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					        Interdomain Routing
Establish routes between autonomous systems (ASes).

                        Verizon

           AT&T                    Comcast

                      Qwest


Currently done with the Border Gateway Protocol (BGP).

                                                   1
Why is Interdomain Routing Hard?
• Route choices are based on local policies.
• Autonomy: Policies are uncoordinated.
• Expressiveness: Policies are complex.
                                               Always choose
Load-balance my                                shortest paths.
outgoing traffic.              Verizon

                    AT&T                 Comcast

                              Qwest
                                            Avoid routes
    My link to UUNET is for                 through AT&T if
    backup purposes only.                   at all possible.     2
      BGP Route Processing (1)
• The computation of a single node repeats the following:
                       Update     Choose
      Receive routes                        Send updates
                       Routing    “Best”
                                            to neighbors
      from neighbors
                        Table     Route

• Paths go through neighbors’ choices, which
  enforces consistency.
• Decisions are made locally, which preserves autonomy.
• Uncoordinated policies can induce protocol oscillations.
  (Much recent work addresses BGP convergence.)
• Recently, private information, optimization, and
  incentive-compatibility have also been studied.            3
          BGP Route Processing (2)
                                      IP Forwarding Table

                                         Install forwarding
                                         entries for best
                                         routes
          Apply Import      Routing            Best Route      Apply Export
           Policies          Table              Selection       Policies


Receive   Apply Policy =                         Based on      Apply Policy =   Transmit
          filter routes      Storage                           filter routes
BGP                                              attribute                      BGP
          & tweak            of routes                         & tweak
updates                                          values                         updates
          attributes                                           attributes

                              Open-ended programming:
                    constrained only by vendor configuration language


                                                                                    4
Example: Convergence
                       Prefer
                    direct route
                        to d


            1       2

 Prefer
  routes
through 2




                d
                                   5
                Example: Oscillation
                            Prefer
                            routes
                           through 1
                                       BGP might oscillate
            1          2                forever between
 Prefer
  routes
                                             1d, 2d
through 2                                     and
                                            12d, 21d


                   d

                                                             6
Example: Convergence
                     Prefer
                     routes
                    through 1


            1       2

 Prefer
  routes
through 2




                d
                                7
            Dispute Wheels

Nodes ui, hub routes Ri, and spoke routes Qi.
Each ui prefers RiQi+1 to Qi.


“No dispute wheel”
       —>
robust convergence



                                                8
 Gao-Rexford Framework (1)
Neighboring pairs of ASes have one of:
  – a customer-provider relationship
    (One node is purchasing connectivity from
    the other node.)
  – a peering relationship
    (Nodes have offered to carry each other’s
    transit traffic, often to shortcut a longer
    route.)
                     peer           providers



                                    customers
                      peer
                                                  9
    Gao-Rexford Framework (2)
• Global constraint: no customer-provider cycles
• Local preference and scoping constraints, which are
  consistent with Internet economics:

    Preference Constraints                          Scoping Constraints

              k1           R                          provider   j              ....
                         ...1..
                             .                  d
     i                                                               peer
                            R                              i                               d
                          . . .2. . .                                       m
                k2
                                                     customer    k
    • If k1 and k2 are both customers, peers,
      or providers of i, then either ik1R1 or       • Export customer routes to all neighbors
      ik2R2 can be more valued at i.                 and export all routes to customers.
    • If one is a customer, prefer the route        • Export peer and provider routes to
      through it. If not, prefer the peer route.      all customers only.



• Gao-Rexford conditions => BGP always converges [GR01]                                         10

				
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