-like behavior?
    bandwidth guarantees needed for audio/video
    still an unsolved problem

                                            Introduction   1-17
Packet-switching: store-and-forward
                  R      R       R

 Takes L/R seconds to       Example:
  transmit (push out)         L = 7.5 Mbits
  packet of L bits on to      R = 1.5 Mbps
  link or R bps
                              delay = 15 sec
 Entire packet must
  arrive at router before
  it can be transmitted
  on next link: store and
 delay = 3L/R

                                                Introduction   1-18
Packet Switching: Message Segmenting

                   Now break up the message
                     into 5000 packets
                     Each packet 1,500 bits
                     1 msec to transmit
                      packet on one link
                     pipelining: each link
                      works in parallel
                     Delay reduced from 15
                      sec to 5.002 sec

                                   Introduction   1-19
Packet-switched networks: forwarding
   Goal: move packets through routers from source to
       we’ll study several path selection (i.e. routing)algorithms
 datagram network:
    destination address in packet determines next hop
    routes may change during session
    analogy: driving, asking directions

 virtual circuit network:
    each packet carries tag (virtual circuit ID), tag
     determines next hop
    fixed path determined at call setup time, remains fixed
     thru call
    routers maintain per-call state

                                                           Introduction   1-20
Network Taxonomy

     Circuit-switched                   Packet-switched
         networks                          networks

 FDM                                Networks         Datagram
                                    with VCs         Networks

 • Datagram network is not either connection-oriented
 or connectionless.
 • Internet provides both connection-oriented (TCP) and
 connectionless services (UDP) to apps.
                                                   Introduction   1-21
Access networks and physical media
 Q: How to connect end
   systems to edge router?
  residential access nets
  institutional access
   networks (school,
  mobile access networks

 Keep in mind:
  bandwidth (bits per
   second) of access
  shared or dedicated?
                                 Introduction   1-22
Residential access: point to point access

 Dialup via modem
    up to 56Kbps direct access to
     router (often less)
    Can’t surf and phone at same
     time: can’t be “always on”
 ADSL: asymmetric digital subscriber line
    up to 1 Mbps upstream (today typically < 256 kbps)
    up to 8 Mbps downstream (today typically < 1 Mbps)
    FDM: 50 kHz - 1 MHz for downstream
          4 kHz - 50 kHz for upstream
          0 kHz - 4 kHz for ordinary telephone
                                                 Introduction   1-23
Residential access: cable modems

  HFC: hybrid fiber coax
     asymmetric: up to 10Mbps upstream, 1 Mbps
  network of cable and fiber attaches homes to
   ISP router
     shared access to router among home
     issues: congestion, dimensioning
  deployment: available via cable companies, e.g.,

                                               Introduction   1-24
Residential access: cable modems

 Diagram:   Introduction   1-25
Cable Network Architecture: Overview

                                   Typically 500 to 5,000 homes

   cable headend

             cable distribution
            network (simplified)

                                                      Introduction   1-26
Cable Network Architecture: Overview

   cable headend

             cable distribution
            network (simplified)

                                          Introduction   1-27
Cable Network Architecture: Overview


   cable headend

                cable distribution

                                            Introduction   1-28
Cable Network Architecture: Overview

                                     V    V   V   V   V   V           N
                                     I    I   I   I   I   I   D   D   T
                                     D    D   D   D   D   D   A   A   R
                                     E    E   E   E   E   E   T   T   O
                                     O    O   O   O   O   O   A   A   L

                                     1    2   3   4   5   6   7   8   9


   cable headend

              cable distribution

                                                                          Introduction   1-29
Company access: local area networks
  company/univ local area
   network (LAN) connects
   end system to edge router
  Ethernet:
     shared or dedicated link
      connects end system
      and router
     10 Mbs, 100Mbps,
      Gigabit Ethernet
  deployment: institutions,
   home LANs happening now

                                  Introduction   1-30
Wireless access networks
 shared    wireless access
  network connects end system
  to router                           router
      via base station aka “access
       point”                           base
 wireless LANs:                      station
    802.11b (WiFi): 11 Mbps

 wider-area wireless access
    provided by telco operator
    3G ~ 384 kbps
      • Will it happen??
    WAP/GPRS in Europe

                                                Introduction   1-31
Home networks
Typical home network components:
 ADSL or cable modem
 router/firewall/NAT
 Ethernet
 wireless access
    to/from                                        laptops
               cable   router/
              modem    firewall
                            Ethernet     point
                                            Introduction   1-32
Physical Media
                                    Twisted Pair (TP)
 Bit: propagates between            two insulated copper
  transmitter/rcvr pairs              wires
 physical link: what lies                Category 3: traditional
  between transmitter &                    phone wires, 10 Mbps
  receiver                                 Ethernet
                                           Category 5 TP:
 guided media:
                                           100Mbps Ethernet
      signals propagate in solid
       media: copper, fiber, coax
 unguided media:
    signals propagate freely,
     e.g., radio

