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109
Opera&ng Systems and Networks Network Lecture 4: Link Layer (2) Adrian Perrig Network Security Group ETH Zürich
Transcript
Page 1: Operang)Systems)and)Networks) Network)Lecture)4:)Link ... · Operang)Systems)and)Networks) Network)Lecture)4:)Link)Layer)(2)) Adrian)Perrig) Network)Security)Group) ETHZürich)

Opera&ng  Systems  and  Networks  

Network  Lecture  4:  Link  Layer  (2)  

Adrian  Perrig  Network  Security  Group  ETH  Zürich  

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Pending  Issues  •  How  to  read  the  course  textbook?  •  How  to  prepare  for  the  exam  given  that  there  is  a  huge  amount  of  material?  

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Where  we  are  in  the  Course  •  Finishing  off  the  Link  Layer!  

–  Builds  on  the  physical  layer  to  transfer  frames  over  connected  links  

Physical  

Link  

Network  

Transport  

Applica&on  

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Topics  1.  Framing  

–  Delimi&ng  start/end  of  frames  

2.  Error  detec&on/correc&on  –  Handling  errors  

Done  

DSL  

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Topics  (2)  3.  Retransmissions  

–  Handling  loss  4.  Mul&ple  Access  

–  Classic  Ethernet,  802.11  

5.  Switching  – Modern  Ethernet  

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Retransmissions  (ARQ)  (§3.3)  •  Two  strategies  to  handle  errors:  1.  Detect  errors  and  retransmit  frame  (Automa&c  Repeat  reQuest,  

ARQ)  

2.  Correct  errors  with  an  error  correc&ng  code  

Done  this  

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Context  on  Reliability  •  Where  in  the  stack  should  we  place  reliability  func&ons?  

Physical  

Link  

Network  

Transport  

Applica&on  

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Context  on  Reliability  (2)  •  Everywhere!  It  is  a  key  issue  

–  Different  layers  contribute  differently  

Physical  

Link  

Network  

Transport  

Applica&on  Recover  ac&ons  (correctness)  

Mask  errors  (performance  op&miza&on)  

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ARQ  (Automa&c  Repeat  reQuest)  •  ARQ  oden  used  when  errors  are  common  or  must  be  corrected  –  E.g.,  WiFi,  and  TCP  (later)  

•  Rules  at  sender  and  receiver:  –  Receiver  automa&cally  acknowledges  correct  frames  with  an  ACK  

–  Sender  automa&cally  resends  ader  a  &meout,  un&l  an  ACK  is  received  

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ARQ  (2)  •  Normal  opera&on  (no  loss,  no  error)  

Frame  

ACK  Timeout   Time  

Sender   Receiver  

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ARQ  (3)  •  Loss  and  retransmission  

Frame  

Timeout   Time  

Sender   Receiver  

Frame  

ACK  

X  

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So  What’s  Tricky  About  ARQ?  •  Two  non-­‐trivial  issues:  

–  How  long  to  set  the  &meout?  –  How  to  avoid  accep&ng  duplicate  frames  as  new  frames  

•  Want  performance  in  the  common  case  and  correctness  always  

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Timeouts  •  Timeout  should  be:  

–  Not  too  big  (link  goes  idle)  –  Not  too  small  (spurious  resend)  

•  Fairly  easy  on  a  LAN  –  Clear  worst  case,  lihle  varia&on  

•  Fairly  difficult  over  the  Internet  – Much  varia&on,  no  obvious  bound  – We’ll  revisit  this  with  TCP  (later)  

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Duplicates  •  What  happens  if  an  ACK  is  lost?  

Frame  

ACK  

X  

Frame  

ACK  Timeout  

Sender   Receiver  

New    Frame??  

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Duplicates  (2)  •  Or  the  &meout  is  early?  

Frame  

ACK  

Frame  

ACK  

Timeout  

Sender   Receiver  

New    Frame??  

