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					CENG 334

Introduction
to the
Internetworking

Part Four



                  1-1
Overview:
 Network Core:
 Packet Switching
 Circuit Switching
 Internet structure
 Performance:
  Loss
  Delay
 Throughput
 Protocol layers, service models
 Security

                                    1-2
The Network Core
 mesh of interconnected
  routers
 the fundamental question:
  how is data transferred
  through net?
    circuit switching:
     dedicated circuit per call:
     Ex: Telephone Net
    packet-switching: data
     sent thru net in discrete
     “chunks”

                                   1-3
Network Core: Circuit Switching

End to End Resources
  reserved for “call”
 link bandwidth, switch
  capacity
 dedicated resources:
  no sharing
 circuit-like
  (guaranteed)
  performance
 call setup required


                                  1-4
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)




                                                          1-5
Circuit Switching: FDM and TDM
                         Example:
FDM
                         4 users

      frequency

                  time
TDM


      frequency

                  time
                                    1-6
 Internet structure: network of networks

 roughly hierarchical
 at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T,
  Cable and Wireless), national/international coverage
    treat each other as equals



  Tier-1
  providers
                         Tier 1 ISP
  interconnect
  (peer)
  privately
                 Tier 1 ISP      Tier 1 ISP



                                                           1-7
Tier-1 ISP: e.g., Sprint
       POP: point-of-presence

           to/from backbone

                       peering
       …                …
                        .
                     …
       …

              …



          to/from customers




                                 1-8
   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
 tier-2 ISP is
customer of
tier-1 provider       Tier 1 ISP        Tier 1 ISP     Tier-2 ISP

                   Tier-2 ISP        Tier-2 ISP


                                                                       1-9
   Internet structure: network of networks

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


                  local
                   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
ISPs
connecting
them to rest
                          Tier 1 ISP             Tier 1 ISP       Tier-2 ISP
of Internet
                                                                        local
                    Tier-2 ISP           Tier-2 ISP
                                                                         ISP
              local         local          local
               ISP           ISP            ISP                                 1-10
 Internet structure: network of networks

 a packet passes through many networks!


           local
            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
                                                                  local
              Tier-2 ISP           Tier-2 ISP
                                                                   ISP
        local         local          local
         ISP           ISP            ISP                                 1-11
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)



A


    B
                           packets queueing (delay)
             free (available) buffers: arriving packets
             dropped (loss) if no free buffers
                                                               1-12
Four sources of packet delay
 1. nodal processing:            2. queueing
    check bit errors                time waiting at output
    determine output link            link for transmission
                                     depends on congestion
                                      level of router


           transmission
 A                           propagation


     B
              nodal
            processing    queueing

                                                               1-13
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!
         transmission
A                         propagation


    B
            nodal
          processing    queueing
                                                           1-14
 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 car (transmission       12*10 = 120 sec
  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

                                                          1-15
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                 See Ethernet applet at AWL
  booth?                                Web site

                                                                1-16
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



                                                           1-17
Queueing delay (revisited)

 R=link bandwidth (bps)
 L=packet length (bits)
 a=average packet
  arrival rate

 traffic intensity = La/R

 La/R ~ 0: average queueing delay small
 La/R -> 1: delays become large
 La/R > 1: more “work” arriving than can be
  serviced, average delay infinite!
                                               1-18
“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


                                                               1-19
 Packet loss
 Queue (Buffer) preceding link in buffer has
  finite capacity
 Packet arriving to full queue dropped (Lost)
 Lost packet may be retransmitted by previous
  node, by source end system, or not at all
                  buffer
               (waiting area)   packet being transmitted
      A


          B
                      packet arriving to
                      full buffer is lost
                                                           1-20
   Throughput
    throughput: rate (bits/time unit) at which
      bits transferred between sender/receiver
        instantaneous: rate at given point in time
        average: rate over longer period of time




     server, with    link capacity
server sends bits pipe that can carry    link capacity
                                        pipe that can carry
 (fluid) of F bits
    file into pipe     Rs bits/sec
                      fluid at rate         c bits/sec
                                           Rfluid at rate
  to send to client    Rs bits/sec)         Rc bits/sec)

                                                              1-21
 Throughput (more)
  Rs   < Rc What is average end-end throughput?

              Rs bits/sec              Rc bits/sec


  Rs   > Rc What is average end-end throughput?

