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Computer Science & Engineering Introduc3on to Computer Networks Applica3on Layer Overview

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Page 1: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

Applica3on  Layer  Overview  

Page 2: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   2  

Where  we  are  in  the  Course  •  We  are  finally  at  the  Applica3on  Layer!  

–  Builds  distributed  “network  services”  (DNS,  Web)  on  Transport  services  

Physical  Link  

Applica3on  

Network  Transport  

Page 3: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   3  

Topic  •  The  DNS  (Domain  Name  System)  

–  Human-­‐readable  host  names,  and  more  –  Distributed  namespace  &  resolu3on  

www.uw.edu?  

Network  

128.94.155.135  

Page 4: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

Names  and  Addresses  •  Names  are  higher-­‐level  iden3fiers  for  resources  •  Addresses  are  lower-­‐level  locators  for  resources  

–  Mul3ple  levels,  e.g.  full  name  à  email  à  IP  address  à  Ethernet  address  •  Resolu3on  (or  lookup)  is  mapping  a  name  to  an  address  

CSE  461  University  of  Washington   4  

Directory  

Name,  e.g.  “Andy  Tanenbaum,”  or  “flits.cs.vu.nl”    

Address,  e.g.  “Vrijie  Universiteit,  Amsterdam”  

or  IPv4  “130.30.27.38”  Lookup  

Page 5: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   5  

Before  the  DNS  –  HOSTS.TXT  •  Directory  was  a  file  HOSTS.TXT  regularly  retrieved  for  all  hosts  from  a  central  machine  at  the  NIC  (Network  Informa3on  Center)  

•  Names  were  ini3ally  flat,  became  hierarchical  (e.g.,  lcs.mit.edu)  ~85    

•  Neither  manageable  nor  efficient        as  the  ARPANET  grew  …  

Page 6: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   6  

DNS  •  A  naming  service  to  map  between  host  

names  and  their  IP  addresses  (and  more)  –  www.uwa.edu.au  à  130.95.128.140  

•  Goals:  –  Easy  to  manage  (esp.  with  mul3ple  par3es)  –  Efficient  (good  performance,  few  resources)  

•  Approach:  –  Distributed  directory  based  on  a  hierarchical  

namespace  –  Automated  protocol  to  3e  pieces  together  

Page 7: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

DNS  Namespace  •  Hierarchical,  star3ng  from  “.”  (dot,  typically  omiied)  

CSE  461  University  of  Washington   7  

Page 8: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

TLDs  (Top-­‐Level  Domains)  •  Run  by  ICANN  (Internet  Corp.  for  Assigned  Names  and  Numbers)  

–  Star3ng  in  ‘98;  naming  is  financial,  poli3cal,  and  interna3onal    J  

•  22+  generic  TLDs  –  Ini3ally  .com,  .edu  ,  .gov.,  .mil,  .org,  .net  –  Added  .aero,  .museum,  etc.  from  ’01  through  .xxx  in  ’11  –  Different  TLDs  have  different  usage  policies  

•  ~250  country  code  TLDs  –  Two  leiers,  e.g.,  “.au”,  plus  interna3onal  characters  since  2010  –  Widely  commercialized,  e.g.,  .tv  (Tuvalu)  –  Many  domain  hacks,  e.g.,  instagr.am  (Armenia),  goo.gl  (Greenland)  

CSE  461  University  of  Washington   8  

Page 9: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

DNS  Zones  •  A  zone  is  a  con3guous  por3on  of  the  namespace  

CSE  461  University  of  Washington   9  

A  zone  Delega3on  

Page 10: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   10  

DNS  Zones  (2)  •  Zones  are  the  basis  for  distribu3on  

–  EDU  Registrar  administers  .edu  –  UW  administers  washington.edu  –  CS&E  administers  cs.washington.edu  

•  Each  zone  has  a  nameserver  to  contact  for  informa3on  about  it  –  Zone  must  include  contacts  for  delega3ons,  e.g.,  .edu  knows  nameserver  for  washington.edu  