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Sequence  Numbers  •  Frames  and  ACKs  must  both  carry  sequence  numbers  for  correctness  

•  To  dis&nguish  the  current  frame  from  the  next  one,  a  single  bit  (two  numbers)  is  sufficient  –  Called  Stop-­‐and-­‐Wait  

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Stop-­‐and-­‐Wait  •  In  the  normal  case:  

Time  

Sender   Receiver  

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Stop-­‐and-­‐Wait  (2)  •  In  the  normal  case:  

Frame  0  

ACK  0  Timeout   Time  

Sender   Receiver  

Frame  1  

ACK  1  

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Stop-­‐and-­‐Wait  (3)  •  With  ACK  loss:  

Frame  0  

ACK  0  

X  

Frame  0  

ACK  0  Timeout  

Sender   Receiver  

It’s  a    Resend!  

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Stop-­‐and-­‐Wait  (4)  •  With  early  &meout:  

Frame  0  

ACK  0  

Frame  0  

ACK  0  

Timeout  

Sender   Receiver  

It’s  a  Resend  

OK  …  

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Limita&on  of  Stop-­‐and-­‐Wait  •  It  allows  only  a  single  frame  to  be  outstanding  from  the  sender:  –  Good  for  LAN,  inefficient  for  high  BD  (bandwidth-­‐delay  product)  

•  Ex:  R=1  Mbps,  D  =  50  ms  –  How  many  frames/sec?  If  R=10  Mbps?  

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Sliding  Window  •  Generaliza&on  of  stop-­‐and-­‐wait  

–  Allows  W  frames  to  be  outstanding  –  Can  send  W  frames  per  RTT  (=2D)  

–  Various  op&ons  for  numbering  frames/ACKs  and  handling  loss  •  Will  look  at  along  with  TCP  (later)  

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Mul&plexing  (§2.5.3,  2.5.4)  •  Mul&plexing  is  the  network  word  for  the  sharing  of  a  resource  

•  Classic  scenario  is  sharing  a  link  among  different  users  – Time  Division  Mul&plexing  (TDM)  – Frequency  Division  Mul&plexing  (FDM)  

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Time  Division  Mul&plexing  (TDM)  

•  Users  take  turns  on  a  fixed  schedule  

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2   2   2   2  

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Frequency  Division  Mul&plexing  (FDM)  •  Put  different  users  on  different  frequency  bands  

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Overall  FDM  channel  

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TDM  versus  FDM  •  In  TDM  a  user  sends  at  a  high  rate  a  frac&on  of  the  &me;  in  FDM,  a  user  sends  at  a  low  rate  all  the  &me    

Rate  

Time  FDM  

TDM  

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TDM/FDM  Usage  

•  Sta&cally  divide  a  resource  –  Suited  for  con&nuous  traffic,  fixed  number  of  users  

•  Widely  used  in  telecommunica&ons  –  TV  and  radio  sta&ons  (FDM)  –  GSM  (2G  cellular)  allocates  calls  using  TDM  within  FDM  

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Mul&plexing  Network  Traffic  •  Network  traffic  is  bursty  

–  ON/OFF  sources    –  Load  varies  greatly  over  &me  

Rate  

Time  Rate  

Time  

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Mul&plexing  Network  Traffic  (2)  •  Network  traffic  is  bursty  

–  Inefficient  to  always  allocate  user    their  ON  needs  with  TDM/FDM  

Rate  

Time  Rate  

Time  

R  

R  

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Mul&plexing  Network  Traffic  (3)  •  Mul&ple  access  schemes  mul&plex  users  according  to  their  demands  –  for  gains  of  sta&s&cal  mul&plexing  

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Rate  

Time  Rate  

Time  

Rate  

Time  

R  

R  

R’<2R  

Two  users,  each  need  R   Together  they  need  R’  <  2R  

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Mul&ple  Access  •  We  will  look  at  two  kinds  of  mul&ple  access  protocols  