                                                     Introduction   1-33
Physical Media: coax, fiber
 Coaxial cable:                   Fiber optic cable:
                                   glass fiber carrying light
  two concentric copper
                                    pulses, each pulse a bit
                                   high-speed operation:
  bidirectional
                                        high-speed point-to-point
  baseband:                             transmission (e.g., 5 Gps)
       single channel on cable    low error rate: repeaters
       legacy Ethernet             spaced far apart ; immune
  broadband:                       to electromagnetic noise
     multiple channel on cable
     HFC

                                                       Introduction   1-34
Physical media: radio
  signal carried in             Radio link types:
   electromagnetic                terrestrial microwave
   spectrum                          e.g. up to 45 Mbps channels

  no physical “wire”             LAN (e.g., WaveLAN)
  bidirectional                     2Mbps, 11Mbps

  propagation                    wide-area (e.g., cellular)
   environment effects:              e.g. 3G: hundreds of kbps

       reflection                satellite
       obstruction by objects       up to 50Mbps channel (or
       interference                  multiple smaller channels)
                                     270 msec end-end delay
                                     geosynchronous versus
                                                    Introduction   1-35
 Internet structure: network of networks

 roughly hierarchical
 at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity,
  Sprint, AT&T), national/international coverage
    treat each other as equals

                                              Tier-1 providers
                                              also interconnect
  Tier-1                                      at public network
                         Tier 1 ISP
                                      NAP     access points
  interconnect                                (NAPs)
                 Tier 1 ISP      Tier 1 ISP

                                               Introduction   1-36
Tier-1 ISP: e.g., Sprint
 Sprint US backbone network

                              Introduction   1-37
   Internet structure: network of networks

  “Tier-2” ISPs: smaller (often regional) ISPs
     Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

                                                            Tier-2 ISPs
Tier-2 ISP pays         Tier-2 ISP                          also peer
                                          Tier-2 ISP        privately with
tier-1 ISP for
connectivity to                 Tier 1 ISP                  each other,
rest of Internet                                  NAP       interconnect
 tier-2 ISP is
                                                            at NAP
customer of
tier-1 provider       Tier 1 ISP        Tier 1 ISP      Tier-2 ISP

                   Tier-2 ISP        Tier-2 ISP

                                                          Introduction   1-38
   Internet structure: network of networks

  “Tier-3” ISPs and local ISPs
     last hop (“access”) network (closest to end systems)

                   ISP     Tier 3                   local
                                         local            local
                            ISP                      ISP
                                          ISP              ISP
Local and tier-            Tier-2 ISP            Tier-2 ISP
3 ISPs are
customers of                        Tier 1 ISP
higher tier                                           NAP
them to rest
                          Tier 1 ISP             Tier 1 ISP       Tier-2 ISP
of Internet
                    Tier-2 ISP           Tier-2 ISP
              local         local          local
               ISP           ISP            ISP                     Introduction   1-39
 Internet structure: network of networks

 a packet passes through many networks!

            ISP     Tier 3                    local
                                   local            local
                     ISP                       ISP
                                    ISP              ISP
                    Tier-2 ISP             Tier-2 ISP

                              Tier 1 ISP

                   Tier 1 ISP              Tier 1 ISP       Tier-2 ISP
              Tier-2 ISP           Tier-2 ISP
        local         local          local
         ISP           ISP            ISP                     Introduction   1-40
 How do loss and delay occur?
packets queue in router buffers
 packet arrival rate to link exceeds output link capacity
 packets queue, wait for turn

                               packet being transmitted (delay)


                              packets queueing (delay)
                free (available) buffers: arriving packets
                dropped (loss) if no free buffers
                                                             Introduction   1-41
Four sources of packet delay
 1. nodal processing:            2. queuing
    check bit errors                time waiting at output
    determine output link            link for transmission
                                     depends on congestion
                                      level of router

 A                           propagation

            processing    queueing

                                                     Introduction   1-42
Delay in packet-switched networks
3. Transmission delay:         4. Propagation delay:
 R=link bandwidth (bps)        d = length of physical link
 L=packet length (bits)        s = propagation speed in
 time to send bits into          medium (~2x108 m/sec)
   link = L/R                   propagation delay = d/s

