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Lecture 11

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Lecture 11 Powered By Docstoc
					CS 164: Global Internet
   Slide Set -- 11
         In this set ...
• More about subnets
• Classless Inter Domain Routing
  (CIDR)
• Border Gateway Protocol (BGP)
• Areas with OSPF
    Forwarding on Subnets
• Resolution of subnet address: Bitwise ANDing
  Host IP address with Subnet Mask gives
  subnet number.
• When a host wants to send an IP packet:
  – Perform BITwise AND between subnet mask and
    destination IP address
     • If result == its subnet no. destination is on same
       subnet (Send ARP etc.).
  – If not, send packet to default router R.
      Router Forwarding Tables
• Table holds entries like
                                                      Subnet mask: 255.255.255.128
                                                      Subnet number: 128.96.34.0



  <SubnetNumber,
                                            128.96.34.15
                                                                     128.96.34.1



  SubnetMask, NextHop>
                                                       H1              R1

                                                     128.96.34.130             Subnet mask: 255.255.255.128 28
                                                                               Subnet number: 128.96.34.128 28

• Router ANDs dest addr                                                                   128.96.34.139

  with subnet mask of each
                                                           128.96.34.129
                                             H3
                                                                             R2
                                                                                         H2

  entry.                                       128.96.33.14
                                                                           128.96.33.1




• Find the right entry (Match
                                                  Subnet mask: 255.255.255.0
                                                  Subnet number: 128.96.33.0


  with subnet no.) and
  forward to Next hop.     Subnet Number               Subnet Mask                            Next Hop
                              128.96.34.0              255.255.255.128                        Int 0

                              128.96.34.128            255.255.255.128                        Int 1
      Router R1’s
                              128.96.33.0              255.255.255.0                          R2
      table
            Other Issues
• Subnet Mask need not align with byte
  boundaries (e.g. 255.255.255.128) -- 7
  zeroes.
• Non contiguous masks are possible --
  255.255.1.0 -- however, this makes
  administration difficult -- not recommended.
• One could have multiple subnets on the same
  physical network ! However, now, hosts on the
  same net would need to go through a router in
  order to talk to each other.
          Outside View
• Routers outside a group of subnets
  see the group as a single network -
  - e.g. 128.96
• However, once packet arrives to
  the group, routers within the group
  need to forward the packets to the
  proper subnet.
   Classless Interdomain Routing

• Abbreviated as CIDR.
• If a network grows to more than 255 hosts, it may
  want a Class B address.
• One possible way of avoiding is to handle many Class C
  routing addresses -- but then, for this one network,
  each router has to maintain multiple routing entries.
• CIDR is an attempt to balance the desire to minimize
  the number of routes that a router needs to know
  versus the need to hand out addresses efficiently.
• Key property: CIDR enables aggregation of routes !
  Removing rigid boundaries
• The idea is to break the rigid boundaries between
  classes.
• As an example, if a network grows to about 16x255
  hosts, assign a contiguous “block” of Class C addresses as
  opposed to a Class B address.
   – Example: 192.4.16 to 192.4.31
• Note -- 16 Class C addresses better than 1 Class B in
  terms of address efficiency.
• In the above example -- the top 20 bits are the same
  for all the addresses and so we have effectively created
  a 20 bit network number !
                The Prefix
• The 20 bit address in our previous example is called
  the “common prefix” for the set of addresses that are
  allocated.
• Observe -- what we did was that we allocated a block
  of Class C addresses that shared a common prefix.
• Now, with this new representation, the network
  numbers are represented by <length,value> -- the
  length represents the number of bits in the prefix.
          Subnets vs CIDR
• The concept is similar but:
  – In a subnet, a single address is shared among
    multiple physical networks.
  – With CIDR, we collapse multiple network
    addresses into a longer network address that is
    typically assigned to an AS (the single AS would
    have a network number or prefix that reflects
    the block of addresses).
• Thus, when we want to route to “any” of
  the networks or even subnets within the AS,
  we route to the AS.
       Route Aggregation
• Specifying simply the prefix associated
  with an AS (as opposed to stating the
  subnet number explicitly) is called route
  aggregation.
• When sending route advertisements (we
  will see how), it suffices to simply
  advertise “common prefixes”.
• Note that for this, careful planning
  would be needed.
                        An Example
• Border gateway advertises the common
  prefix only!

                                                 Corporation X
                                            (11000000000001000001)

    Border gateway
 (adv ertises path to    Regional network
  11000000000001)

                                                 Corporation Y
                                            (11000000000001000000)
       Longest Prefix Match
• Prefixes may overlap:
   – Example 171.69 and 171.69.10 may be found in the
     forwarding table of a single router.
   – Now, if the destination is 179.69.10.7, both the prefixes
     match !
   – Policy -- Choose the longest prefix. why ?
• Choosing the longest prefix the right choice since an
  organization may switch ISPs.




