Introduc3on)to)Computer)Networks)€¦ · CSE)461)University)of)Washington) 4...

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Computer Science & Engineering Introduc3on to Computer Networks Network Layer Overview

Transcript of Introduc3on)to)Computer)Networks)€¦ · CSE)461)University)of)Washington) 4...

Page 1: Introduc3on)to)Computer)Networks)€¦ · CSE)461)University)of)Washington) 4 Shortcomings)of)Switches) 1. Don’tscale)to)large)networks) – Blow)up)of)rou3ng)table,)broadcast Table)for)all)des3naons)in)the

Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

Network  Layer  Overview  

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Where  we  are  in  the  Course  •  Star3ng  the  Network  Layer!  

–  Builds  on  the  link  layer.  Routers  send  packets  over  mul3ple  networks  

Physical  Link  

Network  Transport  Applica3on  

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Why  do  we  need  a  Network  layer?  •  We  can  already  build  networks    with  links  and  switches  

and  send  frames  between  hosts  …  

•  Ques3on:  what  are  the  downsides  of  using  link  layer  solu3ons  (switches)  to  connect  all  of  the  Internet?  

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

–  Blow  up  of  rou3ng  table,  broadcast  

Table  for  all  des3na3ons  in  the  world!  

Broadcast  new  des3na3ons  to  the  whole  world!  

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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!  

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

– Want  to  plan  routes  /  bandwidth  

That  was  lame.  

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Network  Layer  Approach  •  Scaling:  

–  Hierarchy,  in  the  form  of  prefixes  

•  Heterogeneity:  –  IP  for  internetworking  

•  Bandwidth  Control:  –  Lowest-­‐cost  rou3ng  –  Later  QOS  (Quality  of  Service)  

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Rou3ng  vs.  Forwarding  •  Rou3ng  is  the  process  of  deciding    in  which  direc3on  to  send  traffic  –  Network  wide  (global)  and  expensive  

Which  way?  

Which  way?  

Which  way?  

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Rou3ng  vs.  Forwarding  (2)  •  Forwarding  is  the  process  of  sending  a  packet  on  its  way  –  Node  process  (local)  and  fast  

Forward!  packet  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

Network  Services  (§5.1)  

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Two  Network  Service  Models  •  Datagrams,  or  connec3onless  service  –  Like  postal  lehers  –  (This  one  is  IP)  

•  Virtual  circuits,  or  connec3on-­‐oriented  service  –  Like  a  telephone  call  

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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  sta3s3cal  mul3plexing  to  share  link  bandwidth  over  3me  

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Datagram  Model  •  Packets  contain  a  des3na3on  address;  each  router  uses  it  to  forward  each  packet,  possibly  on  different  paths  

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ISP’s  equipment  

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

–  Gives  next  hop  for  each  des3na3on  address;  may  change  

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A’s  table  (ini3ally)          A’s  table  (later)        C’s  Table                    E’s  Table  

B  B  

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

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

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Payload  (e.g.,  TCP  segment)  

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Virtual  Circuit  Model  •  Three  phases:  

1.  Connec3on  establishment,  circuit  is  set  up  •  Path  is  chosen,  circuit  informa3on  stored  in  routers    

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

3.  Connec3on  teardown,  circuit  is  deleted  •  Circuit  informa3on  is  removed  from  routers  

•  Just  like  a  telephone  circuit,  but  virtual  in  the  sense  that  no  bandwidth  need  be  reserved;  sta3s3cal  sharing  of  links  

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Virtual  Circuits  (2)  •  Packets  only  contain  a  short  label  to  iden3fy  the  circuit  

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

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ISP’s  equipment  

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

–  Gives  output  line  and  next  label  to  place  on  packet    

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

1  

1  

Circuit  #1  

Circuit  #2  H3  

H1   F  

F  

5   5  

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

–  Gives  output  line  and  next  label  to  place  on  packet    

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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  

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MPLS  (Mul3-­‐Protocol  Label  Switching,  §5.6.5)  •  A  virtual-­‐circuit  like  technology  widely  used  by  ISPs  

–  ISP  sets  up  circuits  inside  their  backbone  ahead  of  3me  –  ISP  adds  MPLS  label  to  IP  packet  at  ingress,  undoes  at  egress  