                               Note: s and R are very
                                 different quantities!
A                         propagation

          processing    queueing
                                                 Introduction   1-43
 Caravan analogy
                          100 km               100 km
      ten-car     toll                toll
      caravan    booth               booth
 Cars “propagate” at            Time to “push” entire
  100 km/hr                       caravan through toll
 Toll booth takes 12 sec to      booth onto highway =
  service a car                   12*10 = 120 sec
  (transmission time)            Time for last car to
 car~bit; caravan ~ packet       propagate from 1st to
                                  2nd toll both:
 Q: How long until caravan
                                  100km/(100km/hr)= 1 hr
  is lined up before 2nd toll
  booth?                         A: 62 minutes

                                               Introduction   1-44
Caravan analogy (more)
                           100 km              100 km
     ten-car    toll                  toll
     caravan   booth                 booth
                               Yes! After 7 min, 1st car
 Cars now “propagate” at       at 2nd booth and 3 cars
  1000 km/hr                    still at 1st booth.
 Toll booth now takes 1       1st bit of packet can
  min to service a car          arrive at 2nd router
 Q: Will cars arrive to        before packet is fully
  2nd booth before all          transmitted at 1st router!
  cars serviced at 1st

                                               Introduction   1-45
Nodal delay
           d nodal  d proc  d queue  d trans  d prop

 dproc = processing delay
    typically a few microsecs or less

 dqueue = queuing delay
    depends on congestion

 dtrans = transmission delay
    = L/R, significant for low-speed links

 dprop = propagation delay
    a few microsecs to hundreds of msecs

                                                           Introduction   1-46
“Real” Internet delays and routes

 What do “real” Internet delay & loss look like?
 Traceroute program: provides delay measurement
  from source to router along end-end Internet path
  towards destination. For all i:
      sends three packets that will reach router i on path
       towards destination
      router i will return packets to sender
      sender times interval between transmission and reply.

       3 probes        3 probes

            3 probes

                                                       Introduction   1-47
“Real” Internet delays and routes
traceroute: to
                                     Three delay measurements from
1 cs-gw ( 1 ms 1 ms 2 ms
2 ( 1 ms 1 ms 2 ms
3 ( 6 ms 5 ms 5 ms
4 ( 16 ms 11 ms 13 ms
5 ( 21 ms 18 ms 18 ms
6 ( 22 ms 18 ms 22 ms
7 ( 22 ms 22 ms 22 ms trans-oceanic
8 ( 104 ms 109 ms 106 ms
9 ( 109 ms 102 ms 104 ms
10 ( 113 ms 121 ms 114 ms
11 ( 112 ms 114 ms 112 ms
12 ( 111 ms 114 ms 116 ms
13 ( 123 ms 125 ms 124 ms
14 ( 126 ms 126 ms 124 ms
15 ( 135 ms 128 ms 133 ms
16 ( 126 ms 128 ms 126 ms
17 * * *
18 * * *                 * means no response (probe lost, router not replying)
19 ( 132 ms 128 ms 136 ms

                                                             Introduction   1-48
Packet loss
 queue (aka buffer) preceding link in buffer
  has finite capacity
 when packet arrives to full queue, packet is
  dropped (aka lost)
 lost packet may be retransmitted by
  previous node, by source end system, or
  not retransmitted at all

                                       Introduction   1-49
Protocol “Layers”
Networks are complex!
 many “pieces”:
   hosts                      Question:
   routers               Is there any hope of
   links of various      organizing structure of
    media                        network?
   applications
   protocols           Or at least our discussion
   hardware,                   of networks?

                                         Introduction   1-50
Organization of air travel

    ticket (purchase)                 ticket (complain)

    baggage (check)                   baggage (claim)

    gates (load)                      gates (unload)

    runway takeoff                    runway landing

    airplane routing                  airplane routing
                       airplane routing

  a series of steps

                                                         Introduction   1-51
Organization of air travel: a different view

     ticket (purchase)                 ticket (complain)

     baggage (check)                   baggage (claim)

     gates (load)                      gates (unload)

     runway takeoff                    runway landing

     airplane routing                  airplane routing
                        airplane routing

Layers: each layer implements a service
    via its own internal-layer actions
    relying on services provided by layer below
                                                          Introduction   1-52
Layered air travel: services

   Counter-to-counter delivery of person+bags

   baggage-claim-to-baggage-claim delivery

   people transfer: loading gate to arrival gate

   runway-to-runway delivery of plane

    airplane routing from source to destination

                                                   Introduction   1-53
Distributed implementation of layer functionality

                     ticket (purchase)                      ticket (complain)
 Departing airport

                                                                                  arriving airport
                     baggage (check)                        baggage (claim)

                     gates (load)                           gates (unload)

                     runway takeoff                         runway landing

                     airplane routing                       airplane routing

                          intermediate air traffic sites
                            airplane routing       airplane routing

                                         airplane routing
                                                                               Introduction          1-54
Why layering?
Dealing with complex systems:
 explicit structure allows identification,
  relationship of complex system’s pieces
    layered reference model for discussion
 modularization eases maintenance, updating of
    change of implementation of layer’s service
     transparent to rest of system
    e.g., change in gate procedure doesn’t affect
     rest of system
 layering considered harmful?

