More on Internet Routing

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More on Internet Routing

• A large portion of this lecture material comes
  from BGP tutorial given by Philip Smith from
  Cisco (ftp//ftp-eng.cisco.com/pfs/seminars/APRICO
  T2004-BGP00.pdf)

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More on Internet Routing

• What does a router do?
• Find path (routing)
• Forward packet from one interface to another
  interface (forwarding)
• Routing versus forwarding
• Routing building maps and giving directions
• Forwarding moving packet based on the directions.

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• IP Routing
• Path derived from information received from a
  routing protocol.
• Several alternative paths may exist
• Best next hop stored in forwarding table.
• Decisions are updated periodically or as topology
  changes (event driven)
• Decisions are based on
• Topology, policies and metrics (hop count,
  filtering, delay, bandwidth)

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• IP route lookup (part of forwarding)
• Based on destination address
• longest match routing
• More specific prefix preferred over less specific
  prefix.
• Example packet with destination of 10.1.1.1/32
  is sent to the router announcing 10.1/16 rather
  than the router announcing 10/8

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• Explicit versus default routing
• Default
• Simple, cheap
• Low granularity
• Explicit
• High overhead, complex, high cost, high
  granularity
• Hybrid
• Minimize overhead
• Provide useful granularity

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• Autonomous System (AS)
• Collective of networks with same routing policy
• Single routing protocol
• Usually under single ownership and administrative
  control
• Identified by the autonomous system number (ASN),
  which is a 16-bit number.

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• Border Gateway Protocol (BGP)
• Described in RFC 1771
• Routing protocol used to exchange routing
  information between networks
• The AS is BGPs fundamental operating unit
• Used to identify networks with common routing
  polocy.
• BGP-4 is the de facto standard in the Internet

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• BGP terms
• Neighbours ASs which directly exchange routing
  information
• Announce send route information to the neighbor
• Accept receive and use the information sent by a
  neighbour
• Originate insert routing information into
  external announcement
• Peers routers that can directly exchange routing
  and policy information

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Routing flow and packet flow
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• BGP basic
• Runs over TCP port 179
• Path vector protocol (distance vector algorithm
  explicit AS path)
• Two types of updates
• (Route) announcement and withdraw
• BGP update attributes
• ASPATH and some other information

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An example
3210 4210 7610
128.186.0.0/16
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• BGP general operation
• Learn multiple paths via internal and external
  BGP speakers
• Picks the best path and installs in the
  forwarding table
• Best path is sent to external BGP neighbours
• Policies applies by influencing the best path
  selection

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• Policy based routing protocol
• BGP route selection
• Prefer routes with highest local preference
• Prefer routers with shortest AS path
• Prefer routers with lowest MED value
• Prefer EBGP routes over IBGP routes
• Prefer routes via nearest IGP neighbor
• Tie breaking lowest router ID

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Policy disputes

• Global routing system never converge

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Security control plane security

• problems
• Anyone can claim ownership of NLRI
• Anyone can claim en route to NLRI