1.  Randomized.  Nodes  randomize  their  resource  access  ahempts  –  Good  for  low  load  situa&ons  

2.  Conten&on-­‐free.  Nodes  order  their  resource  access  ahempts  –  Good  for  high  load  or  guaranteed  quality  of  service  situa&ons  

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Randomized  Mul&ple  Access  (§4.2.1-­‐4.2.2,  4.3.1-­‐4.3.3)  

•  How  do  nodes  share  a  single  link?  Who  sends  when,  e.g.,  in  WiFI?  –  Explore  with  a  simple  model  

•  Assume  no-­‐one  is  in  charge;  this  is  a  distributed  system  

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Randomized  Mul&ple  Access  (2)  •  We  will  explore  random  mul&ple  access  control  or  medium  access  control  (MAC)  protocols  –  This  is  the  basis  for  classic  Ethernet  –  Remember:  data  traffic  is  bursty  

Zzzz..  Busy!   Ho  hum  

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ALOHA  Network  

•  Seminal  computer  network  connec&ng  the  Hawaiian                islands  in  the  late  1960s  – When  should  nodes  send?  –  A  new  protocol  was  devised  by  Norm  Abramson  …  

Hawaii  

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ALOHA  Protocol  

•  Simple  idea:  –  Node  just  sends  when  it  has  traffic.    –  If  there  was  a  collision  (no  ACK  received)  then  wait  a  random  &me  and  resend  

•  That’s  it!  

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ALOHA  Protocol  (2)  

•  Some  frames  will  be  lost,  but  many  may  get  through…  

•  Good  idea?  

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ALOHA  Protocol  (3)  •  Simple,  decentralized  protocol  that  works  well  under  low  load!  

•  Not  efficient  under  high  load  –  Analysis  shows  at  most  18%  efficiency  –  Improvement:  divide  &me  into  slots  and  efficiency  goes  up  to  36%  

•  We’ll  look  at  other  improvements  

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Classic  Ethernet    •  ALOHA  inspired  Bob  Metcalfe  to  invent  Ethernet  for  LANs  in  1973  –  Nodes  share  10  Mbps  coaxial  cable  

–  Hugely  popular  in  1980s,  1990s  

:  ©  2009  IEEE  

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CSMA  (Carrier  Sense  Mul&ple  Access)  

•  Improve  ALOHA  by  listening  for  ac&vity  before  we  send  (Doh!)  –  Can  do  easily  with  wires,  not  wireless  

•  So  does  this  eliminate  collisions?  – Why  or  why  not?  

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CSMA  (2)  •  S&ll  possible  to  listen  and  hear  nothing  when  another  node  is  sending  because  of  delay  

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CSMA/CD  (with  Collision  Detec&on)  •  Can  reduce  the  cost  of  collisions  by  detec&ng  them  and  abor&ng  (Jam)  the  rest  of  the  frame  &me  –  Again,  we  can  do  this  with  wires  

X  X  X  X  X  X  X  X  Jam!   Jam!  

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CSMA/CD  Complica&ons  •  Want  everyone  who  collides  to  know  that  it  happened  

–  Time  window  in  which  a  node  may  hear  of  a  collision  is  2D  seconds  

X  

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CSMA/CD  Complica&ons  (2)  •  Impose  a  minimum  frame  size  that  lasts  for  2D  seconds  

–  So  node  can’t  finish  before  collision  –  Ethernet  minimum  frame  is  64  bytes  

X  

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CSMA  “Persistence”  •  What  should  a  node  do  if  another  node  is  sending?  

•  Idea:  Wait  un&l  it  is  done,  and  send    

What  now?  

