courtesy: nick mckeown, stanford umar kalim, niit 1 border gateway protocol tahir azim

Courtesy: Nick McKeown, StanfordUmar Kalim, NIIT

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Border Gateway Protocol

Tahir Azim


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Outline

Techniques Flooding Distributed Bellman Ford Algorithm Dijkstra’s Shortest Path First Algorithm

Routing in the Internet Hierarchy and Autonomous Systems Interior Routing Protocols: RIP, OSPF Exterior Routing Protocol: BGP

Multicast Routing


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Routing in the Internet

The Internet uses hierarchical routing The Internet is split into Autonomous Systems

(AS’s) Examples of AS’s: PTCL (17557), HP (71), MCI Worldcom (17373),

Sprint (1239) Try: whois –h whois.arin.net “MCI Worldcom”

Within an AS, the administrator chooses an Interior Gateway Protocol (IGP) Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247).

Between AS’s, the Internet uses an Exterior Gateway Protocol AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771)


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Routing in the Internet

Routing in the Internet Autonomous System ‘A’AS ‘B’AS ‘C’Stub NetworkTransit NetworkStub NetworkInterior Gateway Exterior GatewayProtocolProtocolInterior Gateway ProtocolInterior Gateway Protocol

Stub AS Transit ASe.g. backbone service provider

Stub AS

AS ‘A’ AS ‘B’ AS ‘C’

Interior GatewayProtocol



BGP BGP


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Routing within a Stub AS

There is only one exit point, so routers within the AS can use default routing. Each router knows all Network IDs within AS. Packets destined to another AS are sent to the

default router. Default router is the border gateway to the next AS.

Routing tables in Stub AS’s tend to be small.


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Interior Routing Protocols RIP

Uses distance vector (distributed Bellman-Ford algorithm). Updates sent every 30 seconds. No authentication. Originally in BSD UNIX. Widely used for many years; not used much anymore.

OSPF Link-state updates sent (using flooding) as and when

required. Every router runs Dijkstra’s algorithm. Authenticated updates. Autonomous system may be partitioned into “areas”

corresponding to internal networks. Widely used.


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Exterior Routing Protocols

Problems: Topology: The Internet is a complex mesh of different

AS’s with very little structure. Autonomy of AS’s: Each AS defines link costs in

different ways, so not possible to find lowest cost paths.

Trust: Some AS’s can’t trust others to advertise good routes (e.g. two competing backbone providers), or to protect the privacy of their traffic (e.g. two warring nations).

Policies: Different AS’s have different objectives (e.g. route over fewest hops; use one provider rather than another).


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Border Gateway Protocol (BGP-4)

BGP is not a link-state or distance-vector routing protocol. Instead, BGP uses “Path vector”

BGP advertises complete paths (a list of AS’s). Also called AS_PATH (this is the path vector) Example of path advertisement:“The network 171.64/16 can be reached via the path {AS1, AS5, AS13}”.

Paths with loops are detected locally and ignored. Local policies pick the preferred path among

options. When a link/router fails, the path is “withdrawn”.


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Customers and Providers

Customer pays provider for access to the InternetCustomer may not always need BGP

provider

customer

IP trafficprovider customer


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Customer-Provider Hierarchy

IP trafficprovider customer


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The Peering Relationship

peer peer

customerprovider

Peers provide transit between their respective customers

Peers generally do not provide transit between peers A->C routing must occur through a provider

Peers (often) do not exchange $$$

trafficallowed

traffic NOTallowed

A B C


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Open : Establish a BGP session.

Keep Alive : Handshake at regular intervals.

Notification : Shuts down a peering session.

Update : Announcing new routes or withdrawing previously announced routes.