Page 11: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

DNS  Resource  Records  •  A  zone  is  comprised  of  DNS  resource  records  that  give  informa3on  for  its  domain  names  

CSE  461  University  of  Washington   11  

Type Meaning SOA Start of authority, has key zone parameters A IPv4 address of a host AAAA (“quad A”) IPv6 address of a host CNAME Canonical name for an alias MX Mail exchanger for the domain NS Nameserver of domain or delegated subdomain

Page 12: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   12  

DNS  Resource  Records  (2)  

IP  addresses  of  computers  

Name  server  

Mail  gateways  

Page 13: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   13  

DNS  Resolu3on  •  DNS  protocol  lets  a  host  resolve  any  host  name  (domain)  to  IP  address  

•  If  unknown,  can  start  with  the  root  nameserver  and  work  down  zones  

•  Let’s  see  an  example  first  …  

Page 14: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

DNS  Resolu3on  (2)  •  flits.cs.vu.nl  resolves  robot.cs.washington.edu  

CSE  461  University  of  Washington   14  

Page 15: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   15  

Itera3ve  vs.  Recursive  Queries  •  Recursive  query  

–  Nameserver  completes  resolu3on    and  returns  the  final  answer  

–  E.g.,  flits  à  local  nameserver  •  Itera3ve  query  

–  Nameserver  returns  the  answer  or  who  to  contact  next  for  the  answer  

–  E.g.,  local  nameserver  à  all  others  

Page 16: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   16  

DNS  Resolu3on  

•  What  are  the  implica3ons  of  the  resolu3on  process  presented  above?  

Page 17: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   17  

Itera3ve  vs.  Recursive  Queries  (2)  •  Recursive  query  

–  Lets  server  offload  client  burden  (simple  resolver)  for  manageability  

–  Lets  server  cache  over  a  pool  of    clients  for  beier  performance  

•  Itera3ve  query  –  Lets  server  “file  and  forget”  –  Easy  to  build  high  load  servers  

Page 18: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   18  

Caching  •  Resolu3on  latency  should  be  low  

–  Adds  delay  to  web  browsing  •  Cache  query/responses  to  answer  future  queries  immediately  –  Including  par3al  (itera3ve)  answers  –  Responses  carry  a  TTL  for  caching  

Nameserver  

Cache  query   out  

response  

Page 19: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

Caching  (2)  •  flits.cs.vu.nl  now  resolves  eng.washington.edu  

–  And  previous  resolu3ons  cut  out  most  of  the  process  

CSE  461  University  of  Washington   19  

1:  query   2:  query  

UW  nameserver  (for  washington.edu)  

3:  eng.washington.edu  4:  eng.washington.edu  

Local  nameserver  (for  cs.vu.nl)  

Cache  

I  know  the  server  for  washington.edu!  

Page 20: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   20  

Local  Nameservers  •  Local  nameservers  typically  run  by  IT  (enterprise,  ISP)  –  But  may  be  your  host  or  AP  –  Or  alterna3ves  e.g.,  Google  public  DNS  

•  Clients  need  to  be  able  to  contact  their  local  nameservers  –  Typically  configured  via  DHCP  

Page 21: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   21  

Root  Nameservers  •  Root  (dot)  is  served  by  13  server  names  

–  a.root-­‐servers.net  to  m.root-­‐servers.net  –  All  nameservers  need  root  IP  addresses  –  Handled  via  configura3on  file  (named.ca)  

•  There  are  >250  distributed  server  instances  –  Highly  reachable,  reliable  service  –  Most  servers  are  reached  by  IP  anycast  

(Mul3ple  loca3ons  adver3se  same  IP!  Routes  take  client  to  the  closest  one.)  

–  Servers  are  IPv4  and  IPv6  reachable  

Page 22: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

Root  Server  Deployment  

CSE  461  University  of  Washington   22  

Source:  hip://www.root-­‐servers.org.  Snapshot  on  27.02.12.  Does  not  represent  current  deployment.  