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Datagrams  vs  Virtual  Circuits  •  Complementary  strengths  

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Issue   Datagrams   Virtual  Circuits  

Setup  phase   Not  needed   Required  

Router  state   Per  des3na3on   Per  connec3on  

Addresses   Packet  carries  full  address   Packet  carries  short  label  

Rou3ng   Per  packet   Per  circuit  

Failures   Easier  to  mask   Difficult  to  mask  

Quality  of  service   Difficult  to  add   Easier  to  add  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

Internetworking  (§5.5,  5.6.1)  

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Topic  •  How  do  we  connect  different  networks  together?  –  This  is  called  internetworking  – We’ll  look  at  how  IP  does  it  

  Hi  there!   Hi  yourself  

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

–  Service  model  (datagrams,  VCs)  –  Addressing  (what  kind)  –  QOS  (priori3es,  no  priori3es)  –  Packet  sizes  –  Security  (whether  encrypted)  

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

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Connec3ng  Datagram  and  VC  networks  •  An  example  to  show  that  it’s  not  so  easy  

–  Need  to  map  des3na3on  address  to  a  VC  and  vice-­‐versa      –  A  bit  of  a  “road  bump”,  e.g.,  might  have  to  set  up  a  VC  

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Bump!   Bump!  

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

–  Supports  many  different  links  below  and  apps  above  

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4.  Applica3on  3.  Transport  

2.  Internet  

1.  Link   Ethernet  802.11  

IP  

TCP   UDP  

HTTP  SMTP   RTP   DNS  

3G  DSL  Cable  

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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  lihle  of  lower-­‐layer  networks  –  Gives  lihle  as  a  higher  layer  service    

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

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

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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)  

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Payload  (e.g.,  TCP  segment)  

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

–  Iden3fica3on,  Fragment  offset,  Fragment  control  bits  

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Payload  (e.g.,  TCP  segment)  

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

–  Differen3ated  Services,  Time  to  live  (TTL)  

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Payload  (e.g.,  TCP  segment)  

Later,  with  ICMP  

Later,  with  QOS  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

IP  Forwarding  (§5.6.1-­‐5.6.2)  

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IP  Addresses  •  IPv4  uses  32-­‐bit  addresses  

–  Later  we’ll  see  IPv6,  which  uses  128-­‐bit  addresses  •  Wrihen  in  “dohed  quad”  nota3on  

–  Four  8-­‐bit  numbers  separated  by  dots    

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aaaaaaaabbbbbbbbccccccccdddddddd ↔ A.B.C.D

8  bits   8  bits   8  bits   8  bits  

00010010000111110000000000000001 ↔    

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IP  Prefixes  •  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  

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IP  Prefixes  (2)  •  Wrihen  in  “address/length”  nota3on  

–  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  

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000100100001111100000000xxxxxxxx ↔

↔ 128.13.0.0/16

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Classful  IP  Addressing  •  Originally,  IP  addresses  came  in  fixed  size  blocks  with  the  class/size  encoded  in  the  high-­‐order  bits  –  They  s3ll  do,  but  the  classes  are  now  ignored  

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1

10

110  

0 16   24   32  bits  8  Class  A,  224  addresses  

Class  B,  216  addresses  

Class  C,  28      addresses  

Network  por3on   Host  por3on  

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

CSE  461  University  of  Washington   37  

D  C  B  

A  

Prefix   Next  Hop  192.24.0.0/19   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  des3na3on  address,  i.e.,  the  most  specific  entry  

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

CSE  461  University  of  Washington   38  

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Longest  Matching  Prefix  (2)  

CSE  461  University  of  Washington   39  

Prefix   Next  Hop  192.24.0.0/19   D  192.24.12.0/22   B  

192.24.0.0  

192.24.63.255  

/19  

/22  192.24.12.0  

192.24.15.255  

IP  address  

192.24.6.0            à    192.24.16.32    à  

More    specific  

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CSE  461  University  of  Washington   40  

Flexibility  of  Longest  Matching  Prefix  •  Can  provide  default  behavior,              with  less  specifics  –  To  send  traffic  going  outside  an  organiza3on  to  a  border  router  

