Computer networking layer and datagram networks

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Computer networking layer and datagram networks Powered By Docstoc
					                      Computer	
  Networking	
  

                   	
  Network	
  layer,	
  virtual	
  circuits	
  and	
  
                            datagram	
  networks           	
  




                                             Network	
  Layer




Network	
  layer	
  
❒  transport	
  segment	
  from	
                               application
                                                                transport

     sending	
  to	
  receiving	
  host	
  	
                    network
                                                                 data link
                                                                 physical

❒    on	
  sending	
  side	
                                                       network
                                                                                   data link
                                                                                                network
                                                                                                data link
                                                                     network
     encapsulates	
  segments	
  into	
                              data link
                                                                     physical
                                                                                   physical     physical

                                                                                   network       network
     datagrams	
                                                                   data link
                                                                                   physical
                                                                                                 data link
                                                                                                 physical

❒    on	
  rcving	
  side,	
  delivers	
                                         network           network
     segments	
  to	
  transport	
  layer	
                                      data link
                                                                                 physical
                                                                                         network
                                                                                                   data link
                                                                                                   physical
                                                                                         data link
❒    network	
  layer	
  protocols	
  in	
                                               physical
                                                                                                               application
     every	
  host,	
  router	
                                                  network
                                                                                 data link
                                                                                                               transport
                                                                                                                network
                                                                                               network
                                                                                 physical                       data link
❒    router	
  examines	
  header	
                                 network
                                                                    data link
                                                                                               data link
                                                                                               physical
                                                                                                                physical
                                                                    physical
     fields	
  in	
  all	
  IP	
  datagrams	
  
     passing	
  through	
  it	
  

                                             Network	
  Layer




                                                                                                                             1
 Two	
  Key	
  Network-­‐Layer	
  FuncCons	
  

❒  forwarding:	
  move	
                                analogy:	
  
   packets	
  from	
  router’s	
  
   input	
  to	
  appropriate	
                         ❒  rouCng:	
  process	
  of	
  
   router	
  output	
                                        planning	
  trip	
  from	
  source	
  
                                                             to	
  dest	
  
❒  rou/ng:	
  determine	
  route	
  
   taken	
  by	
  packets	
  from	
                     ❒  forwarding:	
  process	
  of	
  
   source	
  to	
  dest.	
  	
                               geHng	
  through	
  single	
  
                                                             interchange	
  
    ❍  rou/ng	
  algorithms	
  



                                                Network	
  Layer




 Interplay between routing and forwarding

                            routing algorithm


                       local forwarding table
                      header value output link
                                0100   3
                                0101   2
                                0111   2
                                1001   1




        value in arriving
        packet’s header
                              0111                  1

                                            3 2




                                                Network	
  Layer




                                                                                                      2
 ConnecCon	
  setup	
  
 ❒  3rd	
  important	
  funcCon	
  in	
  some	
  network	
  architectures:	
  
      ❍  ATM,	
  frame	
  relay,	
  X.25	
  
 ❒  before	
  datagrams	
  flow,	
  two	
  end	
  hosts	
  and	
  intervening	
  
    routers	
  establish	
  virtual	
  connecCon	
  
     ❍  routers	
  get	
  involved	
  
 ❒  network	
  vs	
  transport	
  layer	
  connecCon	
  service:	
  
     ❍  network:	
  between	
  two	
  hosts	
  (may	
  also	
  involve	
  intervening	
  
        routers	
  in	
  case	
  of	
  VCs)	
  
     ❍  transport:	
  between	
  two	
  processes	
  




                                             Network	
  Layer




 Network	
  service	
  model	
  
 Q: What service model for “channel” transporting
 datagrams from sender to receiver?

Example	
  services	
  for	
                      Example	
  services	
  for	
  a	
  flow	
  
   individual	
  datagrams:	
                        of	
  datagrams:	
  
❒  guaranteed	
  delivery	
                       ❒  in-­‐order	
  datagram	
  
❒  guaranteed	
  delivery	
  with	
                  delivery	
  
   less	
  than	
  40	
  msec	
  delay	
          ❒  guaranteed	
  minimum	
  
                                                     bandwidth	
  to	
  flow	
  
                                                  ❒  restricCons	
  on	
  changes	
  in	
  
                                                     inter-­‐packet	
  spacing	
  


                                             Network	
  Layer




                                                                                               3
  Network	
  layer	
  service	
  models:	
  
                                                   Guarantees ?
   Network       Service                                     Congestion
Architecture     Model           Bandwidth Loss Order Timing feedback

    Internet     best effort none                   no         no    no          no (inferred
                                                                                 via loss)
       ATM       CBR             constant           yes        yes   yes         no
                                 rate                                            congestion
       ATM       VBR             guaranteed         yes        yes   yes         no
                                 rate                                            congestion
       ATM       ABR             guaranteed         no         yes   no          yes
                                 minimum
       ATM       UBR             none               no         yes   no          no




                                            Network	
  Layer




  Network	
  layer	
  connecCon	
  and	
  
  connecCon-­‐less	
  service	
  
  ❒  datagram	
  network	
  provides	
  network-­‐layer	
  
     connecConless	
  service	
  
  ❒  VC	
  network	
  provides	
  network-­‐layer	
  connecCon	
  
     service	
  
  ❒  analogous	
  to	
  the	
  transport-­‐layer	
  services,	
  but:	
  
      ❍  service:	
  host-­‐to-­‐host	
  
      ❍  no	
  choice:	
  network	
  provides	
  one	
  or	
  the	
  other	
  
      ❍  implementaCon:	
  in	
  network	
  core	
  




                                            Network	
  Layer




                                                                                                4
Virtual	
  circuits	
  
    “source-­‐to-­‐dest	
  path	
  behaves	
  much	
  like	
  telephone	
  circuit”	
  