       ISP 1: 223.1.1.0                           ISP 2

                                             ISP 2 would advertise
                                             223.1.1.240
  223.1.1.240                                explicitly
    Revisiting Autonomous Systems
• ASes provide an additional way of hierarchically
  aggregating routing information in the Internet.
• AS --> also called domains and can run their own
  protocols within their administrative regimes.
• In each AS, the amount of routing information
  may be dramatically reduced by using a default
  router.
• If the corporate network is connected to the
  Internet by means of a single border router
  nodes simply send messages to this router.
  Internet View Revisited
                                      Multihomed -- no
                                      transit traffic.
                       Large corporation
                                                              ISP
                                                     “Consumer”

      Peering
      point
                         Backbone service provider        Peering
                                                          point
                 ISP
        “Consumer”


                    Large corporation               I
                                           “Consumer”SP

        Small
      corporation

Stub AS --                              Service Provider Networks
only local
traffic
          Some notation
• Peering Point: Points where different
  providers interconnect.
• Local Traffic: Traffic that begins and
  terminates on nodes within an AS.
• Transit traffic -- passes through various
  Ases -- backbones carry transit traffic.
 Exterior Gateway Protocol
• Abbreviated as EGP
  –   Hierarchical routing
  –   Reach higher level in the hierarchy
  –    Tree structure for routing topology.
  –    No peer-to-peer communications.
  Border Gateway Protocol
• Abbreviated BGP -- currently
  version 4 and is in use.
• Rather complex.
• Goals of BGP are modest:
  – Any loop free path is to be found
   between the source and destination
   (not necessarily min cost or shortest).
   Why the modest goal ?
• Each AS may have a different set of
  nodes, so it is unclear which route is the
  min-cost route!
  – Route aggregation also difficult
• Lots of routing information is required in
  order to guarantee optimality -- may be
  infeasible.
• Trust -- misconfiguration may not yield
  optimal.
                    BGP Details
• BGP supports flexibility -- paths could be chosen
  by a provider based on a policy.
• To configure BGP, each AS admin picks at least
  one node to be the “BGP” speaker -- a
  spokesperson node for the entire AS.
  –    The BGP speaker establishes a BGP session with other
      BGP speakers in other ASes.
• In addition, there are border gateways using which
  packets enter/leave ASes.
• Source advertises complete paths (unlike distance
  vector or link state routing) -- thus loops are
  prevented.
                          An Example
                                                  Customer P   128.96
                                                    (AS 4)     192.4.153
                           Regional prov ider A
                                 (AS 2)
                                                  Customer Q   192.4.32
                                                    (AS 5)     192.4.3
       Backbone network
            (AS 1)
                                                  Customer R   192.12.69
                                                    (AS 6)
                           Regional prov ider B
                                 (AS 3)
                                                  Customer S   192.4.54
                                                    (AS 7)     192.4.23

• AS 2 says 128.96, 192.4.15, 192.4.32,
  192.4.3 can be reached via AS 2.
• AS 1 advertises that these networks can be
  reached via <AS1, AS2> --note full path
  description.
• Loops are avoided.
          AS Numbers
• Each AS is assigned a number --
  16 bits and is unique.
  – The uniqueness requirement has been
   relaxed to some extent --stub ASes
   do not need to be unique.
• One could have up to 65 K AS
  numbers.
            BGP Messages
• BGP has four types of messages
  – OPEN: Establish a connection with a BGP
    peer
    • Note: BGP connection is TCP based ! (Port no.
      179).
  – UPDATE -- advertise or withdraw routes to
    a destination
    • Note --BGP speaker needs to be able to cancel
      previously advertised paths if nodes or links fail.
      This form of negative advertisements are said to
      advertise “withdrawn routes”.
     BGP Messages (cont)
• KEEPALIVE: Inform a peer that
  the sender is still alive but has no
  information to send.
• NOTIFICATION: Notify that
  errors are detected.
     BGP Message Format
• 16 byte fields.
                                  0                        15


• For more detail look at book.
                                      Unf easible routes
                                            length

• Important thing --- BGP             Withdrawn routes
                                          (v ariable)
  updates are of the type
  prefix/length
                                          Total path
                                       attribute length

  – 192.4.16/20                        Path attributes

• Note that forwarding entries
                                         (v ariable)
                                       Network lay er
  can also be similarly               reachability inf o

  represented.
                                          (v ariable)
        Routing with BGP
• For stub AS -- border router injects a
  default route into the intra-domain
  routing protocol.
• If there are more than one border
  router, each injects specific routes that
  they have learned from outside the AS.
• IBGP or Interior BGP is used to
  distribute the information to all other
  routers in the domain (and the speaker).
                 Routing Areas
• Especially used with OSPF.             Area 1
                                                                                    Area 3


• Subdomains of larger domains.
                                                                 Area 0

                                    R9            R7

• One special area called                  R8          R1
                                                                               R3
                                                                                     R4

  backbone area. (Area 0).                                        R2


• Within each area -- link state
                                                                     Area 2



  routing.                                                  R6            R5

• Link state advertisements of
  non border routers do not leave
  area.                                  • A router that is a
• Packet goes from non-backbone          member of both the
  area to backbone area and
  crosses the backbone into the
                                         backbone and a non-
  Internet.                              backbone area (R1) is
                                         called a area router.
          Areas (continued)
• Border routers “summarize” routing
  information and make it available to other
  areas -- act like proxies --reflect costs to
  reach networks from an area.
• When there are many possible routes, routers
  choose cost info to forward packets.
• Trade-offs -- Optimality versus scalability -
  - All packet have to pass through the
  backbone area (may not be optimal).
           Next Time
• IPv6
• Introduction to the transport layer.

				
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