Network Layer - BGPNetwork Layer - BGP

CSC/ECE 573, Sections 001, 002

Fall, 2010

Acknowledgement: Material from various sources used, notably Timothy Griffin (AT&T), Battista et al (Roma Tre U)

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Autonomous SystemsAutonomous Systems An autonomous system (AS) is a region of the Internet that is

administered by a single entity and that has a unified routing policy Each autonomous system is assigned an Autonomous System

Number (ASN).NCSU’s campus network (AS11442) BellSouth Business Systems (AS5002)…

AS numbers between 1 and 65,535 (two bytes)– Numbers greater than 64,511 are “private”

AS numbers may be requested:– Global asn – from your regional internet registry (rir): ripe, arin, apnic– Private asn – from your upstream ISP

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Interdomain and Intradomain Interdomain and Intradomain RoutingRouting

Routing protocols for intradomain routing are called interior gateway protocols (IGP)

– Objective: shortest path Routing protocols for interdomain routing are called exterior gateway protocols

(EGP)– Objective: satisfy policy of the AS

AS 6 AS 7

AS 4

AS 2 AS 5

AS 1

AS 3

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Interdomain Routing Interdomain Routing

Interdomain routing is based on connectivity between autonomous systems

Interdomain routing can ignore many details of router interconnection

AS 1 AS 2

AS 3

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Multiple Routing Protocols Multiple Routing Protocols Multiple routing protocols can

run on the same router Each routing protocol updates

the routing table

routingtable

IP Forwarding

routing tablelookup

incoming IP datagrams

outgoing IP datagrams

routing protocol

routing protocol

RIP Process

OSPFProcess

BGP Process

routing table updates

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Autonomous Systems TerminologyAutonomous Systems Terminology local traffic = traffic with source or

destination in AS transit traffic = traffic that passes through

the AS Stub AS = has connection to only

one AS, only carry local traffic Multihomed AS = has connection to >1 AS,

but does not carry transit traffic Transit AS = has connection to >1 AS

and carries transit traffic

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Stub and Transit Networks Stub and Transit Networks

AS 1, AS 2, and AS 5 are stub networks

AS 2 is a multi-homed stub network

AS 3 and AS 4 are transit networks

AS 3

AS 1

AS 4

AS 2

AS 5

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Selective TransitSelective Transit

Example: Transit AS 3 carries traffic

between AS 1 and AS 4 and between AS 2 and AS 4

But AS 3 does not carry traffic between AS 1 and AS 2

The example shows a routing policy.

AS 2AS 1

AS 3

AS 4

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Customer/ProviderCustomer/Provider

A stub network typically obtains access to the Internet through a transit network.

Transit network that is a provider may be a customer for another network

Customer pays provider for service

AS 5

AS 2

Customer/Provider

AS 6

Customer/Provider

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

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Customer/Provider and Peers Customer/Provider and Peers

Transit networks can have a peer relationship Peers provide transit between their respective customers Peers do not provide transit between peers Peers normally do not pay each other for service

AS 3

AS 5

AS 2Peers

Customer/Provider

AS 6

Customer/Provider

AS 1Peers

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

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Shortcuts through peering Shortcuts through peering

Note that peering reduces upstream traffic Delays can be reduced through peering But: Peering may not generate revenue

AS 3

AS 5

AS 2Peers

Customer/Provider

AS 6

Customer/Provider

AS 1Peers

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

Peers

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Border Gateway Protocol (BGP)Border Gateway Protocol (BGP) Border Gateway Protocol is the interdomain

routing protocol for the Internet for routing between autonomous systems

Currently in version 4 (1995)– Network administrators can specify routing policies– BGP is a distance vector protocol (However, routing

messages in BGP contain complete routes)

Uses TCP to transmit routing messages

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Border Gateway Protocol (BGP)Border Gateway Protocol (BGP) An autonomous system uses BGP to advertise

its network address(es) to other AS’s

BGP helps an autonomous system with the following:

1. Collect information about reachable networks from neighboring AS’s

2. Disseminate the information about reachable networks to routers inside the AS and to neighboring AS’s

3. Picks routes if there are multiple routes available

Who uses BGPWho uses BGP BGP is used by:

– Customers connected to an Internet Service Provider (ISP)

– Customers connected to several ISPs– Transit providers– ISPs that exchange traffic in an exchange point

(NAP)– Customers with very large networks

Two kinds of BGPTwo kinds of BGP

When several organizations join to form the Internet they have to set up links between them

e-BGP: perform interdomain communication i-BGP: allow BGP speakers of same AS to communicate to each

other (full mesh) (not IGP, uses IGP)