              Rs bits/sec              Rc bits/sec


 bottleneck link
link on end-end path that constrains end-end throughput
                                                      1-22
Throughput: Internet scenario

                                  Rs
 per-connection
                          Rs                     Rs
  end-end
  throughput:
                                           R
  min(Rc,Rs,R/10)
 in practice: Rc or      Rc                      Rc
  Rs is often                       Rc
  bottleneck

                          10 connections (fairly) share
                       backbone bottleneck link R bits/sec
                                                        1-23
Protocol “Layers”
Networks are complex!
                                  Question:
 many “pieces”:
                             Is there any hope of
   hosts
                             organizing structure of
   routers                          network?
   links of various media Or at least our discussion
   applications                   of networks?
   protocols
   hardware, software




                                                        1-24
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

                                                          1-25
Layering of airline functionality

ticket (purchase)                                            ticket (complain)   ticket

baggage (check)                                              baggage (claim      baggage

  gates (load)                                                gates (unload)     gate

runway (takeoff)                                              runway (land)      takeoff/landing

airplane routing    airplane routing      airplane routing   airplane routing    airplane routing

   departure                intermediate air-traffic              arrival
    airport                     control centers                   airport



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



                                                                                             1-26
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
  system
    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?


                                                     1-27
Internet protocol stack
 application: supporting network
  applications                         application
      FTP, SMTP, HTTP
 transport: process-process data      transport
  transfer
      TCP, UDP                         network
 network: routing of datagrams from
  source to destination                   link
      IP, routing protocols
 link: data transfer between           physical
  neighboring network elements
      PPP, Ethernet
 physical: bits “on the wire”
                                                     1-28
ISO/OSI reference model
 presentation: allow applications to
  interpret meaning of data, e.g.,      application
  encryption, compression, machine-
                                        presentation
  specific conventions
 session: synchronization,               session
  checkpointing, recovery of data        transport
  exchange
                                          network
 Internet stack “missing” these
  layers!                                   link
    these services, if needed, must      physical
     be implemented in application


                                                     1-29
     message        M
                          source
                        application
                                                 Encapsulation
  segment      Ht   M   transport
datagram Hn Ht      M    network
frame Hl Hn Ht      M      link
                         physical
                                                        link
                                                      physical

                                                                         switch



               destination               Hn Ht   M   network
           M    application           Hl Hn Ht   M     link      Hn Ht   M
     Ht    M    transport                            physical
   Hn Ht   M     network
Hl Hn Ht   M       link                                                  router
                 physical

                                                                             1-30
Network Security
 The field of network security is about:
   how bad guys can attack computer networks
   how we can defend networks against attacks
   how to design architectures that are immune to
    attacks
 Internet not originally designed with
  (much) security in mind
     original vision: “a group of mutually trusting
    users attached to a transparent network”
   Internet protocol designers playing “catch-up”
   Security considerations in all layers!

                                                       1-31
Bad guys can put malware into
hosts via Internet
 Malware can get in host from a virus, worm, or
  trojan horse.

 Spyware malware can record keystrokes, web
  sites visited, upload info to collection site.

 Infected host can be enrolled in a botnet, used
  for spam and DDoS attacks.

 Malware is often self-replicating: from an
  infected host, seeks entry into other hosts


                                                    1-32
Bad guys can put malware into
hosts via Internet
 Trojan horse                  Worm:
    Hidden part of some          infection by passively
     otherwise useful              receiving object that gets
     software                      itself executed
    Today often on a Web         self- replicating: propagates
     page (Active-X, plugin)       to other hosts, users
 Virus                                  Sapphire Worm: aggregate scans/sec
    infection by receiving
                                 in first 5 minutes of outbreak (CAIDA, UWisc data)

     object (e.g., e-mail
     attachment), actively
     executing
    self-replicating:
     propagate itself to
     other hosts, users
                                                                                1-33
     Bad guys can attack servers and
     network infrastructure
  Denial of service (DoS): attackers make resources
      (server, bandwidth) unavailable to legitimate traffic
      by overwhelming resource with bogus traffic
1.   select target
2. break into hosts
   around the network
   (see botnet)
3. send packets toward
   target from                               target
   compromised hosts


                                                              1-34
The bad guys can sniff packets
Packet sniffing:
   broadcast media (shared Ethernet, wireless)
   promiscuous network interface reads/records all
    packets (e.g., including passwords!) passing by

       A                              C


                       src:B dest:A   payload
                                                B
      Wireshark software used for end-of-chapter
       labs is a (free) packet-sniffer
                                                      1-35
The bad guys can use false source
addresses
 IP   spoofing: send packet with false source address
        A                               C


               src:B dest:A   payload

                                             B




                                                         1-36
The bad guys can record and
playback
 record-and-playback: sniff sensitive info (e.g.,
  password), and use later
    password holder is that user from system point of
     view

                          C
     A

                              src:B dest:A   user: B; password: foo


                                                    B

                                                                      1-37
Network Security
 more throughout this part
 Security with Cryptographic techniques
 obvious uses




                                           1-38

				
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posted:8/24/2011
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