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Opera&ng Systems and Networks Network Lecture 5: Network Layer 1 Adrian Perrig Network Security Group ETH Zürich

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Page 1: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Opera&ng  Systems  and  Networks    Network  Lecture  5:  Network  Layer  1  

Adrian  Perrig  Network  Security  Group  ETH  Zürich  

Page 2: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

2  

Pending  Issues  •  Ethernet  performance?  See  Sec&on  4.3.3  in  book.  For  reasonable  parameters,  ~85%  efficiency.  

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Computer  Networks   3  

Where  we  are  in  the  Course  •  Star&ng  the  Network  Layer!  

–  Builds  on  the  link  layer.  Routers  send  packets  over  mul&ple  networks  

Physical  Link  

Network  Transport  Applica&on  

Page 4: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   4  

Why  do  we  need  a  Network  layer?  •  We  can  already  build  networks    with  links  and  switches  and  send  frames  between  hosts  …  

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Computer  Networks   5  

Shortcomings  of  Switches  1.  Don’t  scale  to  large  networks  

–  Blow  up  of  rou&ng  table,  broadcast  

Table  for  all  des&na&ons  in  the  world!  

Broadcast  new  des&na&ons  to  the  whole  world!  

Page 6: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   6  

Shortcomings  of  Switches  (2)  2.  Don’t  work  across  more  than  one  link  layer  technology  

–  Hosts  on  Ethernet  +  3G  +  802.11    …  

Can  we  play  too?   Go  away!  

Page 7: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   7  

Shortcomings  of  Switches  (3)  3.  Don’t  give  much  traffic  control  

– Want  to  plan  routes  /  bandwidth  

That  was  lame.  

Page 8: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   8  

Network  Layer  Approach  •  Scaling:  

–  Hierarchy,  in  the  form  of  prefixes  

•  Heterogeneity:  –  IP  for  internetworking  

•  Bandwidth  Control:  –  Lowest-­‐cost  rou&ng  –  Later  QOS  (Quality  of  Service)  

Page 9: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   9  

Topics  •  Network  service  models  

–  Datagrams  (packets),  virtual  circuits  •  IP  (Internet  Protocol)  

–  Internetworking  –  Forwarding  (Longest  Matching  Prefix)  –  Helpers:  ARP  and  DHCP  –  Fragmenta&on  and  MTU  discovery  –  Errors:  ICMP  (traceroute!)  

•  IPv6,  the  future  of  IP  •  NAT,  a  “middlebox”  •  Rou&ng  algorithms  

This  &me  

Next  &me  

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Computer  Networks   10  

Rou&ng  vs.  Forwarding  •  Rou&ng  is  the  process  of  deciding    in  which  direc&on  to  send  traffic  –  Network  wide  (global)  and  expensive  

Which  way?  

Which  way?  

Which  way?  

Page 11: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   11  

Rou&ng  vs.  Forwarding  (2)  •  Forwarding  is  the  process  of  sending  a  packet  on  its  way  

–  Node  process  (local)  and  fast  

Forward!  packet  

Page 12: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   12  

Our  Plan  •  Forwarding  this  &me  

– What  routers  do  with  packets  

•  Rou&ng  next  &me  –  Logically  this  comes  first  –  But  ignore  it  for  now  

Page 13: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   13  

Network  Services  (§5.1)  •  What  kind  of  service  does  the  Network  layer  provide  to  the  Transport  layer?  –  How  is  it  implemented  at  routers?  

 

Service?  What’s  he  talking  about?  

Page 14: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   14  

Two  Network  Service  Models  •  Datagrams,  or  connec&onless  service  

–  Like  postal  lemers  –  (This  one  is  IP)  

•  Virtual  circuits,  or  connec&on-­‐oriented  service  –  Like  a  telephone  call  

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Computer  Networks   15  

Store-­‐and-­‐Forward  Packet  Switching  •  Both  models  are  implemented  with  store-­‐and-­‐forward  packet  switching  –  Routers  receive  a  complete  packet,  storing  it  temporarily  if  necessary  before  forwarding  it  onwards  

– We  use  sta&s&cal  mul&plexing  to  share  link  bandwidth  over  &me  

Page 16: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Store-­‐and-­‐Forward  (2)  •  Switching  element  has  internal  buffering  for  conten&on  

Computer  Networks   16  

.  .  .  

.  .  .  

.  .  .   .  .  .  