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

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CSE  461  University  of  Washington   41  

Performance  of  Longest  Matching  Prefix  

•  Uses  hierarchy  for  a  compact  table  –  Relies  on  use  of  large  prefixes  

•  Lookup  more  complex  than  table  –  Used  to  be  a  concern  for  fast  routers  –  Not  an  issue  in  prac3ce  these  days  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

Helping  IP  with  ARP,  DHCP  (§5.6.4)  

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CSE  461  University  of  Washington   43  

Topic  •  Filling  in  the  gaps  we  need  to  make  for  IP  forwarding  work  in  prac3ce  –  Gevng  IP  addresses  (DHCP)  »  – Mapping  IP  to  link  addresses  (ARP)  »  

What  link  layer  address  do  I  use?  

What’s  my  IP?  

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CSE  461  University  of  Washington   44  

Gevng  IP  Addresses  •  Problem:  

–  A  node  wakes  up  for  the  first  3me  …  – 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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CSE  461  University  of  Washington   45  

Gevng  IP  Addresses  (2)  1.  Manual  configura3on  (old  days)  

–  Can’t  be  factory  set,  depends  on  use  2.  A  protocol  for  automa3cally  

configuring  addresses  (DHCP)  –  Shims  burden  from  users  to  IT  folk  

Use  A.B.C.D  What’s  my  IP?  

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CSE  461  University  of  Washington   46  

DHCP  •  DHCP  (Dynamic  Host  Configura3on  

Protocol),  from  1993,  widely  used  

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

–  Network  prefix  –  Address  of  local  router  –  DNS  server,  3me  server,  etc.      

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CSE  461  University  of  Washington   47  

DHCP  Protocol  Stack  •  DHCP  is  a  client-­‐server  applica3on  

–  Uses  UDP  ports  67,  68  

Ethernet  

IP  

UDP  

DHCP  

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CSE  461  University  of  Washington   48  

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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CSE  461  University  of  Washington   49  

DHCP  Messages  Client   Server  

One  link  

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CSE  461  University  of  Washington   50  

DHCP  Messages  (2)  Client   Server  

DISCOVER  

REQUEST  

OFFER  

ACK  

Broadcast  

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CSE  461  University  of  Washington   51  

DHCP  Messages  (3)  •  To  renew  an  exis3ng  lease,  an  abbreviated  sequence  is  used:  –  REQUEST,  followed  by  ACK  

•  Protocol  also  supports  replicated  servers  for  reliability  

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CSE  461  University  of  Washington   52  

Sending  an  IP  Packet  •  Problem:  

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

–  How  does  it  get  the  des3na3on  link  address  from  a  des3na3on  IP  address?  

Uh  oh  …   My  IP  is  1.2.3.4  

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CSE  461  University  of  Washington   53  

ARP  (Address  Resolu3on  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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CSE  461  University  of  Washington   54  

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

–  No  servers,  just  asks  node  with  target  IP  to  iden3fy  itself  

–   Uses  broadcast  to  reach  all  nodes  

Ethernet  

ARP  

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CSE  461  University  of  Washington   55  

ARP  Messages  Node   Target  

One  link    

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CSE  461  University  of  Washington   56  

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  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

Packet  Fragmenta3on(§5.5.5)  

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                       Big  packet  

CSE  461  University  of  Washington   58  

Topic  •  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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CSE  461  University  of  Washington   59  

Packet  Size  Problem  •  Different  networks  have  different  maximum  packet  sizes  –  Or  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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CSE  461  University  of  Washington   60  

Packet  Size  Solu3ons  •  Fragmenta3on  (now)  

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

–  Classic  method,  dated  

•  Discovery  (next)  –  Find  the  largest  packet  that  fits  on  the  network  path  and  use  it  

–  IP  uses  today  instead  of  fragmenta3on  

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IPv4  Fragmenta3on  •  Routers  fragment  packets  that  are  too  large  to  forward  •  Receiving  host  reassembles  to  reduce  load  on  routers    

CSE  461  University  of  Washington   61  

Fragment!   Reassemble!  

