assignment - COMP3720 – Computer Networks

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					                                                       COMP3720	
  –	
  Computer	
  Networks	
  
                                                                                              Fall	
  2010	
  
                                                                                          Assignment	
  2	
  

                 	
  
1. [Chapter	
  1,	
  P9]	
  Consider	
  a	
  packet	
  of	
  length	
  L,	
  which	
  begins	
  at	
  end	
  system	
  A	
  and	
  travels	
  over	
  three	
  links	
  to	
  a	
  
   destination	
   end	
   system.	
   These	
   three	
   links	
   are	
   connected	
   by	
   two	
   packet	
   switches.	
   Let	
  di,	
   si,	
   and	
   Ri	
   denote	
   the	
  
   length,	
  propagation	
  speed,	
  and	
  the	
  transmission	
  rate	
  of	
  link	
  i,	
  for	
  i	
  =	
  1,	
  2,	
  3.	
  The	
  packet	
  switch	
  delays	
  each	
  
   packet	
  by	
  dproc.	
  Assuming	
  no	
  queuing	
  delays,	
  in	
  terms	
  of	
  di,	
  si,	
  Ri	
  (i	
  =	
  1,2,3),	
  and	
  L,	
  what	
  is	
  the	
  total	
  end-­‐to-­‐end	
  
   delay	
  for	
  the	
  packet?	
  Suppose	
  now	
  the	
  packet	
  is	
  1,500	
  bytes,	
  the	
  propagation	
  speed	
  on	
  all	
  three	
  links	
  is	
  2.5	
  ·	
  
   108	
  m/s,	
  the	
  transmission	
  rates	
  of	
  all	
  three	
  links	
  are	
  2	
  Mbps,	
  the	
  packet	
  switch	
  processing	
  delay	
  is	
  3	
  msec,	
  
   the	
  length	
  of	
  the	
  first	
  link	
  is	
  5,000	
  km,	
  the	
  length	
  of	
  the	
  second	
  link	
  is	
  4,000	
  km,	
  and	
  the	
  length	
  of	
  the	
  last	
  
   link	
  is	
  1,000	
  km.	
  For	
  these	
  values,	
  what	
  is	
  the	
  end-­‐to-­‐end	
  delay?	
  
   	
  
2. [Chapter	
   1,	
   P30]	
   In	
   modern	
   packet-­‐switched	
   networks,	
   the	
   source	
   host	
   segments	
   long,	
   application-­‐layer	
  
   messages	
   (for	
   example,	
   an	
   image	
   or	
   a	
   music	
   file)	
   into	
   smaller	
   packets	
   and	
   sends	
   the	
   packets	
   into	
   the	
  
   network.	
  The	
  receiver	
  then	
  reassembles	
  the	
  packets	
  back	
  into	
  the	
  original	
  message.	
  We	
  refer	
  to	
  this	
  process	
  
   as	
   message	
   segmentation.	
   The	
   following	
   figure	
   illustrates	
   the	
   end-­‐to-­‐end	
   transport	
   of	
   a	
   message	
   with	
   and	
  
   without	
  message	
  segmentation.	
  Consider	
  a	
  message	
  that	
  is	
  8	
  ·	
  106	
  bits	
  long	
  that	
  is	
  to	
  be	
  sent	
  from	
  source	
  to	
  
   destination	
  in	
  the	
  following	
  figure.	
  Suppose	
  each	
  link	
  in	
  the	
  figure	
  is	
  2	
  Mbps.	
  Ignore	
  propagation,	
  queuing,	
  
   and	
  processing	
  delays.	
  
                                                                                                          	
  




                                                                                                                                                                            	
  
                                                Figure	
  a.	
  End-­‐to-­‐end	
  message	
  transport	
  without	
  message	
  segmentation.	
  




                                                                                                                                                                             	
  
                                                   Figure	
  b.	
  End-­‐to-­‐end	
  message	
  transport	
  with	
  message	
  segmentation.	
  

   a) Consider	
  sending	
  the	
  message	
  from	
  source	
  to	
  destination	
  without	
  message	
  segmentation.	
  How	
  long	
  does	
  
      it	
  take	
  to	
  move	
  the	
  message	
  from	
  the	
  source	
  host	
  to	
  the	
  first	
  packet	
  switch?	
  Keeping	
  in	
  mind	
  that	
  each	
  
      switch	
  uses	
  store-­‐and-­‐forward	
  packet	
  switching,	
  what	
  is	
  the	
  total	
  time	
  to	
  move	
  the	
  message	
  from	
  source	
  
      host	
  to	
  destination	
  host?	
  
   b) Now	
  suppose	
  that	
  the	
  message	
  is	
  segmented	
  into	
  4,000	
  packets,	
  with	
  each	
  packet	
  being	
  2,000	
  bits	
  long.	
  