Input  Buffer   Output  Buffer  Fabric  

Input   Output  

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Store-­‐and-­‐Forward  (3)  •  Simplified  view  with  per-­‐port  output  buffering  

–  Buffer  is  typically  a  FIFO  (First  In  First  Out)  queue  –  If  full,  packets  are  discarded  (conges&on,  later)  

Computer  Networks   17  

(FIFO)  Queue  

Queued  Packets  

Router  Router  

=  

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Datagram  Model  •  Packets  contain  a  des&na&on  address;  each  router  uses  it  to  forward  each  packet,  possibly  on  different  paths  

Computer  Networks   18  

ISP’s  equipment  

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Datagram  Model  (2)  •  Each  router  has  a  forwarding  table  keyed  by  address  

–  Gives  next  hop  for  each  des&na&on  address;  may  change  

Computer  Networks   19  

A’s  table  (ini&ally)          A’s  table  (later)        C’s  Table                    E’s  Table  

B  B  

Page 20: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

IP  (Internet  Protocol)  •  Network  layer  of  the  Internet,  uses  datagrams  (next)  

–  IPv4  carries  32  bit  addresses  on  each  packet  (osen  1.5  KB)  

Computer  Networks   20  

Payload  (e.g.,  TCP  segment)  

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Virtual  Circuit  Model  •  Three  phases:  

1.  Connec&on  establishment,  circuit  is  set  up  •  Path  is  chosen,  circuit  informa&on  stored  in  routers    

2.  Data  transfer,  circuit  is  used  •  Packets  are  forwarded  along  the  path  

3.  Connec&on  teardown,  circuit  is  deleted  •  Circuit  informa&on  is  removed  from  routers  

•  Just  like  a  telephone  circuit,  but  virtual  in  the  sense  that  no          bandwidth  need  be  reserved;  sta&s&cal  sharing  of  links  

Computer  Networks   21  

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Virtual  Circuits  (2)  •  Packets  only  contain  a  short  label  to  iden&fy  the  circuit  

–  Labels  don’t  have  any  global  meaning,  only  unique  for  a  link    

Computer  Networks   22  

ISP’s  equipment  

Page 23: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Virtual  Circuits  (3)  •  Each  router  has  a  forwarding  table  keyed  by  circuit  

–  Gives  output  line  and  next  label  to  place  on  packet    

Computer  Networks   23  

A’s  table                                                          C’s  Table                                                    E’s  Table  

1  

1  

Circuit  #1  

Circuit  #2  H3  

H1   F  

F  

5   5  

Page 24: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Virtual  Circuits  (4)  •  Each  router  has  a  forwarding  table  keyed  by  circuit  

–  Gives  output  line  and  next  label  to  place  on  packet    

Computer  Networks   24  

A’s  table                                                          C’s  Table                                                    E’s  Table  

1  

1  

Circuit  #1  

Circuit  #2  

5  

2   2   2  H3  

H1   1   1   F  

F  

5   5  

Page 25: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

MPLS  (Mul&-­‐Protocol  Label  Switching,  §5.6.5)  •  A  virtual-­‐circuit  like  technology  widely  used  by  ISPs  

–  ISP  sets  up  circuits  inside  their  backbone  ahead  of  &me  –  ISP  adds  MPLS  label  to  IP  packet  at  ingress,  undoes  at  egress  

Computer  Networks   25  

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Datagrams  vs  Virtual  Circuits  •  Complementary  strengths  

Computer  Networks   26  

Issue   Datagrams   Virtual  Circuits  

Setup  phase   Not  needed   Required  

Router  state   Per  des&na&on   Per  connec&on  

Addresses   Packet  carries  full  address   Packet  carries  short  label  

Rou&ng   Per  packet   Per  circuit  

Failures   Easier  to  mask   Difficult  to  mask  

Quality  of  service   Difficult  to  add   Easier  to  add  

Page 27: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   27  

Internetworking  (§5.5,  5.6.1)  •  How  do  we  connect  different  networks  together?  

–  This  is  called  internetworking  – We’ll  look  at  how  IP  does  it  

 Hi  there!   Hi  yourself  

Page 28: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   28  

How  Networks  May  Differ  •  Basically,  in  a  lot  of  ways:  

–  Service  model  (datagrams,  VCs)  –  Addressing  (what  kind)  –  QOS  (priori&es,  no  priori&es)  –  Packet  sizes  –  Security  (whether  encrypted)  

•  Internetworking  hides  the  differences  with  a  common  protocol.  (Uh  oh.)  

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Connec&ng  Datagram  and  VC  networks  •  An  example  to  show  that  it’s  not  so  easy  

–  Need  to  map  des&na&on  address  to  a  VC  and  vice-­‐versa      –  A  bit  of  a  “road  bump”,  e.g.,  might  have  to  set  up  a  VC  

Computer  Networks   29  

Bump!   Bump!  Virtual  circuit  

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Computer  Networks   30  

Internetworking  –  Cerf  and  Kahn  •  Pioneered  by  Cerf  and  Kahn,  the  “fathers  of  the  Internet”  –  In  1974,  later  led  to  TCP/IP  

•  Tackled  the  problems  of  interconnec&ng  networks  –  Instead  of  manda&ng  a  single  network  technology  

©  2009  IEEE   ©  2009  IEEE  

Vint  Cerf   Bob  Kahn  

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Internet  Reference  Model  •  IP  is  the  “narrow  waist”  of  the  Internet  