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CSMA  “Persistence”  (2)  

•  Problem  is  that  mul&ple  wai&ng  nodes  will  queue  up  then  collide  – More  load,  more  of  a  problem  

Now!   Now!  Uh  oh  

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CSMA  “Persistence”  (3)  

•  Intui&on  for  a  beher  solu&on  –  If  there  are  N  queued  senders,  we  want  each  to  send  next  with  probability  1/N  

Send  p=½  Whew  Send  p=½  

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Binary  Exponen&al  Backoff  (BEB)  •  Cleverly  es&mates  the  probability  

–  1st  collision,  wait  0  or  1  frame  &mes  –  2nd  collision,  wait  from  0  to  3  &mes  

–  3rd  collision,  wait  from  0  to  7  &mes  …  

•  BEB  doubles  interval  for  each  successive  collision  –  Quickly  gets  large  enough  to  work  –  Very  efficient  in  prac&ce  

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Classic  Ethernet,  or  IEEE  802.3  •  Most  popular  LAN  of  the  1980s,  1990s  

–  10  Mbps  over  shared  coaxial  cable,  with  baseband  signals  – Mul&ple  access  with  “1-­‐persistent  CSMA/CD  with  BEB”  

48  

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Ethernet  Frame  Format  •  Has  addresses  to  iden&fy  the  sender  and  receiver  •  CRC-­‐32  for  error  detec&on;  no  ACKs  or  retransmission  •  Start  of  frame  iden&fied  with  physical  layer  preamble  

49  

Packet  from  Network  layer  (IP)  

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Modern  Ethernet  

•  Based  on  switches,  not  mul&ple  access,  but  s&ll  called  Ethernet  – We’ll  get  to  it  later  

Switch  

Twisted  pair  

Switch  ports  

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Wireless  Mul&ple  Access  (§4.2.5,  4.4)  •  How  do  wireless  nodes  share  a  single  link?  (Yes,  this  is  WiFi!)  –  Build  on  our  simple,  wired  model  

Send?   Send?  

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Wireless  Complica&ons  •  Wireless  is  more  complicated  than  the  wired  case  (Surprise!)  1.  Nodes  may  have  different  areas  of  coverage  –  doesn’t  fit  

Carrier  Sense  

2.  Nodes  can’t  hear  while  sending  –  can’t  Collision  Detect  

≠  CSMA/CD  

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Different  Coverage  Areas  •  Wireless  signal  is  broadcast  and  received  nearby,  where  there  is  sufficient  SNR  

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Hidden  Terminals  •  Nodes  A  and  C  are  hidden  terminals  when  sending  to  B  

–  Can’t  hear  each  other  (to  coordinate)  yet  collide  at  B  – We  want  to  avoid  the  inefficiency  of  collisions  

54  

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Exposed  Terminals  •  B  and  C  are  exposed  terminals  when  sending  to  A  and  D  

–  Can  hear  each  other  yet  don’t  collide  at  receivers  A  and  D  – We  want  to  send  concurrently  to  increase  performance  

55  

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Nodes  Can’t  Hear  While  Sending  •  With  wires,  detec&ng  collisions  (and  abor&ng)  lowers  their  cost  

•  More  wasted  &me  with  wireless  

Time   XXXXXXXXX  

XXXXXXXXX  

Wireless  Collision  

Resend  X  

X  

Wired  Collision  

Resend  

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Possible  Solu&on:  MACA  •  MACA  uses  a  short  handshake  instead  of  CSMA  (Karn,  1990)  

–  802.11  uses  a  refinement  of  MACA  (later)    

•  Protocol  rules:  1. A  sender  node  transmits  a  RTS  (Request-­‐To-­‐Send,  with  frame  length)  2. The  receiver  replies  with  a  CTS  (Clear-­‐To-­‐Send,  with  frame  length)  3. Sender  transmits  the  frame  while  nodes  hearing  the  CTS  stay  silent  –  Collisions  on  the  RTS/CTS  are  s&ll  possible,  but  less  likely  

57  

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MACA  –  Hidden  Terminals  

•  A!B  with  hidden  terminal  C  1.  A  sends  RTS,  to  B    

D  C  B  A  

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MACA  –  Hidden  Terminals  (2)  

•  A!B  with  hidden  terminal  C  2.  B  sends  CTS,  to  A,  and  C  too    

D  C  B  A  RTS  

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MACA  –  Hidden  Terminals  (3)  

•  A!B  with  hidden  terminal  C  2.  B  sends  CTS,  to  A,  and  C  too    

D  C  B  A  RTS  

CTS  CTS  

Alert!  