      How	
  long	
  does	
  it	
  take	
  to	
  move	
  the	
  first	
  packet	
  from	
  source	
  host	
  to	
  the	
  first	
  switch?	
  When	
  the	
  first	
  packet	
  
      is	
  being	
  sent	
  from	
  the	
  first	
  switch	
  to	
  the	
  second	
  switch,	
  the	
  second	
  packet	
  is	
  being	
  sent	
  from	
  the	
  source	
  
      host	
  to	
  the	
  first	
  switch.	
  At	
  what	
  time	
  will	
  the	
  second	
  packet	
  will	
  be	
  fully	
  received	
  at	
  the	
  first	
  switch?	
  
   c) How	
  long	
  does	
  it	
  take	
  to	
  move	
  the	
  file	
  from	
  source	
  host	
  to	
  destination	
  host	
  when	
  message	
  segmentation	
  
      is	
  use?	
  Compare	
  this	
  result	
  with	
  your	
  answer	
  in	
  part	
  (a)	
  and	
  comment.	
  
   d) Discuss	
  the	
  drawbacks	
  of	
  message	
  segmentation.	
  
      	
  
3) [Chapter	
  2,	
  P4]	
  Consider	
  the	
  following	
  string	
  of	
  ASCII	
  characters	
  that	
  were	
  captured	
  by	
  Wireshark	
  when	
  the	
  
   browser	
  sent	
  an	
  HTTP	
  GET	
  message	
  (i.e.,	
  this	
  is	
  the	
  actual	
  content	
  of	
  an	
  HTTP	
  GET	
  message).	
  The	
  characters	
  
   <cr><lf>	
  are	
  carriage	
  return	
  and	
  line-­‐feed	
  characters	
  (that	
  is,	
  the	
  italized	
  character	
  string	
  <cr>	
  in	
  the	
  text	
  
   below	
  represents	
  the	
  single	
  carriage-­‐return	
  character	
  that	
  was	
  contained	
  at	
  that	
  point	
  in	
  the	
  HTTP	
  header).	
  
   Answer	
  the	
  following	
  questions,	
  indicating	
  where	
  in	
  the	
  HTTP	
  GET	
  message	
  below	
  you	
  find	
  the	
  answer.	
  
   	
  
                  GET /cs453/index.html HTTP/1.1<cr><lf>Host: gai
                  a.cs.umass.edu<cr><lf>User-Agent: Mozilla/5.0 (
                  Windows;U; Windows NT 5.1; en-US; rv:1.7.2) Gec
                  ko/20040804 Netscape/7.2 (ax) <cr><lf>Accept:ex
                  t/xml, application/xml, application/xhtml+xml, text
                  /html;q=0.9, text/plain;q=0.8,image/png,*/*;q=0.5
                  <cr><lf>Accept-Language: en-us,en;q=0.5<cr><lf>AcceptEncoding:
                  zip,deflate<cr><lf>Accept-Charset: ISO
                  -8859-1,utf-8;q=0.7,*;q=0.7<cr><lf>Keep-Alive: 300<cr>
                  <If>Connection:keep-alive<cr><lf><cr><lf>
   	
  
   a) What	
  is	
  the	
  URL	
  of	
  the	
  document	
  requested	
  by	
  the	
  browser?	
  
   b) What version of HTTP is the browser running?	
  
   c) Does the browser request a non-persistent or a persistent connection?	
  
   d) What is the IP address of the host on which the browser is running?	
  
   e) What type of browser initiates this message? Why is the browser type needed in an HTTP request message?	
  