–  Supports  many  different  links  below  and  apps  above  

Computer  Networks   31  

7.  Applica&on  4.  Transport  

3.  Internet  

2/1.  Link   Ethernet  802.11  

IP  

TCP   UDP  

HTTP  SMTP   RTP   DNS  

3G  DSL  Cable  

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Computer  Networks   32  

IP  as  a  Lowest  Common  Denominator  

•  Suppose  only  some  networks  support  QOS  or  security  etc.  –  Difficult  for  internetwork  to  support  

•  Pushes  IP  to  be  a  “lowest  common  denominator”  protocol  –  Asks  limle  of  lower-­‐layer  networks  –  Gives  limle  as  a  higher  layer  service    

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IPv4  (Internet  Protocol)  •  Various  fields  to  meet  straighzorward  needs  

–  Version,  Header  (IHL)  and  Total  length,  Protocol,  and  Header  Checksum  

Computer  Networks   33  

Payload  (e.g.,  TCP  segment)  

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IPv4  (2)  •  Network  layer  of  the  Internet,  uses  datagrams    

–  Provides  a  layer  of  addressing  above  link  addresses  (next)  

Computer  Networks   34  

Payload  (e.g.,  TCP  segment)  

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IPv4  (3)  •  Some  fields  to  handle  packet  size  differences  (later)  

–  Iden&fica&on,  Fragment  offset,  Fragment  control  bits  

Computer  Networks   35  

Payload  (e.g.,  TCP  segment)  

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IPv4  (4)  •  Other  fields  to  meet  other  needs  (later,  later)  

–  Differen&ated  Services,  Time  to  live  (TTL)  

Computer  Networks   36  

Payload  (e.g.,  TCP  segment)  

Later,  with  ICMP  

Later,  with  QOS  

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Computer  Networks   37  

IP  Prefixes  (§5.6.1-­‐5.6.2)  •  What  do  IP  addresses  look  like?  

–  And  IP  prefixes,  or  blocks  of  addresses  –  (This  is  IPv4;  we’ll  cover  IPv6  later.)    

  To:    128.0.32.4  From:  18.31.0.67  

18.31.0.67  

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IP  Addresses  •  IPv4  uses  32-­‐bit  addresses  

–  Later  we’ll  see  IPv6,  which  uses  128-­‐bit  addresses  •  Wrimen  in  “domed  quad”  nota&on  

–  Four  8-­‐bit  numbers  separated  by  dots    

Computer  Networks   38  

aaaaaaaabbbbbbbbccccccccdddddddd ↔ A.B.C.D

8  bits   8  bits   8  bits   8  bits  

00010010000111110000000000000001 ↔    

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IP  Prefixes  –  Modern  •  Addresses  are  allocated  in  blocks  called  prefixes  

–  Addresses  in  an  L-­‐bit  prefix  have  the  same  top  L  bits  –  There  are  232-­‐L  addresses  aligned  on  232-­‐L  boundary  

Computer  Networks   39  

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IP  Prefixes  (2)  •  Wrimen  in  “IP  address/length”  nota&on  

–  Address  is  lowest  address  in  the  prefix,  length  is  prefix  bits  –  E.g.,  128.13.0.0/16  is  128.13.0.0  to  128.13.255.255  –  So  a  /24  (“slash  24”)  is  256  addresses,  and  a  /32  is  one  address  

Computer  Networks   40  

000100100001111100000000xxxxxxxx ↔

↔ 128.13.0.0/16

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IP  Prefixes  (3)  •  More  specific  prefix  

–  Has  longer  prefix,  hence  a  smaller  number  of  IP  addresses  •  Less  specific  prefix  

–  Has  shorter  prefix,  hence  a  larger  number  of  IP  addresses  

Computer  Networks   41  

Prefix  length  

Addresses    in  prefix  

/8   /16   /24   /32  /0  

20  28  216  224  232  

Less  specific   More  specific  

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IP  Address  Classes  –  Historical    •  Originally,  IP  addresses  came  in  fixed  size  blocks  with  the  class/size  encoded  in  the  high-­‐order  bits  –  They  s&ll  do,  but  the  classes  are  now  ignored  

Computer  Networks   42  

0

10

110  

0 16   24     32  bits  8  Class  A,  224  addresses  

Class  B,  216  addresses  

Class  C,  28      addresses  

Network  por&on   Host  por&on  

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Computer  Networks   43  

Public  /  Private  IP  Addresses  •  Public  IP  addresses,  e.g.,  18.31.0.1  

–  Valid  des&na&on  on  the  global  Internet  –  Must  be  allocated  to  you  before  use  –  Now  exhausted  …  &me  for  IPv6!  