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MACA  –  Hidden  Terminals  (4)  

•  A!B  with  hidden  terminal  C  3.  A  sends  frame  while  C  defers  

Frame  

Quiet...  

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MACA  –  Exposed  Terminals  

•  B!A,  C!D  as  exposed  terminals  

– B  and  C  send  RTS  to  A  and  D    

D  C  B  A  

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MACA  –  Exposed  Terminals  (2)  

•  B!A,  C!D  as  exposed  terminals  

– A  and  D  send  CTS  to  B  and  C    

D  C  B  A  RTS  RTS  

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MACA  –  Exposed  Terminals  (3)  

•  B!A,  C!D  as  exposed  terminals  

– A  and  D  send  CTS  to  B  and  C    

D  C  B  A  RTS  RTS  

CTS  CTS  

All  OK  All  OK  

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MACA  –  Exposed  Terminals  (4)  

•  B!A,  C!D  as  exposed  terminals  

– A  and  D  send  CTS  to  B  and  C    

D  C  B  A  Frame  Frame  

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802.11,  or  WiFi  •  Very  popular  wireless  LAN  

started  in  the  1990s  

•  Clients  get  connec&vity  from  a  (wired)  AP  (Access  Point)  

•  It’s  a  mul&-­‐access  problem  ☺    

•  Various  flavors  have  been  developed  over  &me  –  Faster,  more  features    

Access  Point  

Client  

To  Network  

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802.11  Physical  Layer  •  Uses  20/40  MHz  channels  on  ISM  bands  

–  802.11b/g/n  on  2.4  GHz  –  802.11  a/n  on  5  GHz  

•  OFDM  modula&on  (except  legacy  802.11b)  –  Different  amplitudes/phases  for  varying  SNRs  

–  Rates  from  6  to  54  Mbps    plus  error  correc&on  

–  802.11n  uses  mul&ple  antennas;  see  “802.11  with  Mul&ple  Antennas  for  Dummies”  

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802.11  Link  Layer  •  Mul&ple  access  uses  CSMA/CA  (next);  RTS/CTS  op&onal    •  Frames  are  ACKed  and  retransmihed  with  ARQ  •  Funky  addressing  (three  addresses!)  due  to  AP  •  Errors  are  detected  with  a  32-­‐bit  CRC  •  Many,  many  features  (e.g.,  encryp&on,  power  save)  

68  

Packet  from  Network  layer  (IP)  

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802.11  CSMA/CA  for  Mul&ple  Access  •  Sender  avoids  collisions  by  inser&ng  small  random  gaps  

–  E.g.,  when  both  B  and  C  send,  C  picks  a  smaller  gap,  goes  first  

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Time  

Send?  

Send?  

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The  Future  of  802.11  (Guess)  •  Likely  ubiquitous  for  Internet  connec&vity  

– Greater  diversity,  from  low-­‐  to  high-­‐end  devices    

•  Innova&on  in  physical  layer  drives  speed  – And  power-­‐efficient  opera&on  too  

•  More  seamless  integra&on  of  connec&vity  – Too  manual  now,  and  limited  (e.g.,  device-­‐to-­‐device)  

70  

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Conten&on-­‐Free  Mul&ple  Access  (§4.2.3)  •  A  new  approach  to  mul&ple  access  

–  Based  on  turns,  not  randomiza&on  

1  

3  2  

4    

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Issues  with  Random  Mul&ple  Access  •  CSMA  is  good  under  low  load:  

– Grants  immediate  access  –  Lihle  overhead  (few  collisions)  

•  But  not  so  good  under  high  load:  – High  overhead  (expect  collisions)  – Access  &me  varies  (lucky/unlucky)  

•  We  want  to  do  beher  under  load!  