                                              Introduction   1-55
Internet protocol stack
 application: supporting network
  applications                         application
      FTP, SMTP, STTP
 transport: host-host data transfer   transport
    TCP, UDP

 network: routing of datagrams from    network
  source to destination
      IP, routing protocols              link
 link: data transfer between
  neighboring network elements          physical
      PPP, Ethernet
 physical: bits “on the wire”

                                          Introduction   1-56
Layering: logical communication
Each layer:         application
 distributed        network
 “entities”           link
  implement                                network
  layer functions   application              link
  at each node      transport              physical
 entities             link
  perform            physical
                                  application       application
  actions,                        transport         transport
  exchange                         network           network
                                     link              link
  messages with                    physical          physical

                                                Introduction   1-57
Layering: logical communication
E.g.: transport        application
 take data from app
 add addressing,         link
  reliability check     physical
  info to form                          ack      network
  “datagram”           application                 link
 send datagram to     transport     data        physical
 wait for peer to                                             data
  ack receipt                           application       application
                                        transport         transport
 analogy: post                          network           network
  office                                   link              link
                                         physical          physical

                                                      Introduction   1-58
Layering: physical communication
        application               link
        transport               physical
         physical                          data
                       application     application
                       transport       transport
                        network         network
                          link            link
                        physical        physical

                                              Introduction   1-59
  Protocol layering and data
   Each layer takes data from above
    adds header information to create new data unit
    passes new data unit to layer below

               source      destination
        M    application   application             M      message
     Ht M    transport     transport        Ht     M      segment
   Hn Ht M    network       network       Hn Ht    M      datagram
Hl Hn Ht M      link          link     Hl Hn Ht    M       frame
              physical      physical

                                                  Introduction   1-60
Internet History
1961-1972: Early packet-switching principles
 1961: Kleinrock - queueing    1972:
  theory shows                       ARPAnet demonstrated
  effectiveness of packet-            publicly
                                     NCP (Network Control
 1964: Baran - packet-               Protocol) first host-
  switching in military nets          host protocol
 1967: ARPAnet conceived            first e-mail program
  by Advanced Research
                                     ARPAnet has 15 nodes
  Projects Agency
 1969: first ARPAnet node

                                                   Introduction   1-61
    Internet History
    1972-1980: Internetworking, new and proprietary nets
 1970: ALOHAnet satellite        Cerf and Kahn’s
    network in Hawaii               internetworking principles:
   1973: Metcalfe’s PhD thesis        minimalism, autonomy -
    proposes Ethernet                   no internal changes
   1974: Cerf and Kahn -               required to
    architecture for                    interconnect networks
    interconnecting networks           best effort service
   late70’s: proprietary               model
    architectures: DECnet, SNA,        stateless routers
                                       decentralized control
   late 70’s: switching fixed
                                  define today’s Internet
    length packets (ATM
   1979: ARPAnet has 200 nodes
                                                       Introduction   1-62
Internet History
1980-1990: new protocols, a proliferation of networks

 1983: deployment of       new national networks:
    TCP/IP                   Csnet, BITnet,
   1982: SMTP e-mail        NSFnet, Minitel
    protocol defined        100,000 hosts
   1983: DNS defined        connected to
    for name-to-IP-          confederation of
    address translation      networks
   1985: FTP protocol
   1988: TCP congestion
                                             Introduction   1-63
Internet History
1990, 2000’s: commercialization, the Web, new apps
 Early 1990’s: ARPAnet             Late 1990’s – 2000’s:
                                     more killer apps: instant
 1991: NSF lifts restrictions on
                                      messaging, peer2peer
  commercial use of NSFnet
  (decommissioned, 1995)
                                      file sharing (e.g.,
 early 1990s: Web
                                     network security to
    hypertext [Bush 1945, Nelson
     1960’s]                          forefront
    HTML, HTTP: Berners-Lee         est. 50 million host, 100
    1994: Mosaic, later Netscape
                                      million+ users
    late 1990’s:                    backbone links running
     commercialization of the Web     at Gbps

                                                   Introduction   1-64

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