Page 23: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   23  

DNS  Protocol  •  Query  and  response  messages  

–  Built  on  UDP  messages,  port  53  –  ARQ  for  reliability;  server  is  stateless!  –  Messages  linked  by  a  16-­‐bit  ID  field  

Query  

Response  

Time  

Client   Server  ID=0x1234  

ID=0x1234  

Page 24: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   24  

DNS  Protocol  (2)  •  Service  reliability  via  replicas  

–  Run  mul3ple  nameservers  for  domain  –  Return  the  list;  clients  use  one  answer  –  Helps  distribute  load  too  

NS  for  uw.edu?  

A

B

C

Use  A,  B  or  C  

Page 25: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   25  

DNS  Issues  

•  Are  there  any  security  issues  with  DNS?  

Page 26: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   26  

DNS  Issues  •  Recall  that  CDNs  allow  you  to  replicate  content  at  mul3ple  loca3ons  

•  DNS  can  be  used  to  redirect  clients  to  CDN  nodes  based  on  the  resolver  IP  

•  But  this  can  go  wrong  some3mes…  

Page 27: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

HTTP,  the  HyperText  Transfer  Protocol  (§7.3.1-­‐7.3.4)  

Page 28: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

Web  Context    

CSE  461  University  of  Washington   28  

HTTP  request  

HTTP  response  

Page  as  a  set  of  related  HTTP  transac3ons  

Page 29: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   29  

Web  Protocol  Context  •  HTTP  is  a  request/response  protocol  for  fetching  Web  resources  –  Runs  on  TCP,  typically  port  80  –  Part  of  browser/server  app  

TCP  IP  

802.11  

browser  

HTTP  TCP  IP  

802.11  

server  

HTTP  request  

response  

Page 30: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   30  

Fetching  a  Web  page  with  HTTP  •  Start  with  the  page  URL:  

     hip://en.wikipedia.org/wiki/Vegemite  

•  Steps:  –  Resolve  the  server  to  IP  address  (DNS)  –  Set  up  TCP  connec3on  to  the  server  –  Send  HTTP  request  for  the  page  –  (Await  HTTP  response  for  the  page)  –  Execute  /  fetch  other  Web  resources  /  render  –  Clean  up  any  idle  TCP  connec3ons  

Protocol   Page  on  server  Server  

**  

Page 31: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

Sta3c  vs  Dynamic  Web  pages  •  Sta3c  web  page  is  a  file  contents,  e.g.,  image  •  Dynamic  web  page  is  the  result  of  program  execu3on  

–  Javascript  on  client,  PHP  on  server,  or  both    

CSE  461  University  of  Washington   31  

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CSE  461  University  of  Washington   33  

HTTP  Protocol  •  Originally  a  simple  protocol,  with  

many  op3ons  added  over  3me  –  Text-­‐based  commands,  headers  

•  Try  it  yourself:  –  As  a  “browser”  fetching  a  URL  –  Run  “telnet  en.wikipedia.org  80”  –  Type  “GET  /wiki/Vegemite  HTTP/1.0”            to  server  followed  by  a  blank  line  

–  Server  will  return  HTTP  response  with      the  page  contents  (or  other  info)  

Page 33: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

CSE  461  University  of  Washington   34  

HTTP  Protocol  (2)  •  Commands  used  in  the  request  

Method   Descrip.on  GET   Read  a  Web  page  HEAD   Read  a  Web  page's  header  POST   Append  to  a  Web  page  PUT   Store  a  Web  page  DELETE   Remove  the  Web  page  TRACE   Echo  the  incoming  request  CONNECT   Connect  through  a  proxy  OPTIONS   Query  op3ons  for  a  page  

Fetch  page  Upload  data  

Page 34: Introduc3on)to)Computer)Networks)...TLDs)(TopULevel)Domains)) • Run)by)ICANN)(InternetCorp.)for)Assigned)Names)and)Numbers)) – Star3ng)in)‘98;)naming)is)financial,)poli3cal,)and)internaonal