Attributes include: Next hop, AS Path, local preference, Multi-exit discriminator (MED), … Used to select among multiple options for paths

BGP messages exchanged using TCP

BGP announcement = network prefix + path attributes

BGP Messages


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BGP Route Selection Summary

Highest Local Preference

Shortest ASPATH

Lowest MED

Lowest IGP cost to BGP egress

Lowest router ID

traffic engineering

Enforce relationshipsE.g. prefer customer routes over peer routes

Throw up hands andbreak ties

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ASPATH Attribute

AS 7018135.207.0.0/16AS Path = 6341

AS 1239Sprint

AS 1755Ebone

AT&T

AS 3549Global Crossing

135.207.0.0/16AS Path = 7018 6341

135.207.0.0/16AS Path = 3549 7018 6341

AS 6341

135.207.0.0/16

AT&T Research

Prefix Originated

AS 12654RIPE NCCRIS project

AS 1129Global Access

135.207.0.0/16AS Path = 7018 6341

135.207.0.0/16AS Path = 1239 7018 6341

135.207.0.0/16AS Path = 1755 1239 7018 6341

135.207.0.0/16AS Path = 1129 1755 1239 7018 6341

Pick shorter AS path

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Which route shouldFrank pick to 13.13.0.0./16?

AS 1

AS 2

AS 4

AS 3

13.13.0.0/16

Frank’s Internet Barn

peer peer

customerprovider

So Many Choices…

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AS 1AS 2

AS 4

AS 3

13.13.0.0/16

local pref = 80

local pref = 100

local pref = 90

Set appropriate “local pref”to reflect preferences:Higher Local preference valuesare preferred

Frank’s Choices…

peer peer

customerprovider

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


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Prompt> whois -h whois.apnic.net AS17577

as-block: AS17408 - AS18409 descr: APNIC ASN block remarks: These AS numbers are further assigned by

APNIC remarks: to APNIC members and end-users in the APNIC

region admin-c: HM20-AP tech-c: HM20-AP mnt-by: APNIC-HM mnt-lower: APNIC-HM changed: [email protected] 20060517 changed: [email protected] 20071219 source: APNIC

aut-num: AS17557 as-name: PKTELECOM-AS-AP descr: Pakistan Telecom descr: ITI Region PTCL country: PK import: from AS5400 action pref=185; accept ANY import: from AS1239 action pref=185; accept ANY export: to AS5400 announce AS-SETPTCL export: to AS1239 announce AS-SETPTCL admin-c: MH49-AP tech-c: AB242-AP mnt-by: MAINT-PK-ARSHAD mnt-routes: MAINT-PK-ARSHAD changed: [email protected] 20060805 source: APNIC

person: Mansoor ul Hassannic-hdl: MH49-APe-mail: [email protected]: 2nd Floor, International Gateway Exchange Building,address: PTCL Complex, 7th Road, Satellite Town Rawalpindiaddress: Pakistanphone: +92-51-4434470fax-no: +92-51-45-0647country: PKchanged: [email protected] 20060806mnt-by: MAINT-PK-ARSHADsource: APNIC

person: Arshad Buttnic-hdl: AB242-APe-mail: [email protected]: ITI Hall ,Telcom Complexaddress: S/Town Exchange,7th roadaddress: Rawalpindi.phone: +92-51-4455467fax-no: +92-51-4456747country: PKmnt-by: MAINT-PK-ARSHADchanged: [email protected] 20060805source: APNIC


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Who owns an address block?prompt> whois 216.35.221.77

OrgName: SavvisOrgID: SAVVI-2Address: 3300 Regency

ParkwayCity: CaryStateProv: NCPostalCode: 27511Country: US

ReferralServer: rwhois://rwhois.exodus.net:4321/

NetRange: 216.32.0.0 - 216.35.255.255

CIDR: 216.32.0.0/14NetName: SAVVISNetHandle: NET-216-32-0-0-1Parent: NET-216-0-0-0-0NetType: Direct AllocationNameServer: DNS01.SAVVIS.NETNameServer: DNS02.SAVVIS.NETNameServer: DNS03.SAVVIS.NETNameServer: DNS04.SAVVIS.NETComment:RegDate: 1998-07-30Updated: 2004-10-07

# ARIN WHOIS database, last updated 2005-01-17 19:10

# Enter ? for additional hints on searching ARIN's WHOIS database.


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courtesy: nick mckeown, stanford umar kalim, niit 1 border gateway protocol tahir azim

Documents

stanford umar kalim

nick mckeown

algorithm routing

exterior gateway protocol

default routing

border gateway protocol

bgp multicast routing

ospf exterior routing