   	
  
4) [Chapter	
  2,	
  P9]	
  Consider	
  the	
  following	
  figure,	
  for	
  which	
  there	
  is	
  an	
  institutional	
  network	
  connected	
  to	
  the	
  
   Internet.	
   Suppose	
   that	
   the	
   average	
   object	
   size	
   is	
   850,000	
   bits	
   and	
   that	
   the	
   average	
   request	
   rate	
   from	
   the	
  
   institution's	
  browsers	
  to	
  the	
  origin	
  servers	
  is	
  16	
  requests	
  per	
  second.	
  Also	
  suppose	
  that	
  the	
  amount	
  of	
  time	
  
   it	
   takes	
   from	
   when	
   the	
   router	
   on	
   the	
   Internet	
   side	
   of	
   the	
   access	
   link	
   forwards	
   an	
   HTTP	
   request	
   until	
   it	
  
   receives	
  the	
  response	
  is	
  three	
  seconds	
  on	
  average	
  (see	
  Section	
  2.2.5).	
  Model	
  the	
  total	
  average	
  response	
  time	
  
   as	
   the	
   sum	
   of	
   the	
   average	
   access	
   delay	
   (that	
   is,	
   the	
   delay	
   from	
   Internet	
   router	
   to	
   institution	
   router)	
   and	
   the	
  
   average	
  Internet	
  delay.	
  For	
  the	
  average	
  access	
  delay,	
  use	
  ∆/(1	
  -­‐	
  ∆ß),	
  where	
  ∆	
  is	
  the	
  average	
  time	
  required	
  to	
  
   send	
  an	
  object	
  over	
  the	
  access	
  link	
  and	
  ß	
  is	
  the	
  arrival	
  rate	
  of	
  objects	
  to	
  the	
  access	
  link.	
  




                                                                                                                                       	
  
   a) Find	
  the	
  total	
  average	
  response	
  time.	
  
   b) Now	
  suppose	
  a	
  cache	
  is	
  installed	
  in	
  the	
  institutional	
  LAN.	
  Suppose	
  the	
  miss	
  rate	
  is	
  0.4.	
  Find	
  the	
  total	
  
          response	
  time.	
  
          	
  
5) [Chapter	
  3,	
  P16]	
  In	
  the	
  generic	
  SR	
  protocol	
  that	
  we	
  studied	
  in	
  Section	
  3.4.4,	
  the	
  sender	
  transmits	
  a	
  message	
  
   as	
  soon	
  as	
  it	
  is	
  available	
  (if	
  it	
  is	
  in	
  the	
  window)	
  without	
  waiting	
  for	
  an	
  acknowledgment.	
  Suppose	
   now	
  that	
  
   we	
   want	
   an	
   SR	
   protocol	
   that	
   sends	
   messages	
   two	
   at	
   a	
   time.	
   That	
   is,	
   the	
   sender	
   will	
   send	
   a	
   pair	
   of	
   messages	
  
   and	
  will	
  send	
  the	
  next	
  pair	
  of	
  messages	
  only	
  when	
  it	
  knows	
  that	
  both	
  messages	
  in	
  the	
  first	
  pair	
  have	
  been	
  
   received	
  correctly.	
  Suppose	
  that	
  the	
  channel	
  may	
  lose	
  messages	
  but	
  will	
  not	
  corrupt	
  or	
  reorder	
  messages.	
  
   Design	
   an	
   error-­‐control	
   protocol	
   for	
   the	
   unidirectional	
   reliable	
   transfer	
   of	
   messages.	
   Give	
   an	
   FSM	
  
   description	
  of	
  the	
  sender	
  and	
  receiver.	
  Describe	
  the	
  format	
  of	
  packets	
  send	
  between	
  sender	
  and	
  receiver,	
  
   and	
  vice	
  versa.	
  If	
  you	
  use	
  any	
  procedure	
  calls	
  other	
  than	
  those	
  in	
  Section	
  3.4	
  (for	
  example,	
  udt_send(),	
  
   start_timer(),	
   rdt_rcv(),	
   and	
   so	
   on),	
   clearly	
   state	
   their	
   actions.	
   Give	
   an	
   example	
   (a	
   timeline	
   trace	
   of	
  
   sender	
  and	
  receiver)	
  showing	
  how	
  your	
  protocol	
  recovers	
  from	
  a	
  lost	
  packet.	
  