HTTP  Protocol  (3)  •  Codes  returned  with  the  response  

CSE  461  University  of  Washington   35  

Code   Meaning   Examples  1xx   Informa3on   100  =  server  agrees  to  handle  client's  request  2xx   Success   200  =  request  succeeded;  204  =  no  content  present  3xx   Redirec3on   301  =  page  moved;  304  =  cached  page  s3ll  valid  4xx   Client  error   403  =  forbidden  page;  404  =  page  not  found  5xx   Server  error   500  =  internal  server  error;  503  =  try  again  later  

Yes!  

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HTTP  Protocol  (4)  •  Many  header  fields  specify  capabili3es  and  content  

–  E.g.,  Content-­‐Type:  text/html,  Cookie:  lect=8-­‐4-­‐hip  

CSE  461  University  of  Washington   36  

Func.on   Example  Headers  Browser  capabili3es  (client  à  server)  

User-­‐Agent,  Accept,  Accept-­‐Charset,  Accept-­‐Encoding,  Accept-­‐Language  

Caching  related  (mixed  direc3ons)  

If-­‐Modified-­‐Since,  If-­‐None-­‐Match,  Date,  Last-­‐Modified,  Expires,  Cache-­‐Control,  ETag  

Browser  context  (client  à  server)   Cookie,  Referer,  Authoriza3on,  Host  

Content  delivery  (server  à  client)  

Content-­‐Encoding,  Content-­‐Length,  Content-­‐Type,  Content-­‐Language,  Content-­‐Range,  Set-­‐Cookie  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

HTTP  Performance  (§7.3.4,  §7.5.2)  

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CSE  461  University  of  Washington   38  

PLT  (Page  Load  Time)  •  PLT  is  the  key  measure  of  web  performance    –  From  click  un3l  user  sees  page  –  Small  increases  in  PLT  decrease  sales  

•  PLT  depends  on  many  factors  –  Structure  of  page/content  –  HTTP  (and  TCP!)  protocol  –  Network  RTT  and  bandwidth  

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CSE  461  University  of  Washington   39  

Page  Load  Time  •  How  can  we  op3mize  page  load  3me?  – Consider  all  layers  of  the  stack  – Consider  different  kinds  of  web  pages  and  what  they  contain  

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CSE  461  University  of  Washington   40  

Early  Performance  •  HTTP/1.0  used  one  TCP  connec3on  to  fetch  one  web  resource  – Made  HTTP  very  easy  to  build  –  But  gave  fairly  poor  PLT…    

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CSE  461  University  of  Washington   41  

Early  Performance  (2)  •  Many  reasons  why  PLT  is  larger  than  

necessary  –  Sequen3al  request/responses,  even    when  to  different  servers  

–  Mul3ple  TCP  connec3on  setups  to                  the  same  server  

–  Mul3ple  TCP  slow-­‐start  phases  

•  Network  is  not  used  effec3vely  –  Worse  with  many  small  resources  /  page  

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CSE  461  University  of  Washington   43  

Parallel  Connec3ons  •  One  simple  way  to  reduce  PLT  

–  Browser  runs  mul3ple  (8,  say)  HTTP      instances  in  parallel  

–  Server  is  unchanged;  already  handled  concurrent  requests  for  many  clients  

•  How  does  this  help?  –  Single  HTTP  wasn’t  using  network  much  …  –  So  parallel  connec3ons  aren’t  slowed  much  –  Pulls  in  comple3on  3me  of  last  fetch  

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CSE  461  University  of  Washington   44  

Persistent  Connec3ons  •  Parallel  connec3ons  compete  with  each  other  for  network  resources  –  1  parallel  client  ≈  8  sequen3al  clients?  –  Exacerbates  network  bursts,  and  loss  

•  Persistent  connec3on  alterna3ve  – Make  1  TCP  connec3on  to  1  server  –  Use  it  for  mul3ple  HTTP  requests  