Fits  on  first  link  

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IPv4  Fragmenta3on  Fields  •  Header  fields  used  to  handle  packet  size  differences  

–  Iden3fica3on,  Fragment  offset,  MF/DF  control  bits  

CSE  461  University  of  Washington   62  

Payload  (e.g.,  TCP  segment)  

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CSE  461  University  of  Washington   63  

IPv4  Fragmenta3on  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  posi3on  –  Set  MF  (More  Fragments)  on  all  pieces  except  last  

•  Receiving  hosts  reassembles  the  pieces:  –  Iden3fica3on  field  links  pieces  together,  MF  tells  receiver  when  it  has  all  pieces  

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IPv4  Fragmenta3on  (3)  

CSE  461  University  of  Washington   64  

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  

Amer  MTU  =  1500  

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CSE  461  University  of  Washington   65  

IPv4  Fragmenta3on  (4)  •  It  works!  

–  Allows  repeated  fragmenta3on  

•  But  fragmenta3on  is  undesirable  – More  work  for  routers,  hosts  –  Tends  to  magnify  loss  rate  –  Security  vulnerabili3es  too  

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CSE  461  University  of  Washington   66  

Path  MTU  Discovery  •  Discover  the  MTU  that  will  fit  

–  So  we  can  avoid  fragmenta3on  –  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)  

CSE  461  University  of  Washington   67  

   Try  1200   Try  900      

                               

MTU=1200  bytes   MTU=900  MTU=1400  

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Path  MTU  Discovery  (3)  

CSE  461  University  of  Washington   68  

   Try  1200   Try  900      

Test  #2   Test  #3    Test  #1    

MTU=1200  bytes   MTU=900  MTU=1400  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

Error  Handling  with  ICMP  (§5.6.4)  

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CSE  461  University  of  Washington   70  

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  tes3ng  –  Error  is  at  router  while  forwarding  –  Also  tes3ng  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  problema3c  packet;  host  needs  to  rec3fy  

CSE  461  University  of  Washington   71  

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  •  Omen  carry  the  start  of  the  offending  packet  as  payload  •  Each  message  is  carried  in  an  IP  packet  

CSE  461  University  of  Washington   72  

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

CSE  461  University  of  Washington   73  

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

XXXXXXXXXXXXXXX  

Por3on  of  offending  packet,  star3ng  with  its  IP  header  

ICMP  header  IP  header   ICMP  data  

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Example  ICMP  Messages  

CSE  461  University  of  Washington   74  

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

Tes3ng,  not  a  forwarding  error:  Host  sends  Echo  Request,  and  des3na3on  responds  with  an  Echo  Reply  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

IP  Version  6  (§5.6.3)  

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CSE  461  University  of  Washington   76  

•  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  lemover  blocks  held  by  the  regional  registries;  much  3ghter  alloca3on  policies  

CSE  461  University  of  Washington   77  

IANA  (All  IPs)  

ARIN    (US,  Canada)  

APNIC  (Asia  Pacific)  

RIPE  (Europe)  LACNIC  

(La3n  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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CSE  461  University  of  Washington   78  

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  adop3on  incen3ves    

–  Big  push  ~2011  as  exhaus3on  looms  

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IPv6  Deployment  

CSE  461  University  of  Washington   79  

Time  for  growth!  

Source:  Google  IPv6  Sta3s3cs,  30/1/13  

Percentage  of  users  accessing  Google  via  IPv6  

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CSE  461  University  of  Washington   80  

IPv6  •  Features  large  addresses  

–  128  bits,  most  of  header  

•  New  nota3on  –  8  groups  of  4  hex  digits  (16  bits)  –  Omit  leading  zeros,  groups  of  zeros  

32  bits  

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CSE  461  University  of  Washington   81  

IPv6  (2)  •  Lots  of  other,  smaller  changes  

–  Streamlined  header  processing  –  Flow  label  to  group  of  packets  –  Beher  fit  with  “advanced”  features  (mobility,  mul3cas3ng,  security)  

32  bits  

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CSE  461  University  of  Washington   82  

IPv6  Transi3on  •  The  Big  Problem:  

–  How  to  deploy  IPv6?  –  Fundamentally  incompa3ble  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    •  Na3ve  IPv6  islands  connected  via  IPv4  