•  Private  IP  addresses  –  Can  be  used  freely  within  private  networks  (home,  small  company)  –  10.0.0.0/8,  172.16.0.0/12,  192.168.0.0/16  –  Need  public  IP  address(es)  and  NAT  to  connect  to  global  Internet  

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Alloca&ng  Public  IP  Addresses  •  Follows  a  hierarchical  process  

–  IANA  delegates  to  regional  bodies  (RIRs)  –  RIRs  delegate  to  companies  in  their  region  –  Companies  assign  to  their  customers/computers  (later,  DHCP)  

Computer  Networks   44  

IANA  (All  IPs)  

ARIN  (US,  Canada)  APNIC  (Asia  Pacific)  

RIPE  (Europe)  LACNIC  (La&n  America)  

AfriNIC  (Africa)  

ISPs  

Enterprises  

Customers  

Computers  (DHCP)  

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Computer  Networks   45  

IP  Forwarding  (§5.6.1-­‐5.6.2)  •  How  do  routers  forward  packets?  

– We’ll  look  at  how  IP  does  it  –  (We’ll  cover  rou&ng  later)  

 Forward!  

packet  

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Computer  Networks   46  

Recap  •  We  want  the  network  layer  to:  

–  Scale  to  large  networks  •  Using  addresses  with  hierarchy  

–  Support  diverse  technologies  •  Internetworking  with  IP  

–  Use  link  bandwidth  well  •  Lowest-­‐cost  rou&ng  

Next  &me  

More  later  

This  lecture  

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IP  Forwarding  •  IP  addresses  on  one  network  belong  to  the  same  prefix  •  Node  uses  a  table  that  lists  the  next  hop  for  IP  prefixes  

Computer  Networks   47  

D  C  B  

A  

Prefix   Next  Hop  192.24.0.0/18   D  192.24.12.0/22   B  

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Longest  Matching  Prefix  •  Prefixes  in  the  table  might  overlap!  

–  Combines  hierarchy  with  flexibility  

•  Longest  matching  prefix  forwarding  rule:  –  For  each  packet,  find  the  longest  prefix  that  contains  the  des&na&on  address,  i.e.,  the  most  specific  entry  

–  Forward  the  packet  to  the  next  hop  router  for  that  prefix  

Computer  Networks   48  

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Longest  Matching  Prefix  (2)  

Computer  Networks   49  

Prefix   Next  Hop  192.24.0.0/18   D  192.24.12.0/22   B  

192.24.0.0  

192.24.63.255  

/18  

/22  192.24.12.0  

192.24.15.255  

IP  address  

192.24.6.0            à    192.24.14.32    à  192.24.54.0        à  

More    specific  

D  

B  

D  

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Host/Router  Dis&nc&on  •  In  the  Internet:  

–  Routers  do  the  rou&ng,  know  which  way  to  all  des&na&ons  –  Hosts  send  remote  traffic  (out  of  prefix)  to  nearest  router  

Computer  Networks   50  

It’s  my  job  to  know  which  way  to  go  …  

Not  for  my  network?  Send  it  to  the  router  

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Computer  Networks   51  

Host  Forwarding  Table  •  Give  using  longest  matching  prefix  

–  0.0.0.0/0  is  a  default  route  that  catches  all  IP  addresses  

Prefix   Next  Hop  My  network  prefix   Send  direct  to  that  IP  

0.0.0.0/0   Send  to  my  router  

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Computer  Networks   52  

Flexibility  of  Longest  Matching  Prefix  •  Can  provide  default  behavior,  with  less  specific  prefixes  –  To  send  traffic  going  outside  an  organiza&on  to  a  border  router  

•  Can  special  case  behavior,  with  more  specific  prefixes  –  For  performance,  economics,  security,  …  

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Computer  Networks   53  

Performance  of  Longest  Matching  Prefix  

•  Uses  hierarchy  for  a  compact  table  –  Benefits  from  less  specific  prefixes  

•  Lookup  more  complex  than  table  – Was  a  concern  for  fast  routers,  but    not  an  issue  in  prac&ce  these  days  

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Computer  Networks   54  

Other  Aspects  of  Forwarding  •  It’s  not  all  about  addresses  …  

Payload  (e.g.,  TCP  segment)  

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Computer  Networks   55  

Other  Aspects  (2)  •  Decrement  TTL  value  

–  Protects  against  loops  •  Checks  header  checksum  

–  To  add  reliability  •  Fragment  large  packets  

–  Split  to  fit  it  on  next  link  •  Send  conges&on  signals  

–  Warns  hosts  of  conges&on  •  Generates  error  messages  

–  To  help  mange  network  •  Handle  various  op&ons  

Coming  later  

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Computer  Networks   56  

Helping  IP  with  ARP,  DHCP  (§5.6.4)  •  Filling  in  the  gaps  we  need  to  make  for  IP  forwarding  work  in  prac&ce  –  Ge}ng  IP  addresses  (DHCP)  – Mapping  IP  to  link  addresses  (ARP)  

What  link  layer  address  do  I  use?  

What’s  my  IP?  

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Computer  Networks   57  

Ge}ng  IP  Addresses  •  Problem:  

–  A  node  wakes  up  for  the  first  &me  …  – What  is  its  IP  address?  What’s  the  IP  address  of  its  router?  Etc.  –  At  least  Ethernet  address  is  on  NIC  

Hey,  where  am  I?  