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Persistent  Connec3ons  (2)  

CSE  461  University  of  Washington   45  

One  request  per  connec3on  

Sequen3al  requests  per  connec3on  

Pipelined  requests  per  connec3on  

 

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CSE  461  University  of  Washington   46  

Persistent  Connec3ons  (3)  •  Widely  used  as  part  of  HTTP/1.1  

–  Supports  op3onal  pipelining  –  PLT  benefits  depending  on  page  structure,  but  easy  on  network  

•  Issues  with  persistent  connec3ons  –  How  long  to  keep  TCP  connec3on?  –  Can  it  be  slower?  (Yes.  But  why?)  

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CSE  461  University  of  Washington   47  

Web  Caching  •  Users  o�en  revisit  web  pages  

–  Big  win  from  reusing  local  copy!  –  This  is  caching  

•  Key  ques3on:  – When  is  it  OK  to  reuse  local  copy?  

Network  Cache  

Local  copies  

Server  

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CSE  461  University  of  Washington   48  

Web  Caching  (2)  •  Locally  determine  copy  is  s3ll  valid  

–  Based  on  expiry  informa3on  such  as  “Expires”  header  from  server  

–  Or  use  a  heuris3c  to  guess  (cacheable,  freshly  valid,  not  modified  recently)    

–  Content  is  then  available  right  away  

Network  Cache  Server  

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CSE  461  University  of  Washington   49  

Web  Caching  (3)  •  Revalidate  copy  with  server  

–  Based  on  3mestamp  of  copy  such  as  “Last-­‐Modified”  header  from  server  

–  Or  based  on  content  of  copy  such  as  “Etag”  header  from  server  

–  Content  is  available  a�er  1  RTT  

Network  Cache  Server  

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Web  Caching  (4)  •  Pu�ng  the  pieces  together:  

CSE  461  University  of  Washington   50  

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CSE  461  University  of  Washington   51  

Web  Proxies  •  Place  intermediary  between  pool  of  

clients  and  external  web  servers  –  Benefits  for  clients  include  greater  caching  and  security  checking  

–  Organiza3onal  access  policies  too!  

•  Proxy  caching  –  Clients  benefit  from  a  larger,  shared  cache  

–  Benefits  limited  by  secure  and  dynamic  content,  as  well  as  “long  tail”  

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Web  Proxies  (2)  •  Clients  contact  proxy;  proxy  contacts  server  

CSE  461  University  of  Washington   52  

Cache  

Near  client  Far  from  client  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

CDNs  (Content  Delivery  Networks)  (§7.5.3)  

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CSE  461  University  of  Washington   55  

Context  •  As  the  web  took  off  in  the  90s,  traffic  

volumes  grew  and  grew.  This:  1.  Concentrated  load  on  popular  servers  2.  Led  to  congested  networks  and  need      

to  provision  more  bandwidth  3.  Gave  a  poor  user  experience  

•  Idea:  –  Place  popular  content  near  clients  –  Helps  with  all  three  issues  above  

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CSE  461  University  of  Washington   56  

Ques3ons  

•  What  are  good  loca3ons  to  establish  CDN  nodes?  

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CSE  461  University  of  Washington   57  

Before  CDNs  •  Sending  content  from  the  source  to  4  users  takes  4  x  3  =  12  “network  hops”  in  the  example  

Source  

User  

User  

.  .  .  