   	
  
6) [Chapter	
   3,	
   P34]	
   Compare	
   GBN,	
   SR,	
   and	
   TCP	
   (no	
   delayed	
   ACK).	
   Assume	
   that	
   the	
   timeout	
   values	
   for	
   all	
   three	
  
   protocols	
  are	
  sufficiently	
  long	
  such	
  that	
  5	
  consecutive	
  data	
  segments	
  and	
  their	
  corresponding	
  ACKs	
  can	
  be	
  
   received	
   (if	
   not	
   lost	
   in	
   the	
   channel)	
   by	
   the	
   receiving	
   host	
   (Host	
   B)	
   and	
   the	
   sending	
   host	
   (Host	
   A)	
  
   respectively.	
  Suppose	
  Host	
  A	
  sends	
  5	
  data	
  segments	
  to	
  Host	
  B,	
  and	
  the	
  2nd	
  segment	
  (sent	
  from	
  Host	
  A)	
  is	
  
   lost.	
  In	
  the	
  end,	
  all	
  5	
  data	
  segments	
  have	
  been	
  correctly	
  received	
  by	
  Host	
  B.	
  
        	
  
   a) How	
  many	
  segments	
  has	
  Host	
  A	
  sent	
  in	
  total	
  and	
  how	
  many	
  ACKs	
  has	
  Host	
  B	
  sent	
  in	
  total?	
  What	
  are	
  
        their	
  sequence	
  numbers?	
  Answer	
  this	
  question	
  for	
  all	
  three	
  protocols.	
  	
  
   b) If	
  the	
  timeout	
  values	
  for	
  all	
  three	
  protocols	
  are	
  much	
  longer	
  than	
  5	
  RTT,	
  then	
  which	
  protocol	
  
        successfully	
  delivers	
  all	
  five	
  data	
  segments	
  in	
  shortest	
  time	
  interval?	
  
                             	
  
7) [Chapter	
  4,	
  P4]	
  Consider	
  the	
  network	
  below.	
  




                                                                                                                                                 	
  
   a) Suppose	
  that	
  this	
  network	
  is	
  a	
  datagram	
  network.	
  Show	
  the	
  forwarding	
  table	
  in	
  router	
  A,	
  such	
  that	
  all	
  
      traffic	
  destined	
  to	
  host	
  H3	
  is	
  forwarded	
  through	
  interface	
  3.	
  
   b) Suppose	
  that	
  this	
  network	
  is	
  a	
  datagram	
  network.	
  Can	
  you	
  write	
  down	
  a	
  forwarding	
  table	
  in	
  router	
  A,	
  
      such	
  that	
  all	
  traffic	
  from	
  H1	
  destined	
  to	
  host	
  H3	
  is	
  forwarded	
  through	
  interface	
  3,	
  while	
  all	
  traffic	
  from	
  
      H2	
  destined	
  to	
  host	
  H3	
  is	
  forwarded	
  through	
  interface	
  4?	
  (Hint:	
  this	
  is	
  a	
  trick	
  question.)	
  
   c) Now	
  suppose	
  that	
  this	
  network	
  is	
  a	
  virtual	
  circuit	
  network	
  and	
  that	
  there	
  is	
  one	
  ongoing	
  call	
  between	
  H1	
  
      and	
  H3,	
  and	
  another	
  ongoing	
  call	
  between	
  H2	
  and	
  H3.	
  Write	
  down	
  a	
  forwarding	
  table	
  in	
  router	
  A,	
  such	
  
      that	
  all	
  traffic	
  from	
  H1	
  destined	
  to	
  host	
  H3	
  is	
  forwarded	
  through	
  interface	
  3,	
  while	
  all	
  traffic	
  from	
  H2	
  
      destined	
  to	
  host	
  H3	
  is	
  forwarded	
  through	
  interface	
  4.	
  
   d) Assuming	
  the	
  same	
  scenario	
  as	
  (c),	
  write	
  down	
  the	
  forwarding	
  tables	
  in	
  nodes	
  B,	
  C,	
  and	
  D.	
  
      	
  
8) [Chapter	
  4,	
  P25]	
  Consider	
  the	
  network	
  below.	
  Using	
  Dijkstra’s	
  algorithm,	
  and	
  showing	
  your	
  work	
  using	
  a	
  
   table	
  similar	
  to	
  Table	
  4.3,	
  do	
  the	
  following:	
  	
  




                                                                                                                                      	
  
        Compute	
  the	
  shortest	
  path	
  from	
  t	
  to	
  all	
  network	
  nodes.	
  
        a)
        Compute	
  the	
  shortest	
  path	
  from	
  u	
  to	
  all	
  network	
  nodes.	
  
        b)
        Compute	
  the	
  shortest	
  path	
  from	
  v	
  to	
  all	
  network	
  nodes.	
  
        c)
        Compute	
  the	
  shortest	
  path	
  from	
  w	
  to	
  all	
  network	
  nodes.	
  
        d)
        Compute	
  the	
  shortest	
  path	
  from	
  y	
  to	
  all	
  network	
  nodes.	
  
        e)
        Compute	
  the	
  shortest	
  path	
  from	
  z	
  to	
  all	
  network	
  nodes.	
  
        f)
                          	
  
  9) [Chapter	
  5,	
  P14]	
  Consider	
  three	
  LANs	
  interconnected	
  by	
  two	
  routers,	
  as	
  shown	
  below.	
  