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Computer  Networks   58  

Ge}ng  IP  Addresses  (2)  1.  Manual  configura&on  (old  days)  

–  Can’t  be  factory  set,  depends  on  use  2.  A  protocol  for  automa&cally  configuring  addresses  

(DHCP)  –  Shiss  burden  from  users  to  IT  folks  

Use  A.B.C.D  What’s  my  IP?  

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Computer  Networks   59  

DHCP  •  DHCP  (Dynamic  Host  Configura&on  Protocol),  from  1993,  

widely  used  

•  It  leases  IP  address  to  nodes  •  Provides  other  parameters  too  

–  Network  prefix  –  Address  of  local  router  –  DNS  server,  &me  server,  etc.      

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Computer  Networks   60  

DHCP  Protocol  Stack  •  DHCP  is  a  client-­‐server  applica&on  

–  Uses  UDP  ports  67,  68  

Ethernet  

IP  

UDP  

DHCP  

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Computer  Networks   61  

DHCP  Addressing  •  Bootstrap  issue:  

–  How  does  node  send  a  message  to  DHCP  server  before  it  is  configured?  

•  Answer:  –  Node  sends  broadcast  messages  that  delivered  to  all  nodes  on  the  network  

–  Broadcast  address  is  all  1s  –  IP  (32  bit):  255.255.255.255  –  Ethernet  (48  bit):  ff:ff:ff:ff:ff:ff  

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Computer  Networks   62  

DHCP  Messages  Client   Server  

One  link  

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Computer  Networks   63  

DHCP  Messages  (2)  Client   Server  

DISCOVER  

REQUEST  

OFFER  

ACK  

Broadcast  

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Computer  Networks   64  

DHCP  Messages  (3)  •  To  renew  an  exis&ng  lease,  an  abbreviated  sequence  is  used:  –  REQUEST,  followed  by  ACK  

•  Protocol  also  supports  replicated  servers  for  reliability  

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Computer  Networks   65  

Sending  an  IP  Packet  •  Problem:  

–  A  node  needs  Link  layer  addresses  to  send  a  frame  over  the  local  link  

–  How  does  it  get  the  des&na&on  link  address  from  a  des&na&on  IP  address?  

Uh  oh  …   My  IP  is  1.2.3.4  

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Computer  Networks   66  

ARP  (Address  Resolu&on  Protocol)  •  Node  uses  to  map  a  local  IP  address  to  its  Link  layer  addresses  

Source  Ethernet  

Dest.  Ethernet  

Source  IP  

Dest.  IP   Payload  …  

Link  layer  

From  DHCP  

From  NIC  

From  ARP  

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Computer  Networks   67  

ARP  Protocol  Stack  •  ARP  sits  right  on  top  of  link  layer  

–  No  servers,  just  asks  node  with  target  IP  to  iden&fy  itself  –  Uses  broadcast  to  reach  all  nodes  

Ethernet  

ARP  

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Computer  Networks   68  

ARP  Messages  Node   Target  

One  link    

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Computer  Networks   69  

ARP  Messages  (2)  Node   Target  

REQUEST   Broadcast  Who  has  IP  1.2.3.4?  

REPLY  

I  do  at  1:2:3:4:5:6  

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Computer  Networks   70  

Discovery  Protocols  •  Help  nodes  find  each  other  

– There  are  more  of  them!  •  E.g.,  zeroconf,  Bonjour  

•  Osen  involve  broadcast  – Since  nodes  aren’t  introduced  – Very  handy  glue    

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                       Big  packet  

Computer  Networks   71  

Packet  Fragmenta&on  (§5.5.5)  •  How  do  we  connect  networks  with  different  maximum  packet  sizes?  –  Need  to  split  up  packets,  or  discover  the  largest  size  to  use  

It’s  too  big!  Take  that  

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Computer  Networks   72  

Packet  Size  Problem  •  Different  networks  have  different  maximum  packet  sizes  or  MTUs  – MTU  =  Maximum  Transmission  Unit  –  E.g.,  Ethernet  1.5K,  WiFi  2.3K  

•  Prefer  large  packets  for  efficiency  –  But  what  size  is  too  large?  –  Difficult  because  node  does  not  know  complete  network  path  

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Computer  Networks   73  

Packet  Size  Solu&ons  •  Fragmenta&on  (now)  

–  Split  up  large  packets  in  the  network  if  they  are  too  big  to  send  

–  Classic  method,  but  dated  

•  Discovery  (next)  –  Find  the  largest  packet  that  fits  on  the  network  path  and  use  it  –  IP  uses  today  instead  of  fragmenta&on  

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IPv4  Fragmenta&on  •  Routers  fragment  packets  that  are  too  large  to  forward  •  Receiving  host  reassembles  to  reduce  load  on  routers    

Computer  Networks   74  

Fragment!   Reassemble!  