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BGP interactions BGP interactions Router establishes a TCP

connection (TCP port 175) Routers exchange BGP routes

– Offer connectivity – promise to deliver to destination

Periodically send updates BGP is executed between two

routers– BGP session– BGP peers or BGP

speakers Note: Not all autonomous

systems need to run BGP. On many stub networks, the route to the provider can be statically configured

AS 1

AS 2

BGP Session

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BGP interactions BGP interactions

AS 2

AS 1

AS 3

The networks that are advertised are network IP addresses with a prefix, E.g., 128.100.0.0/16

Prefixes reachable from AS 1

Prefixes reachablefrom AS 3

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BGP interactions BGP interactions BGP peers advertise

reachability of IP networks

A advertises a path to a network (e.g., 10.0.0.0/8) to B only if it is willing to forward traffic going to that network

Path-Vector:– A advertises the complete

path to the advertised network

– Path is sent as a list of AS’s

this avoids loops

A

B

Advertisepath to 10.0.0.0/24

10.0.0.0/24

BGP Peer

BGP Peer

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iBGP sessionsiBGP sessions All iBGP peers in the same

autonomous system are fully meshed

Peer announces routes received via eBGP to iBGP peers

But: iBGP peers do not announce routes received via iBGP to other iBGP peers

Update from eBGP session

AS A

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Hot Potato RoutingHot Potato Routing Router R3 in autonomous

system A receives two advertisements to network X

– Which route should it pick?

Hot Potato Rule: Select the iBGP peer that has the shortest IGP route

Analogy: Get the packet out of one’s own AS as quickly as possible, i.e., on the shortest path

Route to X

AS A

R1

Route to X

R2

Route to X Route

to X

R3

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Hot Potato RoutingHot Potato RoutingFinding the cheapest IGP

route: Compare the cost of the two

paths– R3 R1 – R3 R2

according to the IGP protocol

Here: R1 has the shortest path

Add a routing table entry for destination X

Route to X

AS A

R1

Route to X

R2

Cost=6

R3

Cost=23

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Hot Potato Routing can backfire!Hot Potato Routing can backfire! AS1 would serve its customer (source) better by

not picking the shortest route to AS 2 In fact, customer may have paid for a high-

bandwidth service!

AS 2

AS 1

Low bandwidth network

Cost=20

Destination

Source

Cost=5

High bandwidth network

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BGP Message TypesBGP Message Types Open: Establishes a peering session

Keep Alive: Handshake at regular intervals

to maintain peering session

Notification: Closes a peering session

Update: Advertises new routes or

withdraws previously announced routes. Each

announced route is specified as a network

prefix with attribute values

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Content of AdvertisementsContent of Advertisements

BGP routers advertise routes

Each route consists of a network prefix and a list of attributes that specify information about a route

Network Layer Reachability Information (NLRI)

Mandatory attributes:

ORIGIN

AS_PATH

NEXT_HOP Many other attributes

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ORIGIN attributeORIGIN attribute Originating domain sends a route with ORIGIN attribute

ORIGIN attributes also specifies if the origin is internal to the AS or not

AS 1

AS 2 AS 4

AS 5

AS 3

10.0.1.0/8, ORIGIN {1}

10.0.1.0/8, ORIGIN {1}

10.0.1.0/8, ORIGIN {1}

10.0.1.0/8, ORIGIN {1}

10.0.1.0/8, ORIGIN {1}

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AS-PATH attributesAS-PATH attributes Each AS that propagates a route prepends its own AS number

– AS-PATH collects a path to reach the network prefix Path information prevents routing loops from occurring Path information also provides information on the length of a path (By

default, a shorter route is preferred) Note: BGP aggregates routes according to CIDR rules

AS 1

AS 2 AS 4

AS 5

AS 3

10.0.1.0/8, AS-PATH {2,1}

10.0.1.0/8, AS-PATH {3,1}

10.0.1.0/8, AS-PATH {4,2,1}

10.0.1.0/8, AS-PATH {1}

10.0.1.0/8, AS-PATH {1}

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NEXT-HOP attributesNEXT-HOP attributes Each router that sends a route advertisement it includes its own IP

address in a NEXT-HOP attribute The attribute provides information for the routing table of the receiving

router.