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Turn-­‐Taking  Mul&ple  Access  Protocols  

•  They  define  an  order  in  which  nodes  get  a  chance  to  send  –  Or  pass,  if  no  traffic  at  present  

•  We  just  need  some  ordering  …  –  E.g.,  Token  Ring  –  E.g.,  node  addresses  

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Token  Ring  •  Arrange  nodes  in  a  ring;  token  rotates  “permission                to  send”  to  each  node  in  turn  

74  

Node  

Direc&on  of  transmission  

Token  

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Turn-­‐Taking  Advantages  

•  Fixed  overhead  with  no  collisions  – More  efficient  under  load  

•  Regular  chance  to  send  with  no  unlucky  nodes  – Predictable  service,  easily  extended  to  guaranteed  quality  of  service  

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Turn-­‐Taking  Disadvantages  

•  Complexity  – More  things  that  can  go  wrong  than  random  access  protocols!  •  E.g.,  what  if  the  token  is  lost?  

– Higher  overhead  at  low  load  

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Turn-­‐Taking  in  Prac&ce  •  Regularly  tried  as  an  improvement  offering  beher  service  –  E.g.,  quali&es  of  service  

•  But  random  mul&ple  access  is  hard  to  beat  –  Simple,  and  usually  good  enough  

–  Scales  from  few  to  many  nodes  

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LAN  Switches  (§4.3.4,  4.8.1-­‐4.8.2,  4.8.4)  •  How  do  we  connect  nodes  with  a  switch  instead  of  mul&ple  access  –  Uses  mul&ple  links/wires    

–  Basis  of  modern  (switched)  Ethernet  

Switch  

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Switched  Ethernet  •  Hosts  are  wired  to  Ethernet  switches  with  twisted  pair  

–  Switch  serves  to  connect  the  hosts  – Wires  usually  run  to  a  closet  

Switch  

Twisted  pair  Switch  ports  

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What’s  in  the  box?  •  Remember  from  protocol  layers:  

Network  

Link  

Network  

Link  

Link   Link  

Physical   Physical  Hub,  or  repeater  

Switch  

Router  

All  look  like  this:  

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Inside  a  Hub  •  All  ports  are  wired  together;  more  convenient  and  reliable  than  a  single  shared  wire  

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↔  

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Inside  a  Switch  •  Uses  frame  addresses  to  connect  input  port  to  the  right  output  port;  mul&ple  frames  may  be  switched  in  parallel  

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.  .  .  

Fabric  

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Inside  a  Switch  (2)  •  Port  may  be  used  for  both  input  and  output  (full-­‐duplex)  

–  Just  send,  no  mul&ple  access  protocol  

83  

.  .  .  

1  2  3  

4

1  !  4  and  2  !  3  

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Inside  a  Switch  (3)  •  Need  buffers  for  mul&ple  inputs  to  send  to  one  output  

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.  .  .  

.  .  .  

.  .  .   .  .  .  

Input  Buffer   Output  Buffer  Fabric  

Input   Output  

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Inside  a  Switch  (4)  •  Sustained  overload  will  fill  buffer  and  lead  to  frame  loss  

85  

.  .  .  

.  .  .  

.  .  .   .  .  .  

Input  Buffer   Output  Buffer  Fabric  

Input   Output  

XXX  

Loss!  

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Advantages  of  Switches  •  Switches  and  hubs  have  replaced  the  shared  cable  of  classic  

Ethernet  –  Convenient  to  run  wires  to  one  loca&on  –  More  reliable;  wire  cut  is  not  a  single  point  of  failure  that  is  hard  to  

find  

•  Switches  offer  scalable  performance  –  E.g.,  100  Mbps  per  port  instead  of  100  Mbps  for  all  nodes  of  shared  

cable  /  hub  

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Switch  Forwarding  •  Switch  needs  to  find  the  right  output  port  for  the  des&na&on  address  in  the  Ethernet  frame.  How?  – Want  to  let  hosts  be  moved  around  readily;  don’t  look  at  IP  

87  

.  .  .  

