Interdomain Routing and The Interdomain Routing and The Border Gateway Protocol Border Gateway Protocol

(BGP)(BGP)

Today’s Big Picture

Large ISP Large ISP

GPRS AccessNetwork

Small ISP

Stub Stub

Stub

Large number of diverse networks

Internet AS Map: caida.org

Autonomous System(AS)

• Internet is not a single network– Collection of networks controlled by different

administrations

• An autonomous system is a network under a single administrative control

• IANA– An AS owns unique IP prefixes– Every AS has a unique AS number

• ASes need to inter-network themselves to form a single virtual global network– Need a common protocol for communication

Who speaks Inter-AS routing?

R border router internal router

BGPR2

R1

R3AS1

AS2

Two types of routers Border router (Edge)Internal router (Core)

Two border routers of different ASes will have a BGP session

Autonomous Systems (ASes)

An autonomous system is an autonomous routing domain that has been assigned an Autonomous System Number (ASN).All parts within an AS remain connected.

RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System

… the administration of an AS appears to other ASes to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it.

IP Address Allocation and Assignment: Internet Registries

IANAwww.iana.org

RFC 2050 - Internet Registry IP Allocation Guidelines RFC 1918 - Address Allocation for Private Internets RFC 1518 - An Architecture for IP Address Allocation with CIDR

ARINwww.arin.org

APNICwww.apnic.org

RIPEwww.ripe.org

Allocate to National and local registries and ISPs Addresses assigned to customers by ISPs

Whois servers (AS, IP)

• http://www.ripe.net/perl/whois– AS2588

• http://ws.arin.net/cgi-bin/whois.pl– AS701

• http://www.apnic.net/apnic-bin/whois.pl– AS4808

AS Numbers (ASNs)

ASNs are 16 bit values.64512 through 65535 are “private”

• Genuity: 1 • MIT: 3• JANET: 786• UC San Diego: 7377• AT&T: 7018, 6341, 5074, … • UUNET: 701, 702, 284, 12199, …• Sprint: 1239, 1240, 6211, 6242, …• …

ASNs represent units of routing policy

Currently over 20,000 in use.

Partial View of www.cl.cam.ac.uk (128.232.0.20) Neighborhood

AS 786 ja.net

AS 1239 Sprint

AS 4373 Online Computer Library Center

Originates > 180 prefixes, Including 128.232.0.0/16

AS 3356Level 3

AS 6461AboveNet

AS 1213 HEAnet(Irish academic and research)

AS 7 UK Defense Research Agency

AS 5459 LINX

AS 702 UUNET

AS 20965 GEANT

How Many ASNs are there today?

Thanks to Geoff Huston. http://bgp.potaroo.net on October 26, 2004

18,217

12,940origin only (notransit)

IP network assignment process

IANA

RIR

Allo

catio

n

RIR

ISPA

lloca

tion

ISP

BGP

Ann

ounc

emen

t

IETF

IANA

Del

egat

ion

RIR Allocations - Current

IETF Reserved, 20.1, 8%

Multicast, 16, 6%

IANA Pool, 89, 35%

RIR Pool, 14, 5%

Assigned, 116.9, 46%Allocated

BGP Routing Table - Current

IETF Reserved, 20.1, 8%

Multicast, 16, 6%

IANA Pool, 89, 35%

RIR Pool, 14, 5%

Assigned, 42.4, 17%

Advertised, 74.5, 29%

How many prefixes today?

Thanks to Geoff Huston. http://bgp.potaroo.net on October 26, 2004

179,903

Note: numbersactually dependspoint of view…

The Gang of Four

Link State Vectoring

EGP

IGP

BGP

RIPOSPF

EIGRP

17

BGP-4• BGP = Border Gateway Protocol

• Is a Policy-Based routing protocol

• Is the de facto EGP of today’s global Internet

• Relatively simple protocol, but configuration is complex and the

entire world can see, and be impacted by, your mistakes.

• 1989 : BGP-1 [RFC 1105]– Replacement for EGP (1984, RFC 904)

• 1990 : BGP-2 [RFC 1163]

• 1991 : BGP-3 [RFC 1267]

• 1995 : BGP-4 [RFC 1771] – Support for Classless Interdomain Routing (CIDR)

The Border Gateway Protocol (BGP)

BGP = RFC 1771

+ “optional” extensionsRFC 1997 (communities) RFC 2439 (damping) RFC 2796 (reflection) RFC3065 (confederation) …

+ routing policy configurationlanguages (vendor-specific)

+ Current Best Practices in management of Interdomain Routing

BGP was not DESIGNED. It EVOLVED.

19

BGP Operations (Simplified)

Establish session on TCP port 179

Exchange all active routes

Exchange incremental updates

AS1

AS2

While connection is ALIVE exchangeroute UPDATE messages

BGP session

20

Four Types of BGP Messages

• Open : Establish a peering session.