AS 5 AS 1

AS 3

128.100.11.1 128.143.71.21

10.0.1.0/8, NEXT-HOP {128.100.11.1}

10.0.1.0/8, NEXT-HOP {128.143.71.21}

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Connecting NEXT-HOP with IGP Connecting NEXT-HOP with IGP informationinformation

AS 1

AS 3

128.100.11.1/24 192.0.1.2

eBGP

iBGP

R1IGP router

10.1.1.0/8, NEXT-HOP {128.100.11.1}

10.1.1.0/8, NEXT-HOP {128.100.11.1}

Dest. Next hop128.100.11.0/24 192.0.1.2

At R1:

Dest. Next hop10.1.1.0/8 128.100.11.1

Routing table

BGP info

Dest. Next hop128.100.11.0/24 192.0.1.2

10.1.1.0/8 192.0.1.2

Routing table

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Route SelectionRoute Selection Router may get more than one route to an address Rules for selecting a route (in order of priorities):

– Preferences can be advertised as an attribute– Shorter routes are preferred– Close next-hop is preferred

Internal data structures maintained– Local Routing Information Base– Adjacent Routing Information Base – In and Out

Multiple conditions for a route to go into Loc-RIB– Next-hop must be reachable– Only one route to each destination goes into Adj-RIB-In– If preferable over existing, goes into Loc-RIB– External route trumps all i-BGP learned routes– Contributes to main FIB

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Importing and Exporting RoutesImporting and Exporting Routes

An AS may not advertise all routes, or accept all routes that are advertised

Route policies determines which routes are filtered

BGP does not carry policies - carries information that allows local rules to make policy-based inferences

AS A desires some of exported routes to be advertised everywhere, some only in N America

ISP announces convention (outside BGP) that a BGP community identifier will be used for all advertisements meant to be global, and a different one for N America only

A attaches appropriate community attribute tags to exported routes

No enforcement or check is possible

Control Inbound

traffic

AS A

Change export rules

Change import rules

Control Outbound

traffic

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Importing and Exporting RoutesImporting and Exporting Routes

Import and export rules allow control of traffic carried by AS

If an AS wants to have less inbound traffic it should adapt its export rules

If an AS wants to control its inbound traffic, it adapts its import rules

Control Inbound

traffic

AS A

Change export rules

Change import rules

Control Outbound

traffic

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Routing PoliciesRouting Policies

Since AS 5 is a stub network it should not advertise routes to networks other than networks in AS 5

When AS 3 learns about the path {AS1, AS4}, it should not advertise the route {AS3, AS1, AS4} to AS 2.

AS 1

AS 3

AS 5

AS 2

Peers

Customer/Provider

Peers

Customer/Provider

AS 4

AS 6

Peers

Customer/Provider

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Traffic Often Follows ASPATHTraffic Often Follows ASPATH In many cases, packets

are routed according to the AS-PATH

However, in some cases this is not true

(Here: AS 2 filters routes with a long prefix)

AS 2 AS 5AS 3AS 1128.100.0.0/16

128.100.0.0/16, AS-PATH {3,2,1}

AS 2 AS 5AS 3AS 1128.100.0.0/16

128.100.0.0/16, AS-PATH {3,2,1}

AS 4

128.100.22.0/24, AS-PATH {4}

128.100.22.0/24

128.100.0.0/16, AS-PATH {1}

128.100.0.0/16, AS-PATH {2, 1}

Does not advertise /24 networks

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Short AS-PATH does not mean that route is shortShort AS-PATH does not mean that route is short

From AS 6’s perspective – Path {AS2, AS1} is short– Path {AS5, AS4, AS3,

AS1} is long

But the number of traversed routers is larger when using the shorter AS-PATH

AS 1

AS 3

AS 4

AS 5

AS 2

AS 6

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BGP Table GrowthBGP Table Growth

BGP Table Growth - 2BGP Table Growth - 2

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More DetailsMore Details Many other mechanisms

– Minimizing full mesh– Introduce route reflectors– Confederations

Issues with degrees of freedom– Multi-homed AS with multiple connections for stub sharing may routed

packets externally

ConclusionConclusion BGP Issues

– BGP is a simple protocol but it is very difficult to configure – BGP has severe stability issue due to policies BGP is known to not

converge– As of July 2010, 35,000+ AS numbers (of available 64,510) are

consumed– Better understood in detail, but global implications not always obvious– Many quirks remains in practice– http://www.computerworld.com/s/article/9182558/Research_experiment

_disrupts_Internet_for_some

http://www.cisco.com/en/US/products/products_security_advisory09186a0080b4411f.shtml

– Active area for innovation