.  .  .  

.  .  .   .  .  .  

Source  

Des&na&on  

Ethernet  Frame  

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88  

Backward  Learning  •  Switch  forwards  frames  with  a  port/address  table  as  follows:  1.  To  fill  the  table,  it  looks  at  the  source  address  of  input  

frames  

2.  To  forward,  it  sends  to  the  port,  or  else  broadcasts  to  all  ports  

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89  

Backward  Learning  (2)  •  1:  A  sends  to  D  

Switch  

D  

Address Port A B C D

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90  

Backward  Learning  (3)  •  2:  D  sends  to  A  

Switch  

D  

Address Port A 1 B C D

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91  

Backward  Learning  (4)  •  3:  A  sends  to  D  

Switch  

D  

Address Port A 1 B C D 4

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92  

Backward  Learning  (5)  •  3:  A  sends  to  D  

Switch  

D  

Address Port A 1 B C D 4

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Learning  with  Mul&ple  Switches  •  Just  works  with  mul&ple  switches  and  a  mix  of  hubs  assuming  no  loops,  e.g.,  A  sends  to  D  then  D  sends  to  A  

93  

Switch  

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94  

Switch  Spanning  Tree  (§4.8.3)  •  How  can  we  connect  switches  in  any  topology  so  they  just  work?  

Loops  –  yikes!  

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95  

Problem  –  Forwarding  Loops    

•  May  have  a  loop  in  the  topology  –  Redundancy  in  case  of  failures  –  Or  a  simple  mistake  

•  Want  LAN  switches  to  “just  work”  –  Plug-­‐and-­‐play,  no  changes  to  hosts  –  But  loops  cause  a  problem  …  

Redundant    Links  

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96  

Forwarding  Loops  (2)    •  Suppose  the  network  is  started  and  A  sends  to  F.  What  happens?  

Led  /  Right  

A   B  

C  

D  

E   F  

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97  

Forwarding  Loops  (3)    •  Suppose  the  network  is  started  and  A  sends  to  F.  What  happens?  –  A  !  C  !  B,  D-­‐led,  D-­‐right  –  D-­‐led  !  C-­‐right,  E,  F  –  D-­‐right  !  C-­‐led,  E,  F  –  C-­‐right  !  D-­‐led,  A,  B  –  C-­‐led  !  D-­‐right,  A,  B  –  D-­‐led  !  …  –  D-­‐right  !  …  

Led  /  Right  

A   B  

C  

D  

E   F  

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98  

Spanning  Tree  Solu&on  •  Switches  collec&vely  find  a  spanning  tree  for  the  topology  –  A  subset  of  links  that  is  a  tree  (no  loops)  and  reaches  all  switches  

–  Switches  forward  as  normal  but  only  on  spanning  tree  –  Broadcasts  will  go  up  to  the  root  of  the  tree  and  down  all  the  branches  

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Spanning  Tree  (2)  

99  

Topology   One  ST   Another  ST  

Root  

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100  

Radia  Perlman  (1951–)  

•  Key  early  work  on  rou&ng  protocols  –  Rou&ng  in  the  ARPANET  –  Spanning  Tree  for  switches  (next)  –  Link-­‐state  rou&ng  (later)  

•  Now  focused  on  network  security  

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101  

Spanning  Tree  Algorithm  •  Rules  of  the  distributed  game:  

–  All  switches  run  the  same  algorithm  –  They  start  with  no  informa&on  –  Operate  in  parallel  and  send  messages  –  Always  search  for  the  best  solu&on  

•  Ensures  a  highly  robust  solu&on  –  Any  topology,  with  no  configura&on  –  Adapts  to  link/switch  failures,  …  

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102  

Spanning  Tree  Algorithm  (2)  •  Outline:  

