Introduction

• internet connected by thousands of ASes
• routing within AS is intradomain protocols
• between AS is controlled by BGP
• BGP policies can cause routing issues
  • route registry not scalable
• paper proposes a set of guidelines for an AS to follow in setting its routing policies without requiring coordination with other ASes

Interdomain Routing

• ASes connect at IXPs
• physical connection doesn’t imply traffic flow
• BGP speaker must decide whether it should include an advertised route and whether that route should be propagated
• An AS can control how traffic enters/exit the network by using a multiple exit discriminator value on its advertisements to other BGP nodes
• processing advertisement has 3 steps
  • import policies
  • path selection
  • export policies
• vendors and industry in general have settled on how to express routes
  • AS maintainers just need to follow guidelines
• policies can result in a “route oscillation” where routes never settle

BGP Model

• BGP system is a clustered graph
  • \[G = (N, V, A)\]
    • N is ASes
    • V is set of BGP speakers
    • E is eBGP peering sessions
  • each BGP speaker belongs to only one AS.
• Hierarchy
  • customers, providers, and peers
    • customer-provider
      • provider carries customer AS traffic
    • peer-peer
      • traffic shared between two (or more?) peer AS
• BGP export policies
  • export to provider
    • AS export routes and routes of customers, but cannot export routes learned from other providers/peers (no transit service)
  • export to customer
    • AS export routes and routes learned from providers and peers (transit service)
  • export to peer
    • AS exports routes and routes of customers, but not routes learned from other providers or peers (no transit service)
• Guidelines
  • Unconstrained P2P Agreements
    • AS should always prefer a route via a customer over a route via a provider or peer
  • Constrained P2P Relationships
    • same as above, but peer routes can have the same preference as customer routes.
• Guidelines w/ backup links
  • if the route does not container a backup link, use guidelines A/B
  • if the route contains a backup link, the local preference is the backup link preference.