04/22/23 CST 415 - Computer Networks 1

BGP

CST 415

04/22/23 CST 415 - Computer Networks 2

Topics• BGP Defined

• BGP Message Generation

• BGP Message Types

• BGP Limitations

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Problem Definition• A network may consist of many internal networks.• Each of these internal networks is connected via a

router.• The structure of the internal networks is managed

locally, known as an autonomous system.• Any internal network can be connected to an

external network.• This router at this connection point is known as a

boundary gateway.

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Problem Definition• Consider the state of the Internet if the boundary

condition did not exist.– Every router in the Internet would be required to

communicate with an arbitrary set of routers.» The propagation of route updates could overwhelm the

bandwidth of the Internet.

– For routers to communicate directly, they would all need to be connected to a single backbone network.

– Administration of a single Internet would require a single, global, administrative entity.

» This condition is in direct contradiction to the “controlled chaos” growth of the Internet.

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Problem Definition• For the global Internet to work, router interaction

must be limited.• The solution to the routing problem is to have

subsets of routers be locally managed (autonomous systems).– General heuristic

» LAN – up to 5 routers can be involved in getting a packet from source host to destination host.

» WAN - up to 12 routers can be involved in getting a packet from source host to destination host.

– In reality, this number depends on network bandwidth and utilization.

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Problem Definition• Participating Router

– A router that actively participates in a routing scheme (e.g. exchanges routing tables).

– This router must be associated with the system for which it is performing routing.

• Non Participating Router– A router not participating in a routing scheme.– This router may or may not be associated with

the system it is not routing for.

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Problem Definition

Non participating routers and the extra hop problem.

This is a problem occurs because the non-participating router does not exchange routing information with participating routers.

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Problem Definition

Hidden Networks The external network can not exchange information about internal network structure.

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Problem DefinitionThe Upshot…

Routing information must be passed:» From routers participating actively in a routing

group to routers that are passive, yet connected directly to that group.

» Information about hidden networks must be propagated from non-participating routers to routers involved in a group routing scenario.

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BGP DefinedTo effect the communication between

participating groups of routers (autonomous systems) and non-participating routers (boundary gateways), BGP has been defined.

Boundary Gateway Protocol

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BGP Defined

Boundary Gateway Protocol

Purpose - To exchange network reachability information between autonomous systems and from this information determine routes to networks.

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Problem Definition

Internal routers are shown in blue while border routers are red. BGP speakers that communicate within an AS are internal peers. Those that communicate between ASes are external peers. This highly simplified internetwork shows two stub ASes, both of which only connect to the multihomed AS #2

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BGP DefinedBoundary Gateway Protocol

• Each BGP stores information about networks and the routes to them in a set of Routing Information Bases (RIBs).

• This route information is exchanged between BGP routers, and propagated throughout the entire internetwork, allowing each AS (Autonomous System) to find paths to each others AS.

• This enables routing across the entire internetwork.

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BGP DefinedBoundary Gateway Protocol

• BGP supports an arbitrary topology of ASes.

• They can be connected in any manner.• An AS must have a minimum of one router

running BGP, but can have more than one. • BGP can be used to communicate between

BGP routers within the same autonomous system.

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BGP DefinedBGP Provides:• Inter-Autonomous System Communication –

Provide a standard mechanism for autonomous systems to exchange routing information.

• Coordination among multiple BGP speakers – Allow for boundary routers from a single autonomous to exchange routing information.

• Propagation of reachability information – Allow autonomous systems to advertise information related to internal systems as well as systems it is willing to forward packets on behalf of.

• Next-hop information – Provide information related to the “next-hop”.

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BGP DefinedBGP Provides:

• Policy support – support administrative policies related to internal autonomous system privileges as viewed by the outside world.

• Reliable transport – runs on top of TCP

• Routing path information – Provide information related to actual routes available through the autonomous system.

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BGP DefinedBGP Provides:• Incremental route updates – Only update routes

that have changed.• Classless addressing support – Provide support for

the CIDR address masking mechanism.• Route information aggregation – Allow for

multiple route information updates to be sent in a single update packet.

• Authentication – Provide for message authentication before processing.

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BGP State Transition

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BGP Message GenerationBGP peers will exchange messages for:

• Peer acquisition and authentication

• Peer exchange of positive and negative network reachability information,

• Peer network function verification.

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BGP Message TypesOpen Message:• Version – 1 octet: identification of protocol

version.• Autonomous Systems Number – 2 octets:

Identifier for the senders autonomous system.• Hold Time – 2 octets: The maximum amount of

time to wait for a message from the sender.• BGP Identifier – 4 octets: A unique identifier for

identification of the sender.• Optional parameters

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BGP Message

Update Message

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BGP Message TypesUpdate Message:• Withdrawn Length – 2 octets: number of octets for the

destinations that are being withdrawn from the source.• Withdrawn Destinations – variable: the destination

networks that the source is withdrawing from the destination routing tables.

• Path Length – 2 octets: number of octets for destination addresses that are being added to source routing.

• Path Attributes – variable: a descriptor for all destination networks in the update (e.g. list of reachable networks, list of next hops, etc.)

• Destination Networks – variable: the destination networks that are being added to the destination routing tables.

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BGP Message TypesKeepalive Message:

• Inform the receiver to not allow the TCP connection to drop.

• As long as the BGP TCP connection is “kept alive”, the BGP servers will know any new routing information will be up-to-date.

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BGP Message TypesNotification Message:

• Used to inform of error conditions in the BGP peer network.

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BGP Limitations• BGP only advertises networks that can be

reached through the BGP server’s autonomous system.

• BGP does not exchange route cost.– This makes it hard to compute a “least-cost”

route across autonomous systems.

• Because of this, the BGP router should only advertise routes that will be beneficial to outside autonomous systems.