–  Tunnel  carries  IPv6  packets  across  IPv4  network  

CSE  461  University  of  Washington   83  

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

CSE  461  University  of  Washington   84  

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  rou3ng    

CSE  461  University  of  Washington   85  

IPv6  

Link  

User   User  

IPv4  Link  

IPv6  IPv6  

Link  

IPv6  

Link  IPv4  Link  

IPv6   IPv6  

Link  

Na3ve  IPv4  Na3ve  IPv6   Na3ve  IPv6  

Tunnel  

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Computer  Science  &  Engineering  

Introduc3on  to  Computer  Networks  

Network  Address  Transla3on  (§5.6.2)  

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CSE  461  University  of  Washington   87  

Topic  •  What  is  NAT  (Network  Address  Transla3on)?  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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Middleboxes  •  Sit  “inside  the  network”  but  perform  “more  than  IP”  processing  on  packets  to  add  new  func3onality  –  NAT  box,  Firewall  /  Intrusion  Detec3on  System  

CSE  461  University  of  Washington   88  

TCP  

IP  

802.11  

App  

IP  

802.11  

IP  Ethernet  

TCP  

IP  

802.11  

App  

IP  

802.11  

IP  Ethernet  

Middlebox  

App  /  TCP  

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CSE  461  University  of  Washington   89  

Middleboxes  (2)  •  Advantages  

– A  possible  rapid  deployment  path  when  there  is  no  other  op3on  

–  Control  over  many  hosts  (IT)  

•  Disadvantages  –  Breaking  layering  interferes  with  connec3vity;  strange  side  effects  

–  Poor  vantage  point  for  many  tasks  

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CSE  461  University  of  Washington   90  

NAT  (Network  Address  Transla3on)  Box  •  NAT  box  connects  an  internal  network  to  an  external  network  – Many  internal  hosts  are  connected  using  few  external  addresses  

– Middlebox  that  “translates  addresses”  

•  Mo3vated  by  IP  address  scarcity  –  Controversial  at  first,  now  accepted  

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CSE  461  University  of  Washington   91  

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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CSE  461  University  of  Washington   92  

How  NAT  Works  •  Keeps  an  internal/external  table  

–  Typically  uses  IP  address  +  TCP  port  –  This  is  address  and  port  transla3on  

   

•  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  

CSE  461  University  of  Washington   93  

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

NAT  box  

External    des3na3on  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  Des3na3on  IP/port  

CSE  461  University  of  Washington   94  

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  

des3na3on  

Src  =  Dst  =  

Src  =  Dst  =  

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How  NAT  Works  (4)  •  Need  to  enter  transla3ons  in  the  table  for  it  to  work  

–  Create  external  name  when  host  makes  a  TCP  connec3on  

CSE  461  University  of  Washington   95  

Internal IP:port External IP : port 192.168.1.12 : 5523

NAT  box  

External    des3na3on  IP=X,  port=Y  

Internal  source  

Src  =  Dst  =  

Src  =  Dst  =  

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CSE  461  University  of  Washington   96  

NAT  Downsides  •  Connec3vity  has  been  broken!  

–  Can  only  send  incoming  packets  amer  an  outgoing  connec3on  is  set  up  

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

•  Doesn’t  work  so  well  when  there  are  no  connec3ons  (UDP  apps)  

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


Aruba Remote Access Point (RAP) Networks VRD, Version 8Aruba Networks, Inc. Table of Contents | 3 Aruba Remote Access Point (RAP) Networks Validated Reference Design Table of Contents

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Scaling The Edge Bridge Address Table In Datacenter Networks June-2012.

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Table of Contents 2 - F5 Networks...Author Debbie Walkowski Created Date 20161213213407Z

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Energy-Aware Routing in Software-Defined Networks with ... · Defined Networks with Table Compression (using Wildcard Rules). [Research Report] RR-8897, IN- ... Software-de ned Networks

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Creative Europe Networks · 2019-06-02 · 8 creative europe - networks synoptic table synoptic table networks - music networks music acronym organisation name nemo network of museums

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2014...Table 1.1 Historical development of water distribution networks‘ infrastructure and modeling.....2 Table 2.1 Summary of potential definitions of Table 2.2 Applications of

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