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Inter-domain Routing. Outline Border Gateway Protocol. Internet Structure. Original idea. Backbone service provider. Consumer. ”. ISP. “. Consumer. ISP. ”. Large corporation. “. Small. “. ”. Consumer. ISP. “. ”. Consumer. ISP. corporation. Small. Small. Small.

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inter domain routing
Inter-domain Routing

Outline

Border Gateway Protocol

internet structure
Internet Structure

Original idea

Backbone service provider

Consumer

ISP

Consumer

ISP

Large corporation

Small

Consumer

ISP

Consumer

ISP

corporation

Small

Small

Small

corporation

corporation

corporation

CS 640

internet structure1

Large corporation

Consumer

ISP

Peering

point

Backbone service provider

Peering

point

Consumer

ISP

Consumer

ISP

Large corporation

Small

corporation

Internet Structure

Today

CS 640

route propagation in the internet
Route Propagation in the Internet
  • Autonomous System (AS)
    • corresponds to an administrative domain
    • examples: University, company, backbone network
    • assign each AS a 16-bit number
  • Two-level route propagation hierarchy
    • interior gateway protocol (each AS selects its own)
    • exterior gateway protocol (Internet-wide standard)
  • Routes information is propagated at various levels
    • hosts know local router
    • local routers know site routers
    • site routers know core router
    • core routers know everything

CS 640

popular interior gateway protocols
Popular Interior Gateway Protocols
  • RIP: Route Information Protocol
    • distributed with BSD Unix
    • distance-vector algorithm
    • based on hop-count (infinity set to 16)
  • OSPF: Open Shortest Path First
    • recent Internet standard
    • uses link-state algorithm
    • supports load balancing
    • supports authentication

CS 640

bgp 4 border gateway protocol
BGP-4: Border Gateway Protocol
  • BGP-1 developed in 1989 to address problems with EGP.
  • Assumes Internet is an arbitrarily interconnected set of ASs
  • AS traffic types
    • Local
      • starts or ends within an AS
    • Transit
      • passes through an AS
  • AS Types
    • stub AS: has a single connection to one other AS
      • carries local traffic only
    • multihomed AS: has connections to more than one AS
      • refuses to carry transit traffic
    • transit AS: has connections to more than one AS
      • carries both transit and local traffic

CS 640

bgp 4 contd
BGP-4 contd.
  • Each AS has:
    • one or more border routers
      • Handles inter-AS traffic
    • At least one BGP speaker for an AS that participates in routing
    • BGP speaker establishes BGP sessions with peers and advertises:
      • local network names
      • other reachable networks (transit AS only)
      • gives path information
      • withdrawn routes
  • BGP goal: find loop free paths between ASs
    • Optimality is secondary goal
    • It’s neither a distance-vector nor a link-state protocol
  • Hard problem
    • Internet’s size (~12K active ASs) means large tables in BGP routers
    • Autonomous domains mean different path metrics
    • Need for flexibility

CS 640

bgp example

128.96

Customer P

192.4.153

(AS 4)

Regional provider 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 provider B

(AS 3)

Customer S

192.4.54

(AS 7)

192.4.23

BGP Example
  • Speaker for AS2 advertises reachability to P and Q
    • network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2
  • Speaker for backbone advertises
    • networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along the path (AS1, AS2).
  • Speaker can cancel previously advertised paths

CS 640

some bgp details
Some BGP details
  • Path vectors are most important innovation in BGP
    • Enables loop prevention in complex topologies
    • If AS sees itself in the path, it will not use that path
  • Routes can be aggregated
    • Based on CIDR (classless) addressing
  • Routes can be filtered
  • Runs over TCP
  • ASes can apply a variety of policies

CS 640

bgp in practice
BGP in practice
  • 10-20 “backbone” ASs which are fairly richly connected to each other
    • Peers
  • Other “lower tier” ASs hang off the backbone networks -> Customers
    • Some of them may also connect with each other at peering points  Peers
  • Corporations connect as Customers to lower tier ASs or to backbone ASs depending on their need/willingness to pay

CS 640

policy with bgp
Policy with BGP
  • BGP provides capability for enforcing various policies
  • Policies are not part of BGP: they are provided to BGP as configuration information
  • Enforces policies by
    • Choosing appropriate paths from multiple alternatives
    • Controlling advertisement to other AS’s
illustrating bgp policies

peer

peer

provider

customer

Illustrating BGP Policies

AS 4

Frank’s

Internet Barn

AS 3

AS 2

Which route should

Frank pick to 13.13.0.0./16?

AS 1

13.13.0.0/16

policy i prefer customer routing

peer

peer

provider

customer

Policy I: Prefer Customer routing

Route learned from customer preferred over route learned from peer, preferred over route learned from provider

AS 4

local pref = 80

AS 3

local pref = 90

local pref = 100

AS 2

Set appropriate “local pref”to reflect preferences:

Higher Local preference values

are preferred

AS 1

13.13.0.0/16

policy ii import routes

provider route

peer route

customer route

ISP route

Policy II: Import Routes

From

provider

From

provider

From

peer

From

peer

From

customer

From

customer

policy ii export routes

filters

block

Policy II: Export Routes

provider route

peer route

customer route

ISP route

To

provider

From

provider

To

peer

To

peer

To

customer

To

customer

ad