                                                                                     	
  




                                                                                                                                          	
  
    a) Assign	
  IP	
  addresses	
  to	
  all	
  of	
  the	
  interfaces.	
  For	
  Subnet	
  1	
  use	
  addresses	
  of	
  the	
  form	
  192.168.1.xxx;	
  for	
  
           Subnet	
  2	
  use	
  addresses	
  of	
  the	
  form	
  192.168.2.xxx;	
  and	
  for	
  Subnet	
  3	
  use	
  addresses	
  of	
  the	
  form	
  
           192.168.3.xxx.	
  
    b) Assign	
  MAC	
  addresses	
  to	
  all	
  of	
  the	
  adapters.	
  
    c) Consider	
  sending	
  an	
  IP	
  datagram	
  from	
  Host	
  E	
  to	
  Host	
  B.	
  suppose	
  all	
  of	
  the	
  ARP	
  tables	
  are	
  up	
  to	
  date.	
  
           Enumerate	
  all	
  the	
  steps,	
  as	
  done	
  for	
  the	
  single-­‐router	
  example	
  in	
  Section	
  5.4.2.	
  
    d) Repeat	
  (c),	
  now	
  assuming	
  that	
  the	
  ARP	
  table	
  in	
  the	
  sending	
  host	
  is	
  empty	
  (and	
  the	
  other	
  tables	
  are	
  up	
  to	
  
           date).	
  
    	
  
10) [Chapter	
  5,	
  P25]	
  Suppose	
  two	
  nodes,	
  A	
  and	
  B,	
  are	
  attached	
  to	
  opposite	
  ends	
  of	
  an	
  800	
  m	
  cable,	
  and	
  that	
  they	
  
    each	
  have	
  one	
  frame	
  of	
  1,500	
  bits	
  (including	
  all	
  headers	
  and	
  preambles)	
  to	
  send	
  to	
  each	
  other.	
  Both	
  nodes	
  
    attempt	
  to	
  transmit	
  at	
  time	
  t	
  =	
  0.	
  	
  Suppose	
  there	
  are	
  four	
  repeaters	
  between	
  A	
  and	
  B,	
  each	
  inserting	
  20-­‐bit	
  
    delay.	
  Assume	
  the	
  transmission	
  rate	
  is	
  100	
  Mbps,	
  and	
  CSMA/CD	
  with	
  backoff	
  intervals	
  of	
  multiple	
  512	
  bits	
  
    is	
  used.	
  After	
  the	
  first	
  collision,	
  A	
  draws	
  K	
  =	
  0	
  and	
  B	
  draws	
  K	
  =	
  1	
  in	
  the	
  exponential	
  backoff	
  protocol.	
  Ignore	
  
    the	
  jam	
  signal	
  and	
  96-­‐bit	
  time	
  delay.	
  
    	
  
    a) What	
   is	
   the	
   one-­‐way	
   propagation	
   delay	
   (including	
   repeater	
   delays)	
   between	
   A	
   and	
   B	
   in	
   seconds?	
  
           Assume	
  that	
  the	
  signal	
  propagation	
  speed	
  is	
  2	
  ·	
  108	
  m/sec.	
  
    b) At	
  what	
  time	
  (in	
  seconds)	
  is	
  A’s	
  packet	
  completely	
  delivered	
  at	
  B?	
  
    c) Now	
   suppose	
   that	
   only	
   A	
   has	
   a	
   packet	
   to	
   send	
   and	
   that	
   the	
   repeaters	
   are	
   replaced	
   with	
   switches.	
  
           Suppose	
   each	
   switch	
   has	
   a	
   20-­‐bit	
   processing	
   delay	
   in	
   addition	
   to	
   store-­‐and-­‐forward	
   delay.	
   At	
   what	
   time,	
  
           in	
  seconds,	
  is	
  A’s	
  packet	
  delivered	
  at	
  B?	
  

				
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