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CSE  461  University  of  Washington   58  

A�er  CDNs  •  Sending  content  via  replicas  takes  only  4  +  2  =  6  “network  hops”  

Source  

User  

User  

.  .  .  Replica  

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CSE  461  University  of  Washington   60  

Popularity  of  Content  •  Zipf’s  Law:  few  popular  items,  many  unpopular  ones;  both  maier  

Zipf  popularity  (kth  item  is  1/k)  

Rank   Source:  Wikipedia  

George  Zipf  (1902-­‐1950)  

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CSE  461  University  of  Washington   61  

How  to  place  content  near  clients?    •  Use  browser  and  proxy  caches  

–  Helps,  but  limited  to  one  client  or  clients  in  one  organiza3on  

•  Want  to  place  replicas  across  the  Internet  for  use  by  all  nearby  clients  –  Done  by  clever  use  of  DNS  

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Content  Delivery  Network  

CSE  461  University  of  Washington   62  

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Content  Delivery  Network  (2)  •  DNS  resolu3on  of  site  gives  different  answers  to  clients  

–  Tell  each  client  the  site  is  the  nearest  replica  (map  client  IP)  

CSE  461  University  of  Washington   63  

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Consumer        site  

CSE  461  University  of  Washington   64  

Business  Model  •  Clever  model  pioneered  by  Akamai  

–  Placing  site  replica  at  an  ISP  is  win-­‐win  –  Improves  site  experience  and  reduces  bandwidth  usage  of  ISP  

ISP        

User  

User  

.  .  .  Replica  

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CSE  461  University  of  Washington   65  

Topic  •  The  Future  of  HTTP  

–  How  will  we  make  the  web  faster?  –  A  brief  look  at  some  approaches  

request  Network  

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Modern  Web  Pages  •  Waterfall  diagram  shows  progression  of  page  load  

CSE  461  University  of  Washington   66  

webpagetest  tool  for  hip://coursera.org  (Firefox,  5/1  Mbps,  from  VA,  3/1/13)    

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CSE  461  University  of  Washington   67  

Modern  Web  Pages  (2)    Yikes!  -­‐23  requests  -­‐1  Mb  data  -­‐2.6  secs  

webpagetest  tool  for  hip://coursera.org  (Firefox,  5/1  Mbps,  from  VA,  3/1/13)    

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•  Waterfall  and  PLT  depends  on  many  factors  –  Very  different  for  different  browsers  –  Very  different  for  repeat  page  views  –  Depends  on  local  computa3on  as  well  as  network  

Modern  Web  Pages  (3)  

CSE  461  University  of  Washington   68  

Yay!  (Network  used  well)  

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CSE  461  University  of  Washington   69  

Recent  work  to  reduce  PLT  Pages  grow  ever  more  complex!  

–  Larger,  more  dynamic,  and  secure  –  How  will  we  reduce  PLT?  

1.  Beier  use  of  the  network  –  HTTP/2  effort  based  on  SPDY  

2.  Beier  content  structures  –  mod_pagespeed  server  extension  

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CSE  461  University  of  Washington   70  

SPDY  (“speedy”)  •  A  set  of  HTTP  improvements  

– Mul3plexed  (parallel)  HTTP  requests  on  one  TCP  connec3on  

–  Client  priori3es  for  parallel  requests  –  Compressed  HTTP  headers  –  Server  push  of  resources  

•  Now  being  tested  and  improved  –  Default  in  Chrome,  Firefox  –  Basis  for  an  HTTP/2  effort  

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CSE  461  University  of  Washington   71  

mod_pagespeed  •  Observa3on:  

–  The  way  pages  are  wriien  affects                how  quickly  they  load  

–  Many  books  on  best  prac3ces  for                page  authors  and  developers  

•  Key  idea:  –  Have  server  re-­‐write  (compile)  pages              to  help  them  load  quickly!  

–  mod_pagespeed  is  an  example  

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CSE  461  University  of  Washington   72  

mod_pagespeed  (2)  •  Apache  server  extension  

–  So�ware  installed  with  web  server  –  Rewrites  pages  “on  the  fly”  with  rules  based  on  best  prac3ces  

•  Example  rewrite  rules:  –  Minify  Javascript  –  Flaien  mul3-­‐level  CSS  files  –  Resize  images  for  client  –  And  much  more  (100s  of  specific  rules)  


Introduc3on)to)Computer)Networks) · Computer)Science)&)Engineering) Introduc3on)to)Computer)Networks) Overview)of)the)Link)Layer)

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