Fits  on  first  link  

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IPv4  Fragmenta&on  Fields  •  Header  fields  used  to  handle  packet  size  differences  

–  Iden&fica&on,  Fragment  offset,  MF/DF  control  bits  

Computer  Networks   75  

Payload  (e.g.,  TCP  segment)  

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Computer  Networks   76  

IPv4  Fragmenta&on  Procedure  •  Routers  split  a  packet  that  is  too  large:  

–  Typically  break  into  large  pieces  –  Copy  IP  header  to  pieces  –  Adjust  length  on  pieces  –  Set  offset  to  indicate  posi&on  –  Set  MF  (More  Fragments)  on  all  pieces  except  last  

•  Receiving  hosts  reassembles  pieces:  –  Iden&fica&on  field  links  pieces  together,  MF  tells  receiver  when  it  has  all  pieces  

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IPv4  Fragmenta&on  (2)  

Computer  Networks   77  

ID  =  0x12ef  Data  Len  =  2300  Offset  =  0  MF  =  0  

ID  =  Data  Len  =    Offset  =  MF  =  

ID  =    Data  Len  =  Offset  =    MF  =  

Before  MTU  =  2300  

Aser  MTU  =  1500  

(Ignore  length  of  headers)  

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IPv4  Fragmenta&on  (3)  

Computer  Networks   78  

ID  =  0x12ef  Data  Len  =  2300  Offset  =  0  MF  =  0  

ID  =  0x12ef  Data  Len  =  1500  Offset  =  0  MF  =  1  

ID  =  0x12ef  Data  Len  =  800  Offset  =  1500  MF  =  0  

Before  MTU  =  2300  

Aser  MTU  =  1500  

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Computer  Networks   79  

IPv4  Fragmenta&on  (4)  •  It  works!  

–  Allows  repeated  fragmenta&on  

•  But  fragmenta&on  is  undesirable  – More  work  for  routers,  hosts  –  Tends  to  magnify  loss  rate  –  Security  vulnerabili&es  too  

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Computer  Networks   80  

Path  MTU  Discovery  •  Discover  the  MTU  that  will  fit  

–  So  we  can  avoid  fragmenta&on  –  The  method  in  use  today  

•  Host  tests  path  with  large  packet  –  Routers  provide  feedback  if  too  large;  they  tell  host  what  size  would  have  fit  

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Path  MTU  Discovery  (2)  

Computer  Networks   81  

   Try  1200   Try  900      

                               

MTU=1200  bytes   MTU=900  MTU=1400  

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Path  MTU  Discovery  (3)  

Computer  Networks   82  

   Try  1200   Try  900      

Test  #2   Test  #3    Test  #1    

MTU=1200  bytes   MTU=900  MTU=1400  

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Computer  Networks   83  

Path  MTU  Discovery  (4)  •  Process  may  seem  involved  

–  But  usually  quick  to  find  right  size  

•  Path  MTU  depends  on  the  path,  so  can  change  over  &me  –  Search  is  ongoing  

•  Implemented  with  ICMP  (next)  –  Set  DF  (Don’t  Fragment)  bit  in  IP  header  to  get  feedback  messages  

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Computer  Networks   84  

Error  Handling  with  ICMP  (§5.6.4)  •  What  happens  when  something  goes  wrong  during  forwarding?  –  Need  to  be  able  to  find  the  problem  

 Yikes!  What  happened?  

XXXXXXX  

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Computer  Networks   85  

Internet  Control  Message  Protocol  •  ICMP  is  a  companion  protocol  to  IP  

–  They  are  implemented  together  –  Sits  on  top  of  IP  (IP  Protocol=1)  

•  Provides  error  report  and  tes&ng  –  Error  is  at  router  while  forwarding  –  Also  tes&ng  that  hosts  can  use  

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ICMP  Errors  •  When  router  encounters  an  error  while  forwarding:  

–  It  sends  an  ICMP  error  report  back  to  the  IP  source  address  –  It  discards  the  problema&c  packet;  host  needs  to  rec&fy  

Computer  Networks   86  

Report  then  toss  it!  Oh,  now  I  see  …  XXXXXXX  

ICMP  report  

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ICMP  Message  Format  •  Each  ICMP  message  has  a  Type,  Code,  and  Checksum  •  Osen  carry  the  start  of  the  offending  packet  as  payload  •  Each  message  is  carried  in  an  IP  packet  

Computer  Networks   87  

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ICMP  Message  Format  (2)  •  Each  ICMP  message  has  a  Type,  Code,  and  Checksum  •  Osen  carry  the  start  of  the  offending  packet  as  payload  •  Each  message  is  carried  in  an  IP  packet  