• Keep Alive : Handshake at regular intervals.

• Notification : Shuts down a peering session.

• Update : Announcing new routes or withdrawing previously announced routes.

announcement = prefix + attributes values

BGP Attributes

Value Code Reference----- --------------------------------- --------- 1 ORIGIN [RFC1771] 2 AS_PATH [RFC1771] 3 NEXT_HOP [RFC1771] 4 MULTI_EXIT_DISC [RFC1771] 5 LOCAL_PREF [RFC1771] 6 ATOMIC_AGGREGATE [RFC1771] 7 AGGREGATOR [RFC1771] 8 COMMUNITY [RFC1997] 9 ORIGINATOR_ID [RFC2796] 10 CLUSTER_LIST [RFC2796] 11 DPA [Chen] 12 ADVERTISER [RFC1863] 13 RCID_PATH / CLUSTER_ID [RFC1863] 14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen] ... 255 reserved for development

From IANA: http://www.iana.org/assignments/bgp-parameters

Mostimportantattributes

Not all attributesneed to be present inevery announcement

Attributes are Used to Select Best Routes

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

Given multipleroutes to the sameprefix, a BGP speakermust pick at mostone best route

(Note: it could reject them all!)

23

BGP Route Processing

Best Route Selection

Apply Import Policies

Best Route Table

Apply Export Policies

Install forwardingEntries for bestRoutes.

ReceiveBGPUpdates

BestRoutes

TransmitBGP Updates

Apply Policy =filter routes & tweak attributes

Based onAttributeValues

IP Forwarding Table

Apply Policy =filter routes & tweak attributes

Open ended programming.Constrained only by vendor configuration language

Route Selection Summary

Highest Local Preference

Shortest ASPATH

Lowest MED

i-BGP < e-BGP

Lowest IGP cost to BGP egress

Lowest router ID

traffic engineering

Enforce relationships

Throw up hands andbreak ties

BGP Routing Tables

• Use “whois” queries to associate an ASN with “owner” (for example, http://www.arin.net/whois/arinwhois.html)

• 7018 = AT&T Worldnet, 701 =Uunet, 3561 = Cable & Wireless, …

show ip bgpBGP table version is 111849680, local router ID is 203.62.248.4Status codes: s suppressed, d damped, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

. . .*>i192.35.25.0 134.159.0.1 50 0 16779 1 701 703 i*>i192.35.29.0 166.49.251.25 50 0 5727 7018 14541 i*>i192.35.35.0 134.159.0.1 50 0 16779 1 701 1744 i*>i192.35.37.0 134.159.0.1 50 0 16779 1 3561 i*>i192.35.39.0 134.159.0.3 50 0 16779 1 701 80 i*>i192.35.44.0 166.49.251.25 50 0 5727 7018 1785 i*>i192.35.48.0 203.62.248.34 55 0 16779 209 7843 225 225 225 225 225 i*>i192.35.49.0 203.62.248.34 55 0 16779 209 7843 225 225 225 225 225 i*>i192.35.50.0 203.62.248.34 55 0 16779 3549 714 714 714 i*>i192.35.51.0/25 203.62.248.34 55 0 16779 3549 14744 14744 14744 14744 14744 14744 14744 14744 i. . .

Thanks to Geoff Huston. http://www.telstra.net/ops on July 6, 2001

AS Graphs Can Be Fun

27

Policy : Transit vs. Nontransit

AS 701

AS144

AS 701

A nontransit AS allows only traffic originating from AS or traffic with destination within AS

IP traffic

UUnet

Bell Labs

AT&T CBB

A transit AS allows traffic with neither source nor destination within AS to flow across the network

Customers and Providers

Customer pays provider for access to the Internet

provider

customer

IP trafficprovider customer

The “Peering” Relationship

peer peer

customerprovider

Peers provide transit between their respective customers

Peers do not provide transit between peers

Peers (often) do not exchange $$$trafficallowed

traffic NOTallowed

Peering Provides Shortcuts

Peering also allows connectivity betweenthe customers of “Tier 1” providers.

peer peer

customerprovider

Peering Wars

• Reduces upstream transit costs

• Can increase end-to-end performance

• May be the only way to connect your customers to some part of the Internet (“Tier 1”)

• You would rather have customers

• Peers are usually your competition

• Peering relationships may require periodic renegotiation

Peering struggles are by far the most contentious issues in the ISP world!

Peering agreements are often confidential.

Peer Don’t Peer

32

Policy-Based vs. Distance-Based Routing?

ISP1

ISP2

ISP3

Cust1

Cust2Cust3

Host 1

Host 2

Minimizing “hop count” can violate commercial relationships thatconstrain inter-domain routing.