            ❍    performance-­‐wise	
  
            ❍    network	
  acCons	
  along	
  source-­‐to-­‐dest	
  path	
  



 ❒  call	
  setup,	
  teardown	
  for	
  each	
  call	
  before	
  data	
  can	
  flow	
  
 ❒  each	
  packet	
  carries	
  VC	
  idenCfier	
  (not	
  desCnaCon	
  host	
  address)	
  
 ❒  every	
  router	
  on	
  source-­‐dest	
  path	
  maintains	
  “state”	
  for	
  each	
  passing	
  
    connecCon	
  
 ❒  link,	
  router	
  resources	
  (bandwidth,	
  buffers)	
  may	
  be	
  allocated	
  to	
  VC	
  
    (dedicated	
  resources	
  =	
  predictable	
  service)	
  



                                                    Network	
  Layer




VC	
  implementaCon	
  
a	
  VC	
  consists	
  of:	
  
      1.     path	
  from	
  source	
  to	
  desCnaCon	
  
      2.     VC	
  numbers,	
  one	
  number	
  for	
  each	
  link	
  along	
  path	
  
      3.     entries	
  in	
  forwarding	
  tables	
  in	
  routers	
  along	
  path	
  
❒  packet	
  belonging	
  to	
  VC	
  carries	
  VC	
  number	
  
   (rather	
  than	
  dest	
  address)	
  
❒  VC	
  number	
  can	
  be	
  changed	
  on	
  each	
  link.	
  
      ❍      New	
  VC	
  number	
  comes	
  from	
  forwarding	
  table	
  



                                                    Network	
  Layer




                                                                                                           5
 Forwarding	
  table	
                                  VC number

                                                                12            22         32

                                                                  1       3
                                                                      2


Forwarding table in                                 interface
                                                    number
northwest router:
 Incoming interface          Incoming VC #            Outgoing interface           Outgoing VC #

          1                      12                             3                      22
          2                      63                             1                      18
          3                       7                             2                      17
          1                      97                             3                      87
          …                      …                              …                       …


        Routers maintain connection state information!
                                             Network	
  Layer




 Virtual	
  circuits:	
  signaling	
  protocols	
  

  ❒  used	
  to	
  setup,	
  maintain	
  	
  teardown	
  VC	
  
  ❒  used	
  in	
  ATM,	
  frame-­‐relay,	
  X.25	
  
  ❒  not	
  used	
  in	
  today’s	
  Internet	
  



  application
                                                                6. Receive data application
  transport 5. Data flow begins
   network 4. Call connected                                      3. Accept call transport
   data link 1. Initiate call                                    2. incoming call network
                                                                                      data link
   physical
                                                                                      physical



                                             Network	
  Layer




                                                                                                   6
Datagram	
  networks	
  
❒  no	
  call	
  setup	
  at	
  network	
  layer	
  
❒  routers:	
  no	
  state	
  about	
  end-­‐to-­‐end	
  connecCons	
  
    ❍  no	
  network-­‐level	
  concept	
  of	
  “connecCon”	
  

❒  packets	
  forwarded	
  using	
  desCnaCon	
  host	
  address	
  
    ❍  packets	
  between	
  same	
  source-­‐dest	
  pair	
  may	
  take	
  different	
  paths	
  




application
                                                                                  application
transport
                                                                                   transport
 network
 data link 1. Send data                                            2. Receive data network
                                                                                    data link
 physical
                                                                                    physical


                                               Network	
  Layer




                                                                  4 billion
Forwarding	
  table	
                                             possible entries


              Destination Address Range                                       Link Interface	



  11001000 00010111 00010000 00000000
                  through                                                           0
  11001000 00010111 00010111 11111111	



  11001000 00010111 00011000 00000000
                 through                                                            1
  11001000 00010111 00011000 11111111 	



  11001000 00010111 00011001 00000000
                 through                                                            2
  11001000 00010111 00011111 11111111 	



                 otherwise                                                          3	



                                               Network	
  Layer




                                                                                                     7
Longest	
  prefix	
  matching	
  

                      Prefix Match                             Link Interface
       11001000 00010111 00010                                      0
       11001000 00010111 00011000                                   1
       11001000 00010111 00011                                      2
              otherwise                                             3	




     Examples

     DA: 11001000 00010111 00010110 10100001                               Which interface?


      DA: 11001000 00010111 00011000 10101010                              Which interface?




                                           Network	
  Layer




Datagram	
  or	
  VC	
  network:	
  why?	
  

Internet	
  (datagram)	
                        ATM	
  (VC)	
  
❒  data	
  exchange	
  among	
  computers	
   ❒  evolved	
  from	
  telephony	
  
     ❍ “elasCc”	
  service,	
  no	
  strict	
   ❒  human	
  conversaCon:	
  	
  
       Cming	
  req.	
  	
  
                                                    ❍  strict	
  Cming,	
  reliability	
  
❒  “smart”	
  end	
  systems	
  (computers)	
          requirements	
  
    ❍  can	
  adapt,	
  perform	
  control,	
       ❍  need	
  for	
  guaranteed	
  
       error	
  recovery	
                             service	
  
    ❍  simple	
  inside	
  network,	
           ❒  “dumb”	
  end	
  systems	
  
       complexity	
  at	
  “edge”	
  
                                                    ❍  telephones	
  
❒  many	
  link	
  types	
  	
  
                                                    ❍  complexity	
  inside	
  network	
  
    ❍  different	
  characterisCcs	
  
    ❍  uniform	
  service	
  difficult	
  



                                           Network	
  Layer




                                                                                              8

				
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Description: Computer network layer, virtual circuits and datagram networks