Computer  Networks   88  

Src=router,  Dst=A  Protocol  =  1   Type=X,  Code=Y   Src=A,  Dst=B  

XXXXXXXXXXXXXXX  

Por&on  of  offending  packet,  star&ng  with  its  IP  header  

ICMP  header  IP  header   ICMP  data  

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Example  ICMP  Messages  

Computer  Networks   89  

Name Type / Code Usage Dest. Unreachable (Net or Host) 3 / 0 or 1 Lack of connectivity Dest. Unreachable (Fragment) 3 / 4 Path MTU Discovery Time Exceeded (Transit) 11 / 0 Traceroute Echo Request or Reply 8 or 0 / 0 Ping

Tes&ng,  not  a  forwarding  error:  Host  sends  Echo  Request,  and  des&na&on  responds  with  an  Echo  Reply  

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Traceroute  •  IP  header  contains  TTL  (Time  to  live)  field  

–  Decremented  every  router  hop,  with  ICMP  error  if  it  hits  zero  –  Protects  against  forwarding  loops  

Computer  Networks   90  

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Traceroute  (2)  •  Traceroute  repurposes  TTL  and  ICMP  func&onality  

–  Sends  probe  packets  increasing  TTL  star&ng  from  1  –  ICMP  errors  iden&fy  routers  on  the  path  

Computer  Networks   91  

.  .  .    Local  Host  

Remote  Host  1  hop   2  hops   3  hops   N-­‐1  hops   N  hops  

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Computer  Networks   92  

IP  Version  6  (§5.6.3)  •  IP  version  6,  the  future  of  IPv4  that  is  now  (s&ll)  being  deployed  

Why  do  I  want  IPv6  again?  

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Computer  Networks   93  

•  At  least  a  billion  Internet  hosts  and  growing  …  

•  And  we’re  using  32-­‐bit  addresses!  

Internet  Growth  

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The  End  of  New  IPv4  Addresses  •  Now  running  on  lesover  blocks  held  by  the  regional  registries;  much  &ghter  alloca&on  policies  

Computer  Networks   94  

IANA  (All  IPs)  

ARIN    (US,  Canada)  

APNIC  (Asia  Pacific)  

RIPE  (Europe)  LACNIC  

(La&n  Amer.)  AfriNIC  (Africa)  

ISPs  

Companies  

Exhausted  on  2/11!   End  of  the  world  ?  12/21/12?  

Exhausted  on  4/11  and  9/12!  

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Computer  Networks   95  

IP  Version  6  to  the  Rescue  •  Effort  started  by  the  IETF  in  1994  

– Much  larger  addresses  (128  bits)  – Many  sundry  improvements  

•  Became  an  IETF  standard  in  1998  –  Nothing  much  happened  for  a  decade  –  Hampered  by  deployment  issues,  and  a  lack  of  adop&on  incen&ves    

–  Big  push  ~2011  as  exhaus&on  looms  

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IPv6  Deployment  

Computer  Networks   96  

Time  for  growth!  

Source:  Google  IPv6  Sta&s&cs,  30/1/13  

Percentage  of  users  accessing  Google  via  IPv6  

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Computer  Networks   97  

IPv6  •  Features  large  addresses  

–  128  bits,  most  of  header  

•  New  nota&on  –  8  groups  of  4  hex  digits  (16  bits)  –  Omit  leading  zeros,  groups  of  zeros  

 Ex:      2001:0db8:0000:0000:0000:ff00:0042:8329      à        

32  bits  

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Computer  Networks   98  

IPv6  (2)  •  Lots  of  other,  smaller  changes  

–  Streamlined  header  processing  –  Flow  label  to  group  of  packets  –  Bemer  fit  with  “advanced”  features  (mobility,  mul&cas&ng,  security)  

32  bits  

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Computer  Networks   99  

IPv6  Transi&on  •  The  Big  Problem:  

– How  to  deploy  IPv6?  –  Fundamentally  incompa&ble  with  IPv4  

•  Dozens  of  approaches  proposed  – Dual  stack  (speak  IPv4  and  IPv6)  –  Translators  (convert  packets)  –  Tunnels  (carry  IPv6  over  IPv4)  

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Tunneling    •  Na&ve  IPv6  islands  connected  via  IPv4  

–  Tunnel  carries  IPv6  packets  across  IPv4  network  

Computer  Networks   100  

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Tunneling  (2)  •  Tunnel  acts  as  a  single  link  across  IPv4  network  

Computer  Networks   101  

User   User  Tunnel  

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Tunneling  (3)  •  Tunnel  acts  as  a  single  link  across  IPv4  network  

–  Difficulty  is  to  set  up  tunnel  endpoints  and  rou&ng    

Computer  Networks   102  

IPv6  

Link  

User   User  

IPv4  Link  

IPv6  IPv6  

Link  

IPv6  

Link  IPv4  Link  

IPv6   IPv6  

Link  

Na&ve  IPv4  Na&ve  IPv6   Na&ve  IPv6  

Tunnel  

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Computer  Networks   103  

Network  Address  Transla&on  (§5.6.2)  •  What  is  NAT  (Network  Address  Transla&on)?  How  does  it  work?  –  NAT  is  widely  used  at  the  edges  of  the  network,  e.g.,  homes  

I’m  a  NAT  box  too!  