YES

NO

What is Routing Policy

• Policy refers to arbitrary preference among a menu of available routes (based upon routes’ attributes)– Public description of the relationship between external BGP

peers– Can also describe internal BGP peer relationship

• Eg: Who are my BGP peers• What routes are

– Originated by a peer– Imported from each peer– Exported to each peer– Preferred when multiple routes exist

• What to do if no route exists?

Routing Policy Example

• AS1 originates prefix “d”

• AS1 exports “d” to AS2, AS2 imports

• AS2 exports “d” to AS3, AS3 imports

• AS3 exports “d” to AS5, AS5 imports

Routing Policy Example (cont)

• AS5 also imports “d” from AS4

• Which route does it prefer?– Does it matter?– Consider case where

• AS3 = Commercial Internet

• AS4 = Internet2

Import and Export Policies

• Inbound filtering controls outbound traffic– filters route updates received from other peers– filtering based on IP prefixes, AS_PATH, community

• Outbound Filtering controls inbound traffic– forwarding a route means others may choose to

reach the prefix through you– not forwarding a route means others must use

another router to reach the prefix

• Attribute Manipulation– Import: LOCAL_PREF (manipulate trust)– Export: AS_PATH and MEDs

37

ASPATH Attribute

AS7018135.207.0.0/16AS 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

In fairness: could you do this “right” and still scale?

Exporting internalstate would dramatically increase global instability and amount of routingstate

Shorter Doesn’t Always Mean Shorter

AS 4

AS 3

AS 2

AS 1

Mr. BGP says that path 4 1 is better than path 3 2 1

Duh!

Routing Example 1

Routing Example 2

Tweak Tweak Tweak (TE)

• For inbound traffic– Filter outbound routes– Tweak attributes on

outbound routes in the hope of influencing your neighbor’s best route selection

• For outbound traffic– Filter inbound routes– Tweak attributes on

inbound routes to influence best route selection

outboundroutes

inboundroutes

inboundtraffic

outboundtraffic

In general, an AS has morecontrol over outbound traffic

42

LOCAL PREFERENCE

AS 1AS 2

AS 4

AS 3

13.13.0.0/16

local pref = 80

local pref = 100

local pref = 90

Higher Localpreference valuesare more preferred

Local preference used ONLY in iBGP

43

Implementing Backup Links with Local Preference (Outbound Traffic)

Forces outbound traffic to take primary link, unless link is down.

AS 1

primary link backup link

Set Local Pref = 100for all routes from AS 1 AS 65000

Set Local Pref = 50for all routes from AS 1

We’ll talk about inbound traffic soon …

44

Multihomed Backups (Outbound Traffic)

Forces outbound traffic to take primary link, unless link is down.

AS 1

primary link backup link

Set Local Pref = 100for all routes from AS 1

AS 2

Set Local Pref = 50for all routes from AS 3

AS 3provider provider

45

ASpath prepending

AS 1

192.0.2.0/24ASPATH = 2 2 2 2 2 2 2 2 2 2 2 2 2 2

customerAS 2

provider

192.0.2.0/24

192.0.2.0/24ASPATH = 2

AS 3provider

Padding in this way is oftenused as a form of loadbalancing

backupprimary

46

COMMUNITY Attribute to the Rescue!

AS 1

customerAS 2

provider

192.0.2.0/24

192.0.2.0/24ASPATH = 2

AS 3provider

backupprimary

192.0.2.0/24ASPATH = 2 COMMUNITY = 3:70

Customer import policy at AS 3:If 3:90 in COMMUNITY then set local preference to 90If 3:80 in COMMUNITY then set local preference to 80If 3:70 in COMMUNITY then set local preference to 70

AS 3: normal customer local pref is 100,peer local pref is 90

BGP Summary

• BGP4 is the protocol used on the Internet to exchange routing information between providers, and to propagate external routing information through networks.

• Each autonomous network is called an Autonomous System.

• ASs which inject routing information on their own behalf have ASNs.

BGP Peering

• BGP-speaking routers peer with each other over TCP sessions, and exchange routes through the peering sessions.

• Providers typically try to peer at multiple places. Either by peering with the same AS multiple times, or because some ASs are multi-homed, a typical network will have many candidate paths to a given prefix.

The BGP Route

• The BGP route is, conceptually, a “promise” to carry data to a section of IP space. The route is a “bag” of attributes.

• The section of IP space is called the “prefix” attribute of the route.

• As a BGP route travels from AS to AS, the ASN of each AS is stamped on it when it leaves that AS. Called the AS_PATH attribute, or “as-path” in Cisco-speak.

BGP Route Attributes

• In addition to the prefix, the as-path, and the next-hop, the BGP route has other attributes, affectionately known as “knobs and twiddles” -– weight, rarely used - “sledgehammer”– local-pref, sometimes used - “hammer”– origin code, rarely used– MED (“metric”) - a gentle nudge