1.  Elect  a  root  node  of  the  tree  (switch  with  the  lowest  address)  

2.  Grow  tree  as  shortest  distances  from  the  root  (using  lowest  address  to  break  distance  &es)  

3.  Turn  off  ports  for  forwarding  if  they  are  not  on  the  spanning  tree  

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Spanning  Tree  Algorithm  (3)  •  Details:  

–  Each  switch  ini&ally  believes  it  is  the  root  of  the  tree  –  Each  switch  sends  periodic  updates  to  neighbors  with:  

•  Its  address,  address  of  the  root,  and  distance  (in  hops)  to  root  –  Switches  favors  ports  with  shorter  distances  to  lowest  root  

•  Uses  lowest  address  as  a  &e  for  distances  

103  

C  

Hi,  I’m  C,  the  root  is  A,  it’s  2  hops  away     or  (C,  A,  2)  

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104  

Spanning  Tree  Example  •  1st  round,  sending:  

–  A  sends  (A,  A,  0)  to  say  it  is  root  –  B,  C,  D,  E,  and  F  do  likewise  

•  1st  round,  receiving:  –  A  s&ll  thinks  is  it  (A,  A,  0)  –  B  s&ll  thinks  (B,  B,  0)  –  C  updates  to  (C,  A,  1)  –  D  updates  to  (D,  C,  1)  –  E  updates  to  (E,  A,  1)  –  F  updates  to  (F,  B,  1)  

A,A,0   B,B,0  

C,C,0  

D,D,0  

E,E,0   F,F,0  

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105  

Spanning  Tree  Example  (2)  •  2nd  round,  sending  

–  Nodes  send  their  updated  state  •  2nd  round  receiving:  

–  A  remains  (A,  A,  0)  –  B  updates  to  (B,  A,  2)  via  C  –  C  remains  (C,  A,  1)  –  D  updates  to  (D,  A,  2)  via  C  –  E  remains  (E,  A,  1)  –  F  remains  (F,  B,  1)  

A,A,0   B,B,0  

C,A,1  

D,C,1  

E,A,1   F,B,1  

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106  

Spanning  Tree  Example  (3)  •  3rd  round,  sending  

–  Nodes  send  their  updated  state  •  3rd  round  receiving:  

–  A  remains  (A,  A,  0)  –  B  remains  (B,  A,  2)  via  C  –  C  remains  (C,  A,  1)  –  D  remains  (D,  A,  2)  via  C-­‐led  –  E  remains  (E,  A,  1)  –  F  updates  to  (F,  A,  3)  via  B  

A,A,0   B,A,2  

C,A,1  

D,A,2  

E,A,1   F,B,1  

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107  

Spanning  Tree  Example  (4)  •  4th  round  

–  Steady-­‐state  has  been  reached  –  Nodes  turn  off  forwarding  that    is  not  on  the  spanning  tree  

•  Algorithm  con&nues  to  run  –  Adapts  by  &ming  out  informa&on  

–  E.g.,  if  A  fails,  other  nodes  forget  it,  and  B  will  become  the  new  root  

A,A,0   B,A,2  

C,A,1  

D,A,2  

E,A,1   F,A,3  

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108  

Spanning  Tree  Example  (5)  •  Forwarding    proceeds  as  usual  on  the  ST  •  Ini&ally  D  sends  to  F:  

•  And  F  sends  back  to  D:  

A,A,0   B,A,2  

C,A,1  

D,A,2  

E,A,1   F,A,3  

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109  

Spanning  Tree  Example  (6)  •  Forwarding    proceeds  as  usual  on  the  ST  •  Ini&ally  D  sends  to  F:  

–  D  !  C-­‐led  –  C  !  A,  B    –  A  !  E  –  B  !  F  

•  And  F  sends  back  to  D:  –  F  !  B  –  B  !  C  –  C  !  D  (hm,  not  such  a  great  route)  

A,A,0   B,A,2  

C,A,1  

D,A,2  

E,A,1   F,A,3  


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