Internet  

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Layering  Review  •  Remember  how  layering  is  meant  to  work?  

– “Routers  don’t  look  beyond  the  IP  header.”  Well  …  

Computer  Networks   104  

TCP  

IP  

802.11  

App  

IP  

802.11  

IP  Ethernet  

TCP  

IP  

802.11  

App  

IP  

802.11  

IP  Ethernet  

Router  

802.11  

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Middleboxes  •  Sit  “inside  the  network”  but  perform  “more  than  IP”  processing  on  packets  to  add  new  func&onality  –  NAT  box,  Firewall  /  Intrusion  Detec&on  System  

Computer  Networks   105  

TCP  

IP  

802.11  

App  

IP  

802.11  

IP  Ethernet  

TCP  

IP  

802.11  

App  

IP   IP  Ethernet  

Middlebox  

App  /  TCP  

802.11  

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Computer  Networks   106  

Middleboxes  (2)  •  Advantages  

– A  possible  rapid  deployment  path  when  there  is  no  other  op&on  

–  Control  over  many  hosts  (IT)  

•  Disadvantages  –  Breaking  layering  interferes  with  connec&vity;  strange  side  effects  

–  Poor  vantage  point  for  many  tasks  

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Computer  Networks   107  

NAT  (Network  Address  Transla&on)  Box  •  NAT  box  connects  an  internal  network  to  an  external  network  – Many  internal  hosts  are  connected  using  few  external  addresses  

– Middlebox  that  “translates  addresses”  

•  Mo&vated  by  IP  address  scarcity  –  Controversial  at  first,  now  accepted  

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Computer  Networks   108  

NAT  (2)  •  Common  scenario:  

–  Home  computers  use  “private”  IP  addresses  –  NAT  (in  AP/firewall)  connects  home  to  ISP  using  a  single  external  IP  

address  

ISP  

Unmodified  computers  at  home   Looks  like  one    computer  outside  

NAT  box  

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Computer  Networks   109  

How  NAT  Works  •  Keeps  an  internal/external  table  

–  Typically  uses  IP  address  +  TCP  port  –  This  is  address  and  port  transla&on  

   

•  Need  ports  to  make  mapping  1-­‐1  since  there  are  fewer  external  IPs  

Internal IP:port External IP : port 192.168.1.12 : 5523 44.25.80.3 : 1500 192.168.1.13 : 1234 44.25.80.3 : 1501 192.168.2.20 : 1234 44.25.80.3 : 1502

What  ISP  thinks  What  host  thinks  

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How  NAT  Works  (2)  •  Internal  à  External:  

–  Look  up  and  rewrite  Source  IP/port  

Computer  Networks   110  

Internal IP:port External IP : port 192.168.1.12 : 5523 44.25.80.3 : 1500

NAT  box  

External    des&na&on  IP=X,  port=Y  

Internal  source  

Src  =  Dst  =  

Src  =  Dst  =  

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How  NAT  Works  (3)  •  External  à  Internal  

–  Look  up  and  rewrite  Des&na&on  IP/port  

Computer  Networks   111  

Internal IP:port External IP : port 192.168.1.12 : 5523 44.25.80.3 : 1500

NAT  box  

External    source  

IP=X,  port=Y  Internal  

des&na&on  

Src  =  Dst  =  

Src  =  Dst  =  

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How  NAT  Works  (4)  •  Need  to  enter  transla&ons  in  the  table  for  it  to  work  

–  Create  external  name  when  host  makes  a  TCP  connec&on  

Computer  Networks   112  

Internal IP:port External IP : port 192.168.1.12 : 5523

NAT  box  

External    des&na&on  IP=X,  port=Y  

Internal  source  

Src  =  Dst  =  

Src  =  Dst  =  

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Computer  Networks   113  

NAT  Downsides  •  Connec&vity  has  been  broken!  

–  Can  only  send  incoming  packets  aser  an  outgoing  connec&on  is  set  up  

–  Difficult  to  run  servers  or  peer-­‐to-­‐peer  apps  (Skype)  at  home    

•  Doesn’t  work  so  well  when  there  are  no  connec&ons  (UDP  apps)  

•  Breaks  apps  that  unwisely  expose  their  IP  addresses  (FTP)  

Page 114: Operang)Systems)and)Networks) …...Computer)Networks) 4 Why)do)we)need)aNetwork)layer?) • We)can)already)build)networks))with)links)and)switches) and)send)frames)between)hosts)…)

Computer  Networks   114  

NAT  Upsides  •  Relieves  much  IP  address  pressure  

– Many  home  hosts  behind  NATs  •  Easy  to  deploy  

–  Rapidly,  and  by  you  alone  •  Useful  func&onality  

–  Firewall,  helps  with  privacy  

•  Kinks  will  get  worked  out  eventually  –  “NAT  Traversal”  for  incoming  traffic  


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